Composite shear studs on frame elements 1

[Stazz]

It appears that Risa Floor only adds studs to frame members if they have positive moment along the entire member. In most moment frames this is not the case since the ends are fixed. How do engineers handle this?

Do you leave studs off of frame members?

Are there hard and fast rules to determine how many studs you should use on moment frames and what governs that design?

At my last company they said to max the beam out with studs (2 @ 12") which makes sense because don't you want to drag the diaphram force into the frame.

 

[Stazz]

With some more thought, if the beam does have negative moment then won't the studs on the beam crack the concrete in the slab because rather than pushing the concrete into compression as it does under positive moment, it will be pulling the concrete apart as the top flange goes into tension.

 

[ishvaaag]

You might I would say use studs in the negative moment regions as long as proper development of the shear friction forces taken by the stud are properly taken. This is likely difficult in a situation where the beam is not truly a composite beam with a reasonably thick compression head.

In any case in concept, you must take the developed force in shearing at the stud interface between 0 moment point and max moment (be it negative or positive). More exactingly, you would need to develop the whole section capacity taken by the studs at the interface in shear friction, in the same length. You may find codes or books that follow one of these two approaches, which may arguably be valid for the specific situations.

Regarding to doubts, I always have been wary of the whole capacity development bias. This would mean that the authors of the code do not rely themselves in the security of their proposed resulting -by the code- forces, and hence we must provide for more! If the solicitation produced by the code is the maximum we need to care for, it is excessive to require cover for more.

 

[JoshPlumSE]

Composite beams with negative moments are tough for a number of reasons.

1) Can you really rely on the beam to act compositely in the negative moment reason.... Should you?

2) If you do rely on the beam to be composite, then what do you use as your moment of inertia? The newer versions of AISC suggest a method of interpolating between the naked steel and the composite moment of inertia based on how much of the beam is in positive bending and how much is in negative. But, that's not a solution that works very well programatically.
a) It requires you to assume a moment of inertia first, then solve for the moment diagram before you can figure out what you moment of inertial should be.
b) This moment of inertia is somewhat dependent on load combinations because that can change how much of the beam is in positive vs. negative compression.

3) If this beam is really part of a moment frame, then the design should be controlled by the lateral analysis. And, in the lateral analysis, you cannot have any composite action.

For these reasons, design of members with negative bending have been left out of composite design for RISAFloor. However, in the future, we might add it in for situations where there is a short cantilever where the backspan is almost entirely in positive bending.

 

[Stazz]

Josh,

Thanks, That's a big help. Where exactly is "interpolating between the naked steel and the composite moment of inertia" referenced? Is it in a design guides?

I just found in the AISC Spec I-2b that the negative capacity can be used in a composite section provided proper shear connectors and development of reinforcing steel.

Also in Design Guide 8: Partially Restrained Composite Connections, there is a way to use the composite action at negative regions to enhance the moment capacity using negative reinforcing steel in the slab. I'm assuming the section would have to be analyzed as cracked so that the composite section would consist of the W shape and a few reinforcing bars offset from the top flange.

 

[JoshPlumSE]

Stazz -

The first place I saw this was in the commentary to section I of the LRFD 3rd edition there is an equation:

It = a*Ipos + b*Ineg.

In this case Ipos would be the composite beam moment of inertia and Ineg would be the naked steel beam. The a and b values are the fraction of the beam that are in positive or negative bending.

Honestly, this equation still isn't all that good of a solution. Afterall, the a and b values may change based on the moment of inertia used in the analysis.... at least if you include the rotational stiffness of other supporting members or columns.

 

[Stazz]

Yeah, it would be an itterative analysis to hone in on the exact moment of inertia since it would be a function of the moment diagram which is a function of the stiffness. Oh well, at least they provided us with direction. I think they only put that in the code to humor people like me who like exact answers. It's a dead end. Excess steel it is then. Final answer.