Figure 7.3: Stringer bending and torsion modes

• It has been shown that the lower ends of the propagation bands are given by the natural frequencies of the element shown in Fig.7.3b. The modes corresponding to these frequencies are called the stringer torsion modes.


• The lower bounds of the propagation bands may be denoted by with
  
  
• The upper bounds of the propagation bands are given by the natural frequencies of the flexural element shown in Fig. 7.3c. The modes corresponding to these frequencies are called the stringer bending modes and denoted by with
  
  
• Thus, the propagation bands extend from to with . It has been further shown that, for an N-span periodic beam, a periodic element of which is shown in Fig. 7.3a, every successive set of N natural frequencies is clustered within each propagation band given by to . So, for the natural frequencies to be widely separated, the values of 's should be minimum and those of ' s maximum. In other words, the values of should be low and those of high, i.e., the stringer cross-section should be such that it has a low torsional stiffness and a high bending stiffness. This automatically suggests that the cross-section be open, such as Z-section and top-hat section.

Congratulations! You have finished Lecture 7.