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We have a situation where there is a drive located next to the trailing end of a proposed MGS long span installation and are looking for a recommendation (see attachments).  We are unable to install the required 62.5' of guardrail on both sides of the culvert.  The legal speed of the roadway is 55 mph.  Some considerations: we may be able to reduce the length of the long span below 25' to around 15', and the side where we are unable to achieve 62.5' is on the trailing end.  Would additional length of guardrail added to the upstream end of the barrier run make any difference?  Would nesting the long span barrier improve the situation? 

Thanks!

There are not a lot hard guidance we can provide in this situation. As I read your detail, you have a little over 37.5’ downstream of the beginning of the long span. I cannot tell for sure if that is the prior to the CRT posts or prior to the unsupported span. In either case, that is shorter than the tested system which employed 62.5’ from the CRTs to the end of the system or 75’ to the unsupported span. We have not defined a minimum length for the long span. The MGS has been shown to be able to be used at relatively short system lengths. However, the unsupported span of the long span may increase the rail loading and anchorage demand as compared to a standard system.

The lower speed of your installation should help mitigate those concerns to some degree, but we cannot definitely provide a shorter length than what was tested.

Are you using the full 25’ unsupported span? We have provided guidance shortening the minimum lengths if the unsupported span is less than 25’.

https://mwrsf.unl.edu/q&a/view.php?id=677

The dual barrel culvert shown appears to impede 2 posts. If that is to scale in any way, you may be able to omit a single post and shift one post slightly to clear the culvert.

Let me know if that helps or if you wish to discuss further.