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

Question
State WI
Description Text 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.
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Date October 4, 2010
Attachment Hardware Application Memo.pdf


Response
Response

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.

Date October 7, 2010


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