Integrated Buildings & Structures

I am modeling a tall building composed of four vertically-separated towers sitting on a single podium. What are the recommended best practices for: (1) representing tower–podium connectivity in commercial software (ETABS / SAP2000 / OpenSees), (2) performing seismic analysis including modal response-history or time-history, and (3) calculating the fundamental and higher mode time periods for each tower and the coupled system? Any guidance on mass/stiffness coupling, boundary conditions, damping assumptions, and modal combination rules would be appreciated.

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Jaher Wasim S.E., M.ASCE
Managing Director & CEO
D8 Consultants Ltd. (D8CL)
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You might wish to refer to two guideline documents that address this.  One if the Pacific Earthquake Engineering Research Center (PEER) Guidelines for Performance-based Design of Tall Buildings.  The other is the Los Angelese Tall Buildings Structural Design Council Alternative Procedures for Seismic Design of Tall Buildings.  Both are available in PDF form, online, for free.  

The PEER document addresses this in Appendix Section D.3. It recommends modelling the podium together with all buildings supported on it, noting that it may be acceptable to do a detailed model of only one building at a time, while including simplified models (e.g. lollipop models) of the other structures, matching the other structure's significant modal properties.

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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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The best is to model the whole complex in one model and do it with Time history. But it can make the model too big to handle for any system. Simplest approach is to assume the podium as rigid and analyze towers independently. This should be enough in most practical cases where the podium is way more stiff than each of the towers and the transfer slab has no issues distributing seismic loads from the towers onto the podium and additional seismic elements in the podium will have no problem taking these loads. 

The trick is to have a robust transfer slab with as few discontinuities as possible. The forces from towers will act one at a time and resisting system will not experience the sum of all four tower forces simultaneously.  

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Syed Masroor Ph.D., P.E., M.ASCE
Sr Bridge Engineer
Reston VA
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California building codes . Not sure if API has direction . Worked in Mexico and there was an earthquake!

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