Friction under two structural elements in contact

[elfstein123]

Hello, I was having some trouble with quantifying the friction for the scenario shown in the image where the beam is connected to an angle. As we apply the load, if the bolts are loosened, the beam should be free to slide against the angle below which is what I want. The bolts are snug tight originally. How do I calculate the resulting friction? Is it just the self weight of the beam times coefficient of friction or will the additional load also cause some sort of friction (I'm thinking slip critical but not sure if that's relevant). Any thoughts on this would be appreciated.

 

[BridgeSmith]

If you want it to slide, you'll want to leave the bolts loose and lock it with a backup nut. Then, the friction force would just be the weight multiplied by the coefficient of friction, which can vary widely depending on the surface condition, so it can change over time.

If you snug the bolts, the friction could be very difficult to determine, since any rotation (torsion) of the beam will change the force on the contact surfaces.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[elfstein123]

Thanks for your help BridgeSmith! A follow-up question to that, when we have our load still being applied, it'll be transferred via bearing to the lower bracket right? Will that need to be accounted for in the analysis (including friction, if we don't loosen the bolts?).

EDIT: Just to clarify a bit more where my confusion lies, how is the horizontal load applied managed by the system if the bolts are loose? I would expect that there's bearing/contact on the angle but I cannot visual/understand how we would calculate/estimate that.

Thanks again.

 

[human909]

I'm confused by exactly what you have drawn in the supplied drawing. Eg how long are the slots for these bolts.

Also if there force is anything more than minorly eccentric to the shear plane then there is a good chance you'll get rotation as BridgeSmith pointed. The rotation could readily lock up the joint preventing from sliding.

 

[BridgeSmith]

I'm not sure if I understand what you're trying to accomplish. If you ultimately want to restrain it from sliding in the direction of the horizontal force, why would you allow it to slide at all?

Anyway, I'll try to answer with what I do know. The beam will be laterally restrained by friction until the lateral applied force overcomes the static friction, at which point the beam will slide until the bolts are up against the edge of the hole or slot.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[elfstein123]

human909, sorry for that, there are no slots, those were standard bolt hole sizes.
I believe I understand now, my confusion was stemming from how the force would be transferred/resisted with the bolts loose but it would still be the regular friction calculation (weight x coefficient of friction). Thanks for your help, BridgeSmith and human909.

 

[BridgeSmith]

Ok, I'm confused once more. If there are no slots, why does it matter what the resistance due to friction is? The horizontal force applied will be resisted primarily by the the shear strength of the bolt and the capacity on the sides of the bolt holes.

Rod Smith, P.E., The artist formerly known as HotRod10