12.8 Equivalent Lateral Force Method (ELF)
Equivalent Lateral Force Method is only used to determine structural irregularities.
Symbols
Cs = The seismic response coefficient
W = The effective seismic weight
SDS = The design spectral response acceleration parameter in the short period range
R = The response modification factor in Table 12.2-1
Ie = The Importance Factor
SD1 = the design spectral response acceleration parameter at a period of 1.0 s
T = The fundamental period of the structure(s)
TL = The long-period transition period(s)
S1 = The mapped maximum considered earthquake spectral response acceleration parameter
N = Number of stories above the base
Cq =0.0019 ft (0.00058 m)
AB = area of base of structure [ft2 (m2)];
Ai = web area of shear wall i [ft2 (m2)];
Di = length of shear wall i [ft (m)]; and
x = number of shear walls in the building effective in resisting lateral forces in the direction under consideration.
Fi = The portion of the seismic base shear (V) [kip (kN)] induced at level i
Cvx = vertical distribution factor
V = total design lateral force or shear at the base of the structure [kip (kN)];
wi and wx = portion of the total effective seismic weight of the structure (W) located or assigned to level i or x;
hi and hx = height [ft (m)] from the base to level i or x; and
k = an exponent related to the structure period
The Equivalent Earthquake Load Method is based on the principle that the dynamic behavior of the building in the direction of the earthquake considered in buildings with low height is represented by the behavior of the structural system in the dominant vibration mode and the shape of this mode is considered as an approximately inverted triangle.
12.8.1 Seismic Base Shear
The seismic base shear, V, in a given direction is determined with eq. 12.8-1.
12.8.1.1 Calculation of Seismic Response Coefficient
The seismic response coefficient, Cs, is determined with eq. (12.8-2).
The value of Cs computed with Eq. (12.8-2) need not exceed the following:
for T ≤ TL
for T > TL
Cs should not be less than
12.8.2 Period Determination
The fundamental period, T, is not exceed the product of the coefficient for upper limit on calculated period (Cu) from Table 12.8-1 and the approximate fundamental period, Ta, determined with Section 12.8.2.1.
12.8.2.1 Approximate Fundamental Period
The approximate fundamental period (Ta), in seconds, is determined from eq 12.8-7:
12.8.3 Vertical Distribution of Seismic Forces
The lateral seismic force (Fx) (kip or kN) induced at any level is determined from eq 12.8-11 and 12.8-12:
k = an exponent related to the structure period as follows:
• for structures that have a period of 0.5 s or less, k = 1;
• for structures that have a period of 2.5 s or more, k = 2; and
• for structures that have a period between 0.5 and 2.5 s, k shall be 2 or shall be determined by linear
interpolation between 1 and 2.
