View Q&A

(for context, see View Q&A | University of Nebraska–Lincoln (unl.edu))

Remember last fall when I asked about the application of the TL-5 PennDOT PA barrier on a through-truss bridge? Well, I have a follow-up question (just 9 months later).

As it turns out, our posted speed for the proposed truss bridge will be 30 MPH, not 45 MPH. The roadway is fairly low in traffic volume. Anticipated traffic levels for 2025 are AADT = 2900; for 2045 AADT = 3000; and Trucks = 9%. See the attached cross section view.

Since the bridge is over 1700 feet long, we would like to optimize the bridge’s width to the minimum clearance envelope for efficiency. In light of the very low posted speed and low number of trucks, would a reduction of the 4’-4 working width of the PA barrier in TL-5 conditions be acceptable in these less critical conditions? We are hoping to be able to use a 3’-0 setback distance to the superstructure.

As references for this case, our consultant compiled the attached list of recently constructed midwestern bridges with superstructure above the roadway. Of particular note is the South Omaha bridge over the Missouri River, which is a through truss on a roadway posted 45 MPH with just 2’-0 of setback distance and a vertical face TL-4 barrier (see photo). I’ve wondered whether MwRSF was consulted by NDOR on this project with respect to the setback distance question. It seems to me that even if the truss were strengthened to resist collision force by a truck (SUT or TT), questions regarding vehicle redirection and stability, occupant risk factors, etc. would remain unresolved without crash testing or simulation.

I do recall our previous conversation. As you noted, the as tested ZOI of the PennDOT system was 52” based on the working width information from the test. I believe previously we noted that this ZOI was largely due to lean of the truck box. However, we still consider the truck box to have sufficient structure to potentially damage the bridge truss structure if it was contacted.

I appears that you would like to lower the offset to 3 ft. based on lower speeds and lower ADTs.

We would think that that seems reasonable especially in light of the lower speed. The reduction in speed shown would lower the impact severity and kinetic energy by more than a factor of 2 when going from 45 mph to 30 mph. One would expect that this would significantly lower the amount of box lean and potential for contact with the truss structure. I do not have a good way to quantify exactly what that reduction in extension over the barrier would be, but it should be fairly significant.

To further support your argument, NCHRP 12-90 contained recommendations that recommended reducing the offset from bridge piers for barriers taller than 42” to 3.25 ft for MASH TL-5.

Prior guidance indicated that if bridge pier components were located closer than 10.0 ft from the shielding barrier a 54.0 in. tall barrier should be used. Unfortunately, there are at present no MASH TL-5 crash tested rigid barriers that are taller than 42.0 in. While rigid barriers taller than 42.0 in. are likely to reduce the roll of articulated trucks, recent full-scale crash tests and finite element simulations indicate that the modern trailer suspensions result in much more stable impacts than was the case in crash tests of older trailer suspensions. If a trailer does lean over the barrier and contact a pier component, the interaction will not involve the entire mass of the vehicle and is unlikely to develop sufficient impact forces to put the pier system at risk of failure. It is preferable to allow at least 3.25 ft of space between the top traffic face of the barrier and the face of the nearest pier component to minimize contact with a heavy vehicle. In some retrofit situations, however, it may be impossible to provide this amount of space given the existing pier location and arrangement of the roadway. In such cases it is permissible to place the barrier closer to face of at-risk pier components.

Thus your reduction to 3 ft for a lower speed would be supported. Note that this recommendation is based on modeling and not full-scale testing.

In the previous response below, we also noted that our TL-5 test of a 49.25” tall near vertical concrete barrier had a working width of 37.4”. The ZOI when accounting for the offset of the top corner of the barrier from the base due to slope of the barrier face (7.875”) would be 29.525”. Note that in the original email below, I must have miscalculated the offset from the top corner to the base of the barrier which made the ZOI slightly larger. Note that the PennDOT barrier had a dynamic deflection o 20” in the full-scale test. That dynamic deflection likely led to the increased ZOI number observed in that test. However, we would expect the bridge rail deflection to be reduced with the lower speeds as well due to a 63% reduction in the impact severity for the posted speed as compared to the PennDOT test. Again, it is hard to quantify that directly without further analysis. This would tend to further support your use of the 3 ft offset for you lower speed application with a slightly taller barrier.

We did not assist NDOT with the selection of the barrier height and offset to the truss structure for the bridge rail photo that you sent, but states are free to make their own guidance based on test results, speeds, ADTs, etc.

We don’t see any real issues with the offset proposed for the 50” tall PennDOT system.