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Diaphragm Capacities

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I am assessing an existing second floor diaphragm of a steel-framed structure in a zone 4 region to UBC:97. The slab is 3” (76mm) of 3,000psi (20.7MPa) normal weight concrete on top of a profiled metal decking 2” (50mm) deep (gauge 16 – 1.52mm thick). It has 275MPa 12mm bars at 300mm centers (approx #4 @ 12”, fy = 40ksi) both directions, which corresponds to a reinforcement ratio of around 0.49%. If I use ACI318 to calculate the shear strength – then for 2*sqrt(f’c) I get 110psi or 3,960plf (0.76 MPa or 58kN/m), and for the reinforcement I get 196psi or 7,056 plf (1.35 MPa or 102kN/m).


My first (and principal) question is – what phi-factor should I use – 0.85 or 0.6 (see sec. 1909.3.4)? 0.85 gives me 3,370 plf (49kN/m), and 0.6 gives me 2,380 plf (35kN/m). My vote would be for 0.85 J.


I then tried to compare this with values taken from the tables of steel deck manufacturers. Verco (p 59, assuming min. 3,000psi concrete) gives me around 2,205plf (32.2kN/m)(conservatively) for beam supports at 8ft centers, and extrapolating the Vulcraft VLI values would give me something similar. Since these are working loads, if I multiply by 1.4 I get 3,090plf (45.1kN/m). This is less than I would get for the concrete itself if I took phi as 0.85. I understand that the tables do cap the numbers to suit the capacity of the connectors to the beams, so this could account for it.


On revisiting this by referencing Verco p20 (which apparently does not account for any deck strength), I get 5,675plf (interpolated working load) for 3” of concrete over the decking, although I think this includes 2,390plf (interpolated) for wire reinforcement). If I subtract this and then multiply the remainder (3,290plf) by 1.4 I get 4,600plf, which is much higher than even the nominal value according to ACI above (3,960plf). Rather puzzling…


I am assuming that, whatever value I take for the concrete slab (with or without decking), I can still add the effect of the reinforcement (given that I still have to check for transfer mechanism into the beams). Is this other people’s understanding as well?


Incidentally, I don’t know whether the deck was puddle-welded to the supporting beams and whether there are any side-locks (I am assuming not). As a result I would rather be conservative. However, I do know that there are studs and that they are at 200mm centers (7.9”) along the fluting or at 318mm in the troughs (12.5” OC, driven by the trough spacing).


I am inclined to go with the original ACI value of the concrete and reinforcement only, with phi at 0.85 (especially since I am sure that the decking used was not Verco). Still, it is a pity that I cannot take any advantage from all that steel. What do other people do? Any comments welcome. Thanks.

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