Introduction - Unified Theory of Concrete Structures - page 16

Figure 1.4 Knee joint moments and incorrect reinforcement

to the inner tension rebars of the column, while the top compression steel of the beam is
connected to the outer compression steel of the column. This way of connecting the tension
rebars of the beam and the column is obviously faulty, because the bottom tensile force of
the beam and the inner tensile force of the column would produce a diagonal resultant that
tends to straighten the rebar and to tear out a chunk of concrete at the inner corner. In fact, the
strength of such a knee joint is only 10% of the strength of the governing member according
to Swann's tests.
In Figure 1.4(e), the bottom tension rebars of the beam are connected to the outer com-
pression rebars of the column, while the top compression rebars of the beam are connected to
the inner tension rebars of the column. Additional steel bars would be needed along the outer
edge to protect the concrete core of the joint region. Such an arrangement also turns out to be
flawed. The compression force at the top of the beam and the compression force at the outer
portion of the column tend to push out a triangular chunk of concrete at the outer corner of
the joint after the appearance of a diagonal crack. The strength of such a knee joint could be
as low as 17% of the strength of the governing member (Swann, 1969).

1.4.3.1 Knee Joint under Closing Moment

The struts-and-ties models will now be used to guide the design of rebars at the knee joint.
The most important idea in the selection of the struts-and-ties assembly is to recognize the
stress flow in the local region. The concrete struts should follow the compression trajectories