Minimum Cost Versus Minimum Weight of Prestressed Slabs

It is found that for a wide range of unit costs of formwork both approaches lead to approximately similar results provided the unit cost ratio of concrete to steel is above 60 (where cost of concrete is per cubic yard and steel per pound). However, if a cost ratio much below 60 prevails, substantial savings can be achieved by using a minimum cost approach instead of the usual minimum weight approach, which is the aim of the hand design procedure. The minimum weight approach is extended one step further in this study by combining the conditions minimizing the section moduli and the practicality condition which constrains the prestressing force to remain inside the concrete section; this results in a simple algorithm which will systematically lead to the minimum weight feasible slab or rectangular beam. The direct search technique that was used in the computerized optimization algorithm for minimum cost gave satisfactory results.