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Iowa Transition and Curbs

State IA
Description Text

You have alluded several times over the years that the Iowa guardrail transition might not need the 4-inch curb installed below it. Could you please follow up with FHWA on this issue to see if the 4-inch curb requirement can be waived? There are many situations where installation of the curb would be difficult.


Here is a link to our current bridge end post styles and guardrail attachments (Types A, B, C):



And here is a link to our current transition:


  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date April 22, 2011


I have been asked by the State of Iowa to follow up on an old issue pertaining to approach guardrail transitions. In the mid to late 90s, MwRSF successfully developed, crash tested, and evaluated two thrie beam approach transition systems for use in shielding and attaching to the ends of safety shape and vertical concrete parapets. A series of three acceptance letters were prepared on this topic - B-47, B-47a, and B-47b - based on the NCHRP Report No. 350 guidelines. Crash tests were performed on both steel and wood post systems which were spaced on 1 ft - 6 3/4 in. centers near the bridge/parapet end. During the R&D effort, the Pooled Fund States stated that they may desire to utilize a curb at the ends of the bridge rail to better accommodate water drainage. As such, MwRSF incorporated a 4-in. tall wedged shaped concrete curb under the thrie beam region. MwRSF believed that the curb's presence would provide a critical evaluation as impacting front wheel/rim could become wedged under and upward into the tighter space and/or increase vehicular instabilities. If testing with the curb was found to be satisfactory, then MwRSF researchers believed that the non-curb option would also be acceptable.

Later, TxDOT and TTI cooperated to further investigate some changes to the approach guardrail transition, including an elimination of the steel connector plate under the thrie beam end shoe, the elimination of the curb, a different geometry to step back the lower concrete toe near the barrier end using steeper flare rate, and non-use of special chamfer at parapet end.

In the MwRSF testing program, a special connector plate was used to allow the thrie beam and show to remain vertical while attached to a sloped face of the NJ safety shape. In the TTI testing program, the steel connector plate was eliminated, and the rail/end shoe was twisted backward near the top region to lay on the upper sloped face. Subsequently, the modified transition was crash tested under NCHRP 350 using a pickup truck. During this test, the pickup truck rolled over. TTI researchers later concluded that the curb was necessary to provide acceptable safety performance even though several other changes were incorporated. In MwRSF's opinion, the other system changes could also have contributed to the poor barrier performance. Unfortunately, this testing program was stopped and only continued with the development and testing of a TL-2 W-beam transition.

During NCHRP Project No. 22-14(2), MwRSF conducted another pickup truck crash test into the approach guardrail transition system noted above in Paragraph 1 but using the forthcoming MASH criteria. One crash test was performed on an identical transition system which included the curb underneath. Following testing, the barrier system was judged to provide acceptable safety performance. In this effort, higher lateral barrier deflections were observed as compared to those found in the prior successful MwRSF testing program. Once again, MwRSF considered the system to be acceptable both with and without curb. However, State DOTs are unable to use the system without curb due to the language provided in the original B-47 series of acceptance letters.

To date, no full-scale crash testing has been performed on an identical approach guardrail transition system that excluded the wedged-shaped curb.

As such, I am inquiring as to whether FHWA continues to maintain that the approach guardrail transition must be installed with the curb located underneath or whether there has been any changes in stance on this issue.

Regardless, I will forward your FHWA's response to the State of Iowa and the Pooled Fund Program member states. Thanks again for any clarifications on this issue in advance!

Date May 2, 2011


We have reviewed the crash tests videos of subject testing and offer the following appraisal of that review.

Via careful review of crash tests provided by TTI, we conclude the decision which TTI has rendered from their crash test is correct (i.e., transition w/o curbing failed).

We however remain open minded on any additional testing brought forward to our attention in the future.

Therefore and until additional testing is presented, FHWA continues to maintain that approach guardrail transition must be installed with the curb and/or rub-rail located underneath rail element to be considered acceptable installations constructed on NHS system.

Date June 7, 2011

Response Thank you very much for the additional review and consideration on behalf of the State DOTs. If any new crash tests are performed in the future which are believed to potentially alter FHWA's opinion, we will bring this material and results to your attention. Once again, thank you for the additional examination. I will forward FHWA consistence stance on this matter to the Pooled Fund Program member states.
Date June 7, 2011

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