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Guardrail Over Box Culvert

State WI
Description Text

A project has to install a beam guard over a box culvert.  Two options would be to use the long span guard rail or attach to the box culvert using a plate.  Although both alternatives are crash tested, it appears that some modification will be needed to fit the given location.


If the long span detail is used, they will not be able to get the 2' grading behind the post,  Is it possible to use longer post with the long span detail?


If the beam guard post are attached to the box, the post will have to be longer than what was crash tested (see PDF).  At what depth, top of finished surface to top of box, can the plate detail be used? 


At the pooled fund meeting there was some discussion about changing the welding the post to the plate.  Could you forward me an updated detail?


I assume at a certain point if there is adequate soil mass behind the post, could shorter post with decreased post spacing be used?

  • Guardrail
Other Keywords none
Date May 5, 2010
Attachment Bolted Beam Guard Option 2.pdf
Attachment SDD Beam Guard Option 1.pdf


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 24 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.


For some situations, you noted that it may be difficult to provide 2 ft of level, or mostly level, soil grading behind the wood CRT posts. 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 on 2:1 fill slopes. Although unpublished at this time, MwRSF determined that 7.5-ft long wood posts are an acceptable alternative to W6x9 by 9-ft long steel posts when considering the 31-in. tall MGS placed on a 2:1 fill slope using a 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.


Metric-Height W-beam


For the metric-height, W-beam guardrail system configured for long-span culverts, it would seem reasonable to utilize three 7-ft long wood CRT posts adjacent to each end of the box culvert if 2:1 fill slopes are present in this region. If the steep fill slopes continue beyond the location of the CRT posts, then the guardrail would transition to the metric-height, W-beam guardrail system for 2:1 fill slopes using W6x9 by 7-ft long steel posts spaced on 3-ft 1-1/2-in. centers. However, this half-post spacing system resulted in slightly decreased lateral barrier deflections as compared to those observed for standard W-beam barriers with 6-ft 3-in. post spacing. Thus, it would also seem appropriate to provide two 7-ft long W6x9 steel posts at 6-ft 3-in. spacing (i.e., 12 ft - 6 in.) between the last 7-ft long wood CRT post and the start of the half-post spacing. Therefore, all posts beyond the last wood CRT post would be configured as 7-ft long W6x9 steel posts placed at the slope break point of 2:1 fill slopes.


It should be noted that this guidance is provided using our best engineering judgment in the absence of full-scale crash testing, computer simulation, dynamic component testing, or combination thereof. If new information becomes available, MwRSF may deem it necessary to revise this guidance.

Based on the success of MGS Long-Span system, MwRSF now believes that the 1.5 m lateral offset requirement for the Metric-Height, Long-Span, W-beam Guardrail System is overly conservative for culvert slabs covered by mostly level soil fill. As such, it is MwRSF's opinion that the minimum lateral offset between the back side of the CRT wood posts and the front face of the headwall can be reduced from 35 in. to 24 in. while providing comparable safety performance. With this adjustment, the minimum recommended lateral offset between the back side of the rail and the front face of the headwall would be approximately 48 in. or 1.22 m. for the metric-height variation. In addition, it is MwRSF's opinion that the Metric-Height, Long-Span, W-beam Guardrail System has the potential to be placed even closer to the front face of the culvert headwall. However, further reductions in the minimum lateral offset could only be evaluated through full-scale crash testing.

Recently, you requested information regarding the soil fill depth where one would switch from the culvert-mounted, W-beam guardrail system to the standard W-beam guardrail system with posts embedded in soil without special anchorage.


Several years ago, MwRSF developed a metric-height, W-beam guardrail system for attachment to the top slab of a concrete box culvert when structure lengths exceeded 25 ft. The new design utilized an anchored post spaced on 3 ft " 1-1/2 in. centers. Each post was also configured with a welded base plate capable of absorbing energy upon impact. The testing program used a "practical" minimum soil depth of 9 in. Upon completion of the successful testing program according to TL-3 of NCHRP Report No. 350, it was recommended that the back of the posts be placed a minimum of 10 in. from the front face of the culvert headwall. A dynamic barrier deflection of approximately 16.5 in. was observed.


The noted crash testing demonstrated that the barrier system performed in an acceptable manner with 9 in. of soil fill. The researchers also believe that the barrier system would have performed in an acceptable manner with soil fill depths of approximately 43 in., thus replicating the expected safety performance of half-spaced posts used in combination with metric-height, W-beam guardrail systems. For soil fill depths of 43 in. on culvert slabs, it would seem unnecessary to utilize a barrier system with anchored posts or a half-post spacing. Instead, a standard, full-spacing, metric-height, W-beam guardrail system would be used on a culvert if adequate soil fill depth is provided along with a minimum of 2 ft of level (or mostly level) terrain behind the posts.


If the soil fill depth is less than 43 in. but greater than 3 ft, it would seem both desirable and reasonable to use a barrier system that does not require attachment to the culvert slab. Unfortunately, no research has been performed to determine the minimum post length/embedment depth for metric-height, W-beam barriers to meet the TL-3 safety performance guidelines. However, we believe that a W-beam guardrail system with a slightly reduced post length and reduced post spacing would have a high probability for meeting current impact safety standards, especially if configured with the 31-in. top mounting height. However, satisfactory barrier performance can only be determined with the use of full-scale vehicle crash testing.


Date June 11, 2010

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