Bracing is designed to cope with both structural and acoustic requirements. The former are due to the string tension while the latter depend on the frequency and shape of the soundboard vibratio modes. In analogy with the soundboard, the bracing design relies on the measuement of the elastic and mechanical properties of materials and their appropriate selection.
On my instruments, I often use composite bracing that allow me to push beyond the limits of conventional wood bracing. In this way i can obtain very mobile soundboards that can resist over time.