ASME BTH 1-2011

[illini8181]

I am new to ASME BTH 1-2011. I am trying to understand where some of the equations come from. One in particular is confusing me right now. Section 3-3.2, Bolted Connections, states "The allowable tensile stress, Ft', for a bolt subjected to combined tension and shear stress is

Ft' = sqrt(Ft^2-2.60*fv^2)

Ft = allowable tensile stress in bolt
fv = computed shear stress in bolt

Does anyone know how this is derived? Also, should this be compared with the tensile stress in the bolt, or the Von Mises stress in the bolt? BTH seems to imply that you would compare it with the tensile stress in the bolt, but I would think if you have a combined stress state, you would compare it with the Von Mises stress in the bolt.

Andrew

 

[JStephen]

I can't help on the derivation- but I think a lot of ASME BTH is taken from other sources, so similar design codes would be the first guess.
Normally, maximum bolt stress per codes is based on uniform axial tension and uniform shear, with variation of stress within the bolt neglected & presumably accounted for in the code equations. Also, check the definition of area, which is sometimes root area, sometimes gross/shank area (even if the stress is in the threaded part, the allowable load there may be based on gross area rather than root area).

 

[illini8181]

Thanks for the response JStephen.