S = c･W

where:

S = Base shear Force
c = Design coefficient for seismic forces, (see content in Table 1 to the 6). This value can be different for the two directions in which is considered the action of the seismic forces, if the type of structure is different according to each direction.
W = Load or total weight of building

The coefficient “C”varies in 6 seismic zones in which it has been divided the country (see Fig. 1) according to GROUP (see Table 7), TYPE (see Table 8) and GRADE (see Table 9) of the structures. (see Fig. 2)

   The values of the coefficient “C” of Table 1 to the 6 can be modified according to the condition of the soil and the period of the building, affecting it for a reduction factor “D” in the following way:

a) For medium and hard soil condition, when T is greater than 0.5 seconds.

b) For soft soil conditions when T is greater then 0.8 seconds.

See Table 11 to determine soil conditions.