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Shallow Post Embedment

State IA
Description Text

We are utilizing the 31” wood post MGS at this location and we have approximately
37” of soil depth to the top of the box culvert.  Other than attaching
steel posts to the culvert, what options would you suggest?

  • Guardrail
Other Keywords none
Date August 19, 2013
Attachment id=322.pdf
Attachment id=614.pdf


I think that you have a few options.


First, three round, wood-post MGS options are available using Ponderosa Pine, Douglas Fir, and Southern Yellow Pine species in varying diameters based on species. The round-post alternatives were successfully crash tested under the NCHRP Report No. 350 safety standards in combination with a substantial static and dynamic component testing effort. The post embedment depth for each species was 37 in. As such, a round-post option could be used on your culverts now.


The MGS was originally crash tested under NCHRP Report No. 350 when soil strength was not as strong as now provided under the new MASH testing criteria. Per my recollection and from an older MGS R&D bogie testing program in early 2000s, the average post-soil resistance was just under 7 kips through 15 in. of post deflection, which resulted in crashworthy 350 guardrail system. Now and using MASH soil criteria, the average post-soil resistance was around 9.5 kips through 15 in. of post deflection, as reported in “Dynamic Testing of MGS W6x8.5 Posts at Decreased Embedment”, report no. TRP-03-271-12, December 17, 2012. For these efforts, the load height was 24-7/8 in. with an embedment depth of 40 in. In this same guardrail post study, a reduced post embedment depth of 36 in. was investigated to explore post performance in maximum height guardrail systems. For this depth, the average post-soil resistance was about 8.5 kips through 15 in. of post deflection, which is higher than provided for the successful 350 testing of the MGS.


Based on a MASH high-strength soil condition and test noted above, I would expect the standard MGS to meet both the TL-3 NCHRP 350 and MASH safety criteria for using a 36-in. embedment for steel posts spaced on 6’-3” centers. Of course, the safety performance can only be verified through full-scale crash testing. We will be conducting a 2270P test on MGS at SBP of 2:1 with 6-ft long posts in near future. This crash test, along with a successful outcome, would help to support this opinion.


With a reduced post-soil resistance in actual field installations, it would also be acceptable to place the MGS with steel posts spaced at half-post spacing using a 36-in. embedment depth. The extra posts placed in midpoints may provide a factor of safety for the condition of a shallower post in combination with a potentially weaker soil condition than provided by stiff MASH soil.


As such, I think that you have 3 alternatives for consideration – wood posts, 36” embedded posts at standard spacing, and 36” embedded posts at half spacing.


Let me know if you have any questions or comments about the information noted above.


Date August 19, 2013
Attachment TRP-03-271-12.pdf

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