DESIGN BASE SHEAR

The earthquake design of any structure starts with the design base shear. This shear is an estimate of the expected maximum force which occurs laterally to the ground where the structure stands.

Base shear, V, depends on the following parameters:

1. Site soil conditions
2. Proximity to faults or sources of seismic activity.
3. Seismic ground motion probability
4. Structural configurations, its level of ductility and strength
5. Total weight of the structure
6. Structure's fundamental period of vibration when under dynamic loading

1. Simplified Static Force Procedure

                        

2. Static Force Procedure

The total design base shear  shall be computed as follows:

                

This design base shear will govern under the following conditions:

• The computed base shear shall not exceed:

                                  

• The computed base shear shall not be less than:

                                  

• Also, the computed base shear shall not be less than the following if structure is located in Seismic Zone 4

                                 

where:

 = total design base shear
W = total seismic dead load (208.5.2.1)
I   = Importance factor (208-1)
R  = coefficient for the inherent over-strength and global ductility capacity of lateral-force-resisting systems (208-11 or 208-12)
 and = seismic coefficient (208-7 and 208.8)
Z    = seismic zone factor (208-3)
 = near source factor for  (208-4 and 208-6)