Shell roofs are very useful for covering large structures, e.g., assembly halls, recreation centres, libraries, theatres and factories. RCC shell roofs are becoming very popular these days. Very less quantity of materials are required to build up a shell roof as compared to other conventional methods of roofing for the same span. The design of the shell is made as thin as practical requirements will allow, such that the dead load is decreased and the shell acts as a membrane free from large bending stresses. Least quantity of materials is used to the maximum structural advantage.
The following are the common types of shell roof:
- North light shell roof which is used mostly in factories, workshops and places where good daylight is desired.
- If good day lighting is not a requirement, long multiple cylindrical shells with feather edge beams may be useful.
- Double curved shells are structurally more efficient than single curved shells, but it presents more difficulties in preparing the centring for it. Though consumption of materials is less, sometimes the costs of formworks make the shell roofs quite expensive. It proves to be more costly when only a few similar units are to be constructed. Thin shell roofs are economical when many identical units are to be built and the forms can be reused several times. The forms are usually fabricated from timber battens lined with steel sheets or plywood. Sometimes, plastic forms are also used to obtain special surface textures. The materials of formwork and the lining are selected in consideration of the number of reuses in a particular project. Economy may be achieved in two ways for the formwork. Firstly by using moveable formwork when the shell is to be cast in situ. The second way is to use the precast shells.
SHELL ROOF