ECONOMICS OF VERY LARGE SPAN ROOFS

Abstract

Large unobstructed clear space is required for warehouses, aircraft hangars and covered sports stadia etc. Several types of materials and structural systems may be used to roof such large spans. For structures with rectangular plan, the roofs may be with conventional systems like concrete shell roofs, steel space frame or Lamella roofs, cable suspended roofs or relatively unconventional system like a cable reinforced air supported roof. Because of the enormous spanning potentials, cable roofs hold promise. To establish their economical feasibility for Indian conditions, an aircraft hangar of 140 in span has been designed using plane pre-stressed cable trusses and optimisation of the structure has been attempted. The optimised structure has been analysed by a discrete nonlinear method and results compared with approximate analysis. Apart from cable r,,,ofs, only two 'other structural forms are economically feasible. They are concrete catenary shell arch roof and steel Lamella roof and both types have been designed and cost evaluated. Cost comparison indicate that steel roofs are costlier for very large spans (exceeding 100 m). The solution of the structural problem is influenced very much by the ratio of materials to labour costs. Due to availability of cheap labour force in our country, concrete shell roofs using precast techniques may compete with cable roofs upto spans of 150 in but for still larger spans, only cable roofs hold promise and are economically feasible