Connection design forces for seismic condition

[mathcadboy]

Gents,

Can anyone tell me how to get the required moment for connection design in seismic applications for an ordinary steel moment frame using ASCE 7-05 and AISC 360-05? I am using R = 3 so no need to go to AISC 341. (By the way, why would anyone use R = 3.5 when you will be forced to design your connections using AISC 341. Loads will be way way more).

I am used to UBC 97 and when designing structures in low-seismic applications (zone 1 and 2), we take the beam strength OR the moment from the analysis whereine the seismic forces used are multiplied by the seismic force amplification factor as stated in section 2214.4.

I cant find something like that in ASCE 7-05 and AISC 360-05.

Any help will be very much appreciated!

 

[msquared48]

Are you referring to the Omega factor in ASCE7?

Mike McCann, PE, SE (WA)

 

[mathcadboy]

Yes, the omega sub 0.

 

[mathcadboy]

Anyone?

If I take R = 3 for my ordinary steel moment frame in a design category C earthquake design, I would not need to use the overstrength factor in the forces for my connection design?

 

[JAE]

I believe that if you use R=3 you do not need to use Omega factors. I'm away from my ASCE7 and IBC's but I will see if I can check tomorrow.


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[mathcadboy]

Thanks, I will be awaiting your response. I am just surprised that only a matter of 0.5 difference in R value (for ordinary moment frames) can make it possible for you to disregard AISC 341. I am hesitant not to apply the overstrength factor since I have been applying it to all my previous connection designs (seismic).

 

[mike20793]

If it is R=3 you do not need to use the overstrength factor for connection design. That's why it is referred to a steel system not specifically detailed for seismic resistance; ie you are not designing for the same ductility as you would with an ordinary moment frame. Watch your seismic design category, though. SDC C and above require overstrength for collectors and chords in diaphragms. There are many discussions on this forum as to what the purpose of the overstrength factor is, but basically it's to ensure your connection doesn't yield so your plastic hinge forms in the girder.

 

[mathcadboy]

Thanks mike20793. Thats the thing it only mentioned "detailed" in the line (...not specifically DETAILED for seismic resistance). Should I take it as it also meant designed?

It got me thinking that using UBC97 for connection design is overly conservative. As far as I know it doesnt have this feature. It states that you still need to use the overstrength even if its just zone 1 or 2.

 

[PMR06]

The horizontal and vertical irregularity tables (12.3-1&2) can also kick you into the overstrength equations, even with SDC B and even if you are designing R=3 "steel system not specifically detailed for seismic...".

 

[JoshPlumSE]

The may reason to use R = 3.5 is when it is required based on your seismic design category. You're not always allowed to use R = 3.