2 Sided Diaphragm Connection

[sticksandtriangles]

I am struggling to understand double sided diaphragm connections and was looking for some guidance.
Please reference the images below.

Consider this as a flexible diaphragm with a full length shearwall.
Let's also assume that my diaphragm capacity is 1000plf based on a certain deck attachment pattern.

Do you consider section 1-1a and 1-1b to be identical in terms of capacity? I am thinking in terms of:
1) diaphragm shear capacity
2) in terms of ability to deliver force down to the shear wall below.

My intuition tells me something is inherently different between section 1-1a and 1-1b, but I cannot put my finger on it.
My gut tells me that 1-1a and 1-1b has that same ability to resist diaphragm shear, but section 1-1b has less capacity to deliver force to the shear wall below? Then devil's advocate in me tells me how is section 1-1b any different than if they just chopped the deck in half over the shearwall?

The way I see it, Section 1-1a diaphragm shear capacity = 1000plf, section 1-1a capacity to deliver force to shearwall below = 2*1000plf

Do you see any difference between 1-1a and 1-1b in terms items 1 or 2 listed above?

Section 1-1c is what I typically see and I consider diaphragm shear capacity and ability to deliver force down the shearwall below the same (1000plf). In this case, section 1-1c would be inadequate to transfer the total 2000plf demand down to the shearwall below (total jump in the shear force diagram).

Thanks for your thoughts.

S&T

 

[msquared48]

Well, when you assumed a flexible diaphragm, you simplified the connections to have no moment distribution capacity over the beam. That makes all the connections equal, leading to the 2000 plf shear value transmitted to the shear wall.

If this was a rigid diaphragm over the beam, the moment distribution capacity of the diaphragm would increase the diaphragm shear to 1100, and the shear wall force to 2200 plf.

In reality, the actual forces for the continuous steel deck details is somewhere in between as the steel deck without a concrete topping is neither flexible nor rigid, but semi-rigid.

Mike McCann, PE, SE (WA, HI)

 

[KootK]

1) I've pondered similar questions in the past and am relieved to know that I'm not the only one for whom this is not obvious.

2) In general, I think that you've got this down.

Then devil's advocate in me tells me how is section 1-1b any different than if they just chopped the deck in half over the shearwall?

3) It's not. No difference between 1a & 1b other than some abstruse concerns of my own that I'm sure nobody considers in practical design:

a) For 1b, there is a small chance that the two connections could be close enough that they might violate minimum spacing for welds, screws, pins or whatever.

b) The shear buckling mode of failure in corrugated deck is an interesting and complex thing. I kind of wonder if shear coming in from both sides might interact in an additive way somehow. On the other hand, the diagonal compression should be coming to the joint in opposing directions on both sides of the wall which you'd think would actually improve shear buckling capacity. Let's just leave this one on the theoretical mumbo jumbo pile for now.

 

[sticksandtriangles]

Thanks for the feedback, my mind went to the buckling failure mechanism as well Koot.

S&T