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Strength Properties Of Guardrail Posts

Question
State WA
Description Text

We've been asked if we could provide a material strength comparison between steel and wood posts.  This is about the material properties rather than the guardrail system performance.  Dave and I looked at this briefly in a few resources.  One of these we looked at particularly was the "Task Force 13 Standardized Hardware Guide" which lists inertial properties of these post materials and also a stress grade for the wooden posts (see below links). 

 

https://www.aashtotf13.org/Files/Drawings/pwe01-04.pdf

 

https://www.aashtotf13.org/Files/Drawings/pde01-08.pdf

 

We are wondering how we can formulate a meaningful comparison between these post material types and their associated performance.  Is there anything in your past work with pendulum testing etc. that would help simplify a response to this question?  Also, please share any studies/reports you may know of.

 

We predominately use 6 x 8 Douglas fir Grade No. 1 or 6 x 8 Hem Fir Select Structural grade as a comparable post to the W 6 x 9.

 

Thanks for any help.

 

Keywords
  • Guardrail
Other Keywords none
Date January 17, 2012


Response
Response I have some comments and information regarding wood and steel posts with respect to the MGS below. Hopefully it will help you with your decision process.

 

As with all strong-post W-beam guardrail systems, the MGS system dissipates energy through the deflection and deformation of the rail and the rotation of the posts in the soil.  If the posts have do not rotate in the soil and absorb energy, the bulk of the impacting vehicle's energy will be absorbed by the W-beam element, thus increasing the tensile force in the rail.  If the force increases beyond the capacity of the rail, it will fail, allowing the impacting vehicle to pass through.  Therefore, the posts must have sufficient structural capacity to displace founding soils and absorb energy.  Wood and steel posts can both serve this function, but they do have inherent differences.

 

Numerous bogie tests have been conducted on steel, rectangular wood, and round wood guardrail posts in both soil and a cantilever sleeve.  In addition, many full-scale tests have been conducted with both types of posts using standard W-beam and the MGS. I have attached a thesis done in the past here at MwRSF that has a pretty complete literature search on steel and post testing up until 2005 for your reference. The general trend was that the two types of posts behaved very similarly, with some tests suggesting steel posts were better and others suggesting wood posts were better.  So previous research suggests only minimal performance differences.

 

That said, the section and material for the steel and wood posts create distinct differences that should not be ignored. W6x8.5 steel posts have very distinct strong and weak axis bending capacities due to the "I" shape design of the section.  However, steel posts do not fracture and tend to bend and absorb energy when impacted in either the strong or weak axis if the surrounding soil is sufficiently strong. Wood posts tend to generate slightly higher soil rotation forces. Wood posts also tend to fracture if the soil resistive forces exceed the capacity of the post. Wood posts also has a higher degree of material variability due to splits, checks, knots, etc...

 

So how do these differences translate to their performance in guardrails system? That takes further explanation. First, there is a difference in wood post and steel post behavior in the weak axis. In the case of the steel post, the weak axis impact would tend to have some limited rotation in the soil and then yield and bend the post. In a wood post weak axis impact, the post would also rotate in the soil to some degree and then would tend to fracture. Looking at the post capacities, the wood post would likely tend to generate higher peak loads during a weak axis impact, but the energy the post absorbs would be largely dependent on how much the post rotated in the soil prior to fracture. For the steel post, the peak loads would be somewhat lower based on the weak axis capacity of the two posts in question, but the energy absorbed may be higher than the wood post due to the post deformation developing more consistent load over the weak axis deflection. So while the wood post may generate higher weak axis accelerations, the steel post may absorb more energy and create larger changes in velocity. Thus, both posts have some competing negative aspects in their weak axis behavior, but testing has not indicated that either post has a significant advantage or disadvantage or that these effects are detrimental to overall system performance.

 

In a strong axis post impact, the post behavior is different for steel and wood posts. In a strong axis loading of the post, both posts will tend to rotate through the soil. Wood posts have been shown to have slightly higher soil rotation forces in the strong axis, but the effect is minimal on performance. If the soil forces do not exceed the capacity of the post section, then the two posts performance should be fairly similar in the strong axis. If the post in embedded in a very strong soil or frozen soil, then the performance of the post varies more depending on the type of post. When post-soil interaction forces exceed the capacity of a steel post, the steel post yield and deforms. This deformation of the steel post continues to dissipate energy, although the forces and energy are higher than those seen with soil rotation. A wood post will fracture if the post-soil interaction forces  are high enough to exceed the capacity of the post The lack of fracture is important to the performance of the wood post. If post-soil resistance forces exceed the capacity of a wood post, the post fractures and ceases to dissipate energy during an impact. That said, we have run numerous full-scale crash tests with wood posts where the post fractured in the impact area and the performance of the system was still acceptable.  

 

With respect to the MGS system, there are several types of post loading occurring. Some posts are loaded primarily in the lateral direction like the bogie testing. Most of the posts are undergoing a combined load that involves mainly lateral load with some twisting and longitudinal loading of the post. We believe that the majority of the posts in the system are undergoing the combined loading. Finally some posts are being loaded directly along the weak axis of the post due to the vehicle impacting it. In addition, most of the posts impacted along the weak axis will have deflected laterally along the strong axis prior to being impacted by the vehicle.

 

Under combined loading, steel posts tend to twist during impact and fail due to lateral/torsional buckling of the section. When we have conducted simulation analysis in past projects comparing wood and steel post versions of the MGS, we have found that a 10-15 percent reduction in the strong axis moment capacity of the steel posts accounts for the twisting of the steel posts. This reduction has correlated very well with our full-scale crash testing results. We don't see the effect of the steel post twisting as being very different from the wood post when deflected laterally. In addition, the use of a wood post that generates slightly higher lateral resistive forces would not be a concern for the performance of the MGS. We have tested several systems which would bear this out. For example, the original MGS system tested with W6x8.5 posts worked very well and had a dynamic deflection of 43.9" when tested according to MASH. When we tested the MGS with ¼ post spacing, it generated a safe redirection with a much lower deflection of 17.6". Thus, a small increase in post lateral stiffness for wood posts would not be cause for concern.

  

We have tested several wood post MGS systems including round wood posts made from ponderosa pine and Douglas fir, and 6"x8" white pine posts. These systems all performed similarly to the steel post MGS and no issues were observed with occupant risk values or vehicle stability. In addition, conducted testing of the MGS with 6"x8" SYP posts in the past year with both the 1100C and 2270P vehicles. Comparing the 2270P tests with 6"x8" SYP and W6x8.5 posts, we observed very similar performance in terms of vehicle stability and redirection. Dynamic system deflections were 43.9" for the steel post system and 40" for the wood post system. Very similar performance. I have attached videos at the link below of the steel and wood post testing for you to compare.

 

The file 'Wood vs steel MGS.zip' (189.0 MB) is available for download at

http://dropbox.unl.edu/uploads/20120216/ef4e8177e06a0405/Wood%20vs%20steel%20MGS.zip

for the next 14 days.

It will be removed after Thursday, February 16, 2012.

 

In summary, we believe that there is little difference in system performance for the MGS with respect to steel and SYP wood posts. While the post sections have some differences in terms of how the perform, these differences do not seem to have a large effect on the overall performance of the system.

 

The above discussion refers to SYP posts. You had a comment below regarding Douglas fir and Hem fir posts. The Hem Fir Select Structural  and Grade 1 Douglas fir have very similar strength to SYP post, albeit around 11% lower. We have successfully tested the MGS with 6"x8" White Pine posts that were roughly 37% lower strength than the SYP posts. Thus, I would see no issues with using the Douglas fir and Hem fir materials in a 6"x8" wood post in the standard MGS system. Other specialty systems, MGS on slopes, or long span for example, might require further analysis and thought prior to using the alternative posts.

 

Let me know if you have further comments or questions.

Date February 2, 2012
Attachment Hascall-Thesis-Final-12-6-05.pdf


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