Excentrically braced frames - How to limit link rotation

[qcjr]

Greetings,

I am designing a 9-story building with excentrically braced frames in one direction, using the Canadian Building Code.

According to the S16-01 standard (concerning steel design), the inelastic component of the link rotation is limited to 0.08 rad (for link with e < 1.6Mp/Vp). This is my case.

However, after running a dynamic analysis and getting story drifts (to compute the rotation), I am exceeding the rotational limit.

How can I modify my design to obtain a smaller rotation (and/or smaller story drift)? I believe the actual member used in the EBF are irrelevant and do not impact much the story drift. Is it about EBF geometry, i.e. link length mostly?

 

[SethGuthrie]

I find EBF frames to be great in theory, hard in practice. Getting all of the member capacity equations to work, while making sure the link is soft enough and the rotation not too high is exceedingly complicated.

Adjusting the link length, or reconfiguring the braces, is usually what it takes. The link length needs to be short, but not too short and the brace angles not too steep or shallow. Adjusting the beam depth can also help.

I would also double check equivalent lateral force method static analysis results to rule out some anomaly in the results when dealing with a dynamic analysis.

 

[qcjr]

Thanks for the reply It is indeed a difficult bracing type to design and balance. However, it is quite practical to actually allow for some windows and other architectural features

 

[johnbridge231]

Maybe you should increase the cross sectional area of the braces or choose a chord (beam) with larger inertia moment about horizontal axis (if connections are rigid). If not, you can make them rigid (ie bolted endplate connection).

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html