"The majority of Isler's shells are specifically designed to remain in compression under all load conditions, but in the case of the hyperbolic paraboloid [Heilig Geist Kirche (Holy Spirit Church), ] both tension and compression stresses normally occur. To overcome the adverse effects of the tension stresses Isler applied a pre-stress through the shell surface by post-tensioning using eight cables between the wall tops... This he simulated in the workshop model... as usual for Isler's experimental technique, the roof loads were applied by means of small wooden discs distributed evenly over the surface to simulate uniformly distributed load. In turn these were connected to a hanging network of timber spreader bars and strings in such a way that the distributed load could be applied with a single weight or hydraulic jack. The pre-stressing force was applied by small hydraulic jacks. This arrangement allowed vertical, horizontal and pre-stressing loads to be applied in appropriate combinations, simulating the shell self-weight, full snow load, partial snow, etc. Uplift loads induced by the wind were assessed by appropriate factoring and reversing the stresses (derived from electrical strain gauges attached to the model surface) resulting from downward loads."