Some seismic codes have a minimum base shear requirement when applying the modal spectral dynamic method in the seismic design process. CYPECAD carries out this check as of previous versions. More information on this check as well as the implemented codes for which this check is carried out can be found in the “Correction due to base shear” section on the CYPECAD webpage.
When the total results of the dynamic analysis are less than the minimum values prescribed in the seismic code to be applied, the dynamic response parameters have to be adjusted by increasing them using a modification factor, until these minimum values are reached. When the response exceeds the prescribed code values, the parameters of the total dynamic response can be reduced proportionally according to the project designer’s criteria.
Therefore, in the 2016.a version, the program offers different ways of proceeding with this requirement. The modification factor to adjust the results can be defined in different ways:
- Do not carry out the static base shear correction
This option allows users to not apply the base shear check and, therefore, not adjust the results, based on the project designer’s criteria. - Based on the code
This was the only option CYPECAD had to offer in previous versions. Using this option, the program adjusted the results by calculating the modification factor in accordance with what was specified in the corresponding seismic code. The design code specifies that the base shear resulting from the dynamic analysis be greater than a specific percentage of the static shear defined in the code. This percentage depends on how ‘regular’ the structure is. - Specify the static base shear percentage
Using this option, users can define, in accordance with a justified criteria, what static base shear percentage is to be taken as the minimum limit to be reached n the dynamic results. - Specify the modification factors
It is recommended this option be used after an initial analysis has been run and its results analysed. By comparing the dynamic and static shear values, a second analysis can be launched with the modification factor values users consider to be adequate. A factor can be defined for each loadcase and analysis direction. This last option also allows for the maximum base shear condition, contemplated in some seismic codes, to be applied. The modification factor can have a value smaller than one, so that the dynamic results are reduced when these exceed the maximum limit established in the design code.
