Concrete slabs play a crucial role in creating flat surfaces like floors, ceilings, and roof decks in buildings. These slabs bear and transfer loads safely from columns to walls. This article aims to provide an in-depth understanding of the essential features and structural behavior of concrete slabs in building construction.
Concrete slabs are designed to handle uniformly distributed loads, including dead loads (self-weight, floor finishes, walls) and imposed loads. The design considers live loads based on the type of building, with residential structures typically using a live load of 2000 to 3000 N/m² as per Indian standards. Imposed loads for various building types can be referenced from IS 875-Part 2 (1987).
Concrete slabs can be broadly categorized into ground-bearing and suspended slabs. Ground-bearing slabs are constructed directly on the foundation, while suspended slabs span between supports and resist bending moments based on load magnitude and span length. These slabs can be prefabricated off-site or constructed in-situ using formwork.
Conventional slabs, supported by beams and columns, transfer loads to supporting structural elements. They are classified into one-way and two-way slabs. One-way slabs transfer loads along the longer direction, while two-way slabs transfer loads in both directions.
Flat slabs are directly supported by columns without beams, making them easy to construct and requiring less formwork compared to conventional slabs.
Hollow core slabs have voids within them, reducing overall weight and making them suitable for multi-story car parks and office buildings.
Waffle slabs, with a distinctive grid pattern, are ideal for longer spans, offering greater load-bearing capacity compared to conventional slabs.
Composite slabs involve reinforced concrete cast on steel decking, providing superior strength-to-weight and stiffness-to-weight ratios.
The structural behavior of concrete slabs under loads is influenced by their geometry and reinforcement direction. Dead and imposed loads are distributed over the support system.
One-way slabs, with a longer dimension-to-shorter dimension ratio greater than or equal to two, bend along the shorter span when supported at two opposite edges. Main reinforcement is placed along the shorter span, with distribution reinforcement along the longer spans.
Two-way slabs, supported on all four sides with a longer dimension-to-shorter dimension ratio less than two, carry loads in both directions. Main reinforcement is provided in both directions, making them common in multi-story building floors.