Civil Engineering Reference
In-Depth Information
Figure 1.5
Rebar arrangement for closing moment
as closely as possible, and the steel ties should trace the tension trajectories. A model that
observes the stress flow pattern is expected to best satisfy the compatibility condition and the
serviceability requirements.
Figure 1.5(a) shows the struts-and-ties model for the knee joint subjected to closing moment.
First, the top tensile rebars of the beam and the outer tensile rebars of the column are represented
by the ties (solid lines), ab and de, respectively. The centroidal lines of the compression zones
in the beam and in the column are replaced by the struts (dotted lines), og and fo, respectively.
Since the tensile stresses should flow from the top rebar of the beam to the outer rebar of the
column, the top tie, ab, is connected to the outer tie, de, by two straight ties, bc and cd, along
the outer corner. Because of the changes of angles at the node points, b, c and d, the tensile
force in the ties will produce a resultant bearing force on the concrete at each node point
directed toward the point o at the inner corner of the joint. Following these compressive stress
trajectories, three diagonal struts bo, co and do, are installed. The three compressive forces
acting along these diagonal struts and meeting at the node point o must be balanced by the
two compression forces in the struts og and fo. As a whole, we have a stable struts-and-ties
assembly that is in equilibrium at all the node points. The forces in all the struts and ties can
be calculated.
It is clear from this struts-and-ties model that the correct arrangement of the primary tensile
rebar should follow the tension ties abcde, as shown in Figure 1.5(b). The radius of bend
r
of
the rebars has a significant effect on the local crushing of concrete under the curved portion,
