## Deflections Due To Seismic Loads

## Deflections Due To Seismic Loads

(OP)

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.

## RE: Deflections Due To Seismic Loads

## RE: Deflections Due To Seismic Loads

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?

## RE: Deflections Due To Seismic Loads

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

## RE: Deflections Due To Seismic Loads

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.

## RE: Deflections Due To Seismic Loads

Please make it stop!!!

## RE: Deflections Due To Seismic Loads

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.