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I’m working on a project where the designer would like to consider whether to use a long-span system (attached eba211.pdf) or connect to the culvert (attached eba210.pdf). There are a number of culvert locations containing a mixture of headwall heights and widths and the designer has inquired about the deflection distances for each of the long-span layouts. I’ve attached what I could find from recent research, as well as a thought on how we might find another, but I don’t have any inclination as to what deflection distances should be used for the two post layout.



Please review and comment on what I could find and provide guidance on what I couldn’t.



As always, thank you!

We have not done any detailed analysis of intermediate working widths for the MGS system. However, we do have data points at the 25 ft unsupported span as you noted in your table and data points for the standard MGS system. It seems reasonable that our best path to estimating these working width values would be to linearly interpolate between the know values.

I have added a table below that does just that. For the MGS with standard post spacing, I have assumed a working width of 60.3 in. based on the highest working width value we have observed for an MGS system (this is test MGSDF-1 of the MGS with Douglas Fir posts). This should provide a conservative starting value and better represent the CRT posts used in the long span in terms of system stiffness and barrier deflections. I then used your suggested upper bound of 96 in. based on the results of test no. LSC-1. The results come out relatively nice in that the recommended working widths are essentially even foot values.

While this analysis is not substitute for testing, it should provide you with reasonable guidance given our best available information.

Thanks 

Looks like a reasonable approach. Thank you.

As a follow-up question – in the TRP-03-187-07 report on page 101 where it mentions a 2” max height, why would this value be lower than the typical 4” height where we start to view an object as a hazard per Roadside Design Guide?

The 4” you mention below is a stub height requirement that is intended to prevent/reduce damage to the vehicle undercarriage and the underside of the occupant compartment if the vehicle drives over.

The 2” height for the headwall noted in the long span research is listed for a different reason. The is some concern that interaction of the impacting vehicle with headwalls taller than 2” may affect vehicle stability it is redirected. Previous research with curbs have found that 2” curbs affect vehicle motion significantly less than 4” or 6” curbs. Thus, we limited the headwall extension above grade to 2” or less to improve vehicle stability.

Hope that answers your question. Let me know if you need anything else.