Please see my responses below. Thank you for you assistance.
I am trying to get the installation straight in my head.
1. It appears that you have temporary barriers on the far upstream and far downstream ends. Are these free-standing or anchored?
I understand they are not anchored to avoid holes in the bridge deck and they are not close to a drop off.
2. Next you have two types of “special barrier “ sections. Are these temporary barriers as well? Are they anchored or free-standing? What are the connections between the barrier sections.
As above, I think these are not anchored. They are temporary barriers. One section is right in advance of the spanned are and the other one is outside the spanned area. I assume they are similar to our standard F shape barriers. I can ask for connection details if you like.
3. On page 2, the special barrier sections appear to hang of the edge of the road surface? Is this correct or is the road surface only removed at the 6 ft opening?
he road surface is only missing for the 6 foot section through the finger joint and only for half of the roadway.
Thanks for the responses.
I have reviewed the detail you sent. In general, I think that the proposed solution can be made to work. I have a some comments and thoughts.
1. TTI recently designed and tested a median barrier gate that uses a tubular structure that is hinged to protect an opening in a permanent concrete median barrier. This system is somewhat related to what you are proposing, but it was for permanent barrier. The sizing and connection details may be useful.
2. There are other gate systems such as the Armor Guard system that could be applied. However, it sounds like you need some clear area under the opening that these systems will not provide.
3. The tubes in your system are 8"x8"x5/8" tubes. These have slightly lower bending capacity than the TTI design which used 12"x12"x1/4" tubes. However, that is not believed to be an issue due to the shorter span length in your design.
4. The attachment of the tubes near the end of the concrete barrier appears to be done using a bent plate over the front of the tubes and some bolted brackets. It appears that this might be able to be simplified and made safer. The current bent plate bracket would have potential for vehicle snag on the vertical edge of the bracket. I have proposed a revised detail with at bent plate behind the tubes. The tubes would be welded to this plate and the plate could attach to the barrier at several locations. This would reduce vehicle snag and provide for a more positive attachment to the barrier.
5. There will likely need to be additional attachments from the tubes to the barrier than the two shown adjacent to the opening. In order to prevent the tubes from flexing or prying off of the face of the barrier we would recommend additional attachment of the tubes near the tapered ends. You may want to have an additional set of attachments near the start of the tube taper. It is best to be conservative in the attachment scheme given the system is not crash tested. You could use the an attachment similar to the one shown above. Alternatively, you could through bolt through the tubes and barrier in the tapered region as shown below.
6. The current configuration shown has tubes on only the impact side face of the barrier. While this does provide the redirective surface for the impacting vehicle, it is not optimal in terms of developing continuity across the barrier opening. For a system like this, you want to have the barrier act like a continuous unit across the gap. This means development of shear, tension and compression loads. Placement of the tubes on the front side only will handle the shear and compression, but may not be as effective in development of the tensile bending stresses between the barriers. The tubes you have are very strong, so their capacity along may be sufficient to develop continuity as long as they are very effectively anchored to the concrete barriers. However, it may be better to place tubes on the front and back side of the installation or a steel plate across the backside of the installation in order to create a stronger span that engages more effectively with the TCB on each end.
7. Another concern would be snag of the vehicle on the tow of the concrete barriers you have shown. Currently you are transitioning from the sloped face TCB to at partially vertical face for mounting the tubes. However, the barrier tow that remains can be a significant snag hazard that can cause rapid vehicle deceleration and instability. We would recommend removal of the barrier tow and conversion to a purely vertical shape with the tube offset front the barrier sufficiently to prevent snag on the end of the concrete barriers.
8. The steel tubes are currently tapered down at the ends to prevent snag. The taper shown is approx. 4:1. We would taper it more gently. An 8:1 or shallower taper is more appropriate.
9. A simpler option for the design may be a specialized concrete barrier segment in lieu of the steel tubes. You could place the vertical cutout needed at the base of the barrier and not have to deal with all of the attachment concerns with the steel. The concrete section would need to have flared back sections on the ends of the vertical opening to prevent snag on exposed concrete. We do this on open concrete bridge rail posts and approach guardrail transition parapets.
10. Depending on the type of connection used, the size and weight of the barrier segments, and the potential speeds and impact angles in this area, we would expect this type of system to deflect a significant distance when impacted. TL-3 displacements have been over 2 meters for MASH tests of F-shape TCB systems. Thus, you will need to consider the barrier displacement and worker exposure and positioning in the design. If sufficient displacement distance cannot be achieved, one would need to consider anchoring of the barrier system.
Take a look at these comments and let me know if you have questions or want to discuss things further.
Thank you very much. You touched on all of the things that were giving be concern. I will pass this along to our consultant. If they have additional questions I will send you another note. The quick turnaround is greatly appreciated.
I need to shorten the top strap for the MGS mounted to culvert parapet. The NDOR typical parapet is only 8" wide. What is the correct offset behind the threaded rod anchor? Should the cover over the threaded rod be 2"? or centered in the parapet? What should the strap length be?
For the culverts in which the headwall is narrow (yours are 8"), I would not utilize the top-mounted, single-anchor design to attach the socket to the outside of the headwall. For that design, it's important to maintain the 7" anchor offset from the outside face to prevent concrete damage. Unfortunately, that will not leave you enough concrete cover on the inside of the anchor, 2 inches is recommended. We never designed the top mounted attachments for offsets less than 7".
However, you could utilize either the wrap-around design, or the side-mounted design (through bolt). See pages 36-41 of the report (TRP-03-277-14) for the design details of these attachment options. The only difference you would need to make is the length of the strap or bolts to reflect the correct headwall width.
Note, although only 2 of the 5 design concepts were included in the final drawing details, MwRSF has confidence that all five of the concepts provide adequate strength to support the system. Thus, any of the five concepts can be utilized to satisfy the installation needs of existing culverts.
We had project where the contractor supplied unhardened washer for bolting the rail to the post on normal MGS.
I was trying to determine if unhardened washers are acceptable. If they are what material spec should be used?
We do not require any washer under the nut where the guardrail bolt attaches to the post flange. In fact there are no washers at all on the standard steel post versions of the MGS that have been full-scale crash tested. That said, there would be no adverse affect of including a washer between the nut and the flange on the back of the post.
The wood post version of the MGS does use a washer under the nut on the back of the post. The washer used is the ASTM F844 washer typically specified with the A307 post bolt.
In the past, we have considered the use of deeper blockouts in limited cases dependent on system in question. We have used 16" deep blockouts in certain systems, but we have not used 24" deep blockouts in system due to concerns that the additional blockout depth may begin to affect the way the guardrail post is loaded and may increase the potential for later-torsion buckling of the post rather than the desired post loading modes of strong axis bending and rotation of the post through the soil. As such, we have limited these extended blockouts to a single post in a run of guardrail in order to deal with obstacles or other issues.
As you noted in the message you sent, we have allowed deeper blockouts in approach guardrail transitions in the past. The concern for altering the post loading is less prevalent for the transition posts as they tend to be closer spaced and deflect less, which lowers the concern for buckling of the post.
Thus, we believe that it would be possible to use large blockouts for post nos. 2-4 shown in your detail without adversely affecting performance due to the special circumstance you are faced with. However, for general installations we would recommend using the tested configuration as the use of the deeper blockouts has not been formally investigated or tested.
We have conducted research for WisDOT in the past on a related issue of spanning obstacles in a transition and came up with some potential solutions. Take a look at the report below. There is an option in it for deeper blockout posts with a beam spanning the gap that may work for you as well.
A couple of other items to note. First, I am not familiar with the curb section that you are using with the transition. I believe this transition was testing with a 4" wedge curb. As such, other 4" curbs may work with the transition as well, but higher curb sections may require further investigation for use in the transition. The exact dimensions are not listed on the detail.
The detail you have shown also appears to be longer than the transition sections we have tested to MASH with the MGS system. You may have a rational for using a longer transition section, but I wanted you to be aware that the transition may be able to be shortened.
I don't know of any test TL-2 combination bridge rails offhand.
We have done or have seen related research in the past that Illinois is currently using.
In 1998, the MwRSF developed and full-scale crash tested a combination traffic / bicycle bridge railing to TL-4 of NCHRP Report No. 350. The bicycle railing consisted of steel posts and rail segments mounted to a 32-in. tall New Jersey shaped concrete barrier. The barrier system utilized steel cables strung through the longitudinal rail elements to retain fractured railing segments during severe impact events. However, during crash testing, numerous spindles were broken free from the larger longitudinal tubes.
Another research effort was conducted regarding pedestrian railings for Missouri Department of Transportation. Two combination traffic/bicycle bridge railings with horizontal, tubular steel rails for use on a rigid, single-slope, concrete barrier were designed, constructed, and full-scale vehicle crash tested according to NCHRP Report No. 350. The first test consisted of a 2,015-kg (4,442-lb) 1998 GMC C2500 pickup truck impacting at an angle of 25.6 degrees and at a speed of 101.5 km/h (63.1 mph). The pickup snagged on the longitudinal rails during climb and eventually rolled, resulting in test failure. For the second test, modifications were made to the system in an attempt to reduce vehicle penetration and prevent rolling. The second test was also conducted with a 1998 GMC C2500 pickup truck. The pickup weighed 2,029 kg (4,473 lbs), and impacted the system at an angle of 25.6 degrees and at a speed of 102.7 km/h (63.8 mph). Once again, the pickup snagged as it climbed the barrier, resulting in vehicle roll and unsatisfactory results. The results indicated that the barrier system is not suitable for use on Federal-aid highways. However, it was noted that modifications could be made to the system in order to increase its chances of successfully meeting the requirements specified by NCHRP Report No. 350. One change was the use of an increased lateral offset for positioning the posts and rail farther away from the back side of the concrete barrier.
In 2013, the Illinois DOT began to develop a parapet-mounted bicycle railing system. Although Illinois DOT initially sought to utilize the barrier previously developed by MwRSF, concerns about the steel cables and vertical spindles led them to develop a new railing design based that combined the two combination traffic/bicycle rail systems described previously. The new design eliminated both the cables and the spindles while still satisfying AASHTO, FHWA, and Illinois specifications for bicycle and pedestrian railings. The steel rails were mounted and offset from the back of the parapet such that the rail faces were positioned 13-in. away from the front-top corner of the concrete parapet. Since this offset is greater than the Zone of Intrusion for TL-2 concrete barriers, MwRSF recommended its implementation as a TL-2 barrier without full-scale crash testing.
TTI has done several other TL-2 bridge rail tests, but I don't have all of the details for those. You may want to check the TF 13 bridge rail site and the 2006 FHWA bridge rail book (red).
Let me know if you anything else.
We have a single slope barrier from Caltrans that is 56" tall. But I don't believe that it has the vertical reinforcement required for pier protection.
In most cases, we have convinced our structures department and FHWA to hardened new structures for the large truck impact loads. It is cheaper to do and less of a hazard to the driving public.
have any details for a 54 inch single slope barrier they would care to share
It's not quite a single slope barrier, but here are details for the (almost) vertical shape with head ejection criteria that we've used.
I would like this information as well, as we are looking to create a 54" (TL-5) single slope bridge pier protection design for Ohio...
Florida has a 54" safety shape design. http://www.dot.state.fl.us/rddesign/DS/10/IDx/411.pdf
I have attached a link to a previous question on this topic from the Q&A website (ID #360). The drawings are of a 54" F-shaped concrete barrier with a footing for both interior and exterior sections. It was designed specifically for pier protection appilications (hence the footer). This could easily be converted to a single sloped shaped as long as the reinforcement remained the same (bar size, number of bars, and stirrup spacing) and the top with remained the same. I will caution against using this design as a vertical-faced barrier as the base would be narrow and may not provide enough over-turning moment strength.
Our median barrier meets the criteria, but we were looking for a roadside version. Our structures folks also plan to reinforce new structures to avoid the need for such a barrier, but we do still have bridges that don't have redundant piers, so those will require the protection.
What is the intended purposed for the 54" barrier? Is it for pier protection or glare screen/barrier combination?
We are currently working on new standards for single slope barriers and bridge rails. We are using the Texas (10.8 - 11 degree) sloped barrier. The heights will be 36", 42" and either a 54" or a 56".
The purpose of the 54" or 56" height is for a permanent glare screen on top of a barrier, not pier protection. Our current f-shaped concrete median barrier is 56" and our bridge version is 54", so we are currently trying to reconcile the two.
Bottom line, we will have (54" or 56") single slope, bridge rail and median barrier designs to share soon, but they will not be designed for pier protection. We will likely be considering them all MASH TL-4 barriers.
Thank you all for your valuable input. I may have a few questions as the day goes on, but want to answer Mike's question first. This barrier is intended for bridge pier protection. In general, our bridge designers are designing to the LRFD loading (600 Kips I think), but as Maria said, we have many existing structures which are not and some are in vulnerable locations. We have used 42 inch single slope barrier in the past with the Texas slope design, but we are curious if we should switch to either the steeper Caltrans design (9% ??) or Iowa's more vertical face with head ejection criteria. I am not aware of head ejection being an issue with the Texas Design, at least for a 42 inch high barrier, but am curious if the Caltrans design is more at risk for head contact. Maybe Scott could weigh in on this. I am a little concerned about the Iowa design being more difficult to construct, and also if the second, flatter face may allow tankers to slide up over the barrier? Also, it may be harder to transition down to 31 or 32 inches to connect to a crash cushion or MGS barrier. Maybe Scott and Chris could weigh in on these issues.
I would agree that Iowa's vertical-faced barrier is probably more difficult to construct than a single-slope shape due to the multiple angles. For this same reason, it may be slightly more difficult to transition down to a shorter height barrier. Having said that, however, the contractor on our first installation was able to construct the barrier and the transitions in accordance with our plans, and the end result looks good. I can't really comment on the barrier's ability to redirect a tanker truck, as it's my understanding that 90 inches is the minimum height needed to redirect such a vehicle.
Head ejection with the Texas version of the single slope barrier is tough to estimate. Some tests have the vehicle ride a bit up the slope and cause the vehicle to roll away from the barrier. Other tests show the vehicle tires staying down and the vehicle rolling slightly toward the barrier. The risk of head slap is definitely less with the Texas single slope than it would be for more vertical shapes. The magnitude of this reduction… I don't have a good answer for.
Was vehicle stability good in all of the tests you saw as the vehicle comes off the barrier (Texas design)?
I don't recall any vehicle rollovers, only a couple of pickup tests that had >25 degree roll angles. Textured single slope barriers have caused vehicle instabilities for the CA single slope. I would assume the same results would occur for textured TX single slopes.
I am a little late on the response but we do not have the single slope barrier. Below is what our bridge staff uses.
IADOT needs the following:
- 54-in. tall, single-face, reinforced concrete parapet with foundation system for use in shielding bridge piers according to AASHTO 3.?.?
- reinforcement design for the interior and end locations of wall and foundation
- design based on WsDOT report and other more recent TL-5 barriers with reinforced footings/grade beams/slabs
See the attached PDF file for a simplified drawing for a 54" tall, F-shape, TL-5 barrier. A few notes:
Is there a good solution to updating the 2" concrete lip on
Could This be filled with grout? Or steel plates? Cardbored?
When one raises the thrie beam end shoe off of the concrete surface and depending on the method, the threaded bolts could be loaded to a higher stress level due to combined bending and shear. I believe we have in the past used a higher grade of steel when using fabricated steel offset plates on sloped parapets. An offset late could be used but one would want to consider higher grade bolts and hardware that reduces bolt bending and maintains more shear loading. A concrete fill region could be used but may be difficult to cast/bond to old concrete. Reinforcement should be used in this scenario to help anchor the new concrete surfacing. I am not sure how successful this option would be for long-term durability and impact loading that may shatter off concrete patch. The cardboard option is not acceptable.
For new construction, I would eliminate the recessed region.
I have reviewed the attached drawings and have the following comments/edits,
· The spacing between the first S3x5.7 post (in socket on culvert) and the adjacent W6x9 post (standard line post in soil) can be either 37.7" or 75". The wider spacing may allow installations to avoid conflicts with W6x9 posts and concrete structures such as wing walls.
· The cross section view shows a standard line post and blockout configuration, but is labeled as an S3x5.7 post. The line posts (in soil) should remain the standard W6x9 posts. The S3x5.7 posts are placed in sockets which are attached to the culvert, and do not utilize blockouts.
· The notes section call out guardrail bolts for standard line posts. The rail attachment bolts for the bridge rail (and this culvert attachment) are 5/16" dia. A307 bolts. Further a 1.75" square washer (1/8" thick) is utilized between the bolt head and the face of the rail.
· The weak-post bridge rail should have backup plates between the posts and the rail. The original bridge rail was tested with 6" backup plates (6" long sections of W-beam). However, as discussed during the Dec. 17th pooled fund meeting, MwRSF will be recommending utilizing 12" long backup plates for all weak-post guardrail variations due to the consistent occurrence of rail tears forming during crash testing at both TTI and MwRSF. Oversized holes/slots will need to be cut into these 12" backup plates to fit over the splice bolts for the post locations that coincide with rail splices.
· Note section should also indicate that any of the 4 designs on sheets 3-6 are acceptable for use. All utilize the same post, just the attachment to the culvert is different.
· A note should be added to specify an epoxy with a minimum bond strength of 1,300 psi.
· The post length should be 44", not variable
· The post details on the right again show a standard line post to guardrail attachment (12" blockout and 5/8" bolt). This detail needs to be replaced with the 5/16" bolt, square washer, and 12" backup plate as discussed above.
· May want to add total length dimension of 9" (top to bottom) for top mounting plate.
· The 5-13/16" dimension should be from the bottom of the top plate to the center of the hole. The dimension as drawn (bottom of top plate to absolute bottom of plate) should be 7-5/16"
· The welds on the top plate gussets (shown on “top view") should be ¼" fillets. 3/8" welds are too large for ¼" thick plate.
I suggest a change to the bolts on post no 2 on the
low-tension cable system.
Could we install the bolts with the nuts on the top side of
the “slip Base" plates?
This would help with replacement when damaged.
I do not see a difference with the bolts oriented 180 degrees from the current configuration as this change should not affect their ability to slip out of the base.