Code of practice for construction of floors and roofs using precast doubly-curved shell units
1984 Edition

The recommended approach involves mechanized precasting for shells exceeding 1.5 meters incorporating reinforcement within the shell body. The shell surface is calculated using the equations provided, and precise ordinates are laid out to create moulds made from steel, timber, or plastic. These moulds, including the edge beam, are often constructed as a single unit. Casting occurs on a level platform elevated 600 to 1000 mm above ground, with the mould mounted on a trolley equipped with jacks for elevation control. After casting, the edge beam mould is removed after three hours, and the shell mould is demoulded within 24 to 48 hours, extended to 72 hours in cooler climates. For shells with deep ribs over 250 mm and spans above 12 meters, casting is done in two stages with side shuttering up to the shell's edge beam level. This method guarantees accurate shaping, proper reinforcement placement, and controlled curing for durable, lightweight shell units.

According to IS 6332 clauses, openings are best avoided in precast shells to maintain structural integrity. When openings are necessary and their locations are predetermined, they should be incorporated during the precasting stage with adequate stiffening provided by additional reinforcement surrounding the openings. For shell units with multiple openings within the same grid, it is advised to replace that shell grid with a solid reinforced concrete panel featuring properly reinforced openings. Reinforcing the edges of openings with steel bars and avoiding drilling post-casting are crucial steps to retain the load-carrying capacity of the shell elements.

Edge beams must have a minimum thickness of 25 mm in normal atmospheric conditions and 35 mm in corrosive environments, as detailed in IS 6332 Figure 3. Reinforcement requires at least one mild steel bar with a diameter of 6 mm placed along the edge beam. This reinforcement must comply with the provisions of IS 456-1978 for cast-in-place concrete. These requirements ensure that edge beams effectively stiffen the shell perimeter and provide adequate durability and crack control.

For shell units measuring up to 1.5 meters, manual handling without lifting hooks is acceptable. Units larger than 1.5 meters should be handled with mechanical lifting equipment such as cranes or hoists, with lifting hooks or embedded inserts provided during casting for secure attachment. Handling should be performed on level platforms elevated 600 to 1000 mm above ground, equipped with openings matching the shell size. Use of jacking arrangements on trolleys allows controlled lifting and lowering to minimize damage. Careful handling to avoid sudden impacts and ensuring balanced lifting prevents cracking or structural damage to the shells.

For designing supporting elements and concrete mixes in accordance with IS 6332, the following Indian Standards apply: IS 456-1978 for reinforced concrete design and mix proportions; IS 1343-1980 for prestressed concrete; IS 800-1962 for structural steel; IS 883-1970 for structural timber design; IS 875-1964 for load specifications; and IS 3935-1966 for composite construction. Utilizing these codes ensures compliance with Indian regulations for safety, durability, and performance in structural design.