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Would you be able to provide me with the latest drawings of the simplified MGS bridge transition?

Also, I have had a question arise from our parts warehouse regarding 8x8 wood posts and blockouts.  Is there still a need for us to stock these, or can we replace any 8x8 wood post and/or blockout with a 6x8 version instead?  If so, could we replace only one at a time, or would we need to replace all posts and blockouts within the installation at the same time?

I am copying this email to Karla so that she can direct a MwRSF staff member to get you the latest approach guardrail transition detail for the MGS attached to a thrie beam transition that utilizes a half-post spacing. In the future, MwRSF will provide details on how to adapt the noted transition to thrie beam transitions that use a quarter-post spacing.

With regard to wood posts and blockouts, MwRSF is still evaluating the appropriate wood post size and length for use as a substitute in the simplified, steel-post, stiffness transition. Results from this component testing and analysis should be available within 2-3 months.

As noted at the spring Pooled Fund meeting, 6"x8" by 6' long wood posts have been approved for use in the MGS as a substitute for the W6x9 by 6' long steel posts. Although we are confident of their use in the MGS, we were hopeful that the Pooled Fund group would fund the 2270P crash test. Unfortunately, that funding did not come to fruition in the recent meeting due to a prioritization of other projects, such as cable barrier projects. At any rate, Iowa could begin to implement the standard wood post size into the MGS based on the prior FHWA acceptance of the MGS. In addition, Iowa could also implement 6x8 blocks standard guardrail utilizing the 8x8 posts at any time. However, it would be acceptable to use the 6x8 blocks with metric height W-beam rail using 6x8 posts. Finally, you would want to use 6x12 blocks for any MGS installations using W6x8.5 and W6x9 steel posts, or 6"x8" wood posts.

A study funded by the MwRSF Pooled Fund was completed in 2011 which described the use of timber posts in transitions. The study performed numerous component tests to determine minimum and equivalent strengths for posts in the transition region. Subsequently, Barrier VII was used to evaluate approach guardrail transitions and engineers determined the minimum post sizes and strengths to withstand the impact loading according to NCHRP Report No. 350 impact conditions. Timber posts with 8"x10" cross-sections were selected as conservative substitutions for W6x15 posts in the transitions. The report results are located here:

https://mwrsf.unl.edu/reportResult.php?reportId=32

The following conclusions from the study are worth repeating:

A literature study on previous bogie testing and comparisons between wood and steel guardrail posts suggested that 6-ft (1.8-m) long, 6-in. x 8-in. (152-mm x 203-mm) wood posts and W6x9 (W152x13.4) steel posts have similar force vs. displacement characteristics. However, very little component testing had been previously conducted on larger transition posts. Thus, a bogie testing program was undertaken to determine the behavior of W6x15 (W152x22.3) steel posts and wood posts of various cross sections and embedment depths. Early in this bogie-testing program, the propensity for woodpost fracture in stiff soil was observed. As a result, the wood-post replacements were conservatively selected such that the cross section had excess strength capacity to minimize the risk of post fracture. Ultimately, 6.5 ft (2.0 m) long 8-in. x 10-in. (203-mm x 254-mm) wood posts provide similar resistance to rotation and were selected as the replacement for the 7-ft (2.1- m) long W6x15 (W152x22.3) steel transition posts.