Design Ribbed and Flat Slabs

Ribbed Slabs

• Introduction:

• In long span, solid reinforced concrete slabs, e.g. greater than 5 m, the self-weight becomes excessive when compared to the applied dead and imposed loads, resulting in an uneconomic method of construction. One method of overcoming this problem is to use

ribbed slabs which are suitable for longer spans supporting light loading.

• Ribbed slabs are reinforced concrete slabs in which some of the volume of concrete

below the neutral axis (i.e. the area in tension) is removed and replaced with block formers or left as voids, as shown in Figures

• Example: Single Span Hollow-Tile Floor

• A hollow-tile floor, in which the permanent blocks are not jointed in a cement : sand mortar, is to be designed as a series of simply supported spans as indicated in Figures. Using the design data given and assuming that the blocks do not contribute to the strength of the floor, determine:

• i) the suitability of the slab thickness,

• ii) the reinforcement required.

• Design Data: • Characteristic dead load (including self-weight)

5.0 kN/m2 • Characteristic imposed load 4.0 kN/m2 • Characteristic concrete strength (fcu) 40 N/mm2 • Characteristic of reinforcement (fy) 460 N/mm2 • Exposure condition mild • Fire rating Minimum 1 hour • Span of slab 4.0 m • Nominal maximum aggregate size 14 mm

FLAT SLABS

• Definition:

• The flat slab is defined in BS8110: Part 1, clause 1.3.2.1, as a slab with or without drops,

supported generally without beams by columns with or without column heads.

Traditionally, drop caps and panels around columns are used for two principal reasons:

1. Column capitals, or drop caps, are used to improve the punching shear capacity of the

column/slab joint only.

2. Drop panels are used to increase the bending moment capacity of the joint, reduce deflection, and increase the punching shear capacity.

General Code Provisions

1. The ratio of the longer to the shorter span should not exceed 2.

2. Design moments may be obtained by

(a) equivalent frame method

(b) simplified method

(c) finite element analysis

Analysis

• (a) Frame analysis method:

The structure is divided longitudinally and transversely into frames consisting of columns

and strips of slab. Either the entire frame or sub-frames can be analysed by frame analysis

programs

Division of Panels

(b)Simplified method:

• In this method, for structures where lateral stability does not depend on slab-column connections, moments and shears are taken from Table 3.12 of the code for one-way spanning continuous slabs. The total moment across the full width of the panel is calculated and the proportion resisted by the column strip and middle strip are taken from Table 3.18 of the

code.

deformation

Moment distribution

• Punching shear occurs where a two-way concrete fl at slab or plate is connected to a column without beams. It is the tendency of the slab to drop as a unit around the column