4.5.1.1 Construction
This subclause refers to in-situ slabs constructed in one of the following ways:
a) where topping is considered to contribute to structural strength:
1) as a series of concrete ribs cast in situ between blocks that remain part of the completed structure; the tops of the ribs are connected by a topping of concrete of the same strength as that used in the ribs;
2) as a series of concrete ribs with topping cast on forms that may be removed after the concrete has set;
3) with a continuous top and bottom face but containing voids of rectangular, oval or other shape.
b) where topping is not considered to contribute to structural strength: as a series of concrete ribs cast in-situ between blocks that remain part of the completed structure; the tops of the ribs may be connected by a topping of concrete but not necessarily of the same strength as that used in the ribs.
4.5.1.2 Thickness of topping
When a topping is used to contribute to the structural strength, ensure that its thickness, after any necessary allowance has been made for wear, is at least
a) 30 mm for slabs that have permanent blocks as described in 4.5.1.4 and have a clear distance of not more than 500 mm between the ribs;
b) 25 mm for slabs as in (a) above but with each row of blocks jointed in mortar having a cement-sand mixture not weaker than 1:3, or having a cube strength of 11 MPa;
c) the greater of 40 mm or one-tenth of the clear distance between the ribs, for all other slabs
containing permanent blocks;
d) the greater of 50 mm or one-tenth of the clear distance between the ribs, for all other slabs without permanent blocks.
4.5.1.3 Size, spacing and position of ribs
The minimum width of ribs, whether they are rectangular or tapered, should be at least 65 mm and their depth, excluding any topping, should not exceed four times their width.
In-situ ribs should be spaced at centres not exceeding 1,5 m and the edge rib that bears along its length on a beam or wall shall be at least as wide as the bearing, i.e. the block or void shall not be on the bearing.
4.5.1.4 Hollow blocks and formers
4.5.1.4.1 Blocks and formers may be of any suitable material but, when required to contribute to the structural strength of a slab, they should be made of
a) concrete or burnt clay and have a crushing strength of at least 14 MPa measured on the net section when axially loaded in the direction of compressive stress in the slab, or
b) fired briquettes, clay or shale. Amdt 1, Apr. 1994
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4.5.1.4.2 When a slab is constructed in accordance with 4.5.1.2(a) but the topping is not used to contribute to structural strength, the blocks should comply with 4.5.1.4.1(a) or (b). In addition, the thickness of the block material above its void shall be the greater of at least 20 mm or one-tenth of the clear distance between the ribs. The overall thickness of the block and topping (if any) should be not less than one-fifth of the clear distance between the ribs.
4.5.2 Analysis of structure
The moments and forces due to ultimate loads on continuous slabs may be obtained by any of the methods given in 4.4.2 for solid slabs. Alternatively, the slabs may be designed as a series of simply supported spans, provided that they will not be 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.
Rules for the arrangement of reinforcement are given in 4.5.6.
4.5.3 Moments of resistance
The methods given in 4.3.3 for determining the ultimate moment of resistance of beams may be used.
When sections are being analysed, the stresses in burnt clay blocks in the compression zone may be taken as 0,25 times the strength as determined in 4.5.1.4.1(a). However, when evidence is available to show that not more than 5 % of the blocks have a strength below a specified crushing strength, the stress may be taken as 0,3 times that strength.
4.5.4 Shear
In one-way or two-way spanning slabs, the design shear stress v should be calculated from the following equation:
v ' V bvd where
V is the design shear force due to design ultimate loads on a width of slab equal to the centre-to-centre distance between ribs;
bv is the average width of rib; and d is the effective depth.
In the determination of bv, the following shear contribution cases should be taken into consideration:
a) shear contribution by hollow blocks: bv may be increased by the wall thickness of the block, on one side of the rib;
b) shear contribution by solid blocks: when blocks comply with 4.5.1.4, bv may be increased by one-half of the rib depth, on each side of the rib;
c) shear contribution by joints between narrow precast units: bv may be increased by the width of the mortar or concrete joint.
When v is less than vc, where vc is obtained from 4.3.4.1, no shear reinforcement need be provided.
Where v exceeds vc, reinforcement should be provided in accordance with 4.3.4; ensure that v will not exceed the lesser of 0,75 fcu or 4,75 MPa (whatever the reinforcement provided).
Where a critical perimeter (see 4.4.5.2) cuts any ribs, they should each be designed to resist an equal proportion of the applied effective design force. Shear links in the ribs should continue for a distance of at least d into the solid area.
4.5.5 Deflection
The provisions given in 4.4.6 in respect of solid slabs may be applied to the ribs of ribbed slabs. The span/effective depth ratios given in 4.3.6.5 for a flanged beam are applicable, but when the final reduction factor for web width is calculated, the rib width for hollow block slabs may be assumed to include the walls of the blocks on both sides of the rib. For slabs with voids and slabs constructed of box-section or I-section units, calculate an effective rib width by assuming that all material below the upper flange of the unit is concentrated in a rectangular rib having the same cross-sectional area and depth.
4.5.6 Arrangement of reinforcement
4.5.6.1 The provisions given in 4.11.8.2 in respect of maximum distance between bars apply to areas of solid concrete in this form of construction.
4.5.6.2 The curtailment and anchorage of the reinforcement should be as given below.
4.5.6.2.1 Whether the slab has been designed as simply supported or continuous, at least 50 % of the main tension bottom reinforcement should be carried through to the bearing and anchored in accordance with 4.11.7. The tension reinforcement being curtailed in the span will depend on how the moments have been determined, i.e. by analysis or by complying with simplified rules.
If a slab has been designed as simply supported but is continuous over supports, the reinforcement provided in the top of the slab should be at least one-quarter of that required in the middle of adjoining spans. This reinforcement shall extend by at least one-tenth of the clear span into adjoining spans.
4.5.6.2.2 A single layer of mesh should be provided in the topping of all ribbed and hollow block slabs.
The mesh should have a cross-sectional area in each direction of at least 0,12 % of the topping. The spacing of wires should not exceed half the centre-to-centre distance between ribs.
4.5.7 Cover to reinforcement
The side cover to reinforcement in slabs that have permanent blocks shall be at least 10 mm . Similarly, for slabs that have slip tiles under the ribs at least 10 mm thick, the cover to the bars shall be at least 10 mm above the tiles. In all other cases, provide cover in accordance with 4.11.2.