Pretensioned Bolts and Assemblies 6

[volcomrr]

thread507-253463

I normally just browse through eng-tips and rarely ever post (this may be my first post!)

Looking through threads, I saw one from 8 years ago that touched on a questionable debate we were having in the office last week. The question was, if you have pretensioned bolts supporting a hanger, and tension load is applied to the hanger, do the pretensioned bolts see more tension? My thoughts were the bolts would see more tension, 2 other engineers said the bolts wouldn't see more tension, and the last engineer was on the fence. By the end of the discussion, everyone agreed that tension applied to an assembly fastened by pretensioned bolts would create more tension in the bolts.

Then I saw in the above thread, slickdeals quoted someone from Modern Steel or AISC saying that the bolts would not see more tension until the assembly begins to separate. I believe this to be incorrect. I agree the assembly won't separate until the tension force on the hanger overcomes the pretension of the bolts; however, you're adding more tension load to the bolts that are already tensioned. Does anyone have any insight to shed on this? Thanks.

 

[BAretired]

Yes, I agree there would be a small increase in bolt tension to accommodate elastic rebound of the assembly but not simply the sum of the prestress and the added force.

There would be no such increase when stressing a prestressed rod if the tension force applied to the rod is balanced by a compression force applied to the concrete, which is the usual case.

BA

 

[Ingenuity]

It seems as if we've been oversimplifying it a bit. I have to agree with desertfox's statement here. Strain compatibility would dictate that there would be at least a small (granted, typically very small) increase in bolt stress when compression on the plates decreases, since the bolt has to elongate ever so slightly to accommodate the elastic 'rebound' of the plate thickness with the reduced compression stress.

Prof. Geoff Kulak (may he RIP) addresses this in the AISC's Welding and Bolting Primer video at about 8:00 minutes into Part 10: Link

Physical testing of pre-tensioned assemblies typically result in 7% or 8% increase in bolt tension.