|TTI Research Supervisor:
Nauman Sheikh, P.E.
Texas Transportation Institute
Texas A&M University System
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Jeffery K. Petterson, P.E.
Design Policy & Strategic Analysis Estimating Manager
Roadside Safety Engineer
Washington State Department of Transportation (WsDOT)
P.O. Box 47329
Olympia, WA 98504-7246
It is not always easy to obtain the required embedment depth for posts in the field. One common situation is when rock is encountered at the surface or some depth below the surface, preventing the installation of the post at the required embedment depth. Thus guidance is needed for installation of guardrail posts in such scenarios.
The objective of this research was to develop cost effective guidelines for placement of W-beam guardrail posts in rock by optimizing current placement guidelines and by investigating the sensitivity of W-beam guardrail performance to post embedment depth. The guidelines were to be developed for NCHRP Report 350 evaluation criteria.
The researchers evaluated the existing guidelines for coring in rock and performed a geometric optimization with the objective of reducing some of the coring requirements. The greater focus of this research, however, was on evaluating the W-beam guardrail performance to develop guidelines that would eliminate or reduce the coring requirements.
Due to funding limitations, it was proposed to conduct this research in two phases. In the first phase (documented herein), preliminary guidelines would be developed based on simulation analyses aimed at evaluating the W-beam guardrail performance when one or more posts are missing or installed at reduced embedment depth due to the presence of rock. The preliminary guidelines developed through simulation would identify various combinations of reduced post embedment depths or missing posts that would be expected to result in acceptable performance of the W-beam guardrail. Identifying such combinations would allow greater flexibility in installing W-beam guardrail posts when rock is encountered.
To provide data for evaluation of the performance of the W-beam guardrail system when some of the posts are either missing or installed at reduce embedment due to the presence of rock, the researchers performed a series of pendulum tests of soil embedded posts. In these tests, wood and steel posts were installed at various embedment depths. The posts were then impacted by a gravitational pendulum to measure the acceleration-time response resulting from the impact. This enabled the researchers to quantify the post-and-soil interaction behavior at various embedment depths.
The objective of the parametric analysis was to determine if a particular combination of reduced embedment and/or missing posts would “pass” or “fail” under NCHRP Report 350 testing criteria. Based on these results, the preliminary guidelines for W-beam post installation in rock would be developed. For combinations where the guardrail performance is found acceptable, no coring in the rock would be necessary, even when some of the posts are not installed at full embedment depth. For other combinations, some coring may still be required to achieve acceptable performance of the W-beam guardrail. Using this approach would presumably require less frequent drilling in the rock and provide a much greater flexibility in installing W-beam guardrail in rock.
The objective of this research was to develop preliminary guidance for the installation of W-beam guardrail posts when encountered by the presence of rock. The work was proposed to be completed in two phases. In the first phase, reported herein, the researchers were to use simulation analysis to evaluate the performance of the W-beam guardrail under different scenarios where one or more posts were either missing or installed at a reduced embedment due to the presence of rock. The simulation analysis was to be used to determine if a particular configuration would “pass” or “fail” under NCHRP Report 350 testing criteria and thus develop preliminary guidelines for guardrail post installation when rock is encountered. If the members of the pooled fund states were to find the preliminary guidelines acceptable, a second phase would be funded in which crash testing would be performed to verify the preliminary guidelines and develop final guidelines.
To develop the preliminary guidelines through finite element simulation analysis, the researchers conducted a series of pendulum tests involving W-beam guardrail posts installed in soil at reduced embedment depths. The researchers then developed the finite element models for these post and soil configurations. An extensive calibration exercise was performed to determine an appropriate soil material model and properties that would enable the post-soil behavior of the model to match the behavior observed in the tests for different embedment depths.
Once the post-soil response was successfully captured, the researchers developed a full-scale model of a standard strong post W-beam guardrail system and performed a successful validation of the model using data from previously conducted crash tests.
After validation of the standard W-beam guardrail model, a parametric evaluation of W‑beam guardrail performance was performed. In this evaluation, several cases were simulated with one or more posts either missing from the system or installed at reduced embedment depth due to the presence of rock or other below grade obstruction. At this stage, it was determined that the W-beam guardrail model did not exhibit certain modes of failure (such as vehicle pocketing, rail rideover, etc) that would be expected under certain extreme cases (such as when three posts are missing). While the simulation results showed certain trends that suggested improvement or worsening of W-beam performance, the results were not discerning enough to make the “pass” or “fail” judgment needed to develop the preliminary guidelines for post installation in rock. Several modifications and improvements were made to the model to improve its sensitivity in predicting guardrail performance with compromised posts. While some improvements were made in the sensitivity of the model, the issue was not resolved.
Currently used modeling and simulation techniques in the roadside safety community for modeling the W-beam guardrail system are not able to exhibit some of the failure modes necessary for developing the preliminary guidelines. While some new methods such as using alternate material models for incorporating rail rupture could be explored, more resources were needed and it could not be guaranteed that the desired sensitivity of the model would be achieved.
An alternate method of developing the guidelines is through a program of full-scale crash testing. However, this method would require a funding level that currently exceeds the financial capacity of the pooled-fund. It is, therefore, recommended that a problem statement be developed for a national level research project under NCHRP. This would provide the level of funding needed to develop the guidelines for post installation in rock through a program of full-scale crash testing.