Introduction - Unified Theory of Concrete Structures - page 21

First, the design can be very tedious, since the forces in every strut and tie must be calculated
and scaled proportionally. Instead of treating a beam as a whole to simplify the design, the
struts-and-ties model is heading in the opposite direction by treating a beam as an assembly
of a large number of struts and ties.
Second, there are no definite objective criteria for the selection and the proportioning of the
struts and ties, even though recommendations have been made.
Third, and probably of most importance, there are indeed better and more sophisticated
models for the design of beams, which can take into account the compatibility condition
and the constitutive laws of materials. As mentioned in Section 1.2.2, Bernoulli's linear
compatibility can be used for bending and axial loads in the main regions, and Mohr's circular
compatibility can be used for shear and torsion. Detailed study of these better models is the
main objective of this topic.
Nevertheless, struts-and-ties models are well suited to guide the design of local regions,
such as knee joints, for two reasons. First, the strain compatibility conditions in the local
regions are usually too complicated to be employed. We have no choice but to rely on the
stress equilibrium condition. Second, the local regions are of limited lengths compared with
the main regions (see Figure 1.1). Since the number of struts and ties in a local region is
limited, the process of designing struts and ties is relatively easy.