Strut-and-tie model

The distribution of forces can be obtained through intuitive considerations. The distance between the load and the support is quite small, and a direct support can be assumed; in this case, stirrups are not necessary (apart from the minimum reinforcement).

A compression strut is created between the point of application of the load and the support. It is in equilibrium with the horizontal struts and the external forces. The following figure shows the tensile and compressive forces in the form of an arch-and-cable.

Strut-and-tie model

The compressive forces are taken up by the concrete, while the tensile forces are taken up by the reinforcement. To calculate the width of the compressed strut, the force in the strut is divided by the effective strength of the concrete and the thickness of the beam.

The state of stress in the struts is uniaxial, and the tensile strength of the concrete is neglected. However, near the nodes where struts meet, the state of stress is biaxial. It should be noted that in the case of compressive stress fields with constant stresses, the shape of the compression fields is strictly rectangular. This means that the line of discontinuity between the compressed strut and the nodal area is always perpendicular to the direction of the strut.

Strut-and-tie model with thicknesses of the struts