Diaphragm Design Forces ASCE7-05 12.10.1.1

[sedesigner06]

I have a curiosity question as to why the diaphragm forces Fpx from ASCE7-05 12.10.1.1 can produce in the magnitude of four times the seismic design force produced by Cvx vertical distribution. Why should the diaphragm be designed to such a large force if the lateral system cannot take it.

Thanks in advance for any help.

 

[JoshPlumSE]

Seismic forces are designed to fail in a certain ductile manner so that they need not be designed to the full seismic force that they would experience if they had to resist the forces elastically.

I don't think you're detailing out the diaphragm in the same way. Hence, it needs to resist the higher level of forces. Similar in concept to designing for Omega*seismic forces.

 

[sedesigner06]

Ok that make sense, thanks for the explanation. I did my first R=8 gussett plate design for two braces coming in, and that was an experience to say the least. That makes perfect sense now.

 

[Hokie93]

The diaphragm design force (F_px) equation (12.10-1) in ASCE 7-05 accounts for forces resulting from higher modes of vibration, which can induce greater forces at lower floors than are imposed by the story forces, F_x. The vertical distribution of base shear in ASCE 7-05 is based on the first mode of vibration. The subject of diaphragm design is covered in good detail in two articles published in the January 2009 and February 2009 issues of Structural Engineer magazine (

http://www.gostructural.com).

Both articles are written by Rafael Sabelli, Warren Pottebaum, and Brian Dean.

 

[sedesigner06]

Ok thank you very much for the reference.