C_d = k_s･k_g･k_t･α･β_r･ψ

where:

α = the amplification factor
β = the spectral response factor, which depends on the shape of the design response spectrum and on the fundamental period pf the structure:

βr =1	for    T0 ≦ Tz
	for    T0 ≦ Tz

in which
T₀ = the fundamental period of the structure, and
T_z = the characteristic period of the soil at the site.

ψ = the reduction factor, which takes into account the fact that the seismic forces were obtained from an elastic analysis. The reduction factor considers the effect of the potential nonlinear behavior of the structure, ductility, internal force redistribution, and energy dissipation through damping. Numerical values for ψ .
k_s = the seismic zone factor 
k_g = the importance factor 
k_t = the soil profile factor for soil profile types S₁ , S₂ , and S₃.

The characteristics of the soil profile types are as follows:

Soil Profile S₁:
Rock, of any type, or stiff soil where the soil depth is less than 60m and the soil types have overlying rocks that are stable deposits of sand, gravel, or clay.
Soil Profile S₂:
Deep cohesionless soil or stiff clay soil, including sites where the soil depth exceeds 60m and soil types that have overlying rock and stable deposits of sand, gravel, or stiff clays.
Soil Profile S₃:
Soft to medium-stiff clays and sand, characterized by 10m or more of sort to medium-stiff clay with or without intervening layers of sand or other cohesionless soil.