Ss and S1

[Usimo]

Hi all!~
In the IBC,when they write the EQ spectral response acceleration Ss and S1, they use g with Ss and S1;
example Ss=1.4g, then Ss=1.4*9.81?
please explain me
BR
Tharani

 

[JAE]

g is the force via acceleration of gravity. So 1.4g is 1.4 times the force of the acceleration of gravity.

 

[Usimo]

JAE!

For Example
SS = 0.60g, and S1 = 0.40g.

Let's be site classification of ‘Class C’
If we determine Site Coefficients, Fa and Fv. then
Fa = 1.08
Fv = 1.40

So, Adjusted Spectral Response Accelerations:
SMS = Fa(SS) = 1.08(0.6) = 0.65
SM1 = Fv(S1) = 1.40(0.40) = 0.56

So, Design Spectral Response Accelerations:
SDS = (2/3)SMS = (2/3)0.0.65= 0.43
SD1 = (2/3)SM1 = (2/3)0.56 = 0.37

To determine Seismic Design Category. With SDS = 0.43, SD1 = 0.37,

then, What is the Seismic Design Category ?

It's "C" ( from IBC 2000, Table 1616.3(1) and Table 1616.3(2))

Actually question is;
even though SS = 0.60g, and S1 = 0.40g., we don't use this "g" in the rest of the calculations.

Then why is the "g" there?

BR
Tharani

 

[PEStructural]

In the case you're speaking of, they are using "g" more as a unit than a variable. Truly, if you have 0.60g, it is stating that your acceleration is 60% of gravitational acceleration. However, in IBC terms, the g in this regard is being used as a unit. When you discuss earthquakes, it is customary to discuss in terms of g (e.g. 0.4g, 0.6g, etc..). If you didn't use a unit of "g", it wouldn't make since when you stated that there was a 0.6 earthquake. 0.6 what?

Hope this helps.

Mike

 

[Usimo]

Yes Mike!

This makes sense.

But, i don't know ,why, in IBC , They use different formats.
in some Tables, they use 0.xg, in other Tables, they use just 0.x for spectral response accelerations.


thanking you verymuch
BR
Tharani