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54 Inch Barrier Length Necessary Before Pier

State FL
Description Text 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!
  • Bridge Rails
Other Keywords none
Date December 28, 2010

Response 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.
Date February 3, 2011

Date February 3, 2011
Attachment 00410-27of28_v8.pdf

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