Geometric construction of the struts

Since the internal lever arm of the beam is indeterminate, it must be found through iterations until the top of the compressed strut is at the edge of the beam. A direct construction of the stress field can be made geometrically by using the property of the rectangular geometry of compressed struts with constant stresses. By considering that the triangles inscribed in a circle, with the hypotenuse along the radius, are right-angled, the construction of the first compressed field can be done in three steps as described below::

Geometric construction of the inclined strut

After this construction, the thickness of the horizontal compressed strut is graphically deduced from the upper nodal area.

This method, valid only for constant stress fields, allows us to draw the entire stress field without calculations, after determining the support reactions.