Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

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

Donate Today!

Do you enjoy these
technical forums?
Donate Today! Click Here

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

j19 (Structural) (OP)
5 Mar 04 17:38
I am designing a moment resisting, single story frame that is Seismic Use Group II.  In IBC 2000 the ASD basic load combination for seismic is 0.6D + 0.7E  (Formula 16-12).  If I limit the drift of the frame to 0.015h (H/67) should I use the full effect of E or 0.7E?  It doesn't seem to make sense to check deflection for a load that is greater than what I am using to size members with.  I am in a low seismic area but I at least like to run through the numbers.  Thanks.
Taro (Structural)
5 Mar 04 18:41
Use the full value of E and don't forget to include the deflection amplification factor Cd.  0.7E is only used to check stresses at an "allowable" level.
JAE (Structural)
6 Mar 04 0:03
Taro - please clarify this for me.....in the UBC 97, the drift is required to be calculated (under ASD) with the load combos in the early part of chapter 16 - which includes E/1.4 - but the drift limit is based on Delta(M) which is 0.7 x R x Delta(S) where Delta(S) is the E/1.4 drift.

So the code first tells you to use E/1.4, then re-magnifies the deflection by 0.7 x R to get to Delta(M).

So to simply use E isn't consistent with the UBC 97 - yes?
haynewp (Structural)
6 Mar 04 12:46
I was under the impression that for single story buildings, there was no drift limit. IBC 2000, Note "a" under table 1617.3


However, if you are worried about how finishes will respond to the actual deflection in an earthquake, E should not be divided by 1.4 (1617.4.6.1)

UBC 97 looks to me in 1630.9.1 to reference section 1612.2 combinations when using ASD. 1612.2 uses 1.0E not E/1.4
Taro (Structural)
6 Mar 04 14:08
For the 1997 UBC, there was some confusion because conflicting errata were issued.  Section 1630.9.1 originally said to use 1.0E per section 1612.2 for calculating drift (even if ASD is used for design).  The 05-98 errata changed the reference to 1612.3.  Then, the 01-01 errata changed the reference back to the original.  See http://www.iccsafe.org/cs/codes/errata.html#uniform for all the UBC errata.

You must always use full 1.0E force levels to calculate elastic seismic drifts.  Then, an amplification factor (Cd in IBC or 0.7R in UBC) must be applied to determine the inelastic drifts to compare with the code's drift limits.

The purpose of the 0.7E load (or E/1.4 in UBC) is to bring the earthquake forces down to a fictitious "service" load level so that Allowable Stress Design can be used.  These forces are only used for stress checks, not for drifts.

 
JAE (Structural)
6 Mar 04 15:33
Errata for errata !!!

Please make it stop!!!
j19 (Structural) (OP)
9 Mar 04 15:12
I think I found the answer to my question in IBC 2000.  Section 1617.4.6.1 says "Where allowable stress design is used, delta shall be computed using earthquake forces without dividing by 1.4."  

It also says that roe can be taken as 1.0, therefore using the full E may be too conservative.  I am not sure whether or not this is in UBC.

  

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!

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