Long-Span Guardrail – Phase III (405160-8)

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Phase III * (FY2008 – 405160-8):

Problem Statement

There are many instances where it is difficult to install guardrail posts at the normal 6.25 ft spacing due to underground obstacles, such as culverts. Previously, designs for omitting 1 or 2 (12.5 to 18.75 ft span) posts were crash tested in accordance with the NCHRP Report 230 criteria. These designs required 25 ft to 37.5 ft of nested rail to compensate for the increased post spacing. A design that omitted 3 posts (25 ft span) was tested in accordance with NCHRP Report 350. This design was much more complex, requiring 100 ft of nested rail, CRT posts at a reduced spacing on each side of the long span, and deeper block outs. This NCHRP Report 350 design may be over designed for locations where only 1 or 2 posts need to be omitted.

Background

In 1992, Mak, Bligh, Gripne, and McDevitt reported on a study of long span guardrails with wood posts and offset blocks. They used the Barrier VII program in a computer simulation study of several proposed designs of guardrails for spanning culverts. The “best” design identified in the computer study was a nested w-beam with post(s) omitted over the culvert. The nested w-beam guardrail with span lengths of 12 ft 6in and 18ft 9 in was subjected to full scale tests using 4500-lb automobiles in accordance with NCHRP Report 230 and met the associated evaluation criteria. A 25 ft length of nested w-beam was used in the 12 ft 6 in long span design and 37 ft 6 in length of nested w-beam was used in the 18 ft 9 in long span design.

In 1999, Polivka, et. al. reported on design and testing of a 25 ft long span w-beam guardrail that made use of six CRT posts adjacent to the long span. Double depth offset blocks were used on the CRT posts and nested w-beam was used over a 50-ft length. The design was tested with a 2000P (4400-lb) pickup in accordance with NCHRP Report 350 and satisfactorily met the evaluation criteria.

In 2006, TTI tested and reported on a long-span guardrail design under the Roadside Safety Research Program Pooled Fund Study No. TPF(114). The design used 6 in by 8 in wood posts and offset blocks. Two posts were omitted from the long span making it 18 ft 9 in long. Two layers of 12-gage W-beam were nested over a length of 37 ft 6 in and extended over the long span. The nested section extended 12 ft 6 in beyond one end of the long span and 6 ft 3 in beyond the other end of the long span. In test 3-11 of NCHRP Report 350 (2000P vehicle, 100 km/h, 25 deg) the rail element was ruptured and did not contain the vehicle. The rupture occurred in the single layer of w-beam at the splice where the section changed from two layers to a single layer.

In 2007, TTI performed computer simulations of the full-scale crash test mentioned above, and additional simulations on two alternative designs. The alternative designs were:

  1. A single layer 12-gage W-beam with three CRT posts on each side of the long span which was 18 ft-9 in long.
  2. A nested 12-gage W-beam with three CRT posts on each side of the long span which was 18 ft-9 in long. The nested section was 50 ft long.

Simulations were performed with the impact point at the beginning of the long span and at mid-length of the long span.

Results indicated a reasonable probability that both alternative designs would produce acceptable test results when the impact point is at the beginning of the long span. However, when the impact point was at mid-length of the long span, simulation results indicated a reasonable probability both designs would NOT produce acceptable results.

Objective

Develop a strong wood post guardrail design with a 27 inch mounting height that meets NCHRP Report 350 TL-3 and allows for 1 or 2 posts to be omitted.

Benefits

This project is expected to result in a long span guardrail design that meets NCHRP Report 350 evaluation criteria and is significantly less costly than the existing design.

Implementation

TTI will provide all the supporting information and written discussion for submitting a request to FHWA for acceptance of the long span guardrail for use on the National Highway System.

The research will provide information and documentation on testing of long span guardrails so that design and operational standards can be further reviewed and evaluated. Detailed engineering drawings that will facilitate development of standards sheets and specifications will be provided to each participating state.

Drawings provided for Task Force 13 documents will further support implementation of the research.

Work Plan

Task 1 -– Perform Computer Simulations

The finite element code LS DYNA will be used in this project to conduct impact simulations of long-span guardrail.

Task 2 -– Perform Full-Scale Vehicle Crash Test

The researchers will perform test 3-11 of NCHRP Report 350 (2000P vehicle, 100 km/h, 25 deg.) on the selected design. It is argued that this is the critical test for this design and that test 3-10 (820C vehicle, 100 km/h, 20 deg) is not needed. TTI will provide the test facility, test vehicle, instrumentation of the vehicle, high-speed film, video, still photographs, and a final report suitable for submittal to Federal Highway Administration (FHWA).


*At the Novement 2009 meeting, the pooled fund team elected to discontinue this project over concerns about the value of pursuing further research in this system given the predictions demonstrated by the model, and the knowledge that there are systems available that have passed testing.