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Floor or Slab Systems in Building-Features and Types

Reinforced Concrete Buildings Structures are a combination of different structural systems. It involves vertical systems, horizontal systems and lateral systems. Floor or slabs systems are a type of horizontal structural system that are intended to resist the gravity loads like dead and live loads acting over it and to safely transfer them to vertical systems like columns.

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Floor or Slab Systems in Building-Features and Types



Features of Slab Systems in R.C.C Building Structure

A slab system or floor system possess the following features:
  1. Safe transfer of loads to below structural systems
  2. Serve as horizontal diaphragm to connect and stiffen other structural element
  3. Act as a rigid floor diaphragm under the action of lateral loads and distribute safely to vertical frame elements
  4. Maintain floor geometry during lateral load action


Different Types of Floor or Slab Systems for Buildings

In an cast-in-situ R.C.C building, a slab system can be of the following types:
  1. Wall-Supported Slab System
  2. Beam Supported Slab System
  3. Ribbed Slab System
  4. Flat Plate System
  5. Flat Slab System


1. Wall-Supported Slab System

These are slab or floor system supported on load bearing masonry walls. The important features are:
  • Floor slabs have thickness between 100 to 200mm
  • Span range between 3 to 7.5 m
  • Adopted in low-rise buildings
  • Can be supported on opposite spans ( 2 sides) called as one-way slab or on four sides, called as two-way slab.
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These are system in which floor slabs are supported on beams instead of walls. During construction, the slabs are constructed monolithically with the beams in a grid pattern.
  • Used for slabs of span between 3 and 7.5 m
  • Adopted for high-rise building and low-rise framed construction
  • Gravity loads from slabs are transferred directly to the beams and then to columns.
Note: When the beam are directly connected to the columns in the vertical frame, then the beams are called as primary beams or girders. When the beams supporting the slabs are not supported directly by columns, but by other primary beams, then those beams are called secondary beams.

Primary and Secondary Beam


3. Ribbed Slab System

This is a unique slab system in which the slab called as topping is thin say 50 to 100 mm and placed over closely spaced slender beams called as ribs ( spacing between the beams are less than 1.5m ). The thickness of ribs used to support the slabs are not less than 65 mm and a depth three to 4 times the thickness.
  • Also called grid floor
  • Can be constructed as one-way or two way system
  • Two-way rib slab is called as waffle slab system
  • Can be constructed either cast-in-situ or precast
Ribbed Slab System- Both One-Way and Two-way configuration



4. Flat Plate System

In this system, the floor slab is supported directly on the columns without using stiffening beams except at their peripheries as shown in figure below.

Flat plate system without beams ( Beams are only at the periphery)



5. Flat Slab System

These are slab systems in which, flat slab plates are stiffened near the column supports using drop panels or column capitals that are concealed under the drop ceilings. Compared to flat plate system, flat slabs can take higher loads and larger span. This enhanced property is because the drop panels at the column slab intersection improve shear and hogging moment resistance at the junction.

Flat Slab System
  • The flat slab thickness uses a thickness between 125 mm to 300 mm for span of 4 to 9m. 
  • Flat slabs possess highest dead loads per unit area compared to other systems.

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