bolt torque on structural assemblies on the ground vs. in pl

huangyhg

bolt torque on structural assemblies on the ground vs. in pl
we are currently erecting structural steel that will hold refinery equipment and piping.  the contractor is putting sub-assemblies (beam-to-column connections and cross-bracing) together on the ground and performing a final torque.  these 10 ton sub-assemlies are then lifted into place and spliced together in the structure. after these sub-assemblies are lifted into place, will they have to re-torque these bolts?  in other words, has there ever been any case of bolt loosening from the crane erection process?  

montana1,
i think what your contractor is doing should be perfectly acceptable.  a couple of caveats though.
1)if any of the bolts are loaded in tension or in tension+shear, then make sure that the erection process (considering lifting points, any resulting cantilevers, etc will not result in any stress increases beyond what the bolts were designed for.  also that the erection procedures don't put tensile loads on any bolts that they cannot handle.
2)  not bolt related, but consider all member stresses resulting from the same erection conditions- make sure that no members are overstressed.  
that said, from your description, it sounds like the contractor is assembling bents lying on the ground and lifting or tilting them to the upright position.  i would not expect this to cause overstress in bolts or   
if properly tensioned on the ground (either snug or slip critical) and carefully hoisted into place, then it shouldn't be necessary to re-torque the bolts.
dik
be careful about contractors who "torque" bolts. there are different requirements for structural bolts than, say, mechanical pipe flange fasteners.
the pretension in a slip-critical bolt is very very large and the assembly should not loosen under load.