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Allowable exposed offset for PCB to median gate attachment hardware

Question
State WI
Description Text

WisDOT is planning on installing some median gates with temporary barrier.  The manufacturer's drawings indicate that the face of thier gate anchorages and the face of our temporary concrete barrier will not be flush.  One manufacture indicated that the difference could be up to 1".

How much of a difference between the face of our barrier and their anchorage would be considered a snag issue?

I believe that there was some crash testing done at MwRSF indicating that 3/16 or 1/4 inch plate caused a vehicle to roll over. Would this be a good rule of thumb with steel?

Shouldn't manufacturers be able to provide gates that don't have snag issues with out doing a crash test or significant amount of engineering?

 

Thanks

 

Keywords
  • Temporary Barriers
Other Keywords gates
Date November 7, 2012


Response
Response

I have some comments regarding the use of median barrier gates with TCB segments as well as the snag issue you brought up.

First, you are planning to install a median gate system on a run of TCB's. This type of installation poses some concerns as median gate systems were typically designed and tested with permanent concrete parapets. Thus, you will likely need to anchor both sides of the TCB segments adjacent to the gate in order to provide similar deflection performance as compared to a rigid parapet. Currently, we do not recommend anchoring on the front and back sides of a TCB system. However, we have seen in past testing that pins on the backside of a barrier may cause excess rotation and tipping of the barrier which in turn can produce vehicle instability. Thus, we currently do not recommend pinning on both sides of the PCB when placed in the median except for the transition section which we tested. In addition, anchoring both sides of the TCB system will still allow some level of deflection which will be greater than the rigid parapets the median gates were designed and tested with. Thus, there are concerns that the use of a median gate system may be affected in some manner when used with anchored TCB's.

That said, your original question was regarding the level of vertical asperity that can be present on the face the barriers due to the attachment of the steel gate hardware to the TCB segment. Previous research has shown that vertical asperities can affect vehicle stability in a negative manner.

1.       MwRSF conducted testing on an anchored steel temporary barrier section formed by welding together stacked steel H-sections. The H-sections were connected using 3.5"x15"x0.375" vertical steel plates. In the first test of the system, test no. HTB-1, the 2000P vehicle impacted the barrier and rolled. From an analysis of the test results, MwRSF researchers believe that the rollover was caused by snagging of the pickup truck on the barrier. More specifically, the steel rims and sheet metal snagged on the 3/8" thick vertical straps holding the barriers together, the separation between the barrier joints, and the top of the barrier section. This conclusion was based on the damage to the vehicle's right-side sheet metal and steel wheel rims as well as the right-front fender being pulled down during the test as observed on the high-speed film.

 

Following this investigation, MwRSF researchers determined that the safety performance

of the H-section temporary barrier tie-down system (Design No. 1) could be significantly improved by reducing the potential for snag. In order to eliminate the snag potential, two modifications were made to the barrier tie-down system. First, the vertical steel straps positioned on the traffic-side face of the barrier were removed and replaced with a longitudinal seam weld. Second, the anchor bolts used in conjunction with the drop-in anchors were changed from ASTM A325 to ASTM A307 grade bolts. The bolt grade reduction was made in order to reduce the load capacity of the tie-down attachments and allow for a slight increase in the deflection of the system. It was believed that allowing slightly higher deflections would potentially reduce any additional vehicle snag on the top of the barrier section and at the joints.

 

A second test, test no. HTB-2, was performed on the modified system with

a ¾-ton pickup truck and was determined to be acceptable according to the TL-3 safety performance criteria presented in NCHRP Report No. 350.

Based on this research, it would appear that vertical asperities of 3/8" or more can contribute to vehicle instability.

 

The file 'HTB.zip' (23.5 MB) is available for download at

http://dropbox.unl.edu/uploads/20121128/6cb3d21f9c7e7a96/HTB.zip

for the next 14 days.

It will be removed after Wednesday, November 28, 2012.

2.       Previously, MwRSF has provided guidance with respect to the allowable offset for alignment of permanent concrete parapets and temporary concrete barriers.

 

With regards to permanent concrete barrier, we would recommend keeping the lateral offset or alignment offset minimized to eliminate snag. Variations of 1" or less would be preferred.

 

For the temporary barrier installation shown in your photo, we would prefer that the alignment gap be 1" or less, but we believe that gaps as large as 2" are likely permissible. The rationale behind the larger alignment gap allowance is that temporary barrier segments will move when impacted and cause changes in the alignment gap as the impacting vehicle reaches the barrier joint. Thus, a joint that has a given initial alignment will move change alignment as the barrier is impacted. This allows for more tolerance for the temporary barrier gap. Alignments gaps larger than 2" would indicate problems with the temporary barrier joint and would require investigation.

 

Keep in mind that these gaps were specific to concrete barrier overlaps where the concrete would be expected to fracture and give when snagged.

 

3.       TTI conducted research in NCHRP 554 regarding aesthetic barrier design and the size of vertical asperities allowable for concrete barriers. This research found a range of performance for vertical asperities dependent on the angle, depth, and the width between asperities. Crash testing conducted as part of this project found that vertical concrete ridges as deep as ½" could result in failure.  Further simulation analysis found that vertical steps of ¼" were acceptable.

In addition to the above studies, there is some concern that gaps between the sheet metal of the gate system and the concrete barrier could be opened further during impact and increase vehicle snag as well as compromise the structure of the gate connection. As such, we would recommend keeping the gap to a minimum. This should be achievable through using steel plates to bridge from the exposed edges of the gate hardware until it is flush with the surface of the PCB. The thickness of these plates should likely be limited to ¼" based on the issues with 3/8" plate in the HTB testing and the results of NCHRP 554.

Date November 14, 2012


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