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|Description Text|| We have a 7-mile length of road on which we intend to pave the 4 ft. earth shoulders as part of a U.S. bike route. The corridor has an ADT of around 4,000 and nearly 30 box culverts ranging in size from single 5x3 to double 10x10. Due to the traffic volume present, the vast majority of these structures would warrant guardrail based on the Wolford & Sicking model in Transportation Research Record No. 1599.
As treating such a large number of structures is entirely infeasible within the budget of this project, we intend to design except the majority of culverts, treating only the largest as shown below. We will mitigate the exceptions by installing continuous edge line rumble stripes and object markers at each location. There are no excessive ran-off road accident problems today, and we believe that an HSM analysis will predict a further crash reduction over the existing condition.
As this will be a designated bike route, we also intend to provide a 42 in. chain link fence at the edge of shoulder for each culvert, irrespective of the presence of guardrail
Given the existing conditions, I am confident that the direction described above represents a reasonably safe alternative for this corridor. I do have a few questions, however:
1. Do you agree with our general direction?
2. Is the guardrail solution described below adequate? The concern is the face of the rail being flush with the edge of the culvert.
3. Would a 42 in. chain link fence contacting the back side of the W-beam, significantly impair the function of the rail?
|Date||October 25, 2011|
|Attachment||culvert protecion advice.gif|
We have reviewed the supplied information and offer the following questions and comments.
The attached guardrail detail depicts a standard strong-post, W-beam system which appears to be a combination of our metric-height long-span guardrail system and our metric-height guardrail for 2:1 slopes using half-post spacing. However, the detail does not show three CRT posts adjacent to the unsupported span and on each side. These CRT posts were incorporated to prevent concerns with vehicle pocketing. 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 prior to the slope break point.
If it is difficult to provide 2 ft of level, or mostly level, soil grading behind the wood CRT posts, then it may be beneficial to consider lengthening the wood CRT posts 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.
As noted above, it may be necessary to consider options for both metric-height rail and 31" rail. Discussions on each rail height are included below:
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. 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. This configuration allowed for posts to be placed at face of headwall.
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. This configuration utilized a 1.5 m lateral offset between back-side of guardrail and the front face of headwall.
The safety performance of a combination chain-link fence placed directly behind a guardrail system has not been evaluated using full-scale crash testing. As such, it is noted that there exists the potential for some increased risk to motorists and nearby pedestrians, including minor vehicle snag, minor vehicle instabilities, minor guardrail stiffening, and some additional fence debris scattered into the pedestrian region located behind the guardrail system. Of course, the effect that the nearby fence has on guardrail performance would depend on its configuration and relative lateral offset. The closer the fence to the guardrail and/or the more robust the design, the greater its potential influence on guardrail performance and other behaviors noted above.
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. Due to the general need to place posts at the slope break point near culverts, it may be preferred to utilize MGS variations to protect the culvert hazards if culvert grating is not to be considered. If new information becomes available, MwRSF may deem it necessary to revise this guidance.
|Date||October 31, 2011|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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