View Q&A

PennDOT PA Barrier End Section & AGT Configuration

State IA
Description Text

We are employing PennDOT’s PA steel-on-concrete MASH TL-5 barrier on our upcoming Mississippi River bridge replacement project at Lansing. We’ve previously discussed the superstructure setback distance, bicyclist pass-through protection elements, and 2” overall height increase for overlay associated with the barrier’s design. (BTW for your records, we ultimately decided to accommodate the full as-tested working width in our superstructure setback location).


We are now faced with some minor detail issues associated with attachment of guide rail to the PennDOT standard end section of this rail system. The 2 inch overall height increase creates a non-standard condition when compared to the PennDOT standard drawings, and the overall height of guide rail that PennDOT uses is also slightly less (-3/8”) than Wisconsin’s standard 2’-8 guide rail. Please review the attached markups of the PennDOT standard drawings for our proposal (“PennDOT PA Stds MODIFIED.pdf”). Essentially, we’d like to accommodate the 2 inch overall barrier height increase at the AGT by employing the symmetrical thrie beam guide rail transition configuration detailed in TRP-03-387-19, 34-IN. Tall Thrie Beam Transition to Concrete Buttress. Other minor detail adjustments are required in the steel connection plates at the AGT to PennDOT PA end section because of the overall 2 inch height raise, the minor 3/8” discrepancy between PennDOT standards, and the setup shown in TRP-03-387-19. Our posted speed for the affected segment will be 40 MPH (see “Lansing bridge with speeds.pdf”). Also attached are some illustrative 3D views of the proposal.


I believe you’ve already had some correspondence with Erik at Wisconsin DOT regarding the position of the first guide rail mounting post relative to the end of concrete parapet, which does not match the standard dimension in this scenario. Apparently this did not raise any concerns for you about AGT performance. Please reiterate that opinion as part of this review. Also, the AGT shoe in this configuration requires 4 additional bolt holes to accommodate the PennDOT connection detail. Please assess this configuration for us, since both WI and IA typically use a standard 5-hole shoe.


We are also interested in your opinion of the PennDOT standard terminal configuration of the steel tube traffic rails. If you identify any potential concerns with the details for our conditions, please let us know if you have any recommendations for improving the details.


As always, thank you very much for your time and expertise in reviewing these items.



Bridge Rails
Stiffness and Height Transitions

Hybrid Bridge Rails

Transition and Attachment Structures

Date April 21, 2022
Previous Views (64) Favorites (0)
Attachment Lansing bridge with speeds.pdf Attachment PennDOT PA AGT 3D views.pdf Attachment PennDOT PA Stds MODIFIED.pdf Attachment Lansing bridge with speeds.pdf Attachment PennDOT PA AGT 3D views.pdf Attachment PennDOT PA Stds MODIFIED.pdf

We reviewed the proposed modification of the PennDOT system and believe that it looks acceptable for the most part. We do have a few comments.


  1. The overall 34” AGT configuration is generally good. I would note that the system should in your plans uses timber posts at ¼ post spacing. The 34” AGT we tested to MASH used W6x15 posts at ½ post spacing. We have not evaluated the 34” AGT with the configuration shown. However, it likely have similar of slightly lower stiffness than the tested AGT. Given the 40-mph posted, speed. We would believe that it will perform adequately.
  2. The transition and shielding of the upper bridge tube currently consists of using a 30 degree angle vertical taper with lateral flaring of the taper back from the face of the bridge rail and thrie beam. I am not aware if this configuration has been tested. We have tested a similar top tube with an AGT for Illinois and Ohio. In that system we used a 2:1 vertical taper and not lateral flare and it worked acceptably. Note that this is slightly less aggressive than the 30 degrees you have shown (26.6 degrees).
  3. Flaring of the upper bridge tube laterally may expose the termination or end of the lower bridge tube rail to vehicle snag as the top tube is not hidden behind the slope of the top tube due to the flare. This may be a concern for vehicle components extending above the thrie beam. Thus, one may want to consider revising the top tube termination to be closer to the 2:1 vertical taper without lateral flare we tested previously as it should perform safely and shield the lower tube end as well.
  4. Placing the first AGT closer to the end buttress is not an issue as you noted in your email. We would concur.
  5. Currently, there is a 1/2” thick steel mounting place between the thrie beam end shoe and the concrete parapet that may pose a hazard for reverse direction impact snag. In previous testing, we have observed wheel and fender snag that was hazardous on vertical edges that are that thick. In order to alleviate that issue, one could make the plate ¼” thick or less or bevel the vertical plate edge so it does not post the same snag hazard.


Let me know if you have comments or questions.


Date June 15, 2022
Previous Views (64) Favorites (0)

Thanks for the thorough feedback.


A couple of questions come out of a closer look at applying your recommendations. These are illustrated in the attachment.


Q1.  Does extending the PennDOT PA standard ½” thick mounting plate slightly (0.5 inches) downstream in order to get the tapered edge out from behind the AGT shoe create any concerns?

Q2.  The 2:1 steel top tube gets excessively long if the end is to be held entirely below the top of thrie beam. While I don’t think this presents any constructability or fit-up concerns, it’s very unusual to have the steel portion of a combination barrier extend beyond the concrete parapet. Even if we were to move the bend point of the tube closer to the post, I don’t think we can gain the roughly 4.5 inches necessary to keep the end of the steel coincident with the end of the concrete. Any thoughts or suggestions? Must the entire top tube terminus be kept below the top of thrie beam?


Date June 24, 2022
Previous Views (64) Favorites (0)
Attachment PennDOT PA AGT REVISED 3D views 06162022.pdf

In response to your questions:


  1. We don’t see any issues with extending the plate so that the bevel can then make a smoother transition slope to the end shoe. That makes for a more snag resistant setup.
  2. With respect to the upper tube, We would recommend that the tube extend below the thrie beam element to mitigate snag. I don’t see any big issues with extending it past the concrete, but it may be unusual for your typical details. One recommendation would be to extend the lower tube longitudinally to the sloped tube in order to better shield the end of that tube and to better support the sloped tube.

Date June 27, 2022
Previous Views (63) Favorites (0)
Attachment Proposed Modifications.jpg