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Re: Diaphragm Capacities

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Analyze the diaphragm shear strength using the concrete topping thickness (that above the top of the deck for shear parallel to the flutes) and reinforcing (#4 @ 12" EW) and ignore the metal deck.  The Phi factor is 0.6 since it is a diaphragm
 
I don't believe you can combine the strength of the deck with the rebar due to the difference in behavior in bonding interlock of the concrete fill  with metal deck as compared to the bonding of the rebar encased in the concrete fill.  I believe the Verco values, when rebar is added is based upon the concrete topping and reinforcing only...does not include an interaction with the steel deck.
 
Once you use shear headed studs through the deck to the steel beams, the welding of the metal deck is no longer relevant for seismic shear transfer, since all seismic force is designed to go through the shear headed studs.
 
The diaphragm shear strength may be limited based upon the shear studs along the collector.  Verify with the local jurisdiction as to allowable shear capacty of shear head studs used for collector/drag shear transfers.  If I remember correctly, LA City limits 3/4" diameter shear head studs in Lightweight concrete to 3.4 kips per studs for horizontal shear.
 
Michael Cochran SE
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In a message dated 2/21/2007 12:09:33 AM Pacific Standard Time, Andrew.Mole(--nospam--at)arup.com writes:

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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