# AISC Seismic Provisions Question



## ARLORD (Sep 29, 2008)

Question regarding prequalified SMF Reduced Beam Section connection design:

According to AISC Seismic Provisions Section 9.6, in the equation for Mpb for prequalified RBS, it 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?


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## wmmw (Sep 29, 2008)

ARLORD said:


> Question regarding prequalified SMF Reduced Beam Section connection design:
> According to AISC Seismic Provisions Section 9.6, in the equation for Mpb for prequalified RBS, it 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?




Hi ARLORD,

Cpr came from FEMA Section 3.2.4 and AISC 358 2.4.3-1.

Cpr = (Fy+Fu)/(2*Fy) &lt; or = 1.2 (FEMA Eq 3.5.5.1 &amp; AISC 358 Eq 2.4.3-2)

In think for design of the beam-column connection, we have to use Cpr (as per AISC 358-05 which we should use to design the column-beam connection for SMF &amp; IMF). However, for column-beam momnet ratio, we use 1.1 factor. That is my thinking and any other input would be great


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## ARLORD (Sep 29, 2008)

wmmw,

Thanks for your reply. It's clear where Cpr comes from. However in both:

(1) the example in the Steel Provisions, on page 4-59 Examplw 4.11 and

(2) the Prequalified Connections, Section 5.8 Design Procedure;

Cpr is used in the connection design and in the moment ratio check.

I guess my question is why does the Seismic Spec Section 9.6 pg 6.1-33 specifically state for RBS connections use the 1.1 coefficient.


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## ARLORD (Sep 30, 2008)

Thanks wmmw, I don't have FEMA 350 but found a link,

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.


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## wmmw (Sep 30, 2008)

ARLORD said:


> Thanks wmmw, I don't have FEMA 350 but found a link,
> 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
> ...



ARLORD,

I agree with you about using Cpr &amp; 1.1 factor and that what I though in the beginning. I did not know history of 1.1 coefficent.

Good Luck


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