Effect of pre-load on shear strehgth 1

[trainguy]

Hi all.

Typically, (Canadian) steel design codes state that a bolt will fail in shear at a stress of 0.6 * Ftu, over its nominal area.

Is this dependent on pre-load?

I am fully aware of the mechanics of pre-load on tension performance, but I've always wondered about shear.

I have also used the (Canadian) design code equations for combined tension and shear, but when these loads are external...

Any takers?

tg

 

[CoryPad]

I don't think I understand your question. You are aware of the equation for combined loading - how does that not apply?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[trainguy]

I'll rephrase the question.

Let's say you're designing a bolted joint to support a simple gravity loaded beam. Consider, for example, a single angle beam connection. The bolts are torqued during installation, to some certain value, and are therefore in tension.

Assuming a bearing type connection, in sizing these bolts do you use the equations for shear only, or for combined shear and tension?

I have always done this using shear only, and this would suggest that the shear strength equations are independant of pre-load, or that pre-load has been considered in their derivation.

Is this correct?

tg

 

[dimjim]

Your load is the combined load of the dead weight and the increased load caused by the preload.

 

[drawoh]

trainguy,

If the bolt is tightened down hard and is in tension, the joint is held in place by friction, right? Shear does not apply. The shear load works when the bolt is loose for whatever reason. The reason could be weird design, or sloppy installation. Conceivably, there could be some tension on a bolt that is loaded in shear.

Is this a question about code, or about mechanics of materials?

JHG

 

[btrueblood]

The code calculation assumes there is no appreciable tensile stress (preload) on the bolt, and that it is loaded in shear only. Typically, the calculation is done if the load direction of the joint could load the bolt in shear; you then assume that the preload is lost (or never existed), the joint slips, and the bolt is loaded by pure shear.

If the bolt has a tensile preload plus a shear load, you will need to determine the allowable load combination using a combined-stress failure model, e.g. a Mohr's Circle, Tresca, or Von Mises theory. If you are designing a device to take a shear load (e.g. a pivot), consider using shear pins designed specifically for that purpose.

 

[WmacG]

The AISC LRFD code I believe has a section on combined shear and tension on bearing type connections. If the applied shear stress is over a particular value, the "allowable" tension stress is reduced. I guess you could look at it the other way too (for a given tension, shear may be reduced).

In my third edition LRFD manual it is in the Specification, section J3.7.