I have been looking through the SimpleBoltCalc.xls in the Repository. The calculation incorporates a co-efficient of friction of 0.2 (this is the usual steel on steel value I guess). I have also been reading Steel Designer's Manual (UK Version) (US Version)
In here the coefficient of friction used is 0.45 'the permitted value for untreated surfaces', apparently. I am often dealing with bolt slip issues but I am intrigued as to how such a high value can be justified. Have anyone got any suggestions?

I am just pulling a repository comment into the forum to give users an opportunity to answer. It concerns the same calculation SimpleBoltCalc.xls
ericvarhos 2007-06-05 06:52:02
"Simple and easy way to check the bolt calculations. Good job. Just a little curious where the bolt shear values come from, is there perhaps a reference?"

The maximum shear stress failure theory (associated with Tresca and Guest) is relevant to ductile metals. It is conservative and relatively easy to apply. It assumes that failure occurs when a maximum shear strength attains a certain value. This value being the value of shear strength at failure in the tensile test. In this instance it is appropriate to choose the yield point as practical failure. If the yield point = Sy and this is obtained from a tensile test and thus is the sole principal stress then the maximum shear stress Sy is easily identified as Sy/2 (see Mohrs circle). So I can see how the calculation for bolt shear strength based of Sy/2 makes sense.

Yes John that was pretty much the basis of the shear strength calculation. Typically a bolted joint slips before the bolt shears and generally we design against slip because we do not want the joint to loosen. The shear strength of the bolt might be useful if you needed to calculate the post-slip strength of a joint (an ultimate strength calculation).