Analysis of time-dependent internal forces with conventional computer programs - Concrete Structures: Stresses and Deformations - page 178

Figure 6.1 Idealization of members: (a) prestressed member idealized as two bars; (b)
composite member idealized as two bars of different material properties.
whose cross section consists of a precast part and a cast-in-situ part, or of
concrete and steel, is treated as two homogeneous members connected by
rigid arms joining the centroids of the two parts (Fig. 6.1(b) ).
With the idealization using short rigid arms as shown in Figs. 6.1(a) and
(b), the actual member should be divided into a number of short members
(say 10; see Example 6.5). The internal forces obtained by analysis should be
considered representatives of the actual structure only at mid-length of the
short members. If the external loads are applied only at the nodes, the bend-
ing moment at mid-length of a member is the average of the two bending
moment values at the two ends and the shearing force and the axial force are
constants.