Statics: Prestress

Prestressed wooden blocks forming a beam and a column

This model demonstrates the effect of prestressing which makes separate wooden blocks act as a beam or column.

(a)                                                                                                                 (b)
Fig. 11-5: Effect of prestressing (1)

Fig. 11-5a shows a number of separated wooden cubes which are linked using a metal wire through small holes at their centres. One end of the wire passes over a support post while the other end is constrained horizontally by a metal post preventing the cubes from falling down. The structure formed cannot support even its own weight and the cubes simply hang on the wire. If weights are attached to the right hand end of the wire, the wire is tightened. The metal post and the increased compressive prestress between the cubes enable them to form a structure that can now carry an external load as shown in Fig. 11-5b.

(a)                                                             (b)                                                             (c)
Fig. 11-6: Effect of prestressing (2)

A loose elastic string, with one end fixed to a base, passes through the central holes of a pile of wooden blocks as shown in Fig. 11-6. The effect of prestress can be demonstrated as follows:

  1. Push the column from one side, it will topple as shown in Fig. 11-6a.
  2. Reform the column and tighten the elastic string anchoring it to the top block (Fig. 11-6b)
  3. Hold the base of the model and again push one side of the column. This time the blocks act as a single member which cannot be easily toppled as shown in Fig. 11-6c.