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Use of Wood Posts with limited fill behind Posts

Question
State WY
Description Text

MwRSF has done some testing on SYP as well as white pine posts for Wisconsin.  Some time ago the pooled fund issued recommendations for the use of 7' long W6X9 posts at 3'-1 1/2" spacing for conventional w-beam guardrail when the fill slopes off directly behind the post at a 1V:3H or steeper slope.  Can wood posts be substituted for the W6X9 steel posts and of what length?  Typically Wyoming permits the use of Ponderosa Pine which is probably similar to white pine, although we do get some southern yellow pine occasionally.

Keywords
  • Guardrail
Other Keywords none
Date February 17, 2012


Response
Response

We have successfully tested the MGS with both SYP and white pine posts. We have also successfully tested round ponderosa pine posts with the MGS. These tests were all conducted on flat ground.

 

MGS has also been successfully tested with steel posts on slopes as steep as 2:1. This system used 9' long W6x9 posts at standard post spacing. The posts were located at the slope break point.

 

As part of that effort, a series of seven bogie tests were conducted on 6-in. x 8-in. (152-mm x 203-mm) SYP wood posts of 7.5 and 8 ft (2.29 and 2.44 m) lengths and 9-ft (2.74-m) long, W6x9 (W152x13.4) steel posts placed at the break point of a 2H:1V fill slope. The results from these tests were evaluated and compared. The results found that the 7.5-ft (2.29-m) long, 6-in. x 8-in. (152-mm x 203-mm) SYP wood post provided the best possible performance and the closest correlation to the 9-ft (2.74-m) long, W6x9 (W152x13.4) steel post used in the original design. Thus, it is recommend that the MGS system may be installed adjacent to a 2H:1V fill slope with either 9-ft (2.74-m) long, W6x9 (W152x13.4) steel posts or 7.5-ft (2.29-m) long, 6-in. x 8-in. (152-mm x 203-mm) SYP wood posts.

 

We addressed the issue of weaker post materials in the white pine MGS report. See the text below:

 

"Wood posts are often utilized in longitudinal barrier systems that are configured for special applications, such as in stiffness transitions, barriers adjacent to steep slopes, or barriers to shield the ends of transverse culverts. Within these special barrier applications, the dynamic behavior of an embedded post can greatly affect its safety performance. For example, premature fracture of wood posts within an approach guardrail transition may lead to an increased propensity for vehicle pocketing and/or snag on a bridge end. As such, MwRSF researchers have concerns regarding degraded barrier performance when considering the use of the weaker, 6-in. x 8-in. (152-mm x 203-mm), white pine wood posts in lieu of standard, SYP or DF rectangular wood posts in stiffness transitions and special MGS applications. However, it is possible for white pine posts to be used within approach guardrail transitions, guardrail end terminals, or guardrail anchorage systems. First, the geometry (i.e., width, depth, and length) of white pine posts could be modified to provide equivalent stiffness and strength to that provided by the original SYP or DF wood posts. Second, the post spacing could be modified to provide equivalent barrier capacity and energy dissipation characteristics to that provided by the original SYP or DF wood posts. Finally, full-scale vehicle crash testing may be used to demonstrate that unmodified, standard-size white pine posts provide acceptable barrier performance when used in combination with stiffness transitions or other special MGS applications.

As noted previously, W-beam guardrail systems have been developed for use in shielding various roadside hazards, such as fill slopes equal to or greater than 2H:1V and transverse culvert openings. Previously and based on full-scale crash testing, the Midwest Guardrail System (MGS) was successfully adapted for use at the slope break point of a 2H:1V fill slope using 9-ft (2,743-mm) long, W6x9 (W152x13.4) steel posts spaced on 6 ft - 3 in. (1,905 mm) centers. Later and based on dynamic component testing, a wood post version of the MGS system was configured with 7.5 ft (2,286-mm) long, SYP posts and for use in shielding a 2:1 fill slope. For the SYP wood post variation, the embedment depth was 58 in. (1,473 mm).

Unfortunately, WP posts would likely fracture prior to rotating in soil when installed with a 58-in. (1,473-mm) embedment depth on a 2H:1V fill slope, thus resulting in reduced energy absorption, increased system deflections, and a greater propensity for vehicle instabilities. As such, the post geometry would need to be altered in order to mitigate concerns for post fracture. For example, the post length and associated embedment depth could be decreased to reduce the post-soil resistance. Alternatively, the post's cross section could be modified to provide increased capacity and greater resistance to post fracture when using a 58-in. embedment depth. Further, full-scale crash testing could be used to demonstrate that the MGS with white pine posts would perform in an acceptable manner even with the fracture of a greater number of wood posts.

Based on the desire to maintain a standard cross section for 2H:1V fill slope applications, a reduction in post length was deemed more desirable. Unfortunately, a decreased embedment depth would result in a reduction in the lateral stiffness and strength of the MGS. Thus, the post spacing would likely need to be reduced to provide comparable barrier capacity and energy dissipation characteristics to that provided by the steel post and SYP wood post variations of the MGS for use on 2H:1V fill slopes. Further analysis, as shown in Appendix F, revealed that a white pine MGS system located adjacent to a 2H:1V fill slope should utilize 6.5-ft (1,981-mm) long, 6-in. x 8-in. (152-mm x 203-mm) wood posts at half-post spacing, or on 37½ in. (953 mm) centers. All other features of standard MGS remain the same."

 

With respect to conventional W-beam, we have not determined the proper post length for wood posts adjacent to 2:1 slopes. For non-MGS systems (i.e., standard metric height W-beam guardrail), the center of the W6x9 steel post is to be placed at the slope break point using 7-ft long posts spaced 3-ft 1 1/2-in. on centers. In order to make the argument for wood posts, I believe that it would be important to perform a limited number of dynamic bogie tests using 6, 6.5, and 7-ft wood post lengths in a sloped soil pit, similar to what was done with the MGS adjacent to slopes.

 

Let me know if this answers you questions or if you need more information or discussion.

 

Thanks

Date February 18, 2012


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