Summary
Highlights
The video introduces the linear static analysis procedure, which uses formulas based on NSCP 2015 and a design response spectra (Figure 208-3). This spectra features the structure period over Ps on the x-axis and spectral acceleration (ranging from CA to 2.5 CA) on the y-axis.
The base shear (V) is calculated as CS multiplied by the seismic weight (W). CS, the seismic coefficient, is determined by the spectral acceleration multiplied by the importance factor (I) and divided by the response modification factor (R).
The importance factor (I) reflects a structure's required performance after an earthquake, increasing the base shear for critical structures. The response modification factor (R) is a system-based factor from Table 208-11, which is higher for structures with greater ductility.
Two methods for computing periods of vibration are discussed: Method A (approximate periods of vibration, Ta) and Method B (Raley method, Tb). Method A uses formulas based on structure type and height (hn). Method B is more detailed, requiring seismic weight and rigidity per level.
When calculating hn (height above ground), it's generally advised to use the lower reasonable value, as a lower period results in a higher base shear. Method B, while more accurate, is often superseded by Method A in practice due to its complexity and code limitations on period values.
The base shear is distributed vertically as lateral forces (FX) at each level. The force is greater at higher levels and lesser at lower levels, reflecting the increased displacement at the top of a building during seismic events. An additional force, Ft, may be added at the top for structures with periods greater than 0.7 seconds.
Various formulas for design base shear are presented, including V = CVI/RT * W, Vmax = 2.5 Cai/R * W, Vmin = 0.11 CaiW, and, specifically for Zone 4, V = 0.8 ZNVI/R * W. These formulas correspond to different sections of the NSCP.