Base Shear - Uniform Building Code (UBC)


The UBC addresses the probability of significant seismic activity in various locations by categorizing geographic regions of the U.S. as Seismic Zones 0 through 4 (See UBC Figure 16-2). Seismic Zone 0 indicates a geographic location where no seismic activity is expected to occur. Seismic Zone 4 indicates a geographic location with a high probability of significant seismic activity.


The equivalent static force procedure in the Uniform Building Code (UBC 1630.2) specifies the following formula for calculating base shear (V):


V = Cv I W / R T (UBC Equation 30-4)

The UBC also specifies the following upper and lower bounds for V:


Upper bound: V < 2.5 Ca I W / R (UBC Equation 30-5)

Lower bound: V > 0.11 Ca I W / R (UBC Equation 30-6)

An additional lower bound applies in seismic zone 4 (major portions of California, as well as regions in Alaska, Arizona, Hawaii, Idaho, Montana, Nevada and Wyoming):


V > 0.8 Z Nv I W / R (UBC Equation 30-7)


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