Response Spectrum Analysis

Question

Dear Sir, 
 We have a model where the units is set to Mton and cm . Our questions are; 
 a.) Do we have to convert accleration scale factor to 9.805*100? 
 b.) Do we have to convert the Period Vs accleration curve *100? ( Max 0.18, so 0.18*100?) 
 C.) When doing the analysis, my CQC shear is 4 times less than the 85% equivalent static shear specified by code. Already i am using 1 as my direction factor. 
 So, how to increase the base shear? 
 We are following american code ASCE7-05 with client specific Response spectrum curve. 
 Our Staad Pro version; STAAD.Pro V8i(SS4) 
 20.07.09.11 
 
 regards 
 Bubalan Sundaram 
 Tokyo

Kris Sathia · Accepted Answer

Answer to Q1: 
 Yes, provided the spectral accelerations are normalized values, meaning, they are divided by "g". In other words, if the spectral acceleration is 490.25 cm/sec^2, and you specify it as 0.5, then, the scale factor is 980.5 
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 Answer to Q2: 
 No if you have provided the spectral accelerations as normalized values. 
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 First, instead of specifying the direction factor as 1, specify it as I/R where 
 I = Occupancy Importance Factor 
 R = Response Modification factor 
 The source of this information is section 12.9.2 of ASCE 7-05, which is what the IBC 2006 implementation in STAAD is based on. 
 Then, run the analysis again and compare the base shears. 
 In ASCE 7-05, section 12.9.4 states that "Where the combined response for the modal base shear (Vt) is less than 85 percent of the calculated base shear (V) using the equivalent lateral force procedure, the forces, but not the drifts, shall be multiplied by 0.85 V/Vt : 
 where 
 V = the equivalent lateral force procedure base shear, calculated in accordance with this section and Section 12.8 
 Vt = the base shear from the required modal combination 
 Since the multiplication by 0.85 V/Vt is to be done only for the forces and not the displacements, you could create a load combination in which the factor of 0.85 V/Vt is applied to the spectrum case, and use that combination case for steel or concrete design.