IBC - Base Shear

Base Shear - International Building Code (IBC)

The IBC addresses the probability of significant seismic ground motion by using maps of spectral response accelerations (S_s and S₁) for various geographic locations (see IBC Figures 1615(1) through 1615(10)). These mapped spectral response accelerations are combined with soil conditions and building occupancy classifications to determine Seismic Design Categories A through F for various structures. Seismic Design Category A indicates a structure that is expected to experience very minor (if any) seismic activity. Seismic Design Category F indicates a structure with very high probability of experiencing significant seismic activity.

The equivalent static force procedure in the International Building Code (IBC 1617.4) specifies the following formula for calculating base shear (V):

V = C_sW

(IBC Equation 16-34)

where the seismic response coefficient, C_s, is defined as:

C_s = (2/3) F_vS₁I_E / R T

(IBC equations 16-36, 16-17, and 16-19)

The IBC specifies the following upper and lower bounds for C_s:

Upper bound:

C_s < (2/3) F_a S_s I_E/ R

(IBC Equations 16-35, 16-16, and 16-18)

Lower bound:

C_s > (0.044) (2/3) F_a S_s I_E

(IBC Equations 16-37,
16-16, and 16-18)

An additional lower bound applies for structures in Seismic Design Categories E and F, or structures with a large spectral response acceleration for one-second period of vibration, S₁ > 0.6g:

C_s > 0.5 S₁ I_E / R

(IBC Equation 16-38)

The upper bound value for C_s tends to govern for relatively stiff structures that exhibit a small (short) fundamental period of vibration (T). The lower bound values for C_s tend to govern for relatively flexible structures that exhibit a large (long) fundamental period of vibration (T).