How to select the seismic response coefficient (Cs) in ASCE/SEI 7-05

[chenbaowei]

Hi All

I have been implementing seismic analysis following ASCE7/SEI 7-05 and I meet a big confusion these days when I plan to calculate the equivalent lateral force procedure base shear to scale design values of combined response for modal response spectrum analysis (Section 12.9 of ASCE/SEI 7-05).
According to Section 12.8.1.1 of ASCE/SEI 7-05 (Page 129) we can get this relationship shown in the following:
When T<=TL,S1>=0.6g
0.5S1/(R/I)<=CS<=SD1/T/(R/I);CS=SDS/(R/I)
Now we have a project:

Ss=1.981;S1=0.926;SDS =1.32;SD1 =0.93:R=1.5;I=1.0;T=2.807s;TL=8s

So we can get these following results:
0.5S1/(R/I)=0.5*0.926/(1.5/1)=0.309

SDS/(R/I)=1.32/(1.5/1)=0.88

SD1/T/(R/I)=0.93/2.807/(1.5/1)=0.221

That means Csmax <Csmin,it is very clear that there is a conflict.0.5S1/(R/I)=0.309<CS=0.88>SD1/T/(R/I)=0.221
Then I am confused which number should select for Cs,0.221? or 0.309?

This is an urgent project for me.Could you please help me clarify this confusion? I am very appreciated for your quick reply!


BR


Henry

 

[BENDOG]

My interpretation is the following:

Cs is the minimum value of 12.8-2 and 12.8-3(or 12.8-4). But in addition where S1>0.6, Cs cannot be less than 12.8-6.

So in your case 12.8-6 would govern.