Transfer Level Design for Lateral Loads 1

[DC_Engr]

Hi All, I am working on a 'podium/transfer' level design that has columns and walls landing on its slabs and beams from floor above. The entire podium is going to be topped with 4" concrete topping. The columns and walls transfer not just the usual gravity loads but seismic loads that are shown to act in both vertical and horizontal direction. If we assume the lateral (horizontal) loads, at the base of upper story columns & walls, are taken out in diaphragm and therefore beams & slabs only need to be sized for gravity loads plus vertically directed seismic loads; would that be correct assumption? Note that podium is not CIP, so members are typically connected at ends with simple roller or pin type connections. Let me know your thoughts, thanks.

 

[Tomfh]

There have been some bad failures associated with the design assumption that certain walls and columns carry gravity loads only.

All connected parts of the structure will resist lateral loads in proportion to their stiffness, and should be designed accordingly.

Regarding your particular case, maybe provide a sketch so it’s clearer how it all works.

 

[milkshakelake]

I think you're correct. The vertically directed seismic loads should consider overstrength factor because there is a massive change of stiffness (like a shear wall not continuing below the podium). The shear transfer from the upper lateral system to the podium diaphragm should be taken into account through its load path (i.e. shear wall to supporting member on podium -> supporting member to collectors and/or new diaphragm). Also look up the "backstay effect," which might be relevant to your configuration.

 

[DC_Engr]

Thanks @milkshakelake & @Tomfh. Partial framing plan is attached. It's not a thought thru layout that's been handed down to us to work with, as you will notice. Let me know your thoughts or if you have further questions.