Concrete diaphragm shear strength
Concrete diaphragm shear strength
(OP)
I'm almost embarrassed to admit that after a little over four years on the job I've never "designed" a concrete diaphragm.
I'm working on a large building that has a lot of openings near the braced frames and I want to make sure that we have enough actual diaphragm left at these large openings.
I'm looking in the SDI Diaphragm Design Manual and all of the tables at the back limit the concrete fill to 2 1/2" over the top of the rib. Why is that? I haven't read through the entire manual, but I'm not finding anything where I did read.
Also, why is the fastener layout important for a concrete filled deck? Is that just for the contribution from the actual deck?
I'm working on a large building that has a lot of openings near the braced frames and I want to make sure that we have enough actual diaphragm left at these large openings.
I'm looking in the SDI Diaphragm Design Manual and all of the tables at the back limit the concrete fill to 2 1/2" over the top of the rib. Why is that? I haven't read through the entire manual, but I'm not finding anything where I did read.
Also, why is the fastener layout important for a concrete filled deck? Is that just for the contribution from the actual deck?






RE: Concrete diaphragm shear strength
Regarding the 2.5" - that is sortof the industry standard for diaphragm tables, not sure of the origination, but generally you have a good bit more than that so the tables are conservative. The best way I have found to calculate the actual capacity is using Hilti's Profis DF diaphragm design software which, in addition to calculating diaphragm strength using Hilti Pin's of course, will calculate strengths for standard puddle welds etc. with whatever concrete thickness you want. I verified it against the Diaphragm Design manual and it matches very well: http://ww
Regarding the fastener layout - yes it just affect the contribution from the actual deck, which can be fairly minimal compared to the concrete itself.
Also if you are in a high seismic area you must check provisions of ACI Chapter 21.11 (in 318-08) for maximum compression forces in the chords etc.
RE: Concrete diaphragm shear strength
RE: Concrete diaphragm shear strength
The side seam and puddle welds limit racking of the individual panels in lateral action, adding to the diaphragm capacity.
If shear studs are not used, the only mechanism for transferring any composite action between the concrete and steel deck to any drag strut/shear collectors, is the welding of the deck to those members.
Vulcraft, and similar steel decking manufacturers, have tables of allowable shear for variations of infill, metal gage, rib depth, concrete strength, and fastener type and spacing.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Concrete diaphragm shear strength
Think of the concrete in a composite deck as the "stiffening component" of the composite deck. This resists the in-plane shear, similar to how a web resists shear in a wide flange. However, the concrete does nothing for you with regard to your edges; i.e. transfer shear from your diaphragm to your drag beams ("collectors").
If your diaphragm shear is really high, ignore the table values from the deck manuf and use conventional reinforced concrete calculations to arrive at higher shear capacities (just like a thin concrete shear wall). For this condition, the deck is just formwork, and you rely on studs for both composite beam behavior and shear transfer.