IBC SEISMIC FOOTING RESPONSE MODIFICATION FACTOR

[bertzm]

Hi all,
can someone help me? I don't understand if i have to use response modification factor when i design foundations (overturning, soil pressure....) or if i need to work in elastic condition (no ductile-dissipative foundations).

 

[JoshPlumSE]

Generally, the basis for the seismic force will be dependent on WHAT you are supporting. If you are supporting a structure, then you use the R factor for the structure. If you are supporting some rigid equipment, then your foundation should resist the Fp forces for the equipment.

 

[bertzm]

Thanks for your answer but my post was not clear.
I'm designing a building with ordinary shear walls. I calculated the Sms Sm1, Sds Sd1, The base shear, the R factor (5) and overstrngth factor (4.5). I use the modal analisys and I reduce the spectrum with R factor for section design.
My question is: For the overturning and for the soil design have I to use the reduced spectrum or i have to increase it with the overstrenght factor?.
I make this question because I'm italian and in Italy we have to design the foundations with the elastric spectrum, and it sounds strange to design a foundation with a reduced spectrum.
thank you

 

[JoshPlumSE]

The overstrength is really intended for critical portions of the system (like connections) that must achieve a higher strength for the overall system to behave in a ductile manner (and hence justify the R modification to your forces).

You do not generally have to increase the foundation design forces by the overstrength factor. I've seen folks design the attachment of their piles to the cap per the overstrength. But, I've never seen someone design the entire slab per the overstrength.

 

[bertzm]

ok,
thanks very much

 

[lutein]

the overstrength factor does not apply to foundation design. in fact, recent research has shown that little uplift in the foundation may actually suggest a better ductility.

The foundation just need to be designed for conventional load combination, make sure you include the siesmic vertical component and redundency factor (if applicable).