The seismic design coefficient Cs is calculated as

S_a = value of the accelection abstained from the design spectra corresponding to the fundamental period of the building (see Fig. 1)
R = response modification factor for the structural system and material detail requirements as given in Table 1
The building must be designed to resist a horizontal design base shear given by

V = horizontal design base shear
W = total vertical weight of the building. W must be equal to the total weight of the structure plus the weight of all other elements such as partitions, permanent equipment, tanks, and their contents, etc. In ware houses, 25% of the live load on the floor must be included.

S_a = maximum horizontal acceleration, as a fraction of the acceleration of gravity, that a single-degree-of- freedom system with a vibration period. 
T will sustain during the design earthquake
A_a = effective peak ground acceleration given in Fig. 2, as a fraction of the acceleration of gravity
A_v = effective peak ground velocity, in terms of acceleration, given in Fig. 3, as a fraction of the acceleration of gravity
S = site coefficient, dimensionless, given in Table 2
I = importance coefficient, dimensionless, given in Table 3
T = fundamental period of the system in seconds
For Soil Profile S₃ with A_a greater than or equal to 0.30, the value of S_a need not exceed 2.0A_aI  Fig. 4 shows the elastic design acceleration spectra in rock for four major cities in Colombia.