CYPECAD allows users to define the Fundamental period of the structure (used to calculate the base shear) in two ways:
- Based on the code (only way possible in previous versions)
- Specified by the user (implemented in the 2014.c version)
Either of the two options can be selected in the section Estimation of the fundamental period of the structure within the Code for the calculation of seismic loading dialogue box (General data ˃ Select “with seismic loading” from the “Loads” section ˃ select Mexico as the country and CFE 2008 or NTC 2004).
The value of the fundamental period of a building must be obtained based on the properties of its seismic resistance system, in the direction being considered, in accordance with structural dynamic principles. Alternatively, most seismic codes allow for the fundamental period to be estimated:
- Using empirical formulas provided within their articles
- Using other methods, as long as these are adequately sustained analytically or experimentally
The estimated fundamental period is applied in the calculation of the static shear at the base of the structure (base shear) which:
- Adjusts the dynamic results to minimum values prescribed in the code, if the dynamic method is applied
- Generate the distribution of the equivalent static lateral forces, if the static method is applied
Article 3.3.5.1 of the CFE 2008 code establishes the calculation of the base shear force by taking the bottom period limit of the plateau of the design spectrum (Ta) as the estimated fundamental period; and article 8.1 of the NTC-2004 code takes any period which coincides with the design spectrum plateau (Ta ≤ T ≤ Tb).
The indicated values are limits established in the codes that can be used in the absence of more precise data. If users have values of the fundamental period which are more adjusted to their structure (calculated using their methods or using software such as CYPECAD, which calculates the fundamental period of the structure in each direction – the values can be consulted after the analysis in the “Justification of seismic action report”: File ˃ Print job reports), they may specify them by selecting the Specified by the user option, located in the Code for the calculation of seismic loading dialogue box, as has been indicated previously.
