Our question is: What is the correct amount of torque to apply to the MGS guardrail rail bolt and nut when fastening the rail bolt to the guardrail post and block during MGS rail installation?
We’re working with our Maintenance group and this topic came up about properly tightening the rail bolts to the posts and blockouts. The bolt head rests against a curved surface (not flat) of the rail element and often the maintenance forces are over-tightening the rail bolt. This causes deformation of the curved rail slot area, and can also cause the bolt head to start to push through the rail slot. It is thought that over-tightening of the rail bolt may be part of the reason that allows snow load placed against the back of the MGS guardrail to pull the rail away from the post and block on MGS guardrail systems located in Washington State mountain passes.
We’re hoping that since MwRSF developed the MGS guardrail system they may have the correct torqueing specification that we can use so that our maintenance forces and installers can follow the correct specification and not over-tighten the rail bolts in the future.
There is not a formal bolt torque specification for the post-to-rail bolts that I am aware of. This has not been previously specified as a torque wrench is not something state DOTs or contractors typically employ when installing guardrail. We typically install the bolts in our tested system by torquing the bolts until the guardrail slot around the bolt head begins to deform slightly and the head of the bolt bites into the blockout. In a previous study, we looked at the torque required to tighten bolts to that level and found it to be 92 ft-lb.
Details on this investigation are in Chapter 8 of the report linked below.
Please let me know if you need anything else.