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We had  project where the contractor supplied unhardened washer for bolting the rail to the post on normal MGS.

I was trying to determine if unhardened washers are acceptable.  If they are what material spec should be used?

We do not require any washer under the nut where the guardrail bolt attaches to the post flange. In fact there are no washers at all on the standard steel post versions of the MGS that have been full-scale crash tested. That said, there would be no adverse affect of including a washer between the nut and the flange on the back of the post. 

The wood post version of the MGS does use a washer under the nut on the back of the post. The washer used is the ASTM F844 washer typically specified with the A307 post bolt.