Statics: Shear and Torsion

Open and closed sections subject to torsion with warping

This demonstration shows the difference in the torsional stiffness of two circular members, one with a closed section and the other with an open section where warping can be observed.

(a)                                                                                  (b)
Fig. 5-9: Open and closed sections subject to torsion with warping

Fig. 5-9 shows two foam pipes that are used for insulation and each of the two pipes has a length of 450 mm. The sections have machined slits and in one of the pipes the slit is sealed using tape and glue. One pipe thus effectively has an open circular section and one has a closed circular section. The sections have been analysed in Example 5-2 where it was predicted that the torsional stiffness of the closed section was 3.5 times that of the open section. By twisting the two foam pipes it is possible to feel the significant difference in their torsional stiffnesses.

When twisting the two foam pipes with a similar effort it can be seen from Fig. 5-9 that:

  • It is much easier to twist the open section than the closed section.
  • The effect of warping can be observed (Fig. 5-9a) at the right hand end of the pipe which has a slit along its length.
  • There is little warping effect on the pipe with the closed section (Fig. 5-9b).