1st Floor Diaphram

[helmueller]

I'm designing the foundation for a 5 story steel braced frame structure. Earthquake forces control the lateral design. There is unbalanced soil pressure on the west foundation walls. Currently, the RAM model has the 1st floor diaphram attached to the braced frames and foundation walls which results in the walls taking the majority of the lateral forces at and below the 1st floor. Due to some MEP/Arch/Struct/ coordination issues, I'm contemplating disconnecting the 1st floor diaphram from the braced frames. Thus, the walls would resist lateral earth pressure and a much smaller lateral force from earth quake forces while the interior steel bracing continues to carry the vast majority of the earth quake forces to the foundation. Does anyone have any comments/experience about this issue? Any detailing concerns?

 

[ThatEngineerDude]

helmueller (Structural)

In designing the foundation you may find that resisting the overturning moments by utilizing the walls are much easier and cheaper than beefing up the braced frame to resist by itself. The braced frame will increase substantially by the increased story and lack of continuity for lateral load transfer at first floor, i.e., unbraced lengths increase, overall ht. increases, etc.
The unbalanced soil pressure may be from the model trying to resist the unbalanced weight of a partial or "walk-out basement. This can add considerable shear but can be reduced if retaining walls are allowed to deflect slightly rather than pursue a life of resistance. A good diaphram at the first floor will help to transfer this shear to the sidewalls, provided it is capable and aspect ratio is good (not too narrow).
The 1st floor diaphram attached to the braced frames and foundation walls which results in the walls taking the majority of the lateral forces at and below the 1st floor is a good thing. If connecting the 1st floor diaphram from the braced frame is difficult due to shafts and openings or coordination problems try floor bracing in the plane of the first floor in the form of a horizontal truss.
My experience with this saved $100k in one project by using a concrete diaphram vs. plywood which could not provide this level of resistance.
As for detailing concerns, just follow the load path from where the loads are generated to the place of resistance.

 

[mjohan]

Where can I find design examples for calculating a concrete decks shear capacity at supports for wind or seismic loading? Any information on hollow core slabs would also be very helpful.

Thanks

 

[Althalus]

Regarding your Concrete deck question:
If you are using a steel deck filled with concrete, the
deck manufacturer will have test data that you may use
for the diaphragm strength. It will also vertical load
capacities for given spans and thicknesses.