Introduction - Unified Theory of Concrete Structures - page 19

1.4.3.2 Knee Joint under Opening Moment

A struts-and-ties model (model 1) for the knee joint subjected to an opening moment is shown
in Figure 1.6(a). This model is essentially a reverse case of the model for a closing moment
(Figure 1.5a). This means that the ties and the struts are interchanged. Such a model should
also be stable and in equilibrium. According to this model, the correct arrangement of the
primary tensile rebars should follow the tension ties, as shown in Figure 1.6(b). It is noted that
this set of radially oriented rebars should, in reality, be designed as closed stirrups because of
anchorage requirements.
A second struts-and-ties model (model 2) for opening moment is given in Figure 1.6(c).
The rebar arrangement according to this model is given in Figure 1.6(d). This model explains
why the bottom tension rebar of the beam should be connected to the inner tension rebar of
the column by first forming a big loop around the core of the joint area. The connection of the
rebars may be achieved by welding.
Design of rebars according to a combination of models 1 and 2 is given in Figure 1.6(f). The
big loop of the tension rebar is shown to be formed by splicing rather than welding. The closed
stirrups in the radial direction could also serve to control cracking. The direction of the crack
is shown in Figure 1.6(e). The final rebar arrangement also includes the diagonal rebars, ab,
perpendicular to the diagonal line connecting the inner and outer corners. The effectiveness
of such diagonal rebars is explained by the two struts-and-ties models in Figure 1.7. The first
one (Figure 1.7a), is an extension of model 1 in Figure 1.6(a), while the second (Figure 1.7b),
is a generalization of model 2 in Figure 1.6(c).

Figure 1.6 Rebar arrangement for opening moment