Structural behaviour of cantilever beams.

 

Famous photograph by Benjamin Baker, in which through a living model the cantilever structural principle on which the solution to the Firth Bridge over the Forth was based was depicted.

"To illustrate the structural theory of a cantilever beam (Gerber), a human demonstration was carried out. The loads were represented by the person sitting in the middle of the span of the suspended span. The arms of the persons seated on both sides represented the traction at the joints; the wooden bars, the compression at the lower elements, and the bricks, the anchor points located at the pylons. The chairs represent the granite piles. Imagine the chairs 500 m apart and the men's heads as high as St. Paul's cross (London church, 104 m) their arms represented by steel beams and the canes by 3.5 m diameter tubes at the base you get a good notion of the structure."

Heinrich Gerber applied in 1866 a theory that consisted in subdividing the continuous beam through ball-and-socket joints "which define a point of zero bending moment". This type of beam with intermediate joints is what is now known as a Gerber beam. The position where these hinges are located allows influencing the behavior of the beam; therefore, it is not only used directly in some structures, but in some projects a real beam can be idealized by assimilating it to a Gerber beam, whose isostatism allows a simple approximate calculation.