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I have several questions regarding details on the Nested W-Beam Long Span Guardrail for Low Fill Culverts from the attached acceptance letter I hope you can clarify so we can finalize our drawings and implement the system:

Question No. 1

With a 1450 dynamic deflection, should the 0.6m ledge behind the guardrail posts be increased to (1.0m or 1.5m) to accommodate the increased deflection. I assume the foreslope beyond the ledge can be 2H:1V or flatter? Typically for w-beam guardrail systems, our practice in Ontario is to permit the 3H:1V or flatter foreslope behind the guardrail to start immediately behind the posts - 2H:1V slopes not permitted within deflection zone of system. 

Question No. 2

With respect to the use of 7,620-mm long, W-beams, would it still be acceptable to use 3,810-mm long, W-beams throughout? If it is necessary to use any 7620mm long w-beams, we would likely specify their use from post 5 through post 18.

Question No. 3

What was the rationale for the 7,620-mm long, W-beam between posts 18 and 22? I would have thought the installation (for the nested W-beam segments) would have been symmetrical about the mid span point between posts 11 and 12? 

Question No. 4

What is the size of the holes for Posts 9 to 14?

Question No. 5

Any recommendations on how close post 11 and 12 could be placed to a culvert. We were thinking of specifying 0.5m min. clearance from post 11 and 12 to the culvert, as we understand the posts rotate mainly in a lateral direction in an impact.

Question No. 6

Any thoughts on how you will shift the post spacing to tie into MGS with posts at the 1/4 span points, or will it involve a complete retest of the low fill culvert design at the MGS height of 31"?

Question No. 7

Any chance of obtaining an mpeg of the crash test through e-mail? I have a copy of the 12 minute VHS tape from you on the Ohio Long Span Guardrail which I could convert to a 30 second or 1 minute segment into an mpeg, but if you have an mpeg or avi ready to go, that would be great.

Response No. 1

For standard, strong-post, W-beam guardrail systems, it is recommended that the guardrail posts be placed in level or mostly level roadside shoulders, say 10H:1V or flatter. Behind the guardrail posts, it is generally recommended that 24 in. of soil fill be provided so that adequate post-soil forces can be generated to ensure the proper vehicle containment and redirection by the barrier system. At this time, we do not have any information indicating that a width of level fill greater than 0.6 m (24 in.) be provided behind the CRT posts used in long-span guardrail systems and adjacent to the unsupported length. Beyond the 0.6 m (24 in.) ledge, a fill slope of 3H:1V or steeper could be located, that of which is shielded by the roadside barrier system.

Additional research performed after this response on an MGS long span application utilized a 25-ft long unsupported length and three timber CRT posts on each side of the unsupported length. In these tests, a 24-in. flat distance was located behind the CRT posts adjacent to the long span leading to the slope break point of a 3:1 slope. System details and crash test results are discussed in:

https://mwrsf.unl.edu/reportResult.php?reportId=109

Response No. 2

 MwRSF crash tested the (NCHRP Report No. 350) long-span guardrail system using 3,810-mm (12-ft 6-in.) long, W-beam rail sections over the unsupported length as well as directly adjacent to the long span in order to create a worst-case test condition in the middle of the unsupported span. This selection positioned a rail splice at the center of the 7,620-mm (25-ft) long span. In summary, you can choose to either implement or not utilize a rail splice within the unsupported length of nested guardrail.

 Response No. 3

For crash testing, the NCHRP Report No. 350 G4(1S) long-span guardrail system was constructed with 30.48 m of nested W-beam rail. On the crash-tested installation, two 7.62-m long, single, W-beam rails or 15.24 m total were placed upstream of the nested region, while one 7.62-m long, W-beam rail was placed downstream of the nested region. This configuration provided an asymmetrical configuration about the centerline of the system which was believed to be more common in actual field installations (i.e., more length on the upstream side than on the downstream side). Typically, longer guardrail runout lengths would be required on the upstream end of the obstruction. The system could be installed in a symmetrical manner with a standard guardrail terminal placed beyond each end of nested W-beam rail. For two-way traffic, a guardrail terminal could be located on the downstream end starting at the nested location. Adequate runout length to accommodate reverse direction traffic would still need to be determined.

 During evaluation of the MGS long span system to MASH TL-3 impact conditions, the unsupported rail was not nested and this question would not apply. 

Response No. 4

 MwRSF uses 90-mm (3 1/2-in.) diameter holes for CRT timber posts.

Response No. 5

 The NCHRP Report No. 350 TL-3 crash tests described in the test report and TRR journal paper were performed on a test installation that did not include a concrete box culvert, headwall, and wingwall. In actual field applications, a concrete headwall would typically extend above the low-fill soil, run parallel to the roadway, and prevent the soil from eroding over the culvert end. In this situation, if the headwall is placed too close to the guardrail, the potential exists for the vehicle's wheel or fractured CRT posts to contact the headwall. If significant wheel contact occurs with the headwall or post debris striking the headwall, vehicular instabilities or rollover may result. Analysis of the OLS-3 crash test results revealed a maximum lateral dynamic rail deflection of 1.45 m. During this event, the vehicle's right-front wheel was also found to protrude under the deformed guardrail. In order to minimize or eliminate the potential for wheel contact on the culvert headwall or post debris wedged between the headwall, the back face of the guardrail should be positioned a minimum of 1.5 m away from the front face of the headwall.

Future research is planned and funded within the Midwest States Regional Pooled Fund Program to adapt the Midwest Guardrail System to long-span applications and determine whether the barrier system can be positioned closer to the curlvert headwall.

 The MGS long span which was evaluated in accordance with MASH TL-3 impact conditions was located approximately 18 in. behind the back side of the W-beam. During test designation 3-11, the vehicle's impacting front wheel partially extended into the culvert and impacted the culvert headwall, disengaging the wheel from the vehicle and causing the vehicle to lift upward. All MASH evaluation criteria were satisfied. Therefore, the MGS Long Span system with 25 ft unsupported guardrail length and without nesting was determined to be successful according to MASH evaluation criteria.

Response No. 6

MwRSF researchers believe that The entire guardrail system utilized before, within, and after the culvert structure will consists of the Midwest Guardrail System spaced on 1,905-mm (75-in.) centers. For This taller system with an unsupported length and closer placement near the curlvert headwall, it is believed that full-scale vehicle crash testing with a 2000P vehicle will be required. was full-scale tested according to MASH impact conditions and determined to be successful.

 Response No. 7

MwRSF has several *.avi files for the successful full-scale vehicle crash test (test no. OLS-3). However, due to the size of those files, I will copy those files to a CD-ROM and mail them to you.Video, photo, and research report results are located on the MwRSF Research Hub website.

https://mwrsf.unl.edu/researchhub.php