29.1 General
This covers the slabs constructed in one of the ways described below:
a) As a series of concrete ribs with topping cast on forms which may be removed after the concrete has set;
b) As a series of concrete ribs between precast blocks which remain part of the completed structure; the top of the ribs may be connected by a topping of concrete of the same strength as that used in the ribs; and
c) With a continuous top and bottom face but containing voids of rectangular, oval or other shape.
29.2 Analysis of Structure
The moments and forces due to design loads on continuous slabs may be obtained by the methods given in Section 5A (C) for solid slabs. Alternatively, the slabs may be designed as a series of simply supported spans provided they are not exposed to weather or corrosive conditions; wide cracks may develop at the supports and the engineer shall satisfy himself that these will not impair finishes or lead to corrosion of the reinforcement.
29.3 Shear
Where hollow blocks are used, for the purpose of calculating shear stress, the rib width may be increased to take account of the wall thickness of the block on one side of the rib; with narrow precast units, the width of the jointing mortar or concrete may be included.
29.4 Deflection
The recommendations for deflection in respect of solid slabs may be applied to ribbed, hollow block or voided construction. The span to effective depth ratios given in 22.2 for a flanged beam are applicable but when calculating the final reduction factor for web width, the rib width for hollow block slabs may be assumed to include the walls of the blocks on both sides of the rib. For voided slabs and slabs constructed of box or I-section units, an effective rib width shall be calculated assuming all material below the upper flange of the unit to be concentrated in a rectangular rib having the same cross-sectional area and depth.
29.5 Size and Position of Ribs
In-situ ribs shall be not less than 65 mm wide. They shall be spaced at centres not greater than 1.5 m apart and their depth, excluding any topping, shall be not more than four times their width. Generally ribs shall be formed along each edge parallel to the span of one way slabs. When the edge is built into a wall or rests on a beam, a rib at least as wide as the bearing shall be formed along the edge.
29.6 Hollow Blocks and Formers
Blocks and formers may be of any suitable material. Hollow clay tiles for the filler type shall conform to accepted standard [6-5A(40)]. When required to contribute to the structural strength of a slab they shall,
a) be made of concrete or burnt clay; and
b) have a crushing strength of at least measured on the net section when axially
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE
loaded in the direction of compressive stress in the slab.
29.7 Arrangement of Reinforcement
The recommendations given in 25.3 regarding maximum distance between bars apply to areas of solid concrete in this form of construction. The curtailment, anchorage and cover to reinforcement shall be as described below:
a) At least 50 percent of the total main reinforcement shall be carried through at the bottom on to the bearing and anchored in accordance with 25.2.3.3.
b) Where a slab, which is continuous over supports, has been designed as simply supported, reinforcement shall be provided over the support to control cracking. This reinforcement shall have a cross-sectional area of not less than one-quarter that required in the middle of the adjoining spans and shall extend at least one-tenth of the clear span into adjoining spans.
c) In slabs with permanent blocks, the side cover to the reinforcement shall not be less than 10 mm. In all other cases, cover shall be provided according to 25.4.
29.8 Precast Joists and Hollow Filler Blocks
The construction with precast joists and hollow filler blocks of concrete or clay shall conform to good practice [6-5A(41)].