Which seismic force do you use for connections(steel anchor stud) between diaphragm and collectors? 1

[ALEXZ0622]

All,

Given case here:
Steel special moment frame structure with concrete filled composite deck as rigid diaphragm. Which seismic force would you use to design the steel anchor studs between diaphragm and collectors?


After I searched ASCE 7-10 and 16, it seems that option 1 will be my answer.To confirm this, I went through 2012 IBC SEAOC structrual/seismic deisgn manual Vol.4 - Examples for steel-framed buildings. Page 250 uses option 1 to design anchor studs.

However, I'm not 100% sure, since the collector elements will be designed based on Option 2 - ASCE7-10 12.10.2.1, which includes overstrength factor. If the steel anchor stud on top of collectors does not have guaranteed capacity to transfer overstrength level seismic forces into collectors, why do we need to design collector elements for overstrength level forces? Can I explain this as such: seismic forces will somehow transfer not only through steel anchor studs, but also the bearing of column face to collectors/SFRS(seismic force resisting system), etc.?

Option 1: ASCE 7-10 12.10.1 Diaphragm design forces (the only possible amplification is due to irregularity (25%) from 12.3.3.4)
Option 2: ASCE 7-10 12.10.2.1 Load combinations with overstrength factor

Thanks.

 

[Shu Jiang PEng SE]

Combinations with overstrength factor are only applicable to structure with Design Category C through F.

 

[ALEXZ0622]

Shu Jiang,

I see, so what you're saying is that we should use diaphragm design loads since it applies to all SDC,
But think about this, the collector elements are required to be designed for overstength level forces for SDC C through F, then why won't the steel anchor studs being required to design for overstrength level? I do know the Chapter 12.10.2.1 applies to collector elements including their connections to vertical elements, which do not include steel anchor studs between diaphragm and collectors.

I agree those anchor studs should be designed for diaphragm design forces. The only thing I'm quite confused is why it's not required to use overstrength level forces, even overstrength level forces are transferred to collectors by those anchor studs.

 

[Hokie93]

This issue is addressed in an article ("Diaphragms for Seismic Loading - Part 2") by Sabelli, et al, in the February 2009 issue of Structural Engineer magazine. The authors recommend the shear studs in a composite diaphragm collector member be designed using the load combinations from ASCE 7-10 Section 12.4.3.2 (that is, with the over-strength factor) since the shear stud connection will generally be less ductile than the diaphragm.