Headed Stud / Shear Friction

[slickdeals]

Folks,
If there is an embedded plate with headed studs on the soffit of a concrete beam, can the shear-friction design method be used for tension loads in lieu of Appendix D?

The logic I am thinking is that the headed studs will transfer tensile loads through bearing of the head in concrete. Once this load is transferred into concrete, any concrete cone breakout is prevented by adding longitudinal steel in the beam in addition to what is needed for flexure. Calculate As = Tu/ phi fy. In addition, you can always add closely spaced shear ties (say at 3" o.c.) to prevent the cone breakout on the sides.

Does this make sense?

 

[ishvaaag]

It is obvious that shear-friction is a mechanism dealing with shear at some interface. The plate at the soffit with embedded studs in the beam or slab I understand is to be used to sustain vertical (hanging) loads, hence its function is not related to the notional shear-friction mechanism. You will be dealing with the problem of direct pullout of the plate-studs gadget by the force, and so the main problem is getting enough anchor as to deal with the pulling force.

 

[slickdeals]

Question on headed studs / channel connectors:
When a headed stud in a composite beam is in shear, the shear force pushes the stud laterally which is in turn resisted by the concrete on the other side. Hence, there is no rotation on the stud and hence, carries only a shear force and no bending?

However, the code requires the welds for channel connectors to be designed for the eccentricity of the load if the weld is smaller than 3/16". Why is that so?

 

[ishvaaag]

I venture that because at limit states the strut that notionally stabilizes the head of the stud will have by then undergone some destruction, most likely crushing and slippage at the ends, then allowing for some degree of bending to concurr.

 

[paddingtongreen]

Slick, there is bending, the concrete is not infinitely stiff. They want to be sure that the stud, if it is going to fail, does so gradually, they want the weld to be stronger than the stud.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[par060]

So suppose you have a stirrup or tie next to the stud so that on the bottom of the beam it passes transversly through the cone...and say the stirrup extends 12" past the cone on either side....how does the force know that the stirup is developed vertically 12" past the cone.....it doesn't so the force applied at the failure plain on the horizontal portion of the stirrup is the same as a longitudanal bar passing through the same cone at 90 degrees.

I've never done it this way but it seems right to me.

 

[bones206]

Even with additional flexural reinforcement, I don't think that you could provide enough stiffness in the direction of the tension load to provide satisfactory restraint of the breakout cone. Once the cone develops, the longitudinal rebar would give you some ductility but its more efficient to transfer the vertical load into vertical stirrups.

I agree with StructuralEIT, I don't think the shear friction model fits this failure mode. My interpretation of Appendix D is that if you want to avoid being limited by the concrete cone breakout capacity, you must provide anchor reinforcement that satisfies the requirements of D.5.2.9, which specifically defines what can be considered as anchor reinforcement (stirrups, ties or hairpins at a certain spacing relative to the anchors). Since normal flexural reinforcement doesn't qualify, it can't be used in lieu of concrete breakout strength per D.4.2.1.

Even if you design the stirrups to take all the load, you still have to check anchor steel strength per Appendix D against the stirrup capacity.