Use of Deflection Amplification Factor Cd on vertical seismic

[JAE]

I know that C_d is used to factor up the "elastic" lateral story drifts to provide a more realistic lateral drift under a seismic event. The IBC indicates the use of C_d with [δ]_xe in section 12.8.6 of the ASCE 7.

However, there is also a vertical component to seismic effects required using 0.2S_DS to be added to the dead loads of a member.

The question is raised as to whether the C_d factor can be or should be applied to this vertical seismic load to get a more accurate measure of vertical deflection under dead and seismic loads.

I can't think of a significant reason to be concerned with vertical DL deflection plus vertical magnified seismic deflection except under some cases where a vertical sag might be a concern for clearances etc.

Also, ASCE 7 only speaks of C_d in the context of story drifts, not vertical beam deflections.

So the question is - do we ever include the C_d factor on the 0.2S_DS?

 

[WillisV]

Yes.

 

[JAE]

WillisV.
Thanks for the reply. Do you have some reference or article that discusses this?

The reason I'm suspect that you do this is that the 0.2S_DS factor doesn't have "R" in it. It appears to be somewhat empirical.

 

[vaquers]

I think that it isn't appropiate to include the Cd factor for vertical deflections.

The Cd factor takes into account the fact that we are reducing the seismic force assuming damage, which means that the structure doesn't remain elastic and the deflections are bigger. We don't reduce the vertical seismic forces with R o other factor, so we don't need to account Cd. Regards,

Sebastian

 

[JAE]

vaquers - that is what I was thinking. The behavior of a flexural beam vertically is nothing like the behavior of a frame system laterally using the R's.

I just don't know where the 0.2S_DS comes from.

 

[vaquers]

Dear JAE,
assuming that the vertical PGA is 2/3 (experimental) of the horizontal PGA and using the SRSS (squareroot of the sum of the squares) we have SQRT(1^2+(2/3)^2)=1.20. Regards,

Sebastian

 

[sandman21]

No, Cd for vert. seismic is meaningless