MIDWEST STATES POOLED FUND PROGRAM
MGS Guardrail with an Omitted Post
Sponsoring Agency Code
TPF-5(193) Supplement 80
Post conflicts within a run of guardrail are a common occurrence. For very short underground obstacles, the obstruction may potentially be avoided by using modified post spacing. However, the only approved method for avoiding obstacles longer than 4.25 ft is to install a long-span system. The MGS long-span system was developed for situations where one, two, or three consecutive posts are omitted to create unsupported spans of 12.5 ft, 18.75 ft, and 25 ft, respectively.
The MGS long-span system was designed with requirements for minimum upstream and downstream length as well as a requirement for three CRT posts on each side of the unsupported span to prevent pocketing and increased rail loading. These requirements were based on full-scale crash testing of the MGS long-span with a 25-ft long unsupported span. Prior recommendations have been given to state DOTs regarding locations with only one or two posts omitted in a run of standard guardrail. These recommendations have tended to err on the conservative side and require the application of CRT posts adjacent to even a single omitted post due to lack of analysis and crash testing. However, the potential exists to develop more aggressive guidance for omission of one or two posts in a guardrail installation to avoid obstacles if further analysis and testing is performed.
Thus, a need exists to develop more accurate guidance for omission of a single post in a run of MGS guardrail. The research should seek to evaluate the omitted post without the use of adjacent CRT posts. In addition, the research should provide guidance regarding whether multiple omitted post treatments could be utilized within a long run of continuous guardrail, and if so, the minimum required separation distance between them.
The MGS long-span system was developed to span transverse culverts measuring 24 ft wide or less. In this circumstance, three posts would be removed from the system for an unsupported span length of 25 ft. This system also utilized three CRT posts on each side of the culvert structure. For culverts measuring less than 24 ft wide and where one or two posts are omitted, it still would be necessary to utilize the CRTs on each side of the unsupported segment of rail. Although the required number of CRT posts may decrease when omitting only one or two posts, MwRSF has been unable to recommend this option without further analysis and testing to alleviate concerns due to the potential for increased rail deflections, pocketing, and vehicle snag.
Additional research efforts have been conducted in areas related to the issue of omitted posts. A previous MwRSF study of circular cross-section, alternative-species timber posts investigated the effect of several weak timber posts fracturing early with low force and energy levels during a LON impact on the MGS guardrail system. In this study, computer simulation with BARRIER VII was conducted with as many as four consecutive weak posts in the impact area of an MGS system. While the results of the simulations were not conclusive, it was clear that the inclusion of weak posts increased the rail deflections considerably. As such, it is reasonable to assume that similar increases in deflection could be expected with omitted posts which would provide no or limited energy dissipation.
Another related research effort on W-beam guardrail posts in rock conducted by TTI produced a preliminary investigation of the performance of a W-beam guardrail system when some of the posts are omitted or installed at reduced embedment. For this study, TTI performed a parametric evaluation of W-beam performance when one or more posts were missing using LS-DYNA computer simulations. TTI conducted simulations of W-beam guardrail installations with one, two, or three posts omitted and compared them to baseline simulations of a standard W-beam guardrail. The results from the models with as many as three omitted posts demonstrated increased lateral deflection as expected, but all of the models displayed successful redirection with no signs of vehicle pocketing or override. TTI researchers were skeptical of the simulation results as previous W-beam long-span studies had determined that unsupported spans of more than two or more omitted posts would require special design changes, such as CRT posts or nested guardrail. TTI concluded that their current LS-DYNA simulation models were not sophisticated enough to capture the required failure modes, such as pocketing or rail rupture. While certain indicators of decreased performance were identified, they were not sufficient to determine the potential for safe redirection. As a result, TTI recommended that full-scale crash testing would be required to accurately determine the effect of omitted posts.
The MGS system, which has proven to be far more robust in full-scale crash testing than older W-beam guardrail designs, would have an increased potential for successful vehicle redirection with omitted posts as compared to the modified G41S system investigated by TTI. In addition, an MGS system with omitted posts may be able to redirect vehicles without special modifications, such as nested rail or CRT posts.
The objective of this research effort is to develop guidelines for MGS installations with a single omitted post for clearance of obstacles. The research would attempt to focus on the omission of a post without the use of CRT posts adjacent to the unsupported span. Full-scale crash testing would be conducted to evaluate the use of a single omitted post according to the TL-3 impact safety requirements in MASH. Following successful full-scale crash testing, additional analysis would be conducted to evaluate the potential for omission of a single post in multiple locations in a run of guardrail and the corresponding minimum spacing between the omitted posts.
The research effort will begin with the construction of the MGS with a single omitted post at the MwRSF Outdoor Test Facility for evaluation. The system will be evaluated according to the MASH guidelines for test designation no. 3-11 with the 2270P pickup truck vehicle. It is believed that the 1100C vehicle test can be waived for this system because the 2270P vehicle will provide a more stringent test of the failure modes expected in with the omitted posts such as excessive dynamic deflection, pocketing, vehicle snag, and rail rupture. The CIP for this test will be selected based on maximizing the potential for vehicle pocketing and post snag using the CIP charts in MASH and the researchers engineering judgment. The full-scale vehicle crash test will be conducted, documented, and evaluated by MwRSF personnel and in accordance with the MASH guidelines.
Following the successful full-scale crash testing, results from the crash testing will be applied to estimate potential concerns associate with multiple single omitted posts that are spaced apart in a run of MGS guardrail. Results from the full-scale test would also be analyzed to provide further guidance on allowable spacing between omitted posts based on the behavior of the guardrail system during the test.
After completion of the full-scale crash testing, a summary report of the research project will be completed detailing the tested barrier system, full-scale crash test results, evaluation of barrier performance, additional analysis, and recommendations for implementation and barrier system installation.
The successful development and evaluation of a MGS guardrail with omitted posts would provide states with a potentially simpler and less-costly alternative for dealing with post conflicts within a run of guardrail.
Snapshot of Recent Developments
130 Whittier Research Center
2200 Vine Street
Lincoln, Nebraska 68583-0853
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