flat slab in zones 3 &4

[CHAKKOUR]

Hi guys,

Why does the UBC 97 forbid the use of flat slabs in high seismicity zone ?? since it will work as a diaphragm transmitting the forces to the vertical columns, where is the problem ?? Is it because the vertical earthquake ??

Thanks a lot

 

[Ron]

Why are you using UBC 97? Is this a forensic or retrofit project?

 

[HSIII]

There's no more recent codes for construction where you are than UBC '97? Or is that what you have available and you're trying to make do with what you have?

Or, as Ron asked, is this a forensic investigation?

 

[CHAKKOUR]

In the specifications, we shall use the UBC 97. So, back to the main question, what do u think ?? since the vertical earthquake is included in the 0.5 x Ca x I, why is it forbidden to use flat slab system in high seismicity zone ?
Thanks

 

[Ron]

CHAKKOUR...that code is outdated. There has been significant research and develop in seismic codes since that time. It would be imprudent to offer new design done to an old code without more validity than that it was "specified".

I do not routinely deal with seismic design, so others here can offer more insight.

 

[ishvaaag]

Certainly by providing enough stiffness through shearwalls, the relatively weaker slabs would see manageable moments coming from seismic action, so I essentially agree with your view.

Normally I have seen them not recommended in the context of column and slabs only systems against lateral forces, when over some number of floor levels the nodes of the lower floors become unmanageable strained.

 

[CHAKKOUR]

ok, i agree, but since u will check ur slab against earthquake (punshing stress + vertical earthquake), do i have to check it for something else (other than gravity loads of course) ??
Thanks for ur help.
2nd question :
I dont know if u're familiar with etabs, when u assign stiffness modifier to a shear wall, i generally assign 0.7 to f11, f22, f12 (axial rigidity). The question is : do i use these stiffness to have the reinforcement also or just to check the drift ?? i mean, i was intending to put the modifiers for the slabs and beams and 1.0 for walls. It will make it the wall more dtiff and the reinforcement will be more than if u put 0.7.
What do u think ??
thanks for ur help

 

[rapt]

Chakkour,

Even though the framing system is taking out your earthquake effects in strength design, there is still inter storey drift between floors that puts extra stress on the slab/column connections for punching shear You have to allow for this as well.