Rigid Diaphragm - Decking Connections 1

[mferg318]

I'm designing a building with a rigid diaphragm (reinforced concrete slab with non-composite deck) and masonry shear walls. My design approach has been to have steel floor beams supporting the slab that carry gravity loads out to pilasters in the masonry walls. My slab is tied directly into the shear walls. Here's my question: the assumption in this design is that the floor framing only handles the gravity loads, and the rigid diaphragm carries the lateral loads out to the shear walls. But if I'm fastening my decking to the floor beams with even the minimum number of fasteners, isn't the lateral load (or some of it) going into the framing? And if so, don't I have axial loads in the floor beams?

 

[KootK]

And if so, don't I have axial loads in the floor beams?

You'll have some, non zero axial in the beams. That said, I would expect that axial to be very small and I believe that the overwhelming majority of engineers would disregard it in this situation.

 

[mferg318]

You'll have some, non zero axial in the beams. That said, I would expect that axial to be very small and I believe that the overwhelming majority of engineers would disregard it in this situation.

What's the mechanism that explains this? Are we saying that once the concrete cures, the slab is so rigid that any connection of the decking to steel is negligible? In other words, decking connections are mostly for construction in this scenario? This was my design initial design assumption, but the more I thought, the more I confused myself.

 

[KootK]

What's the mechanism that explains this?

That's going to vary a bit from system to system. Feel free to keep firing off questions until you're satisfied though.

Are we saying that once the concrete cures, the slab is so rigid that any connection of the decking to steel is negligible?

In this situation, I feel that the quality of the connections is near irrelevant. Rather, it's the stiffness of the rest of the load path down to the foundations. Where that path is overturning shear walls... pretty stiff. Where it's overturning wall piers... less stiff. Combine that with the expectation that the slab will be wildly stiff in plan, one would expect most of the load to wind up at the shear walls.

In other words, decking connections are mostly for construction in this scenario?

Yeah. The connections are there to perform a bunch of jobs such as:

1) uplift maybe.

2) bracing the beam top flanges against LTB.

3) Worker safety prior to slab curing.

4) Probably some other stuff...

This was my design initial design assumption, but the more I thought, the more I confused myself.

That's altogether healthy. The best place to be technically is:

1) Have obsessed over every ridiculous thing ad nauseum AND;

2) Recognize where simplifications are justified / routinely made.

Not having #1 makes you a dumb ass.

Not having #2 make it impossible to do the work.

It's the folks that don't question things that we worry about.

 

[mferg318]

That's going to vary a bit from system to system. Feel free to keep firing off questions until you're satisfied though.



In this situation, I feel that the quality of the connections is near irrelevant. Rather, it's the stiffness of the rest of the load path down to the foundations. Where that path is overturning shear walls... pretty stiff. Where it's overturning wall piers... less stiff. Combine that with the expectation that the slab will be wildly stiff in plan, one would expect most of the load to wind up at the shear walls.



Yeah. The connections are there to perform a bunch of jobs such as:

1) uplift maybe.

2) bracing the beam top flanges against LTB.

3) Worker safety prior to slab curing.

4) Probably some other stuff...



That's altogether healthy. The best place to be technically is:

1) Have obsessed over every ridiculous thing ad nauseum AND;

2) Recognize where simplifications are justified / routinely made.

Not having #1 makes you a dumb ass.

Not having #2 make it impossible to do the work.

It's the folks that don't question things that we worry about.

Thanks Koot, I appreciate the detailed response and encouragement. I think I've satisfied any doubts I had about my design approach/assumptions. I've already designed my shear transfer reinforcement into the shear walls, so minimum required decking attachment to satisfy the items you listed above should be enough.