Dual Resisting Frame Analysis and Design in Etabs 1

[Ganesh Persaud]

Hello everyone,

I need help in understanding how to run an design of an Etabs model with a dual frame resisting system.
1. Do I use one model with the resisting frame being specified in one direction and add a percentage of stiffness to the other direction?
2. Do I run two separate models with the two frame system respectively and run two different design for each frame and check if both resisting frame passed design?

Thank you.

 

[Agent666]

What do you mean exactly by dual frame resisting system, can you explain or post a drawing of the structure you are discussing?

Do you mean a two way moment frame (moment frames in both orthogonal directions)?
Or dual structure (wall + moment frame in the same direction)?

Also are you talking about modelling in a 2D model vs 3D model?

Sorry, the post isn't to clear to me, which may be why you have not received any responses yet?

 

[wannabeSE]

More than likely, I would use 2 models,
1. Complete model with moment frames and braced frames or walls
2. Model with moment frames alone and base shear scaled to 25% of complete model.
But, there is no rule on how to analyze the dual frame. Depending on the actual layout and framing, I might choice a different method.

Agent666,
In the US, ASCE 7-10 includes "dual systems" which are a combination of moment frames and braced frames or moment frmaes and shear walls. Dual Systems have there own section of seismic coefficients in the table of coefficients. The code requires the moment frames be capable of resisting 25% of the seismic force.

 

[Enrique2015]

This depends of what type of dual system material etc. It can be done in one model, in a nut shell:
1) Run the model for 100% of seismic force and design your walls/braces
2)Run the model for 25% of your seismic (i.e a SF of 0.25 on your load case) loads and assign a small stiffness modifier to f12 if you have walls or to your axial stiffness of braces if you ware using braces
3) Design your moment frame s for these forces

Make sure you keep the seismic loads at same level, this means that if you are using auto seismic loads , the fundamental period will change if you reduce stiffness . If this is the case note the fundamental period on step 1 and use it as user defined for the reduced stiffness model

http://www.structuralapps.com

http://www.Tilmanllc.com

 

[Ganesh Persaud]

Thank you very much, this was very helpful. For example, if I’m using the a flanged cruciform column. Does etabs recognize this as being moment frames in both directions?