Drag steel to Elevator Core in PT slab?

[randomizer]

Hello fellow Structural Engineers,
Thank you for your help,

GIVEN:
200x90 15" concrete PT slab.

The 200 ft direction is split 40-120-40 by two stairway cores.

In the 90 ft direction, the stairway cores split the length 45-45.

FIND:
So, in the concrete rigid diaphragm, do you put collector steel to the elevator core walls if they are in the middle (as described) of the PT slab?


POSSIBLE SOLUTION:
It would seem unnecessary in a case where you have a very deep diaphragm, and the core walls were at the midline.

As, I think it might get there by the strut & tie method? See Appendix A, pg 353 of ACI 318-05.

Multiple concrete struts in a 15" slab can take a lot of load.... but it would be a very long struct, i.e. column.

So, I am thinking I may have to use drag struts, with the OMEGA factor of 2.5, which adds quite a bit of steel, but at least I can go to #8's given the thickness of the slab.

RECOMMENDATIONS:
Any thoughts?

Thank you!

Mike in Seattle

 

[lkjh345]

We will use tension drag strut bars out of core walls and into slab (PT, mild reinforced and concrete fill on metal deck) when our shear cores are offset to one edge of the building or we have an expansion joint next to a core. We usually use dowel bar subsitutes in these locations so as not to require holes in the formwork.

When the cores have a sufficient amount of floor on all 4 sides, we generally rely on bearing of the edge of the slab (diapragm)into the core wall to transfer the diaphragm forces into the core walls. It doesn't take much bearing area to transfer quite large loads.