Diaphragm Design Loads 1

[3doorsdwn]

Question: the procedures for coming up with a diaphragm load (i.e. in section 12.10.1.1 of ASCE 7-05) are only applicable for the design of the diaphragms themselves correct (and the associated elements like chords, etc.)? I've always thought they just served as a minimum design load.

The reason I ask is: I was making sure that this load was not then to be distributed to shear walls, etc. for design......I've wondered this sometimes because after all the trouble you go to calculate this load, for a flexible diaphragm (with an irregular shape) you'd have to go back and figure the load to the shear walls based on a tributary area method. Thanks in advance.

 

[WillisV]

Your interpretation is correct. The diaphragm loads apply to diaphragms, collectors, chords, and connections to shearwalls etc. The shearwalls themselves are designed for force distribution via flexible, rigid, or semi-rigid action based on force levels from the Equivalent Lateral Force method (or a dynamic method if you so choose). Note for a 1 story building the ELF method and the diaphragm loads are the same thing.

The reason behind the different set of loads is to try to better capture the local demands on a diaphragm if it happens to have a good bit more mass than the others. The ELF forces capture this in the shearwall design, because the effect of the mass is thrown up higher in the building in a mode 1 type distribution which increased overturning moments etc. However for the local diaphragm the ELF distribution might underestimate the inertial forces thus the separate checks.