This article highlights the capacity of Advance Design for scaling up the modal spectral analysis seismic load cases according to base shear values obtained by the equivalent lateral force method of US and Canadian seismic codes.
In US and Canadian seismic codes (ASCE and NBC), the Modal Spectral Analysis (MSA) results are scaled up to produce at least 80%, 90%, or 100% (depending on the code and structure) of the Equivalent Lateral Force (ELF) method base shear. For these codes, Advance Design is capable of calculating the ELF base shear and automatically scaling the MSA results.
Calibration option
To activate the option for automatically scaling the MSA results, the user must enable the calibration in the MSA load case.
Calibration example for NBC 2015 seismic load case
Scaling details
Details of MSA scaling according to ELF base shear can be found in report generator tables on the following path “Finite Element Analysis” → ”Loading data” → “Seismic Analysis Settings”.
Details of MSA scaling in report generator
Example of scaling calculation details according to NBC 2015
In the image above:
- Ta emp is the empirical fundamental period obtained by code formulas;
- Ta dyn is the fundamental period obtained from the modal analysis;
- Ta is the period used for ELF base shear calculation (Advance Design is trying first to use Ta dyn, however, the seismic codes impose an upper limit that should be respected. In the case presented in the image above the upper limit is 1.5xTa emp);
- V is the base shear calculated by ELF;
- Ve and Vd are respectively the elastic and design base shears calculated by MSA.
As we can see in the image “Example of scaling calculation details according to NBC 2015”, The MSA was scaled up to meet at least 80% (code requirement) of the ELF base shear. So, the minimal base shear used for MSA is 80% of 114.54 kN which is 91.63 KN.
For US and Canadian seismic codes, Advance Design is capable of calculating the ELF base shear and scaling up the MSA results to meet a certain percentage or the totality (according to code specifications) of the ELF base shear.