First of all, we have to know that we have two different scaling; the scaling of forces and the scaling of drifts, so the MRSA forces used to design the elements may be differ from forces used to check drift (as I said; may be) The scaling of forces starts from initial scaling ( gI/R) to take the effect of importance factor and response modification factor , then we have to do scaling up 100% of ELF (scaling down is not permitted) because the computed fundamental period may be the result of an overly flexible (incorrect) analytical model (In other words; flexible building may results in very low base shear). Scaling of Drifts Displacements from the modal response spectrum are only scaled to the ELF base shear where Vt is less than CsW and Cs is determined based on Equation (12.8-7). For all other situations, the displacements need not be scaled, because the use of an overly flexible model will result in conservative estimates of displacement that need not be further scaled. So finally we have; ELF (Forces) for strength design ELF (Forces) for drift check MRSA (Forces) for strength design MRSA (Forces) for drift check
Hind Hammada: Why does the ACSE code require scaling dynamic forces for seismic design, and what is the specific scaling factor? Almost all dynamic analysis results are less than of static ones (such as ELF). So the values like Base Shear tend to be smaller in dynamic analysis than ELF results. In ASCE7 you scale up to 100% ELF so you get little bump if things go your way. I agree in principle, though, that the choice to go RSA/NLA is usually driven more by having identified the need to capture complex dynamic behavior than by a desire to save on construction costs. ASCE 7-22 12.9.1.4 #RekanDaho
Yes, I agree, often designs in dynamic analysis tend to be smaller than static analysis, until now I haven't understood, why is there a scale factor? #cmiiw
Senior Structural Engineer and Lecturer
7moDear Eng Anas, Thank you for sharing useful information as usual. Here we have a big question mark, why should I scale also displacements between static and dynamic? Displacements/Drifts are results of the applied forces, so, In case the dynamic forces were scaled up, I have to consider the deformations corresponding to those forces. The static EQ_Drift force was introduced to serve from the accurate method of period calculation instead of the approximate one, whereas the MRSA originally is based on the modal analysis. On other hand, what if the Dynamic base shear was greater than the static base shear, but the static EQ_Drift was less than the dynamic base shear, what should we do? Lastly, what would be starting value of the scale factor to be used for drift scaling? Is it gI/R or the obtained scaling factor from dynamic force scaling? I think dynamic base shear should be scaled with respect to the static one ( seismic drift or seismic only) and used for whatever comes after ( design, checks,...) Thank you 😊