mass m equal to the mass of the roof shown and its lateral stiffness k is equal to the sum of the stiffnesses of individual pipe columns. A similar idealization, shown in Fig. I.l.3b, is appropriate for the tank when it is full of water. With sloshing of water not possible in a full tank, it is a lumped mass m supported by a relatively light tower that can be assumed as massless. The cantilever tower supporting the water tank provides lateral stiffness k to the structure. For the moment we will assume that the lateral motion of these structures is small in the sense that the supporting structures deform within their linear elastic limit.
We shall see later in this chapter that the differential equation governing the lateral displacement u(t) of these idealized structures without any external excitation-applied force or ground motion-is