Statics: Shear and Torsion

Open and closed sections subject to torsion without warping

This demonstration shows the difference in the torsional stiffness of two non-circular members, one with a closed section and the other with an open section where warping is restrained.

(a)                                                                      (b)                                                                      (c)
Fig. 5-10: Open and Closed sections subjected to torsion without warping

Fig. 5-10a shows two 500mm long steel bars, one with a square hollow section and the other with an I section which is made by cutting a square hollow section into two halves along its length and welding the resultant channel sections back to back. Handles are welded to the ends of the bars to allow end torques to be easily applied. The sections have been analysed in Example 5-3. Due to the addition of the handles, the warping that occurs in open sections as shown in Section 5.3.4 is now restrained. In other words, the model with the I section would be stiffer than that analysed in Example 5-3.

By applying torques at the ends of the two bars it is readily felt that the bar with a closed section is much stiffer than that the bar which has an open section.