Prequalified SMF RBS

[ARLORD]

Question regarding prequalified SMF Reduced Beam Section connection design:

According to Seismic Provisions Section 9.6, in the equation for Mpb for prequalified RBS includes 1.1Ry*Fyb*Zrbs, however, in the AISC 358 Section 5.4, it directs you to EQ 5.8-5 where Mpr = Cpr*Ry*Fy*Ze.

Why are these two equatons using different values, 1.1 vs. Cpr?

 

[ARLORD]

Per Fema 350, June 2000, Commentary Section 3.2.4(see link below), in calculating Mpr the coefficent 1.1 was used in the 1997 AISC Seismic Provisions, however, FEMA testing determined that for most connection types, the peak moment developed is higher than using 1.1, therefore the Cpr factor was developed, limited by 1.2.

http://www.fema.gov/plan/prevent/earthquake/pdf/fema-350.pdf

I am not sure, but I think the latest AISC Seismic Provisions incorporates FEMA also. So I guess they added the Cpr factor for calculating Mpr, but left the 1.1 in the moment ratio section as an alternate based on previous standards.

I guess the moral of the story is to use Cpr to calculate Mpr in all cases unless, the code states or you can show that 1.1 can be used instead of Cpr.