In a guitar, the braces act as beams to help support the string loads. The bracing on the underside of a guitar top is an array of beams designed to strengthen the top and control the static deflections due to the string tension, whilst allowing sufficient vibration to couple the strings’ oscillations to the surrounding air; quite a tricky balance to achieve.
The basic principles of beam mechanics that guitar designers need to know are:
Deeper sections make stiffer beams than broader sections. If we double the width, we double the stiffness. If we double the depth, the stiffness rises by a factor of eight, i.e. by the factor cubed. If we double the length, the stiffness of the beam reduces by a factor of eight, i.e. the factor cubed.