MIDWEST STATES POOLED FUND PROGRAM
Standardized Concrete Parapet for Attachment of Thrie Beam AGTs
Sponsoring Agency Code
TPF-5(193) Supplement 81
During the last 20 years, several thrie beam approach guardrail transitions were developed and successfully crash tested according to the Test Level 3 (TL-3) requirements published in NCHRP Report No. 350. These approach guardrail transitions (AGTs) were used to attach strong-post, W-beam guardrail to various bridge rails and median barriers, including reinforced concrete parapets as well as beam and post systems. Although most AGTs look similar, each AGT has a unique combination of features including rail thickness, post size and spacing, and use of a hydraulic curb. However, due to the sensitivity of transition regions, these variables are not interchangeable between AGTs. Thus, each AGT is specific to its own features as well as the bridge railing or parapet to which it is anchored.
Crash testing has illustrated the sensitive nature of these AGT designs with recent failures occurring due to an alteration of an AGT feature (e.g., addition/removal of a curb or changes to the rigid parapet geometry and attachment hardware). The majority of these failures have been the result of excessive vehicle contact on the lower, upstream corner of the rigid parapet. This result indicates that the parapet toe and end geometry may be even more critical than previously believed. Thus, there exists a need to develop a standard concrete parapet end geometry for use with all thrie beam AGTs. Further, the new parapet design should be configured to allow the use or removal of curbs with any given AGT.
As part of a Pooled Fund consulting effort in early 2014, the MwRSF compiled a list of thrie beam AGTs that have been tested to NCHRP Report No. 350 and/or MASH standards. This list included a description of various features for each AGT as well as the general results from the crash test. This list provides the backbone for the literature review required to begin this project.
Within this historical testing are numerous examples of how slight alterations to an AGT can change the outcome of full-scale tests. The difference between a success and a failure can be as small as a 3-in. difference in post embedment depth, the geometry of the thrie beam attachment plate, or the addition/removal of a curb. One example corresponds with testing of a commonly-used transition consisting of nested thrie beam rail supported by posts spaced at 18Â¾ in. and installed with a 4-in. tall wedge shaped curb. This transition was originally developed at MwRSF for the Iowa DOT and successfully crash-tested to NCHRP Report No. 350 standards. During NCHRP Project 22-14 and the development of MASH, this transition was successfully crash-tested to MASH safety standards. However, a recent test of this system with the curb removed resulted in excessive vehicle snag and vehicle rollover. By developing a standardized parapet end section, MwRSF hopes to eliminate this issue and allow for the use of thrie beam AGTs with or without a curb.
The objective of this research effort is to develop a standardized concrete parapet end section for attachment of thrie beam AGTs with or without a concrete curb. The concrete parapet would be compatible with a wide variety of thrie beam AGTs, including those with variations in post type, post spacing, rail nesting, and use of a hydraulic curb.
Development of the standardized concrete parapet end section will include an updated review of existing thrie beam transitions to concrete parapets that were tested to either NCHRP Report No. 350 or MASH standards. Fortunately, a table containing many of these tests and their geometric characteristics was already formulated as part of a Pooled Fund Consulting effort in early 2014. The new parapet geometry will be based on the previous AGTsâ€™ crash testing results, dynamic deflections, and additional design and analysis. The design of the new parapet will then be submitted to the states for feedback and comment. The proposed parapet design will then be full-scale crash-tested with a selected critical AGT that provides the greatest risk of vehicle snag. Testing will be conducted according to MASH test no. 3-21 with a 2270P vehicle. Finally, a summary report documenting all design, testing, analysis, and conclusions will be written.
A single design for the concrete parapet end section at the downstream end of AGTs will simplify state design standards. No longer will transitions be associated with only a single concrete parapet shape. All thrie beam transitions will be able to connect to the new parapet. The designer then only needs to transition the parapet to the proper shape and height of the bridge rail.
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