Horizontal Base Shear 
The equivalent lateral forces applied at each floor
level of the building are given by where: C_{d
}=_{ }the design seismic coefficient
defined
= the distribution factor at level i of the
building corresponding to the first mode of vibration
W_{i} = the seismic design weight attributed to the ith
level The distribution factor
at level i for mode r is given by in
which h_{i}
= the height of the ith level measured from the base
Y_{ir}
= the normal mode displacement at level i in the mode r.

Basic Seismic
Coefficient 
The design seismic coefficient C_{d}_{
}is given as the product of several factors:
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:
β_{r}
=1 
for
T_{0} ≦ T_{z} 

for
T_{0} ≦ T_{z} 
in which
T_{0} = 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 S_{1 }, S_{2 },
and S_{3}.
The characteristics of the soil profile types are as
follows:
Soil Profile S_{1}:
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 S_{2}:
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 S_{3}:
Soft to mediumstiff clays and sand, characterized by 10m or more of sort to
mediumstiff clay with or without intervening layers of sand or other
cohesionless soil.

Structural
Performance
Factor 
Table 4
Reduction Factor (ψ)
Type of
Building

･Multistory
steelframed building
･Reinforced
concrete multistory buildings without frameshear wall interaction
･Reinforced
concrete multistory frame buildings with structural walls
･Reinforced concrete buildings with
walls, columns, and slab floors but without beams 
