Impact of Seismic Site Classification

[Okiryu]

Hi !... I do geotech so, sorry if this is a dumb question: what are the implications in the structural design if we change the class from D to E or F? Say, we have a mid size building (5 story) so the period may be about 0.5 seconds. My understanding is that for LOW periods, there is a trend for the spectral accelerations to become lower as the site class become worse.

 

[BridgeSmith]

Not sure about the other codes, but for AASHTO it doesn't work that way. Going from D to E increases the design acceleration coefficents, resulting in higher design forces and displacements for structures of all periods. Site Class F requires site-specific analysis, which may result in lower forces, but can be quite costly.

 

[skeletron]

For the NBCC 2015:
Going from Site Class D to E is going from stiff soil to soft soil. Going from Site Class E to F is going from soft soil to "other soils" that require a site specific evaluation. You can view the trend of the F(T) values in Tables 4.1.8.4. Typically, for T>0.5 it appears the F(T) value will increase as Site Class goes from C to F.

There is also Note 103 in Commentary J where the "fundamental period of vibration" =< 0.5s for structures built in Site Class F, then the values for F(0.2) and F(0.5) can be determined assuming that liquefaction will not occur. This only applies for determining the site class and determining the site coefficients.

 

[Okiryu]

Thanks much for the responses. So, if T is less than 0.5 sec, then the site class and the site coefficients can be determine considering no liquefaction but the actual design of the structural elements should follow the code assuming that the site is class F?