Deflections Due To Seismic Loads

[j19]

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]

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]

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]

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]

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]

Errata for errata !!!

Please make it stop!!!

 

[j19]

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.