INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Use of Deflection Amplification Factor Cd on vertical seismic

Use of Deflection Amplification Factor Cd on vertical seismic

(OP)
I know that Cd 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 Cd with δxe in section 12.8.6 of the ASCE 7.

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

The question is raised as to whether the Cd 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 Cd in the context of story drifts, not vertical beam deflections.

So the question is - do we ever include the Cd factor on the 0.2SDS?

RE: Use of Deflection Amplification Factor Cd on vertical seismic

Yes.   

RE: Use of Deflection Amplification Factor Cd on vertical seismic

(OP)
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 0.2SDS factor doesn't have "R" in it.  It appears to be somewhat empirical.

 

RE: Use of Deflection Amplification Factor Cd on vertical seismic

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

RE: Use of Deflection Amplification Factor Cd on vertical seismic

(OP)
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 0.2SDS comes from.

 

RE: Use of Deflection Amplification Factor Cd on vertical seismic

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

RE: Use of Deflection Amplification Factor Cd on vertical seismic

No, Cd for vert. seismic is meaningless

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close