Seismic Design of Granular Bins (Cd?)

[aggman]

We are working on the design of a 300 ton elevated rock storage bin. The seismic design category is D. The bin sites on elevated legs, braced in one direction and moment frames in the transverse direction. We are designing to IBC 2006 (ASCE 7-05) and have selected the appropriate R factors per table 15.4-2. My question is in regards to applying Cd. Per section 15.7.10.3 it states that the design drift, the elastic lateral displacement of the stored mass center of gravity, shall be increased by the factor Cd for evaluating the additional load in the support structure. Normally (at least to my understanding) that the Cd is simply a magnification factor to the elastic deflection to limit our story deflections, but is not actually used to calculate loads. In the above section it reads to me that we should actual calculate the seismic shears, apply the forces and get the elastic (first order) deflections, and then amplify them by 2.5 (Cd) and size all the members to that load. Am I reading this incorrectly?

 

[MiketheEngineer]

Not sure - but "mechanical" objects have much less criteria than habitat buildings. Keep reading.

 

[aggman]

Thanks for the response Mike. I think we got it resolved. It was an interpretation issue. We were losing track that we were trying to account for the P-delta effects. Sometimes it seems that I can read the same section 5 times and come out with a different answer every time.

 

[MiketheEngineer]

We all suffer from that!!