asce 31-03 seismic evaluation of existing buildings

[sennafan]

Any one out there have experience with ASCE 31-03. I am repairing a steel frame parking structure (substantial alteration due to scope) which has a braced frame lateral system that does not meet current code, but is close. The building department allows evaluation using ASCE 31-03 as an alternate to meeting IBC 2003, but I am getting worse results for some elements. I have reread the 31-03 code 3 times looking for an error on my part, but get the same conclusion. Obviously since the lateral system is not too far from meeting the current code, I was hoping to save the expense of seismic retrofit and assumed the standard would be somewhat relaxed compared to cuurrent code.

I am getting a pseudo force of 6 times the design force required by IBC 2003/ASCE 7-02.

Any ideas about what I missed or confirmation of the result would be appreciated.

Thanks

 

[civilperson]

How did you develop the ASCE 7-02 force? What are the site parameters for 1 second and for short term accelerations? What live load mass is assumed and at what level?

 

[sennafan]

Ss = 1.51, S1 = .527 for both codes. ASCE 7 uses R=5 and rho = 1.5 and calculates a multiplyer of 0.33W. ASCE 31-03 loads included 10 psf of live load, which was not included in the ASCE calculation, but calculates a pseudo seismic force of approximately 1.2W based on C=1.2. Structure period is estimated at approximately 0.12 sec by both methods.

I forgot the effect of rho and the difference in mass in the ASCE 7-02 calcs when I wrote the post, so the difference is more like a factor of 2.5 applied for equivalent mass (E = 0.3W versus Quf = 1.01W). Still, this is a much higher load than the use of Fye = 1.25Fy and Phi = 1.0 make up for.