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JohnB965 (Structural) (OP)
5 Jan 08 15:12
I am currently using ETABS and UBC Code to model buildings in the middle east. Although ETABS calculates lateral drifts I am not sure of the following :-

1. ETABS calculates lateral drifts based on the SRSS earthquake load case. Does this value need to be multiplied by 0.7 R to obtain maximum inelastic displacements, or is this already included in the analysis. The maximum value of this drift has an upper limit of 0.02

2. This is the part I am not sure about. I also want to limit service drifts to H/500. Do I simply divide the drift by 1.4 to get service loads or do I need to multiply the ETABS drifts by 0.7 R / 1.4
Helpful Member!  drile007 (Structural)
5 Jan 08 16:03

I'm not familar with UBC code, but
1. I think you shouldn't multipied the drifts with 0.7 R (I supposed this is ductility of the structure), since drifts are   independent from ductility
2. Same as 1.

Drile007

Helpful Member!  slickdeals (Structural)
9 Jan 08 8:03
John, It is very difficult to control your building deflections to H/500 for a seismic load case. H/500 is more for wind load at service level. For seismic, you can use the allowable interstory drift in ASCE which can of the order of a few inches.
Helpful Member!  ahmedhegazi (Structural)
11 Jan 08 16:23
John
the values you get from Etab should be multiplied by 0.7R to get the maximum inelastic response displacement (UBC 1630.9.2) and it is interstory drift not story drift

the value for H/500 is used only for wind service limit and it should not be multiplied by 0.7R

Upper limit for seismic drift is function in building period (UBc 1630.10.2)
slickdeals (Structural)
11 Jan 08 19:52
Please note that for seismic loads the displacement is always  calculated at design levels per ASCE. You should not reduce your it to service levels. In addition the displacement should be multiplied by deflection amplification factor (Cd) to get the maximum inelastic deflections. HTH
drile007 (Structural)
19 Jan 08 13:54

Sorry, my claim in previous post was wrong (slickdeals and ahmedhegazi are right)...you should multiplied the lateral drifts with ductility to get real (inelastic) drifts which one you should check by the code.

Drile007

Helpful Member!  Murali27 (Structural)
23 Jan 08 10:00
John

With UBC code,

1. Lateral drifts (displacements) are to be multiplied with 0.7 * R to obtain the maximum inelastic displacement, which would be useful in checking pounding of structures, expansion joint, etc,.

2. Seismic story drift demand is required to allow the movement in cladding design. I am not sure of H/500, which is really uneconomical to achieve.

UBC gives the limit 0.02 x story height, (as calculated above). If your story drift ratio is less than 0.02, then it is fine.

If you look at Eurocode, there is a limit of story drift ration 0.005 at service level, i.e., H/200

Hope this helps

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