Hi.
I've modeled 2D RC moment frame. Then I used diagonal braces in that frame (I've used equal DOF 1 2 for introducing hinge connection between brace and frame, braces are buckling restrained braces, I used truss element for yielding portion and elastic beam column for connecting portion).
the calculated natural period is equals for two models, and even when I use too big cross sectional area for braces the period does not change any way; Why?
Sahar,
Your general question is fairly easy. However, your specific problem requires perusing your model and we don't even know what computer program you are using, at this point.
It's almost always preferable to use braced frames. Moment frames are something we have to resort to whenever the space for the braces is occupied by something else, such as equipment or building openings (ie, windows, doors). Moment frames can require lots of welding or rather large bolted brackets. This choice is usually exercised by the architect.
Hi Sahar
It's not really easy to find the error in your code this way. But based on what you described, if two natural periods are exactly the same, so it is possible that you have defined the coordination of nodes (or bracing members) wrong. In other words, OpenSees can not recognize the bracing members as a part of RC structure's domain, hence does not include their stiffness in stiffness matrix.
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