Basically, every system has a set of frequencies in which it responds to vibrate when set in motion by some disturbance such as seismic or wind event. This response is based on the mass and stiffness of the system. The shortest form of frequency is more known as the natural frequency and it is just the inverse of the fundamental building period.
In seismic analysis and design, if a structure's frequency is close to the frequency of the earthquake, more energy is introduced in the structure. Shorter fundamental periods attract higher seismic vibrations.
The elastic fundamental period of structures, T, can be computed in two ways.
METHOD A: APPROXIMATE FUNDAMENTAL PERIOD
The most straightforward method for determining the building period involves using empirical formulas based on information from several instrumented buildings.
where: Ct for steel- moment-resisting frames =0.0853
Ct for reinforced concrete moment-resisting frames and eccentrically braced frames = 0.0731
Ct for all other buildings = 0.0488
Ct for structures with concrete or masonry shear walls =
hn = total height of the building from the support or base to the top in each direction.
METHOD B. PROPERLY SUBSTANTIATED ANALYSIS USING EIGENVALUE ANALYSIS AND RAYLEIGH'S METHOD
Fundamental period, T, may be calculated using the structural properties and deformational characteristics of the elements.
where: fi = any lateral force distributed approximately
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