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One of our districts is in the process of removing several damaged crash cushions (NCHRP-350 systems) and replace them with new ones.  Single-slope concrete barrier ends will be protected by new crash cushions (preferably with MASH systems) at 3 different gore areas. 42” were measured at the base of all single-slope concrete barrier. As observed on the attached pictures traffic would be present on both sides of the system. We would like to know what width the system should has to shield adequately the barrier ends in case there is not an exact 42” width system. Guidelines or key information on this connection aspect is appreciated.

• Having a wider than 42” width system would be needed?

• Would be it possible to have a narrower backstop width than 42” (let’s say 24”) and transitioning it out to the concrete barrier (avoiding wider/longer concrete pads)? Or the system would be better be transitioned itself as observed below (if it has been considered crashworthy):        

• Since the current contract doesn’t include any modifications with the concrete barrier (such as cutting the concrete to leave more room for the MASH product or adding concrete pad), could there be possibilities to install a MASH crash cushion within the existing area?

Thanks,

I have some preliminary thoughts.

When we consider shielding a wider barrier end like your examples, we generally want to limit the exposure of the end of the parapet. This means that we want the crash cushion to shield the edges of the parapet. As such, we would generally want the crash cushion to be as wide as the parapet at the point where it attaches to the parapet. Additionally, we would want to limit exposure of the parapet above and below the crash cushion as well. Thus, we are also concerned with the portion of the parapet toe that extends out below the crash cushion or extends above the top of the crash cushion.

In terms of functionally meeting those goals, different crash cushion designs may be more or less effective depending on their width, structural stiffness, and other factors. There are crash cushions with increased width. Additionally, there are crash cushions with transitions from the rear of the crash cushion that can widen out to the increased width. I have attached an example from SCI installation manual of how they widen it out to other structures. I believe that other manufacturers have similar setups.

As far as modifications to the concrete barrier end itself, that is certainly an approach that can help. Tapering the width and height of the parapet down to meet the end of the crash cushion is effective, but can be difficult to accommodate with existing installations due to space and cost. We have suggested a research topic in the pooled fund to develop a standard parapet end for crash cushion attachments for new installations. This would be similar to the standardized AGT buttress we did for approach guardrail transitions. However, this research has not yet moved forward.

Those are some initial thoughts. Take a look and let me know if you wish to discuss it further.