UBC Lateral Story Forces

Lateral forces that counteract the base shear, V, are assumed to act at each story level of the structure. The magnitude of each story force,Fx, is determined from the following formula:

(UBC Equation 30-15)


where:


hx is the height from the base of the structure to level x

wx is the portion of the building weight assumed to be “lumped” at level x. wx typically includes the total weight of the floor or ceiling/roof system at level x, plus half the weight of the vertical elements (walls; columns) located immediately below level x and half the weight of the vertical elements located immediately above level x.

Ft is an additional lateral force assumed to act at the top of a structure. This force is intended to approximate the effects of higher modes of structural vibration. The magnitude of Ft is determined based on the natural (fundamental) period of vibration of the structure, T:

Ft = 0 when T < 0.7s
Ft = 0.07 T V when 0.7s < T < 3.57s (UBC Equation 30-14)
Ft = 0.25 V when T > 3.57s


Since Ft = 0 when T < 0.7s, it is apparent from UBC Equation 30-8 that Ft = 0 for buildings less than 114.5 ft tall. Thus, designers are not required to consider an additional lateral force at the top (Ft) for most wood buildings.