Post -by Gautam Shah
Roof and floor systems are called elements mediating across the load-bearing systems, such as walls and beams. Roofs by themselves may transmit the load, such as in Flat slabs, Shell structures, Domes, etc. Roofs can take various shapes, but Floors are flatter and only occasionally slightly inclined. The distinction of floor and beam or wall may or may not be very apparent. In some structures both are well integrated, so act coherently (e.g. waffle slabs). In other structures the identity of each is distinct yet they may operate interdependently. In assembled structures the components are independent, and easy to identify (a stone plate on a wall is simply supported). In cases where the structural support system and covering elements are integrated, the stress transfer is very efficient. This also results into a system that is lean and lightweight, when compared to system of independent (simply supported) components.
Covering elements in Compressive roof systems are composed of stiff materials. These elements have a comparatively substantial thickness in proportion to the span. The thickness requirements vary depending on the composition of structure and the stress resistance of the material, the shape configuration of the covering element, the mode of stress transfer (across or along the section) and the end (support) conditions.
Covering elements in Tensile roofing systems are composed of stiff or flexible materials. The later have very small thickness in comparison to the size of the span. The material being very thin and flexible takes on the shape the way it is stressed to, and also depending on the gravity induced stress forms (catenary). Covering elements of a roofing system, when pre-stressed (usually pre-stretched), show greater resistance to various stresses
For predominantly compressive elements, the shear capacity increases several fold. For tensile elements, the capacity to deal with local loads increases.
Covering material may be a homogeneous mass, a composition of smaller units or a layered mass. Covering materials in a roofing system are wood planks (shingles), plywood, laminated paper composites, fibre bitumen composites, fibre cement composites, ceramics, stones (slates, marbles, metamorphic rocks), glass, fibre-glass, cast in situ and pre-cast RCC, hollow blocks of cement and terracotta, cellular blocks, air entrained or expanded foam blocks, plastics, plastic foams, metal and alloy sheets, leaves, grass, sticks, natural fibres, mats, textiles, wool, synthetic woven materials, tarpaulins, impregnated fabrics, clay blocks, bricks, ice, ferro and magnesium cement castings, rubber sheets, nylon, Teflon, etc.
A transparent roof cover material that would allow light and permit vision through was a dream, that every builder tried to realize. Romans used small glass disks inserts for roof illumination. Gothic period saw tall perpendicular windows almost merging into the roof. Paintings and murals with extensive skies as the theme were painted on the ceilings. However, it was the Conservatory at Derbyshire in 1836, Crystal Palace at London in 1850, both designed by Paxton, and the Palm house at Botanical Garden, Kew in 1845, designed by Burton and Turner, the dream of a transparent roof was realized. Transparent roofs are now made of acrylics and other plastics for solarium, green houses, passive solar heating systems, etc. Kenzo Tange designed the Japan Olympic stadium roof as a catenary – flexible structure. French museum Louvre at Paris has an extension wing with a glass pyramid.