ASCE 7 Seismic clarification?
ASCE 7 Seismic clarification?
(OP)
Hi, I am just looking for some opinions on my interpretation of the code on diaphragm forces.
I have seen around the web, and heard from others, that diaphragm forces (Fpx, eq 12.10-1) will always be higher than analysis forces (Fx, eq 12.8-11 & 12.8-12).
However, according to ASCE 7-10 12.10.1.1:
Along these lines, some diaphragms will be designed with rho = 1.3 and some with rho = 1.0. If the collector is designed to overstrength, some levels will be reduced by 1.3 first and some would not reduced. Light frame collectors are not designed for overstrength, but in high seismic, irregular structures, the design forces determined from section 12.10.1.1 (diaphragms) are increased 25% increase according to 12.3.3.4. I assume this 25% increase would apply to design forces from 12.8 where those govern (roof and upper floors), but it is not explicitly stated.
Does anyone have any more insight than the slightly ambiguous wording of ASCE 7-10?
I have seen around the web, and heard from others, that diaphragm forces (Fpx, eq 12.10-1) will always be higher than analysis forces (Fx, eq 12.8-11 & 12.8-12).
However, according to ASCE 7-10 12.10.1.1:
"Floor and roof diaphragms shall be designed to resist seismic design forces from the structural analysis, but shall not be less than that determined in accordance with Eq.
1210-1"
To me this indicates that analysis forces from Eq. 12.8-11 & 12 will govern diaphragm design in some cases. I see one case where this could happen if rho = 1.3. Also according to section 12.10.1.1:1210-1"
"...For inertial forces calculated in accordance with Eq. 12.10-1, the redundancy factor shall equal 1.0."
Therefore, at the roof, Fx will be 30% higher than Fpx, and at upper floors, Fx could govern diaphragm design as well. So to me it is unconservative to just apply Fpx to diaphragms.Along these lines, some diaphragms will be designed with rho = 1.3 and some with rho = 1.0. If the collector is designed to overstrength, some levels will be reduced by 1.3 first and some would not reduced. Light frame collectors are not designed for overstrength, but in high seismic, irregular structures, the design forces determined from section 12.10.1.1 (diaphragms) are increased 25% increase according to 12.3.3.4. I assume this 25% increase would apply to design forces from 12.8 where those govern (roof and upper floors), but it is not explicitly stated.
Does anyone have any more insight than the slightly ambiguous wording of ASCE 7-10?






RE: ASCE 7 Seismic clarification?