The structure shown in Fig 1 (a) is taken from a 20-item test for an understanding of structural behaviour. In the preamble to the test, it is explained that roller supports, in the structure at point D, can sustain a reaction in either direction, normal to the direction of the roller. The test was the basis of a 15-year study, the control group for which was the graduate entry to the Ove Arup Partnership.
D.M. Brohn