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CRT Posts Adjacent to Slopes for MGS Long Span

Question
State WA
Description Text

I found your e-mail on the "Midwest Guardrail System for Long Span Culvert Applications" and was hoping you could offer some quick advice.


I'm using the Washington State Design Manual which provides the following installation cases (See Attached Figure 1.jpg).


I'm spanning 25' and need to install 3 CRT posts on each side of the culvert.
What are your thoughts on using 11' long CRT posts on each side as shown in CASE 6 above? I'm trying to stay within my R/W and not have to spend money on a retaining wall.

Keywords
  • Guardrail
Other Keywords none
Date April 14, 2011
Attachment Figure 1.jpg


Response
Response

MwRSF has successfully developed and crash tested two W-beam guardrail systems to span across long concrete box culverts, such as those measuring up to 25 ft in length. For the first system, the metric-height W-beam guardrail was configured with a 27-3/4-in. top mounting height, while the Midwest Guardrail System (MGS) was utilized for the second configuration with a 31-in. top mounting height. For both designs, three 6-in. x 8-in. by 6-ft long wood CRT posts were placed adjacent to the long span using the 6-ft 3-in. post spacing. Beyond the CRT wood posts, the guardrail system was transitioned into a steel post, wood block, semi-rigid barrier system which also used 6-ft long posts and a 6-ft 3-in. post spacing. For both crash-tested systems, a region of level, or relatively flat, soil fill was provided behind the CRT wood posts.


We recommend providing 2 ft of level, or mostly level, soil grading behind the wood CRT posts. However, we understand that this can be difficult. As such, your inquired as to whether the wood CRT posts could be lengthened to account for the reduction in soil resistance resulting from an increased soil grade behind these six posts, especially when placed at the slope break point of a 2:1 fill slope.


Recently, MwRSF performed limited research to determine an acceptable MGS post length for a 6-in. x 8-in. solid wood post installed at the slope break point of a 2:1 fill slope. MwRSF determined that 7.5-ft long wood posts are an acceptable alternative when considering the 31-in. tall MGS placed at the slope break point of a 2:1 fill slope using 6-ft 3-in. post spacing.


The MGS Long Span system utilizes six CRT wood posts. A CRT post's moment capacity about its strong axis of bending is approximately 81 percent of that provided by the standard wood post. In the absence of dynamic component test results, it is believed that the six CRT wood posts could also be fabricated with the 7.5-ft length when used in the MGS Long Span system. If the steep fill slopes continue beyond the location of the CRT posts, then the guardrail would transition to the MGS for 2:1 Fill Slopes using either 6-in. x 8-in. by 7.5-ft long wood posts or W6x9 by 9-ft long steel posts.


Thus, for the cases you sent, we believe Case 2 is acceptable and that Case 1 and Case 3 would be acceptable if 7.5 long CRT posts were used. We cannot recommend the use of extended length CRT posts on steep slopes as you have shown in Cases 4-6. Determination of proper CRT post lengths for this type of installation would require additional analysis and testing in order to ensure proper function of the CRT's in the long span system.

Date April 15, 2011


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(402) 472-0965
Email: mwrsf@unl.edu
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