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RE: Braced frame + concrete shear wall Dual system

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Anantha

 

The effect of ramp on the distribution of lateral loads is a complex issue. Most people in seismic design use a 3D frame program and include the ramp is their model to get as realistic a distribution as possible.

 

Combination of braced frame and shear wall is not a dual system. Dual system combines a rigid system (i.e. braced frame) with a more flexible system such as a moment frame. It also requires that the moment frame be able to resist 25% of the lateral load. In your case you are basically combining two different system and you need to use the lower R for your overall system design.

 

Drift ratio is a unitless number. Therefore, it doesn’t matter if you are using the metric system.

 

Ben Yousefi, SE

Santa Monica, CA

 

 


From: Anantha Narayan C.K. [mailto:anant_27(--nospam--at)yahoo.com]
Sent: Tuesday, August 15, 2006 6:11 AM
To: seaint(--nospam--at)seaint.org
Subject: Braced frame + concrete shear wall Dual system

 

Gentlemen,

 

I am designing a parking garage for a project in Trinidad. It has a smooth ramp (for parking) and a flat transition for turning and going up. There is an induced torsion due to the fact that the first flight of the ramp is connected to the ground. The building's center of rigidity is closer to the point where the ramp begins. I think that would make sense, but please offer your comments on this. Also, will the ramp act as a vertical truss and provide lateral bracing?

 

The job is in Seismic Design category D with Ss = 1.71 g and S1 = 0.68 g. The building is 14.6 m tall. and has plan dimensions of 39 m x 70m.

 

We are using (thinking of using) chevron braces in addition to concrete shear walls at the ends. Is this dual system uncommon as I did not find any reference to the same in IBC 2003. Will using an R value of 5 (for the SCBF) be very conservative? If not, what is a means to calculate a more realistic R value? Also, can ASCE 7-02 drift table value of 0.02 Hsx be used in metric since there are no metric values mentioned in parantheses.

 

I am fairly new to seismic design and have mostly been engineering for wind (work in florida). Please feel free to provide your experiences and knowledge with regards to seismic design in the Caribbean and in particular Trinidad.

 

Thanks
Anantha

 

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ANANTHA NARAYAN, E.I.

Structural Engineer

Bliss and Nyitray Inc.

Miami, FL - 33134

 


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