Introduction - Unified Theory of Concrete Structures - page 9

Figure 1.2 Plane truss model of a concrete beam with bottom longidutinal rebars and stirrups resisting
shear and bending
steel tension ties form a truss that is capable of resisting applied loadings. The struts-and-ties
model has been used, intuitively, by engineers to design concrete structures since the advent of
reinforced concrete. At present, it is used primarily for the difficult local regions (D regions).
Examples of the struts-and-ties model will be given in Section 1.4.
When the struts-and-ties concept was applied to a main region (B region), it is known as a
truss model . For a reinforced concrete beam, truss model can be applied, not only to bending
and axial loads, but also to shear and torsion. Two examples will be discussed, namely, a beam
subjected to shear and bending as shown in Figure 1.2, and a beam subjected to torsion as
showninFigure1.3.
The first application of the concept of truss model to beam shear was proposed by Ritter
(1899) and Morsch (1902) as illustrated in Figure 1.2. In their view, a reinforced concrete beam
acts like a parallel-stringer truss to resist bending and shear. Due to the bending moment, the
concrete strut near the upper edge serves as the top stringer in a truss and the steel bar near the
lower edge assumes the function of the bottom stringer. From shear stresses, the web region
would develop diagonal cracks at about 45 ◦ inclination to the longitudinal steel. These cracks
would separate the concrete into a series of diagonal concrete struts. To resist the applied shear