Midwest States Pooled Fund Program Consulting Quarterly Summary

Midwest Roadside Safety Facility

10-01-2010 to 12-31-2010


Guardrail Placement in a Cut Area

Question
State: IL
Date: 10-04-2010

Here is a question that has been posed to me by a designer in one of our Districts. They want to minimize cut of the existing back slope (virtually not touch it), while squeezing in a curb and guardrail along the roadway. This results in the earth slope rising steeply behind the curb and within the deflection space of the guardrail system.

I am suggesting to them to use a concrete barrier, if a barrier is needed here.

However, can you see any way the guardrail might work? I think this probably is not possible because the increased embedment of the posts would lead us to a shorter post in order to compensate for the increased fill. However, there is not room for deflection before encountering the back slope and the deflecting system will be interfered with by that slope. Also, the vehicle itself will encroach into the back slope area, contributing lifting and/or snagging potential.

Attachment: https://mwrsf-qa.unl.edu/attachments/fa18d95533e003dd793697416c139003.jpg


Response
Date: 10-05-2010
Both the 1:1 and 1.5:1 cut or back-slopes on the upper side of the road would be potentially hazardous and provide an increased propensity for impacting vehicles to climb the unprotected slope and result in vehicle rollover. As such, your group has accurately identified the need to shield the hazard if it cannot be removed, flattened, etc. assuming traffic volumes, speeds, other factors, etc. warrant shielding it.

Placing a standard MGS directly in front of the steep slope would result in the impacted vehicles contacting the slope under the rail as the barrier deformed backward. The guardrail system would likely be more stiff as the built-up soil would provide increased soil resistance for the steel posts in addition to that already provided by the increased fill height located behind the curb section. The back side of the guardrail system would also likely make contact with the back-slope as it deformed during the high-energy impact event.

Although there would exist the possibility for this system to perform in an acceptable manner, full-scale testing would likely be needed to demonstrate satisfactory performance for the MGS with a back-slope starting under the rail and at the post locations. If 12 in. of clear and level terrain (33 in. from rail face) were provided behind the posts, I think the system would likely perform in an acceptable manner with the adjacent 1:1 back-slope shown in the plans.

Unfortunately, it does not appear as though the clear and level terrain can be provided behind the guardrail system. For such situations, it may be necessary to utilize a more rigid barrier system at the base of the back-slope.


Response
Date: 06-11-2013
Could you provide guidance for flatter backslopes?  Would the same 12-inch offset behind the posts be recommended for 3:1, 4:1, 6:1, 8:1 backslopes?  Or at some degree of slope, could the toe of the slope be located closer to the post or face of rail?

Response
Date: 09-12-2014


Wood Blockout Splitting

Question
Date: 10-04-2010

On a recent contract we received a concern from the field because brand new 12" wood blockouts have significant splitting. Some of the blocks have splits on each side of the block that almost meet in the middle.

The question is whether or not this will affect the performance of the block or is it essentially the same as using 2 blocks to achieve the 12" dimension? It would seem that once the blocks are in place and clamped between the rail and post they cannot go anywhere.

Should these blocks be rejected?

Is there any concern that they will not perform as expected?

has this come up as an issue in the past?

Is there an acceptable amount of splitting that can be allowed?


Attachment: https://mwrsf-qa.unl.edu/attachments/50b45ec383f53e154811a6d66ae699c4.JPG

Attachment: https://mwrsf-qa.unl.edu/attachments/2c55bf64be1403080056f676f80a1980.JPG

Attachment: https://mwrsf-qa.unl.edu/attachments/eace217f3e8c2898034ed6fb3fc4d64c.JPG


Response
Date: 10-08-2010
I do not believe that the noted checking/splitting within the timber blockouts is a major concern in terms of guardrail performance. The primary load direction is compression from the rail being pushed back toward the blocks, then posts. The cracking should not significantly affect this behavior. However, for timber blocks located upstream and downstream from the impacted region, the tensile action will pull the rail in front of the blocks, thus causing a twisting action for the posts and blocks. This tensile load in rail will accentuate blockout fracture away from the impact region in blocks with significant cracking on their side faces. Thus, more blockout damage may occur during vehicular impacts, but this compounded damage is not believed to degrade barrier performance within the impacted region.

I have also sent copies of the supplied photos to my contact at the Forest Products Laboratory. I have yet to hear back from him regarding this request.



FHWA Application and Installation of Roadside Hardware Memo 10-1-10

Question
State: WI
Date: 10-04-2010

FHWA has sent out a memo titled: "Application and installation of Roadside Hardware".

My first scan of the document found some things that I have concerns on . For an example:

Q: OUR GUARDRAIL CROSSES A CULVERT AND WE CAN'T DRIVE A POST. CAN WE OMIT THE POST?

A: The Midwest Guardrail System (31-inch rail height) has been successfully tested with three posts omitted, leaving a span of 25 feet. Special posts are used at either end of the gap but the rail does not have to be doubled up, or "nested" over the gap. Standard strong-post w-beam rail (minimum 27-3/4 inch rail height) can be installed with one or two posts omitted but the rail needs to be nested across the gap as well as up- and down-stream from the gap.

What the FHWA memo leaves out is that special post and grading is also needed on the standard beam guard installation when a long-span system is in use.

There are some other things also listed in this memo that I have questions on.

If MwRSF could review the document an provide comments it would be greatly appreciated.

Attachment: https://mwrsf-qa.unl.edu/attachments/2af5abe0ace47231e56add4da2228fb2.pdf


Response
Date: 10-07-2010

I have reviewed the email memo that you attached from FHWA regarding commonly asked safety hardware questions and FHWA's response. I would agree that some of the information in the memo is incomplete or misleading. I have copied items that were needed addressing below along with comments in red.

Q: WHEN CAN I USE A NON-REDIRECTIVE CRASH CUSHION?

A: Care must be used in applying a non-redirecting, gating crash cushion. They are designed to decelerate a vehicle impacting head-on on the nose. Vehicle penetration is likely to occur for angle hits from the nose to near the mid-point of the array. Vehicle penetration / override of the system is possible for high speed, high angle impacts near the rear of the device.

All gating, non-redirective crash cushions should be applied to hazards that are not likely to be impacted at an angle on the side at any significant velocity. They are appropriate on low speed facilities, and in work zones with higher speeds where lane widths are constrained and the potential for high angle hits is limited. Potential problems with these non-redirecting attenuators include vaulting over the nose of the attenuator into the work area, and inadequate clear run out areas behind the devices. All users of these devices should be made aware of the factors that contribute to proper performance as outlined in the crash test report. Examples of

non-redirecting, gating crash cushions include all sand barrel arrays, the Triton CET (Concrete End Treatment) and the ABSORB 350 (which was specifically designed for use with the Quickchange Moveable Barrier.)

It should also be noted that non-redirective crash cushions such as sand barrel arrays can pose a hazard if impacted in the reverse direction on the heavy barrels adjacent to the rigid hazard. Impact in the reverse direction at this point in the array is untested and the large mass of the final barrels could cause rapid and violent deceleration of the impacting vehicle that would exceed our occupant risk limits.

Q: WHY IS THE W-BEAM CONSTRUCTION TOLERANCE NOW ONLY ONE INCH?

A: Crash testing has shown that the standard strong post w-beam guardrail without rub rail is acceptable in the range from 27-3/4 inches to 30 inches above the ground. When the rail was tested at a lower height the pickup truck vaulted over the rail. A taller rail without rub rail can cause significant wheel snagging on small cars. This leaves a very narrow range of installation heights, and FHWA recommended 29 inches +/- one inch.

The Midwest Guardrail System (MGS) tolerance is greater at +/- 3 inches. The MGS was initially tested at its design height of 31 inches with 12-inch blockout with no rub rail. It was known that the performance would be acceptable down to 27-3/4 inch just like the G4(1S) but we wanted to encourage the taller initial height so we recommended a construction tolerance of just one inch. A subsequent crash test (in July 2010) of the MGS at a height of 34 inches using the small passenger car was successful, and now validates the MGS tolerance is plus or minus 3 inches.

The height tolerance for the MGS cannot be listed as + 3" at this time. As you noted we have conducted testing at 34" that worked with the small car. In addition, we have recently conducted an acceptable small car test at 36" top of rail height. While this would suggest that there is potential for safe application of the MGS at higher rail heights, there are still some issues to resolve before we would recommend the upper tolerance higher than 1". First, we have not tested this system with the 2270P vehicle. While we believe that the higher guardrail heights can contain the 2270P vehicle, we do not have full-scale testing to verify this, nor do we k now what effect the higher rail heights would have on the working width and deflections of the system. Second, by raising the rail height, we significantly change the loading of the end anchorages in the system. The increased height changes the angle of the cable anchorage and can affect system performance. This effect was noted in the development of the MGS system when we first tested with a 2000P at the 31" height. Thus, in order to allow the MGS system to be used at higher heights would require analysis of the effects on the anchorage system, potential redesign of the end anchors, and full-scale testing.

Based on these concerns, we would not recommend a top end tolerance of more that 1" until such time as we can more fully research the 2270P impact and conduct full-scale testing.

Q: HOW DO WE HANDLE THE HEIGHT TRANSITION BETWEEN G4(1S) AND MGS AND THEIR TERMINALS?

A: You should transition from a 27-3/4 inch tall barrier or terminal to a 31-inch tall barrier over the span of two 12-foot, 6-inch pieces of w-beam rail. When replacing or repairing long portions of a damaged rail the new rail should be installed at the proper design height, transitioning down to the existing rail over the length of two 12 foot, six inch, pieces of rail at either end. W-Beam to Thrie-Beam bridge transitions may need to use the non-symmetric W-to-Thrie connector that keeps the top height of the entire rail at approximately 31 inches.

It should be noted that there is no need to transition in height to a 27 ¾" high terminal design. The SKT, FLEAT, and ET end terminals have all been tested and approved at the 31" rail height and provide the benefits of 31" guardrail without transitioning in height down to a lower system.

Q: OUR GUARDRAIL CROSSES A CULVERT AND WE CAN'T DRIVE A POST. CAN WE OMIT THE POST?

A: The Midwest Guardrail System (31-inch rail height) has been successfully tested with three posts omitted, leaving a span of 25 feet. Special posts are used at either end of the gap but the rail does not have to be doubled up, or "nested" over the gap. Standard strong-post w-beam rail (minimum 27-3/4 inch rail height) can be installed with one or two posts omitted but the rail needs to be nested across the gap as well as up- and down-stream from the gap.

The FHWA memo is unclear as to the required details for the MGS long-span and standard W-beam long-span systems. For MGS, three CRT wood posts are required adjacent to the unsupported length. For the standard W-beam system with long-span, three CRT wood posts are also required along with 100 ft of nested W-beam. Both systems work with three posts omitted over the culvert length! No comment was provided as per the lateral placement of the posts/rail relative to the face of the culvert headwall. The MGS system is allowed to be placed closer to the headwall than the nested W-beam long span system.

Q: CAN WE PAVE A MOW STRIP UNDER OUR GUARDRAIL?

Q: CAN WE PLACE GUARDRAIL POSTS IN A CONCRETE SIDEWALK OR MEDIAN?

A: A two-inch thick asphalt pavement should not adversely affect the crash performance of w-beam guardrails as it will break up when the post moves backwards in the soil. Concrete under the guardrail would have to be constructed with a gap behind the post and backfilled with a loose material to allow the post to move when the rail is struck. There are also various commercial products that can be placed under the w-beam to block weeds. Check with the manufacturer to see that they have designed the product with post deflection in mind.

TTI has conducted a considerable amount of research into the development of safe and effective mow strip designs. There reports (FHWA/TX-04/0-4162-2 and 405160-14-1) contain the best current guidance for installation of posts in mow strips and concrete surfaces.

Previous research by MwRSF and TTI has suggested that installation of posts in concrete is not safe. Further, installation of posts in asphalt, as recommended above, is not recommended due to the expected increase in the forces required to rotate the post in the soil and develop the proper energy absorption by the post. This is especially critical for wood post systems because the wood posts would have a tendency to fracture and absorb very little energy. TTI conducted limited testing of posts in asphalt and found that it was not a suitable material for placing post in.

Q: MANY OF OUR GUARDRAIL TERMINALS HAVE A STEEL BEARING PLATE ON THE FIRST POST THAT SOMETIMES ROTATES UNTIL IT IS UPSIDE-DOWN. IS THIS OK?

A: No. This bearing plate (8 x 8-inch square with an off-center hole) must be installed with the longer dimension upright (5" dimension up and the 3" dimension down). If the cable slackens over time traffic vibrations may allow this plate to rotate downward due to gravity. If this happens the ability of post #1 to fracture in a head-on impact (thus preventing a snag point) is severely compromised. On wood posts, a nail can be driven to prevent this rotation. A solution that works on both wood and steel breakaway posts is to specify that this steel plate be fabricated with tabs on either side that will wrap around the side of the post an inch or so to prevent rotation. This is an acceptable modification to all crashworthy terminals that use this 8 x 8-inch bearing plate. Of course, it is still critical to install the bearing plate with the 5" dimension up and the 3" dimension down.

The statement above suggests that the bearing plate in question serves to facilitate the fracture of the first post in the anchorage. This is NOT the function of the bearing plate. The bearing plate functions to transfer longitudinal loads from the rail to the end anchorage to develop tension in the guardrail for redirective impacts near the terminal end. It serves no purpose in the fracture of the first post.

Q: WHAT KIND OF FOUNDATION DO WE NEED FOR OUR CONCRETE MEDIAN BARRIER?

Many variations exist between highway agencies regarding reinforcing and footing details for concrete median barriers; however there have been few reported problems with any particular design and a need for a standard detail is not apparent. Research by the California Department of Transportation has shown that a concrete footing is not necessary; the concrete can be cast directly on asphaltic concrete, Portland cement concrete, or a well-compacted aggregate base.

The statement above is misleading in that it considers only foundation design (or lack of it) with no regard to the barrier design. Concrete median barriers develop loads as a function of the barrier capacity and the foundation capacity. While it is true that some median barrier designs have been show to work with minimal foundation design, this does not suggest that any median barrier design can be installed in this manner. Thus, it falls on the designer to consider the combination of barrier and foundation that meets the design impact loading safely.

Q: SHOULD WE USE BREAKAWAY BASES FOR SIGN AND LIGHT POLES MOUNTED ON CONCRETE MEDIAN BARRIERS?

A: No, breakaway bases should not be used. Mounting any pole on top of a median barrier should be avoided because trucks will lean over the barrier upon impact and hit whatever is on top. A rigid pole may or may not break off, but there is no safety advantage in making it easier for the pole to break away and fly into the opposing travel lanes.

The potential for a pole being struck by the box of the truck can be minimized by making the barrier wider. If you transition to a vertical face and/or taper the width of the barrier you can provide additional offset to the pole. The point is to minimize the potential for broken poles to fly into the opposite roadway. Work zone signs may be mounted on barriers if you use roll up signs on fiberglass supports as they have less potential for causing serious damage.

In addition to the concerns for the impact of large truck boxes on sign and light poles mounted on median barriers, there are further concerns regarding the Zone Of Intrusion (ZOI) for small cars and pickup trucks as well as concerns regarding occupant head ejection from the vehicle that may impact such devices. Thus, these devices mounted on median barriers may pose a significant risk to passenger vehicles a well.

Q: WE WANT TO ADD LIGHTS, A BATTERY, AND A SOLAR PANEL TO OUR SCHOOL ZONE SIGN. DOES THE COMBINATION HAVE TO BE CRASH TESTED?

A: There are four factors that determine the acceptability of breakaway supports:

1) Stub height (Must be 4 inches or less. As this will not change with the addition of ITS hardware it will not be discussed further.)

2) Vehicle velocity change / occupant impact forces

3) Windshield penetration

4) Roof crush

2) The addition of flashing lights and solar panels or other ITS equipment will not likely affect the change in velocity experienced by the vehicle or its occupants unless it becomes substantial compared to the mass of the pole. Additional hardware attached at or above the sign will raise the center of gravity of the system slightly but since it is away from the base the breakaway features will still perform as intended. The overall mass of the pole, sign, and auxiliary equipment should not exceed 600 pounds.

3) Windshield damage was not a formal pass/fail criterion under the 1985 AASHTO Sign and Luminaire spec and we did not change this when we adopted Report 350 in 1994. However, windshield damage will be pass/fail evaluation criteria under the AASHTO MASH. If the auxiliary hardware is at or above the sign, the effect should be minimal.

NCHRP 350 does include windshield damage in the evaluation of signs. The guidance in NCHRP 350 is somewhat subject and not rigorously defined, but it is an evaluation criteria and should be considered when evaluating sign performance under NCHRP 350.

Safe placement of these types of devices on the sign depend on more than placing the hardware at or above the sign. It would also depend on the structure of the sign, the sign height, the type of vehicle impacting the sign, and the deformation or breakaway of the sign support when it is impacted. Thus, effective placement of the auxiliary hardware on the sign would require further analysis than simply placing the hardware at or above the sign.

4) Roof crush up to 5 inches was permitted under NCHRP Report 350, but very few sign installations even approached that amount. (Luminaire poles weighing 1000# or more could easily fail this test.) The addition of more hardware could increase the risk under low speed impacts, but roof crush can be controlled by following the 600 pound weight limit mentioned above. Under MASH, roof crush will be limited to 3 inches maximum.



H-Barrier

Question
State: OH
Date: 10-07-2010

ODOT is talking about making this design a standard in MOT situations on structures that have limited cross sections.
This predates me as a member of the pooled fund, but this barrier looks promising to us.
Can you provide us with any addition information and the history of this project?
Did you select this width because of the availability of H-piles?
Do you think the width of this barrier could be reduced even more?
Do you know any state are using this design?

http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware//barriers/pdf/b117.pdf





Response
Date: 10-08-2010

From my recollection, MwRSF crash tested a free-standing version of Iowa's Steel H-Pile Temporary Barrier system in the late 80s. The Iowa DOT developed the H-Pile system and then hired MwRSF to conduct the compliance testing according to the AASHTO Guide Specifications for Bridge Railings. As I recall, one pickup truck test was successfully performed at the target impact conditions of 60 mph and 20 degrees. The joint detail between sections was both cumbersome but very strong. Steel angled plates were used to interconnect the sections using a large number of bolts which did not allow for much construction tolerance for misalignment and/or uneven surfaces. However, the joint detail did transfer load well across the joints to adjacent barrier segments. For free-standing applications, this barrier system with rigid joint detail resulted in dynamic barrier deflections of approximately 18 in.

Many years later, the IA DOT and the Pooled Fund program had MwRSF do some follow-on research with this general barrier system. This later study included a simplification of the joint detail as well as the incorporation of a tie-down system, then followed by full-scale crash testing according to NCHRP Report No. 350. Although the joint detail was simplified, the full-scale crash tests were only performed for the tied-down design variation. Upon completion of the testing, the modified design was shown to greatly reduce barrier deflections when subjected to a 2000P pickup truck impact at the target conditions of 62 mph and 25 degrees.

Two research reports have been prepared over the years to document the findings noted above. If desirable, MwRSF could send electronic copies of these to you. The later report would likely have been sent to Dean Focke shortly after the project was completed.

I am only aware of this barrier system being used in the State of Iowa. In order to obtain further information of its use, I have copied this email to our colleagues within the Iowa Department of Transportation with the hope that further light could be shed on its current use.

As noted previously, the sizing of the steel H-sections was made by the Iowa DOT. As I recall, the system width was approximately 14 in. wide. Thus, I am reasonably confident that a new or revised steel system (possibly with concrete ballast similar to this design) could be developed with a system width 1 to 2 in. narrower than used in the current Iowa design.



MGS Installed on 2:1 Slopes

Question
State: WI
Date: 10-01-2010

I've been working on developing WisDOT's standard detail drawings for the MGS system and I have a question. I was reviewing MwRSF's crash test report, "Development and Evaluation of the Midwest Guardrail Systems (MGS) Placed Adjacent to a 2:1 fill Slope" (TRP-03-185-10).

In this report, MGS with standard post spacing, 6' 4" post embedment was tested at two rail heights (27 ¾" and 31"). The 31" height passed MASH, but the 27 ¾" rail height failed MASH.

What would be the lower height limit for MGS on a 2:1 slope? What modifications should a designer do a MGS system with standard post spacing and 6'4" of embedment that has had the 31" rail mounting height lessened by overlays (e.g. install a normal length post at the midspan)?



Response
Date: 10-08-2010
Initially, the development and crash testing program began with the plan to demonstrate satisfactory safety performance for the MGS installed on 2:1 fill slopes using the same lower height tolerance obtained for standard MGS. After the first test failed MASH test designation 3-11, we chose to then re-test the modified barrier system at the same nominal height used for the standard MGS. Although we believe that the MGS installed on 2:1 fill slopes would perform in an acceptable manner at heights below 31 in. (such as for 29 or 30 in.), we are unable recommend these lower height tolerances without first demonstrating acceptable safety performance through the use of full-scale vehicle crash testing.


For situations where pavement overlays are placed adjacent to the MGS installed on 2:1 fill slopes, one possible option would include raising the guardrail post and rail height by an amount approximately equal to the thickness of the adjacent pavement overlay.


We have discussed options for your prior question regarding overlays next to MGS for 2:1 fill slopes. Our options are listed below.


(1) Utilize two bolt holes in the 9-ft long steel posts placed at full-post spacing. If an overlay causing the rail height to drop 2 to 3 in., the W-beam rail and blockout can be mounted to each post using the upper bolt hole in order to retain 31-in. top rail height.
(2) Implement Option 1 plus also install 7-ft long posts at half-post spacings. This conservative option may provide most safety.
(3) Install MGS at 33 in. near 2:1 fill slopes and taper barrier ends to 31 in. after extending beyond 2:1 sloped region. After overlay is placed, the rail height would be about 31 in.


A distant 4th option is noted below but it carries more risk of vehicle override with lower rail height.
(4) Install 7-ft long posts at half-post spacings and between existing 9-ft long posts. Leave rail height at 28 to 29 in. after overlay placed.

Traffic Signal Pole Exemption and TCB Transition Cap

Question
State: WI
Date: 10-13-2010

Wisconsin had some questions about traffic signal pole exemption and TCB transition cap which are addressed in the response below.



Response
Date: 10-13-2010

I have some information on the items we discussed on the phone today.


First, you requested If we had any information on the reasoning behind traffic signal poles in the median having an exemption from requiring they be protected. While it would be best to protect all median hazards, there are some arguments that have been made to exempt traffic signals.


1.First, traffic signals serve to control accidents at intersections. Thus, if the signals were allowed to breakaway or disengage when impacted, the loss of the signal could cause secondary collisions that were more severe that impact of a single vehicle with the traffic signal support.

2.Second, if the intersection is functioning properly, the speeds of vehicles approaching the signal support would often be decreasing or reduced as compared to remainder of the roadway.

3.Third, traffic signal supports located in the median can only truly be shielded from one direction of traffic, shielding from both directions would require that shielding extend from the support into the intersection. This is not feasible.

I also looked into your questions regarding the overhanging piece of steel on the TCB median barrier approach transition cap. On the tested system, we designed the cap to match the slope of the sides of the PCB section. Because the tops of the PCB section and the median barrier were not aligned, the cap had an overhanging piece of steel on the oncoming traffic side. This piece is not required for cap designs that match up to barriers of different widths or heights. Changes in the height of the median barrier will require a different cap design. We would require the following.


1.All cap designs use the same vertical slope for the cap as the tested design.

2.The sides of the cap should match the side slope of the F-shape PCB segment.

3.If the cap is longer than the tested cap design, intermediate anchorage should be provided on the side of the cap at the midspan length to provide additional anchorage.

I have attached details for a representative cap for a transition from 32" PCB to the Wisconsin 56" single slope.


Attachment: https://mwrsf-qa.unl.edu/attachments/de23ff3147ed00dc5307135837f9426a.pdf


MGS Height Tolerance

Question
State: KS
Date: 10-19-2010

Recently you crash tested the MGS at 34" that passed. Is it appropriate for me to say +- 3" for all the applications?

Also what metric height did you test the 34" system at? I want to be consistent with actual measurement units in the RDG.



Response
Date: 10-20-2010

MwRSF successfully crash tested the MGS at heights of 34 and 36 in. with the 1100C small car at TL-3 of MASH. This crash testing was at interior locations away from the end terminals. In addition, no 2270P crash testing has been performed. Our current concern is that the current end anchors may not be fully capable of handling the tensile loading imparting under TL-3 MASH conditions both at interior and end regions at the raised heights. Note that we already had increased foundation length in our existing anchorage hardware when the rail height increased from 27.75 to 31 in.



INDOT F-shape PCB

Question
State: IN
Date: 10-20-2010

We need to get an anchored temporary barrier wall 350 approved for INDOT. Because time is of the essence, our concept is to take a 350 approved anchored system such as the Drop Pin or Kansas anchored system and modify the design to fit within the 350 approved Indiana "‹Ã...“F' shape. That is we would use the reinforcing and anchoring system from the approved wall and modify it to fit within the INDOT barrier wall.


I have attached the approval letter and INDOT standard for your review. I will also try to retrieve the report completed for the INDOT wall.


In the meantime, please review and provide any comments you have concerning our approach.
Attachment: https://mwrsf-qa.unl.edu/attachments/7645c0b578c843e369fa9b3f80557d7e.pdf


Response
Date: 10-29-2010

We have reviewed your TCB details. I have attached details for the F-shape barrier used by Kansas developed at MwRSF.

We would recommend the following with respect to modifying your barrier section.

  1. The Indiana barrier would need to be modified to accommodate the anchor holes and additional reinforcement for the anchors holes present in the MwRSF design.
    1. It should be noted that the toe of the Indiana barrier is 1" shorter than the toe of the MwRSF barrier, thus it may be difficult to fit the reinforcing steel and anchor holes in the barrier with appropriate concrete cover.
  2. The Indiana barrier is 2' shorter than the MwRSF barrier. This should not be an issue, but we would still require the same number of anchor points as the MwRSF design.
  3. The Indiana barrier would need to be modified to have equal or greater barrier reinforcement throughout the barrier as the MwRSF design. Our testing of these barrier in their anchored configuration has shown that we are very close to the capacity of the barrier section we have. We would also like to see the shear steel extended into the toe of the barrier in a manner similar to the MwRSF design.
  4. We would recommend that the Indiana barrier switch to a 6 loop end connection configuration similar to the MwRSF design. The 6 loops design tends to be stiffer than traditional 4 loop designs, reduces loads on the connection pin, and help prevent barrier rotation when the system is used with anchors.
  5. The connection pin used in the Indiana barrier is listed as a 1 3/16" bolt. No grade is listed for this bolt. The MwRSF design used a 1 .25" diameter A36 steel pin. The connection pin would need to have equal or greater strength and ductility to the pin used in the MwRSF barrier.


Response
Date: 12-20-2010

Attached preliminary drawings show modified INDOT F-shape TCB with Kansas F-shape TCB reinforcing bars and anchor bolt details.


  1. The drawing sheet I of 2 shows modified shape of the INDOT TCB to accommodate the anchor holes and additional reinforcement for the anchor holes same as Kansas barrier. The drawing also shows the comparisons between the two barrier shapes and the barrier steel and concrete strength details which are same as Kansas barrier. Note that INDOT barrier toe is 2" but has same concrete cover and clearances as Kansas barrier.
  2. The Indiana barrier is 2'-6" shorter than Kansas barrier but will have same number of anchor points (anchor bolts on traffic side only).
  3. Indiana Barrier will have equal barrier reinforcement similar to the Kansas barrier but closely spaced due to the shorter length of the Indiana barrier as shown on sheet 2 of 2 of the attached drawing.
  4. The sheet 2 of 2 of the drawing shows six loops design as you have recommended.
  5. The connection pin between barriers will be 1 ¼" diameter, A36 steel minimum.


Please review these modifications to the Indiana F-shape anchored barrier and provide your feedback and let us know if this anchored barrier can be qualified and approved to NCHRP 350, TL3 without test. We are planning to prepare final drawings of the Indiana modified barrier after we hear from you. We will send you the final drawings again for your review.



Attachment: https://mwrsf-qa.unl.edu/attachments/1afa27a3877c597322a5422d4d9d497e.pdf


Response
Date: 12-21-2010

I have reviewed the details you sent and have the following comments.


1.  You list loop bars as ¾" diameter smooth bars with Fy = 60 ksi. This is not the correct spec. The proper spec for the loop bar steel is, "The loop bars (6d1, 6d2, and 6d3) shall be A706 Grade 60 or A709 Grade 70 0.75" [19] smooth or deformed steel bars. Alternative steel chemistry may be used as long as the alternative material provides a minimum yield of 60 ksi [420 MPa], a tensile strength of not less than 1.25 times the yield strength but a minimum of 78 ksi [550 MPa], a minimum 14% elongation in 8" [203], and passing a 180 degree bend test using a 3.5D pin bend diameter. The loops shall be installed within 0.12" [3] of the plan dimensions."

2. We are concerned that the height of the barrier is only 31". All previous testing of the MwRSF F-shape barrier was conducted with a 32" high barrier. In F-shape barrier impacts we have observed a tendency for the vehicle to climb the barrier face. Thus, the height of the barrier is critical in achieving proper redirection. As such, we would recommend that the barrier be 32" high.

3. With the shorter toe on your proposed barrier, there is slightly less cover for the loops used to retain the anchor bolts. This may adversely affect the longevity of the barrier segment and potentially it capacity.

4. On one of your details, you show the three loop connection attached to your current F-shape barrier with two loops. While we believe that this type of connection can work, we would only recommend mixing barrier segments in free-standing barrier installations. Also, when connecting barriers with dissimilar loop connections, we would recommend that a ½" dia. x 10" Grade 5 Hex bolt and 2.5"x4"x1/2" keeper plate be used at the bottom of the connection pin to insure the pin does not pull out of the loops under load. This bolt and keeper plate were part of the original barrier connection, but were eliminated after switching to the 3 loop connection. However, if you connect the three loop connection to your current F-shape barrier with two loops, we would recommend that the bolt and keeper plate be used.


Other than the four issues above, we see no other problems with the barrier as shown. I should also note that we cannot determine if this barrier can be qualified and approved to NCHRP 350, TL3 without test. The acceptance of the design must be done by FHWA through their approval process. We can help you get in contact with Nick Artimovich if need be. I am not certain at this time that the barrier could be approved to NCHRP 350 as the deadline for all NCHRP 350 approvals was 12/31/10. As such, you may need to seek approval under MASH. This may be possible as the MwRSF F-shape has undergone several full-scale tests using the MASH criteria.



MGS Questions

Question
State: GA
Date: 10-28-2010

We are considering using the 31" Midwest guardrail system. We have been using the test data and drawings to develop our own standards, but I have a few outstanding questions about the applicability of this guardrail.

How much shoulder is needed behind the 6' post to provide sufficient support? I trying to get a handle on appropriate uses for the 6' post vs. the 9' post.

Has there been any testing of type 12 anchors for the 31" guardrail?

Has there been any testing of double faced guardrail for median applications?



Response
Date: 10-29-2010

How much shoulder is needed behind the 6' post to provide sufficient support? I trying to get a handle on appropriate uses for the 6' post vs. the 9' post.


For MGS guardrail:


1. Standard MGS guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.


2. For MGS guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.


3. For MGS guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.


4. For MGS guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 9' W6x9 posts at standard spacing are recommended.



Has there been any testing of type 12 anchors for the 31" guardrail?


I am not sure what the type 12 anchor is without more details. I have attached the anchorage that we tested with and that we recommend for non-terminal locations.


Has there been any testing of double faced guardrail for median applications?


We have an FHWA approved median barrier version of the MGS. See the attached details.


Attachment: https://mwrsf-qa.unl.edu/attachments/628c438b1990eaf0dcf1e1c22b39c01f.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/ce5f7b38aa2efde95278a239f5aa8536.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/e406b6484a82f0a81d64735ab752cb1c.pdf


Minnesota TL-4 Combination Bridge Rail

Question
Date: 11-01-2010

The conclusions/recommendations of the researchers in this report ( ..Minnesota TL-4 Combination Bridge Rail... ) are as following:


"Finally, the authors believe that this combination traffic/bicycle bridge railing can be adapted to other safety shape bridge railings (i.e., F-shape and single slope) or vertical parapets of similar height and top width with only minor modifications. Additionally, it is believed that no further testing will be required since the F-shape and single-slope barriers are considered to behave slightly better than the New Jersey shape in crash testing (6.8.23)."

As far as you know, can this railing be successfully installed at the top of a 1070 mm (42 in.) F-Shape barrier?




Response
Date: 11-02-2010

We believe that this railing with vertical spindles would perform better when placed on a 42" parapet versus a 32" parapet. The increased height should reduce vehicle engagement with the bike-pedestrian rail. Of course, the only certain way to evaluate its safety performance is through full-scale vehicle crash testing.



MGS Working Width and Dynamic Deflection

Question
State: WI
Date: 11-01-2010

I was reading the consulting summaries and had a few questions.

My first question is on the discussion that Scott, Dr. Faller and I had about the bridge rail retro fit. During one of our phone conversations it was mention that MwRSF would talk to FHWA about this retro fit option. Has there been any progress with FHWA?

My second question is on the table of MGS guard rail. On the standard post spacing, W6x9 mash tests, there is a test where the working width and the dynamic deflection distance is differ only by 0.3 inches.

If working width is barrier system width plus maximum deflection or maximum vehicle lean and dynamic deflection is measured from back of system, shouldn't the difference between the two measurements be greater than 0.3? Most of the other deflections and working widths are greater than 0.3" except for the round DF test.

If this result is correct, you may wish to provide a note to discuss why it is correct.



Response
Date: 11-01-2010

The working width and dynamic deflection for test 2214-MG1 was 57.3 in. and 57.0 in., respectively. The working width is the maximum lateral displacement of a post, rail, vehicle, etc. away from the front face of the barrier. The dynamic deflection pertains to the distance between a rail or post location before and during the test. If a rail midspan location has the greatest lateral deflection and the rail becomes partially flattened at this location, then it could be possible for the working width to be only slightly greater than the dynamic deflection However, I would think that this would be approximately 50% of the rail thickness added to the max. D.D. or 58.6 in. versus 57.3 in. We will have someone re-check the high-speed film to determine where the W.W. value of 57.3 in. was obtained.



Response
Date: 11-02-2010

The working width point was determined at a midspan between two posts. There was a sign error in the calculation of the working width data. The working width should be 1489 mm or 58.6 inches.



South Dakota Snow Gate Modifications

Question
State: KS
Date: 11-03-2010

Our district staff would like to install a modified version of the South Dakota snow gate (See attached "SD Snow Gate.pdf" and "TRP-03-44-94.pdf"). However, they are proposing some modifications to the base:

1. They would like to use a screw-in Chance Lighting Foundation (See attached "11242ng4 Model (1).pdf").

2. They would like to use the Transpo Pole-Safe " Model 4100 frangible bolts. (See attached "Pole-Safe 4100 Details.pdf" and "Pole-Safe Skirt Details.pdf").

They have some other thoughts regarding signing and lighting attachments as well as an alternate method of storing the gate in its road open condition. I will pass along information for those items as I receive it, but they would like to begin installing the foundation next Monday.

Would you please review the attached "SD_SnowGate_Mod.pdf" and verify that these modifications will not affect the crashworthiness of the system? Please comment also on the grading as shown. (Sorry for the short notice).

Let me know if you have any questions.

We received the below response from Transpo. Do you have any comments on the information they've provided?

"The original model PoleSafe 201 had a tensile strength of 24 kips and the new 4100 has a tensile strength of 49.8 kips. Both models had a restrained shear strength of 5.5 kips. With this said what it means is that the new PoleSafe couplings are twice as strong in tension and have the same breakaway strength as the 201 couplings. The new couplings should actually perform better than the old 201 couplings in that they will be able to support the gate in the open position with much less stress on the couplings.
Attachment: https://mwrsf-qa.unl.edu/attachments/986e2826317c8784ddfb5a101fe60fc7.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/ab485ffc4910a8a291fb5d773b408806.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/82deceeeb97dbe56dad490a65f50ef8c.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/71cbb8803638c75d536442e5eb0e4d5c.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/1ebe210a6b00ebce2209a5167f4935b9.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/138d7c70224d0fb1f61238a4da74c52e.pdf


Response
Date: 11-08-2010

I have reviewed your proposed changes to the snow gate design. The changes you propose create two main issues with respect to the performance of the snow gate.


  1. Coupler equivalency
    - You are proposing to change from the Pole-Safer 201 couplers that were used in the tested design to the Pole-Safe 4100 design. In order to replace the couplers from the original tested design, we need to verify that the Pole-Safe 4100 couplers perform in a similar manner and would break away without increasing the occupant risk limits of the design. I have looked up the approval letters for both coupler designs, but they do not contain sufficient information to evaluate the couplers. As such, we would recommend that you contact Transpo for guidance on the replacement of the coupler. If they can show that the coupler design would work similar to the Pole-Safe 201 for a short, heavy break away device, then use of the Pole-Safe 4100 would be acceptable.


It appears that Transpo has stated that the Polesafe 4100 and 201 models have equivalent breakaway loads. Thus, it would be acceptable to use the 4100 model in your modified design. The only way to further evaluate this would be to compare the velocity change for tests of short, heavy poles from testing of both the 201 and 4100 couplers.


  1. Foundation design " Your new design also replaces the original 2' diameter x 3' deep concrete footing with a 8.6" diameter x 5' long Chance Lighting foundation. The concern here is that the small diameter foundation may rotate in the soil prior to developing sufficient loads to cause the couplers to break. The original foundation was significantly wider and thus would be expected to develop the soil forces more quickly. There is some concern that movement of the foundation would affect the coupler performance and potentially impede the proper breakaway mechanism. In order to alleviate these concerns, we would recommend a foundation analysis (LPILE or some other method) to determine the relative stiffness of the two foundation alternatives. This should provide enough confidence to use the new foundation if the results are similar.


The use of the 10:1 grading shown is not a cause for serious concern in our eyes.




Guardrail for Fill Slope Applications

Question
State: IL
Date: 11-03-2010

Here is a question regarding options for a roadside barrier in a location where there has been an embankment failure and subsequent repair using a soil nail repair procedure. Also, the roadway top width is narrow, such that we would not have 2' of embankment behind the back of guardrail posts. Records show that the soil nails are present at 4.26 to 5.16 feet below ground surface. If we were to use the 9' posts recommended for guardrail (MGS) in this location, the posts would extend about 6 feet below ground surface, and interference with soil nails would be a concern.


We are wondering if you are aware of any other alternatives or variations of roadside barrier systems that might be considered?


Also, would use of standard length 6' posts, on 3' 1 ½" spacing be considered under all these constraints? I understand that MwRSF prefers the longer posts for more uniform, reliable results, but we would be open to ideas for mitigating this (soil plates?). Terminals for the guardrail could be placed outside the slope repair.

Response
Date: 11-03-2010

Recall, MwRSF has developed two barrier options for use on 2:1 fill slopes. Below, you noted the MGS option which utilizes 9-ft long steel posts spaced on 6 ft " 3 in. centers. The 31-in. tall, MGS option was developed under the MASH safety performance criteria. The maximum dynamic deflection was found to be approximately 58 in.

Several years ago, MwRSF also developed a metric-height, W-beam guardrail system for 2:1 fill slopes using 7-ft long steel posts spaced on 3 ft " 1½ in. centers. This 27¾-in. tall, W-beam guardrail option was developed under the NCHRP Report No. 350 safety performance criteria. The maximum dynamic deflection was found to be approximately 32 in.

Based on the successful performance of the MGS system with 9-ft long posts in conjunction with 58 in. of dynamic deflection, it would seem reasonable that the metric-height system could raised to a 31-in. height and converted to a MGS system. With this modification, the post embedment depth would be reduced by only 3¼ in. from that used for the noted development and crash testing program. In addition, the maximum dynamic barrier deflection would likely fall between 32 and 58 in. With 7-ft long posts, the embedment depth would be 52 in. or 4.33 ft.

Based on the information noted above, it is my opinion that MGS should perform in an acceptable manner when installed at the SBP of 2:1 fill slopes if configured with 7-ft long steel posts spaced on 3 ft " 1½ in. centers. If necessary, it would seem reasonable to also construct the MGS at the 32-in. upper height tolerance using 7-ft long posts. With this variation, the post embedment depth would be 4.25 ft.



Inserts for Bridge Approach Section

Question
State: NE
Date: 11-04-2010

Is there design for Inserts for attaching bridge approach sections rather than having to drill all the way through the bridge rail?


This is an existing bridge rail where we did not get a precast insert into the off-end of the rail.
Now we are switching to head to head traffic where it now will have a chance of getting hit from the other direction.


What size/ length of insert is required here?

Response
Date: 11-09-2010
I am enclosing a copy of an excel table that I have used over the years to obtain capacities for threaded bolts and anchor rods. Please note that the contents of the table do not reflect reduction factors of any kind. Based on your proposed rod size, it would appear that five ¾-in. diameter, 36 ksi steel threaded anchor rods would not meet your shear load requirement. However, five ¾-in. diameter, 92 ksi steel threaded anchor rods would meet the requirement if no reduction factor is utilized, but it would not meet the 80-kip requirement if the 0.75 reduction factor is used.

If we considered 7/8-in. or 1-in. diameter anchor rods of 92 ksi (Grade 5, 325, 193-B7, etc.) steel material, then both anchor rod sizes would provide adequate shear capacity, even with using the shear reduction factor.

Can you provide details for the swedge fittings?

Attachment: https://mwrsf-qa.unl.edu/attachments/fbbcbd03c15e129e3d56b3ce131fc490.xls

Attachment: https://mwrsf-qa.unl.edu/attachments/fbbcbd03c15e129e3d56b3ce131fc490.pdf


Additional Blockout Depth

Question
State: WI
Date: 11-05-2010

FHWA's NHI Roadside Design Class allows for extra block outs to be installed within a given run of beam guard. WisDOT has adopted this within its current standard detail sheet for regular beam guard.

With MGS using larger blockouts, would the same guidance that is in the NHI class still apply?


Response
Date: 11-05-2010
Historically, MwRSF may have allowed the use of up to triple 8-in. deep blocks at a few locations within a run of metric-height W-beam guardrail, thus resulting in an offset of 24 in. However, it is uncertain as to whether the use of three 8-in. deep blocks may be too excessive when used continuously with metric-height W-beam guardrail. In the metric-height W-beam long span guardrail, MwRSF incorporated the use of double, 8-in. deep wood blockouts with the three CRT posts adjacent to the long span.

The MGS utilizes 12-in.deep blocks for standard applications as well as for special applications. For example, the MGS long span design utilizes one 12-in. wood block with three CRT posts instead of two stacked 12-in. deep blocks. For the MGS, it would seem reasonable that the use of two 12-in. deep stacked blocks could be accommodated at a few locations as well, thus also resulting in a rail offset of 24 in. However, it is uncertain as to whether the use of two 12-in. deep blocks may be too excessive when used continuously with the MGS.

Thus, based on previous testing of systems with deep or extended blockouts and an analysis of the contact lengths of typical MGS testing, MwRSF would recommend the following:

1. Double standard blockouts or combinations of blockouts up to 16-in. deep may be used continuously in a guardrail system.
2. Triple standard blockouts or combinations of blockouts up to 24-in. deep should be limited to one in any 75 ft of guardrail.


Response
Date: 11-05-2010

Attachment: https://mwrsf-qa.unl.edu/attachments/1226e740c1be066cfe9173776b640b2c.pdf


NDOR Temporary Concrete Barrier

Question
State: NE
Date: 11-08-2010

We are trying to implement the KsDOT Barrier in the next week.

I question how to raise the bottom of the barrier 2" (preferred).

Bar 4a1 is a stirrup which usually comes within 2" of the bottom of the barrier/ ground.

The rise on the left half of the barrier is preferred, the right is an odd option I think will lose the 3" base piece.

Can the stirrups labeled 4a1 be cut to keep the steel from being closer than 2" to the bottom when we raise it like the right half?

Or can 4a1 be bent slightly different to meet our 2" distance to the outside of the barrier?



Response
Date: 11-09-2010

I have reviewed your email questions below and have a few comments. See below!

We are trying to implement the KsDOT Barrier in the next week.

I question how to raise the bottom of the barrier 2" (preferred).

**I am not sure that I understand the concept of raising the bottom of the barrier by 2 in. However, I assume that you are referring to the need to add increased hydraulic drainage flow to specific locations on the bottom of the barrier.

Bar 4a1 is a stirrup which usually comes within 2" of the bottom of the barrier/ ground.

**In the original TCBs, the vertical stirrups and lower longitudinal rebars were placed close to the barrier's base with an acceptable concrete cover for the steel bars. Later, the Pooled Fund member states met in St. Joseph, MO to discuss the standardization of the TCB as well as the modifications/addition of some other features, including fork lifting slots. When the fork lifting slots were incorporated, the shape and placement of the vertical stirrups remained the same but the lower longitudinal rebars had slightly moved upward to provide cover above the fork lifting holes. The affect of these changes can be garnered by viewing the original details and comparing them to those details listed for the KS/FL (Midwest) TCB free-standing and tied-down systems.

The rise on the left half of the barrier is preferred, the right is an odd option I think will lose the 3" base piece.

**The modifications on the left side of the barrier depict a distinct fork lifting slot and a separate drain slot. However, on the right side, there exists a fork lifting slot that it integrated into the drain slot, thus requiring a modification to the vertical stirrup and reduced barrier contact with the ground. Personally, I like the detail on the left side more than that on the right side. If provision for drainage is really needed, I would almost rather see four fork lifting slots centered between the vertical stirrups such that the vertical stirrups do not require modification and barrier support is better distributed. With this change, there would not be narrow, 3-in. wide segments. On another note, are you counting on the 3-in. wide drainage slots between the barrier ends? If not, would the two fork lifting slots be sufficient?

Can the stirrups labeled 4a1 be cut to keep the steel from being closer than 2" to the bottom when we raise it like the right half?

**If the bottom side is raised under vertical stirrups, the bottom of the bars would be bent inward at a higher elevation to fit under the lower longitudinal bars. However, it is not recommended to have this change occur near the tie-down locations. Thus, the proximity of the drainage slots near the outer tie-down locations could result in increased concrete fracture when the barriers are anchored to a paved surface or bridge deck.

Or can 4a1 be bent slightly different to meet our 2" distance to the outside of the barrier?

**As noted above, it would be possible to bend the lower stirrups ends inward at a slightly higher elevation to meet concrete cover. However, the drain slots should be integrated such that they do not pose concerns for increased concrete fracture at the tie-down locations.



Response
Date: 11-10-2010

The Nebraska Department of Roads (NDOR) has recently contracted with a contractor to provide 25,000 Lin. Ft. of new concrete barrier and FHWA-NE is requesting the NDOR to update its design to something similar to the Kansas Portable Concrete Barrier (PCB) which has incorporated a pin and six connection loop system and some of these low-cost improvements to which Mr. Horne alluded. The Kansas PCB has been crash tested and is accepted by FHWA in letter HSA-10/B-122. As we have previously discussed, NDOR has determined that instead of directly adopting the Kansas design, we prefer to adopt a modified design that incorporates features that Nebraska has found to be beneficial.

  NDOR is requesting that MwRSF review the following changes to the Kansas design and advise if the barrier will continue to perform satisfactorily with the desired changes.


Modifications to:

Loop Steel:

The Kansas plan calls for "1.25 times the yield strength but a minimum of 80 KSI" The ASTM standards for A706 steel include this. Both plans call for yield strength of 60 KSI. The minimum bending diameter for ASTM A706 steel is 4 x (3/4" dia.) = 3" our plan shows this in the bending diagrams.

Six Loop System " Connection Pin/ Retaining Bolt:

The six loop system does not require a retaining bolt at the bottom of the connection pin used to connect adjacent barriers; NDOR's barrier has this detail and we have elected to keep the detail only requiring it to be used when using the strap near a drop-off.

Anchor Bolt Block Out:

The Kansas plan shows a standard detail for an anchor bolt block out to allow the barrier to be bolted to the substrate; NDOR elected to make the anchor bolt block out optional and to be built at the discretion of NDOR since it is not required for all projects. The U-shaped steel bars labeled 6A2 required for the anchor bolts to transfer load are also omitted when the anchor bolt blockout is absent.

Tie-Down Strap:

The Kansas barrier plan does not have an alternate of using a tie-down strap to mount the barriers to the substrate. The tie-down strap was tested with the NCHRP 350 testing procedures and is an accepted detail for the 12.5' barrier. NDOR desires to retain the option of using the tie-down strap.

Foot Print - bottom of barrier in contact with the ground:

The Kansas plan has a foot print of 12.9 square feet the proposed Nebraska design has 14.4 square feet. NDOR prefers that there be additional lifting slots for drainage conveyance under the barriers to reduce ponding on the roadway and allow movement by larger forklifts.


The modification is shown on the elevation view as being an additional 1' of barrier on each half elevated 3" from the ground for the width of the barrier and results in a reduction of 3.75 square feet of foot print. To mitigate this decrease in the area of the barrier in contact with the ground the NDOR plan removed the 7" wide x 1" high inverted V-shape on the bottom of the Kansas plan, shown on Section B-B of the Kansas plan, this had held 6.12 square feet from contact with the ground.


NDOR requests that MwRSF review this information along with the attached plans and advise if the Kansas barrier, modified as proposed will continue to function as tested and accepted or include further suggested modifications to perform satisfactorily.


NDOR further requests an opinion on whether the Nebraska PCB designs (both 4-loop & 6-loop) and the modified Kansas PCB can be pin connected together and be considered to perform satisfactorily to NCHRP Report 350 or MASH Test Level 3 evaluation criteria.



Response
Date: 11-12-2010

I have reviewed the enclosed NDOR materials and have the following comments.


(1)   The reinforcing steel for loop bars are shown to conform to ASTM A706 Grade 60, which infers a minimum yield strength of 60 ksi, a minimum tensile strength of 80 ksi, and a minimum % elongation in 8 in. equal to 14% for no. 6 or ¾-in. diameter bars. A footnote also reads that the tensile strength shall not be less than 1.25 times the actual yield strength. It is acceptable for NDOR to denote that the loop bars conform to ASTM A706 Grade 60.


(2)   Historically, the loop bars and reinforced concrete barriers have been fabricated and crash tested using a 2¾-in. pin diameter to achieve the specified loop geometry. Florida, Iowa, Missouri, and Kansas all utilize a 2¾-in. pin diameter. NDOR has depicted a 3-in. pin diameter. In order to maintain the same drop pin and rebar loop clearances, it would be recommended that NDOR utilize the 2¾-in. pin diameter.


On another issue, Iowa and Kansas specify that the steel rebar for loop bars pass the 180-degree bend test using a 3½-in. pin diameter, while Florida specifies that a 2¾-in. diameter pin be used for the 180-degree bend test. Missouri does not identify a bend-test requirement. NDOR does not currently identify a 180-degree bend test requirement. ASTM A706 denotes the bend test to demonstrate that the bar can be bent around the pin without cracking on the outside radius of the bent portion. Thus, if a bend test were to be performed, it would seem appropriate to run the 180-degree bend test using the same diameter that would be used in the final loop configuration.


(3)   A six-loop rebar connection system with drop pin is shown in the NDOR CAD details. At the base of the drop pin, a horizontal retainer bolt was originally configured for use with the four-loop rebar connection system as well as for the tie-down strap anchor system. However, the retainer bolt is not required in free-standing TCB configurations that utilize the six-loop rebar connection.


(4)   An alternative tie-down system was originally developed for the Midwest F-shape temporary concrete barrier which consisted of vertical bolts or rods penetrating the barrier's toe. At these anchor locations, horizontal rebar loops were incorporated to strengthen the TCB at the attachment locations. The NDOR temporary concrete barrier does not include these additional rebar loops in all sections, unless the barrier section will later be used in tied-down applications. It is acceptable to leave out these 6A2 bars if the TCB will only be used in free-standing applications or anchored using the tie-down strap.


(5)   NDOR noted that Kansas does not utilize the alternate tie-down strap with the F-shape TCBs. However, I reviewed the Kansas standard plans and found detail RD622B which depicts the tie-down strap anchor method.


(6)   NDOR has proposed to increase the length of the lateral openings on the underside of the TCB to allow for improved water drainage flow from the roadway to travel under the barrier, thus reducing concerns for water ponding near the travel lanes. The detail with four separate drainage slots is acceptable. After considering alternatives, it would also be acceptable to combine the two slots on each half of the barrier into one slot measuring 2 ft - 3 in. long and shifting the outer edge inward slightly to provide additional concrete cover near the outer tie-down holes. For this second alternative, the 4A1 bars above the slot would need to be modified slightly. The 2-ft long middle support section would be maintained.


Based on the features identified in Item Nos. (1) through (6), it is our opinion the modified NDOR F-shape TCB will provide an acceptable safety performance when used in similar applications to those approved for the Iowa, Kansas, Florida, and "Midwest" TCBs. The aforementioned barrier versions have been previously crash tested in free-standing and tied-down applications according to either the NCHRP Report No. 350 or MASH impact safety standards.


Lastly, NDOR requested that MwRSF provide comment regarding the safety performance of a free-standing, TCB system which utilizes one end of a four-loop connection to attach to another end of a six-loop connection. As noted previously, the four-loop, Iowa TCB system was successfully crash tested under the NCHRP Report No. 350 impact safety standards. Later, the six-loop, Midwest/Kansas/Florida TCB system was successfully crash tested under the MASH impact safety standards. When the six-loop connection was integrated into the F-shape TCB section, the geometry of both loop connections was considered to ensure that the two designs could be attached to one another. Therefore, it is our opinion that a TCB barrier system which contains joints where both loop connections attach to one another would be considered crashworthy and capable of meeting the Test Level 3 impact conditions.


For informational purposes, I have attached PDF copies for the TCB CAD details for Kansas, Iowa, Florida, and Missouri.


Attachment: https://mwrsf-qa.unl.edu/attachments/db03ebe2903ddd2cb14ade15bfd6989d.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/6f3b2f4b7b3044c30f38e1f8a4d3bd51.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/48f3d14af150e291e93bc8818df59326.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/7aeb70325e395b85334a52e7ccdeb2d2.pdf


Retrofit Transition

Question
State: WI
Date: 11-09-2010

I was doing some digging around and found this a TTI crash test for a TL-2 beam guard transition to rigid barrier. I was thinking of adding this as a retro fit option for some location where beam guard is directly attached to a rigid barrier.


I was thinking of using the following restrictions for its use:

45 mph or less

Rigid barrier does not slope downward.

Rigid barrier is NJ, F or vertical.

No curb and gutter is installed under the beam guard transition, or between the face of rail and edge of lane.



I was also wondering if WisDOT could use this transition as a retro fit for higher speed facilities (55 mph or less), under the assumption that providing a TL-2 system until a facility is fully reconstructed is a more viable option than tearing the whole thing out and installing a TL-3 transition.



What input MwRSF could provide would be appreciated.


Attachment: https://mwrsf-qa.unl.edu/attachments/39019b89b43b6a0348fab29690757ad8.pdf


Response
Date: 11-10-2010

I am somewhat familiar with the TL-2 approach guardrail transition that you have identified. I am attaching a copy of a technical paper that was published on this topic in a Transportation Research Record. The general guidelines that you have noted seem reasonable. There may be situations where this design could be used on higher speed roadways if traffic volumes were sufficiently low, thus warranting the use of a TL-2 barrier system.

I believe that we provided a test level chart in one of our prior reports as a function of traffic volume and speed. That chart was based on work performed for NCHRP project no. 22-8. We may have published that chart in the ZOI-Barrier Attachment Report or the Guidelines for Bicycle-Pedestrian Facilities Report.



Texas HT Barrier ZOI

Question
State: IA
Date: 11-15-2010

I am trying to establish a TL-5 ZOI chart for the Texas HT barrier, based on the 80,000 pound truck test that was performed by TTI back in 1984. Would you be able to provide me with such a chart, or supply me with the materials I would need to develop the chart on my own?


Response
Date: 11-29-2010
The crash test for the 50 in. tall combination rail (test no. 2416-1) provides limited answers to the ZOI question. First, the barrier installation was not long enough to redirect the truck or allow it to stabilize before it reached the end of the system. As such, the cargo box continues to roll/ lean onto the barrier more and more until the barrier ends and the vehicle rolls onto its side. Thus, the maximum lateral extent can only be observed during the time of contact. Also, the camera views are not ideal as the overhead does not capture the downstream end of the barrier and the downstream camera is not directly in line with the barrier. This makes getting accurate distances from the test difficult and getting the vertical locations of the box as it rotates very difficult.


What I was able to gather from the test was a maximum lateral distance behind the barrier of approximately 4.5 ft. ZOI has typically been shown by dimension based on the top front of the barrier, so this extension would be 5.5 ft from the front of the steel tube (to be conservative you could use 6 ft).
Also, the height of the barrier prevented the bottom of the box from getting over and behind the rail, so the ZOI would not need to extend below the top of the rail as was recommended in the TL-4 ZOI.

Response
Date: 11-30-2010
Should I assume the 5.5-foot ZOI extends all the way up to 13.5 feet above the ground? Or do you think the maximum lateral extent of the truck occurred at some height below 13.5 feet?


We may consider limiting intrusion by raising the height to the top of the railing. Do you have any "rules of thumb" that we could use to estimate how much the intrusion would be reduced for each additional inch of barrier height?


Response
Date: 11-30-2010
The maximum lateral extent was lower than the top height of the box when the vehicle was at rest (I'm assuming that distance is the 13.5 ft you are referring to). However, the vertical position of the box is not easily obtainable from the test video. Also, since the box slid off the barrier and rolled onto its side before it had a chance to right itself (return to an upright position), the path of the box on its return is unknown " and very well could be more critical than the path to maximum extent. Therefore, I would recommend treating the max lateral distance as the boundary for all heights " ZOI would be a box of width 5.5 ft. We just don't have enough data to better define the ZOI.


There is not a rule of thumb to adjust the lateral extent of the box. We simply do not have the data from multiple TL-5 tests at multiple heights to establish such a relationship. Also, be carefull raising the height of the barrier. Extending the height of the steel rail without raising the height of the concrete parapet with it (extending the rail support posts) will reduce the strength of the rail and possibly lead to failure of the support posts and/or anchor bolts.


Alternative F-shape Barrier Connection Pin Detail

Question
State: MN
Date: 11-22-2010

Per our discussion last week, I am sending you a request for your consideration and approval of two Portable Concrete Barrier (PCB) Connecting Pin designs.

Minnesota uses an F shaped, 12.5' long, pin and loop, portable concrete barrier system. The design was developed by Midwest Roadside Safety Facility. The supporting FHWA acceptance letters are, B-41 for the original design, and B-122 for the current design. Our design matches the current design, as proposed for the Barrier and Hardware Guide (SWC09) through task force 13. See attached (SWC09 10-29-08.pdf).

The current connector pin is located at

http://aashtotf13.tamu.edu/Guide/Hardware/Components/FMW02.pdf

We have been told by our construction office that the current connector pin design is difficult to work with when installed. Especially when there is tension in the barrier system, thus having the effect of locking the pins into the loops. Construction personnel often use hammers to tap the pins loose, which in turn causes damage to the upper plate of the connecting pin design (FMW02).

Our two proposed options are a "T" shaped pin and a "Cane" shaped pin. See the attached drawing (pin_11_22_10.pdf). Both proposed designs provide the same 1.25" diameter and 25" long vertical pin design as FMW02. The proposed changes are to the top configurations of the bars only. The "T" shaped top is the preferred design, however the "Cane" shaped top is less expensive to make, and still provides the necessary durability in the field.

Also attached is our proposed standard 8337C plate (StandardPlateReviewForm_8337C_Draft.pdf). Our intention is to allow all three connecting pin types within our standards provided you approve. Our Proposed 8337C plate 3 of 3, will be revised to include all three options.


Attachment: https://mwrsf-qa.unl.edu/attachments/0f4aede369cc02db5d6c5370dde44892.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/5a573c754ae3bb7cf93cf8cf463ddaa7.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/0ef802a7541df607a511c30c406f717f.pdf


Response
Date: 11-23-2010

We have looked through your proposed pin designs and we have a couple of comments/concerns.

1. We believe that the T-handle design would work acceptably, but we are concerned with the weld between the main pin and the T-handle. The current pin design has a ¼" fillet weld on the top and bottom of the plate. This is a weld length of approximately 7 7/8" and a weld area of 1.39 in2. The top of the pin can be loaded with significant vertical loads as the barriers rotate adjacent to one another, especially in a tie-down or anchored configuration. Thus, we are concerned that the T-handle pin does not have sufficient weld area to handle vertical loading similar to the tested pin and plate design. Our experience in welding round sections perpendicular to one another has found it very difficult to develop load capacity.

2. We also have concerns with the cane type pin. The concern here is that under high loads, the short extension on the cane pin could be pulled into the loops and compromise the joint. The bent end of the pin would be free to rotate when installed and could be in a position that allows it to be pulled into the loops when loaded, or large barrier and joint deflections could pull the relatively short bent end into the loops.

3. If the issue at hand is damage to the plates at the top of the pin, increasing the plate thickness should address that.



Response
Date: 11-24-2010

For furthered consideration is the attached proposed detail combining both request of MNDOT and MwRSF concerns on satisfying weld length & area requirements.


Attachment: https://mwrsf-qa.unl.edu/attachments/78a760ec0c07a06ba4a61085494e92bc.pdf


Response
Date: 11-24-2010

We would like to pursue some type of "T" bar option. I would like to propose that you consider taking out the 2 ½ " of bar between the 4"x2.5"x0.5" plate and the top 6" horizontal bar. The 4"x2.5"x0.5" plate could be welded on both sides, but on the top of a 2' 1-1/2" bar, and then the 6" long horizontal "T" top could be welded to the plate, extending 1" beyond either side of it. The 6" top could be round or square stock.


There seems to be a discrepancy with the drawings. The AASHTO link and B-122 (2003) show only a one sided weld for the plate to the pin. B-41 (1997) shows welding on both sides of the plate. Do you know which one is correct since one gives twice the weld area as the other?


If only one side needs to be welded, the plate could be brought up to the T handle and welded on the bottom without worrying about welding the handle for retrofit use if the pin length is acceptable.


Minnesota uses an F shaped, 12.5' long, pin and loop, portable concrete barrier system. The design was developed by Midwest Roadside Safety Facility. The supporting FHWA acceptance letters are, B-41 for the original design, and B-122 for the current design. Our design matches the current design, as proposed for the Barrier and Hardware Guide (SWC09) through task force 13. See attached (SWC09 10-29-08.pdf).
Attachment: https://mwrsf-qa.unl.edu/attachments/2d7c20fdd8c05dc4974feb02594d380e.pdf


Response
Date: 02-03-2011

I have given some additional thought to the T-top connection pin for the F-shape barrier. I have included some additional comments below.


  1. First, I have reviewed the T-pin design that you have proposed which includes a 2.5"x4"x1/2" plate welded to the top of the 1.25" diameter connection pin. The T-pin is then welded to the top of this plate. I don't see any issues with this design. The F-shape barrier was originally tested to NCHRP 350 with a top plate mounted exactly as you propose. I have attached details. If the restraining plate at the top of the pin is attached with lower capacity welding than the original design that was tested, there concern that the top cap could disengage from the pin and allow the pin to exit the connection loops. This in turn would eliminate the integrity of the connection. However, because you are welding the top plate with the same weld used in the tested design, there should be no strength issues and attachment of the T-pin should be acceptable. Thus, I believe that your proposed design should function acceptably.
  2. We also discussed the T-pin design that has currently been made by your barrier fabricators. This design consists of a 1.25" diameter T-pin welded directly to the top of the 1.25" diameter connection pin. Again, the concern here is that the T-pin may not be connected to the connection pin with sufficient weld to have similar capacity to the tested design and ensure that the T-pin does not disengage from the connection pin during an impact. We cannot determine exactly what the loads were on the top plate during testing of the original pin. Thus, we must require that any modification of the connection pin must have similar or greater capacity.


I do not believe that it is possible to get sufficient weld area (and corresponding weld capacity) in the fabricators design to match the tested pin. The strength and capacity of a given weld is determined by the throat area of the weld. Weld throat area can be determined by the formula At= .707hl. In this formula, At is the throat area, h is the height of the weld, and l is the weld length. The tested pin cap was attached to the connection pin with a throat area of 1.39 in2. Thus, we would require that the attachment of the T-pin to the connection pin have similar throat area and weld capacity.

  1. It may be possible to retrofit the existing T-pins that have been fabricated. I have attached a detail for a proposed retrofit. This retrofit would attach the tested pin plate to the pin using the standard ¼" fillet weld on the bottom. The plate could be slid up the pin from the bottom. Then the plate would be welded to the T-pin on top with a flare bevel weld along the length of the plate. This would require checking to make sure the retrofitted pin still extended into the barrier loops (had the same effective length) as the tested design. Let me know what you think.


With respect to the weld details, there are different weld details floating around out there. There are currently three details.


  1. The original pin cap was welded with the cap flush with the top of the 24.5" long pin. The cap was welded to the pin with a ¼" fillet weld on the bottom of the cap and the top of the cap was welded to the pin with a flare bevel weld. This pin design was used when the free-standing barrier was originally tested to NCHRP Report 350.
  2. The pin cap weld configuration was used when the steel strap tie-down was developed for the F-shape PCB. At that time, we used a 27 ¾" pin that mounted the cap plate 1" below the top of the pin. This cap was attached with ¼" fillet welds on both the top and bottom of the plate.
  3. The remaining F-shape PCB testing was conducted with a 28" long pin with the pin cap mounted 2.5" below the top of the pin. The pin cap for this design was welded with a ¼" fillet weld on the top of the pin cap only. This pin was a design originally submitted directly to us by KsDOT when we switched from the two loop to three loop connection design. It was used in both the MASH testing and the other tie-down and transition testing conducted at MwRSF.


Based on the different configurations above, we have typically recommended that the second configuration with top and bottom fillet welds be used. However, the single fillet weld design has passed the free-standing barrier MASH test, and it was used in all of the tie-down and transition designs excluding the steel strap tie-down. Thus, it would be okay to use the third pin configuration as long as you did not plan to use the steel strap tie-down. The steel strap tie-down would still require the second pin design.


My previous weld areas were calculated based on the second pin design. If you went with the third option, then your revised T-pin design would require ½ the weld area. This would be a throat area of 0.694 in2. I don't believe that you can get that much weld area with the welding of the T handle directly to the pin. Thus, some form of retrofit would still be needed. However, the retrofit I proposed could be simplified by only using the fillet weld on the underside of the pin cap and then welding the T-handle to the top of the pin cap plate. No filler weld needed on the top of the pin cap to attach it to the pin.
Attachment: https://mwrsf-qa.unl.edu/attachments/190dd679cd20782f77a221933fd62236.PDF


Response
Date: 02-14-2011

We have put together a design which is similar to what Bob had suggested below. Please see the attached PDF.


We are proposing that the plate be attached with the ¼" fillet weld on the underside of the pin plate. We are not proposing any additional welding on the top side of the plate. The proposed modified pin design does state that this design is not to be used with the steel strap tie down.


Attachment: https://mwrsf-qa.unl.edu/attachments/43fabd9e455d343cc84e8a8a78072db8.PDF


Response
Date: 02-14-2011

The detail looks consistent with our discussions, and I have no issues using this pin.




Concrete Barrier Profile

Question
State: MO
Date: 11-29-2010

On the six laning of Route 65 -- there is concrete barrier being installed in the median to divide NB from SB

There is a curve on that job where the median is wide and the top of barrier on the high side of super was up to 3 ft more than the top on low side


We asked bridge office about this and they had a retaining wall/stepped barrier designed by the consultant


The contractor has poured the high side retaining wall portion of this median barrier and they are now trying to slip form in the half on the low side --( there is a base that was poured with the high part already for it to set on)


The contractor is having trouble with the slip form machine kicking out at the top potion as they pour this front face . They are ending up with an approx 2 inch gap behind the barrier at the top-- looks flush at the bottom


the contractor wants to fill this gap with expansion grout -- which construction thinks would be ok .


The question is : if the face of the barrier ends up more vertical -- by approx 2 inches, than standard , is that a problem from the crash/safety standpoint?

I don't believe this is a problem since the more vertical a face is, the safer it usually is. I am concerned about the fact that this is not how this particular barrier was tested.



Response
Date: 11-29-2010
I am not concerned with a barrier shape with a front face that ranges between the single slope and vertical cross sections. I also agree that it would likely be beneficial to fill the back-side gap to prevent water from penetrating into this region and causing damage during freeze-thaw cycles.


Thrie Beam Guardrail Transition

Question
State: KS
Date: 12-01-2010

I am trying to upgrade our drawings to the MGS. I have some questions on the transition section as shown on our new drawing, RD613A (see att.). Initial thoughts were to keep our similar thrie beam section with the 6'6" post as shown on RD613 (original drawing) for the 31'-3" total length of transition. Note: all of our current post lengths are 6'6"; thrie or w-beam sections as shown on RD613. For our new MGS drawing (RD613A), do we need to update the post in the thrie beam section to 7' similar to your testing? At the thrie to w beam section of rail, your post were shown as 6' long and we are still showing 6'6". Should this change to 6'? I recall some conversations that states could keep their particular thrie beam transition section but wanted to verify some of these thoughts. Any help would be appreciated.


Attachment: https://mwrsf-qa.unl.edu/attachments/41624b4b5bbf0b0c78df0bafa5a4cf7b.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/c62aabd24629803048b11dea249617ed.pdf


Response
Date: 12-09-2010

The original Kansas DOT thrie beam approach guardrail transition (RD613) utilized 12 ft " 6 in. of nested thrie beam and 12 ft " 6 in. of single thrie beam between the bridge end and the thrie beam side of the W-beam to thrie beam transition section. A 6 ft " 3 in. long W-beam to thrie beam transition segment was then used, followed by standard W-beam guardrail. A 1 ft " 6¾ in. post spacing was used through the nested thrie beam section, while a 3 ft " 1½ in. post spacing was used over the single thrie beam as well as the transition element. Steel and wood posts were denoted to be 6 ft " 6 in. long through and including the post at the midpoint of the transition element.

In comparison to several other NCHRP 350-approved thrie beam approach guardrail transitions, detail RD613 is a relatively long design which was intended to provide a gradual change in lateral stiffness for impacts near to and upstream from the bridge end.

In recent years, the Pooled Fund program sponsored the development and testing of a stiffness transition for the upstream end of the original thrie approach guardrail transitions. From this research, it was determined that the shorter transition designs may need to be extended in order to provide a more gradual change in lateral stiffness. However, we also observed a vehicle pocketing/rollover propensity when changes in thrie beam nesting, post spacing, and/or post type coincided.

In detail RD613A, it appears as though several posts were added beyond the W-beam to thrie beam transition section using a half-post spacing. Based on some of our prior examples for adapting the new stiffness transition to existing approach guardrail transitions, it would seem that several post could be removed beyond the W-beam to thrie beam transition segment such to have only 1 or 3 half-post spacings in this W-beam region. RD613A shows a 12 ft " 6 in. long segment of single thrie beam. Based on our adapted design variations, it would seem reasonable to utilize a 6 ft " 3 in. segment of thrie beam. Further, I may recommend that 6 steel posts be configured with a length of approximately 6 ft " 6 in. using a quarter-post spacing, similar to what is depicted in our draft transition report. It would also be worthwhile to use four shorter 6-ft long steel posts at quarter-post spacings and starting at your current post 7. All remaining steel posts would also use the 6-ft length. Section C-C depicts a 7-ft 6-in. long steel post. I believe you intended to depict it as a 6-ft 6-in. long steel post. In summary, it would seem reasonable to more closely match your detail to that proposed in Figure 96 of the draft transition report.



ADOT Long Span

Question
State: AZ
Date: 12-07-2010

Attached is our draft of the MGS Long Span Guardrail. We basically allow for one, two or three posts to be eliminated within the 25' maximum span. We call for the 3 CRT posts on each end when any posts are eliminated- would this still be prudent when only one or two posts are skipped? We also allow for 25' guardrail sections so the splice would be eliminated at midspan wherever it happens to fall in the guardrail run. We assume this also applies to the normal MGS guardrail runs when 25' rail elements are used. We would appreciate if you could take a look and provide any comments you feel appropriate.


Attachment: https://mwrsf-qa.unl.edu/attachments/d8679fa2e5471f5360ed661637163da2.pdf


Response
Date: 12-08-2010

When one, two, or three posts are removed in a row of MGS, MwRSF still recommends that three MGS CRT posts be utilized on the upstream and downstream sides of the 12.5 ft, 18.75 ft, and 25 ft unsupported lengths. The span could be accommodated with 25 ft long guardrail segments where the rail splice does not occur at the post locations. The CRT posts were modified for MGS Long-Span applications by raising the hole locations to account for higher rail heights. For metric-height guardrail, different CRT posts were used for long-span applications. As such, CRT posts for the MGS long-span uses different hole locations than those placed in the CRT posts for metric-height W-beam long-span applications. It may have been more clear if we had referred to the posts as MGS CRT posts or 31" guardrail CRT posts. As long as you provide dimensions, it should not be an issue. You might consider showing the two holes in the six posts adjacent to the long span in PLAN and ELEVATION views.

As a minor point, the guardrail posts appear to be square in the PLAN view. Also, there are some extra dashed lines in PLAN view across vertical surface between post and blockout " not sure if those are nails or something else. May also want to add barrier height to ELEVATION view.



Thrie Beam Height Guidance

Question
State: IA
Date: 09-21-2010

Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?

Response
Date: 12-13-2010

I am not aware of a documented height tolerance for thrie beam guardrail systems. Initially, one may attempt to argue that the minimum height could be as low at that corresponding to W-beam guardrail systems. However, I would suspect that mounting thrie beam with a top height of 27¾ in. would potentially increase the propensity for vehicle climb, barrier override, and/or rollover upon redirection due to the increase face below normal W-beam rail with same top height.

At this time, the roadside safety community has considered the minimum top height for W-beam rail to be approximately 27¾ in., while the maximum top height for the MGS is 32 in. At the minimum W-beam top height, a thrie beam element would extend downward to 7¾ in., thus potentially creating new safety risks. Selected thrie beam guardrail systems have successfully met crash testing guidelines when installed with a top height of 34 in. As such, it is my opinion that the minimum height tolerance for modified thrie beam guardrail may be somewhere around 31 in. for NCHRP 350, while the top height tolerance may be closer to 39 to 40 in. at the TL-3 impact conditions.

Thrie beam has been successfully crash tested over the years. Below, I have provided a few of the test results but not those for the T-39 thrie beam guardrail system.

Test No. System Description Top Rail Height Result

404211-5a Modified Thrie Beam w/ 81" Steel Post & Tapered Block 34" Passed 8000S TL-4 test

404211-11 Strong-Post Thrie Beam w/ 81" Wood Post & Wood Block 31.65" Passed 2000P TL-3 test

404211-10 Thrie Beam w/ 81" Steel Post and Routed Wood Block 31.65" Passed 2000P TL-3 test

471470-31 Thrie Beam (G9) w/ 78" Steel Post and Steel Block 32" Failed 2000P TL-3 test

471470-30 Modified Thrie Beam w/ 81" Steel Post and Tapered Block 33.6" Passed 2000P TL-3 test

Recent Test Thrie Beam (G9) w/ 78" Steel Post and Full-Depth Wood Block 31.5" Failed 2270P TL-3 test (MASH)

Although I have yet to see the results of the recent failed MASH test, it would seem reasonable that improved safety performance could be obtained by using a shortened wood blockout or the modified steel tapered (collapsible) blockout " both of which reduced climb and allow the lower corrugation to fold back.



Anchored PCB and Expansion Joints

Question
State: MO
Date: 12-15-2010

Do you have a piece of hardware that will handle the transition between two pinned-down temporary installations of pin & loop F-barriers over an expansion joint. The joint probably has a 7 to 10 inch throw. (See attached)



I spoke to Rory Meza and she showed me the Texas standard plan in which they simply cantilever one of the segments out over the joint. This would certainly allow for movement, but at full contraction, there would be a sizeable gap/snag point between the barriers, nor could the barriers be connected to one another. We've seen the state of Illinois use steel plates which cover the gap while one of the barriers slides independently. The plate is cast with studs into the other barrier.


This seems like a reasonable solution but I wondered how you have seen it handled.


Attachment: https://mwrsf-qa.unl.edu/attachments/c4f5c0c0a6e5cb6468396b8719228003.jpg


Response
Date: 12-15-2010
MwRSF has not developed a stiffened cover plate system for protecting a large gap formed between two rigid barrier ends with an interior expansion joint in the deck surface. However, I do recall that the Kansas DOT has developed a system but not sure of the details. We provided some feedback years ago, but their bridge division personnel performed the FEA analysis and design work. I suggest that you contact Rod Lacy or Scott King to acquire their details for such a connection. However, note that longer lengths of the gap could greatly degrade performance of the existing design.

Response
Date: 02-16-2012
Yes, we have an expansion joint barrier detail that was modeled by our bridge department like Ron indicated. Attached is that drawing in Microstation and Adobe formats. If you want to view the other concrete safety barrier details, you can download them for free at the website below in PDF or DGN format. Hope this helps!


KART service (free login):
http://kart.ksdot.org

Attachment: https://mwrsf-qa.unl.edu/attachments/a28d64af23e1d2174c9fc27dbd580c5f.pdf


54 Inch Barrier Length Necessary Before Pier

Question
State: FL
Date: 12-28-2010

Our team is currently designing a 54 inch barrier wall for abutting or intruding bridge piers. We plan on providing a transition length for the change from a 32 to a 54 inch high barrier. However, we would like to know if you could offer an opinion on the length of 54 inch barrier required before encountering the pier? I have attached a sketch to illustrate the location of the length desired by our team. We would sincerely appreciate any assistance that you could offer to us on this matter!

Response
Date: 02-03-2011
We have been able to discuss the FLDOT situation for shielding a bridge pier/abutment with a Test Level 5 (TL-5) highway barrier system. From your sketch, it is apparent that the FLDOT is seeking guidance regarding the recommended length of 54-in. tall, TL-5 barrier in advance of the tall hazard (i.e., critical pier/abutment).

To date, there is virtually no specific guidance for reasonably determining the length-of-need barrier protection for tractor-trailer impacts into bridge piers. Currently, AASHTO requires that 54-in. tall barriers be used to shield piers when placed close to the pier. Alternatively, 42-in. tall barriers have been recommended in situations when sufficient lateral clearance is provided between the barrier and pier. These recommendations have been made to prevent high-energy, tractor-trailer vehicles from impacting piers and causing catastrophic damage.

We understand that this AASHTO requirement can be quite costly to the DOTs, especially when considering the infrequent number of tractor-trailer impacts and high number of piers requiring shielding. As such, we have prepared our best guidance based on engineering judgment and experience with the understanding that a more refined recommendation would require further research.

In any event, we start with the assumption that the TL-5 impact condition involves a tractor-trailer vehicle striking a barrier at 50 mph. A TL-5 barrier would be used within the length-of-need to shield the pier and prevent a tractor-trailer vehicle from striking the pier. In your situation, the 54-in. tall barrier would be used per its limited lateral clearance. Upstream from the 54-in. tall barrier, a TL-3 rigid, reinforced concrete barrier with structurally-adequate anchorage would be connected to the TL-5 barrier and used to prevent errant passenger vehicles from encountering the pier/abutment structure.

TL-3 barriers measuring 32 in. tall are not capable of containing and redirecting tractor-trailer vehicles impacting at the TL-5 condition. However, we believe that these TL-3 barriers would be capable of dissipating significant energy to slow down the heavy vehicle, thus greatly reducing the severity and potential for tractor-trailer impact events into bridge piers. In addition, these TL-3 barriers would likely scrub-off speed during the initial contact with the front and upper barrier faces, and then again after the heavy vehicle had rolled onto its side behind the barrier and continued to slide toward the pier structure. As such, it was our goal to greatly reduce the tractor-trailer vehicle's impact speed with the pier under situations involving TL-3 barrier override or penetration in advance of the TL-5 barrier and pier structure.

It is our hope that the severity of the vehicle crash into pier could be greatly reduced, such as that occurring with a speed reduction from 50 to 25/30 mph. For an initial speed of 50 mph, we would expect to scrub off at least 5 mph prior to landing on the back side of the barrier. With the vehicle on the barrier's back side and potentially on its side, a trailer-trailer vehicle would then be further slowed with friction losses through vehicle drag (i.e., sliding and/or soil plowing). Using a coefficient of friction of 0.5 and a reduced initial speed of 45 mph, we calculated the distance over which the vehicle's speed would be further slowed to 25 to 30 mph. From this simple analysis, the required distance ranged from 75 to 94 ft. As such, we selected a distance of 85 ft for the full-height, 54-in. tall TL-5 barrier found upstream from the pier. Adjacent to the barrier, a 14-ft 8-in. long sloped transition segment would be utilized to transition the concrete barrier from 54 to 32 in. using a 8:1 slope, thus resulting in a total upstream combined length of approximately 100 ft excluding the TL-3 barrier.

In summary, we have utilized engineering judgment and experience to configure the length of a TL-5 tall concrete barrier system for protecting bridge piers " 84 ft of full-height barrier and 15 ft of transition to sum to 100 ft. Please note that this length-of-need guidance is likely conservative and is not based on any economic analysis.


Response
Date: 02-03-2011

Attachment: https://mwrsf-qa.unl.edu/attachments/fdd8fdc36c55fd7cd16f96df2526fba1.pdf