Concrete Joist Diaphragm Strength

[Corey7320]

I am analyzing an existing building with concrete joist floor system. The joists are 5in wide by 8in below the slab. The slab is only 2 or 2.5 in thick. The joist spacing is 20in and the spans are 10-12ft. I am in a high seismic area and the diaphragm strength is being evaluated. The slab alone has minimal strength due to the small thickness.

What is the typical practice with a system such as this? Can you take an effective depth for evaluation which would take advantage of the joists? If possible, can this approach only be used perpendicular to the joists?

In the direction parallel to the joists can you take advantage of supporting beams and bridging between joists to add to your diaphragm strength? There is a bridging member or beam every 5-6 ft.

Any input on this topic is appreciated.

 

[hokie66]

Why do you say the slab has minimal strength? A 2" or 2.5" concrete slab is quite a stiff element. I suppose there could be issues with chord reinforcement normal to the ribs, but if there are supporting beams and bridging in that direction, they should serve as chords.

 

[Corey7320]

I calced the shear strength from ACI (phi=1.0)and for the 2in slab a capacity of 7.24k/ft was found. ASCE41 m factor used was 1.5 for Imediate occupancy. The expected f'c is 2500psi the expected fy =49.5ksi and there are only 1/4" dia bars at 12" o.c.

For example at a 32 ft long perimeter shear wall i have roughly 500k coming in from the diaphragm based on deformation control and 2/3 MCE (IO level) (15k/ft) which is twice my capacity considering only 2in slab.

 

[Ron]

Check your concrete strength...I would bet it's higher than 2500 psi.

 

[Corey7320]

I am awaiting concrete test results, but until then ASCE 41 gives ranges for what can be assumed. Thats where 2500psi came from

 

[rscassar]

Is there WWF in the 2" slab. I don't see why a 2" slab cannot be used as a diaphragm. What are the diaphragm stresses you are trying to transfer and to what element. If 23/32" plywood can be used as a diaphragm I would have no issues with using 2" reinforced concrete.