story drift 2

[h-badawy]

dear experts

I have confusion for two issue regarding story drift:
1-as per ASCE 7-10 Section 12.8.6 requires: “Where allowable stress design is used, Δ shall be computed using the strength level seismic forces specified in Section 12.8 without reduction for allowable stress design.” this meaning that only the case of allowable design Δ will be computed from ultimate load combination or in all cases of design i have to get Δ form ultimate load combination ?.
2-in the same item above figure 12.8.2 (story drift determination) he said "elastic displacement computed under strength-level design earthquake forces" the meaning from (elastic)that all of the concrete elements on the 3D model should be un-cracked (without any modifiers) to get the elastic displacement ?

Thank you

 

[JoshPlumSE]

1) In all cases, the seismic drift calculations are based on the strength level design forces.

2) A level of cracking consistent with strength level seismic forces should be assumed. Look at ACI section 10.10.4.1 (I've got the 2008 edition in front of me). The suggestion there is to use 0.7*Igross for columns and 0.35*Igross for beams. Of course, you can always look at your members in more detail (ACI equations 10-8 and 10-9) to come up with more accurate numbers.

 

[h-badawy]

thanks Joshplum

1) ok
2) ok but after that the value which i got from the cracked model i will multiplied by ( Cd/I ) to transferred from elastic to inelastic displacement so the value now increased twice first when i put the modification factors to the section and second time when i multiplied by ( Cd/I) my question is what is the benefit of the modifiers if i will multiply the value by the factor (Cd/I)?

Thank you

 

[JoshPlumSE]

ASCE's assumption is that you're going to use an elastic analysis for your members in general. And, under this analysis you want to make sure that the stiffness of the structure is adequately represented in a way that will result in good analysis results. Member forces and moments, base reactions et cetera. This is actually required per ASCE section 12.7.3 which requires the consideration of cracked sections.

That section doesn't give you a ton of guidance. So, you could read the commentary or the NEHRP commentary for more information on exactly what they're looking for regarding the cracked stiffness.

But, at the end of the day, your analysis results are still likely to be based on an assumed ELASTIC stiffness where the loading has been divided by R. The inherent assumption here is that the loading you applied is reduced because the structure will absorb the seismic energy through inelastic displacement / hinging of some sort. The Cd/ I amplification is meant to convert this to the total inelastic deflection that your structure would experience under the full earthquake loading not the reduced load (1/R) that we use for the basic analysis.