Bs8110

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  • Topic: Tensile stress, Shear stress, Shear strength
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Section 3

BS 8110-1:1997

3.5.5.2 Shear stresses The design shear stress v at any cross-section should be calculated from equation 21: V v = -----bd equation 21

In no case should v exceed 0.8Æfcu or 5 N/mm2, whichever is the lesser, whatever shear reinforcement is provided. 3.5.5.3 Shear reinforcement Recommendations for shear reinforcement in solid slabs are given in Table 3.16. 3.5.6 Shear in solid slabs under concentrated loads The provisions of 3.7.7 may be applied. 3.5.7 Deflection Deflections may be calculated and compared with the serviceability requirements given in Section 3 of BS 8110-2:1985 but, in all normal cases, it will be sufficient to restrict the span/effective depth ratio. The appropriate ratio may be obtained from Table 3.9 and modified by Table 3.10. Only the conditions at the centre of the span in the width of slab under consideration should be considered to influence deflection. The ratio for a two-way spanning slab should be based on the shorter span. 3.5.8 Crack control In general the reinforcement spacing rules given in 3.12.11 will be the best means of controlling flexural cracking in slabs, but, in certain cases, advantage may be gained by calculating crack widths (see Section 3 of BS 8110-2:1985).

3.6 Ribbed slabs (with solid or hollow blocks or voids)
3.6.1 General 3.6.1.1 Introduction The term “ribbed slab” in this sub-clause refers to in-situ slabs constructed in one of the following ways. a) Where topping is considered to contribute to structural strength (see Table 3.17 for minimum thickness): 1) as a series of concrete ribs cast in-situ between blocks which 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 which 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 which remain part of the completed structure; the tops of the ribs may be connected by a topping of concrete (not necessarily of the same strength as that used in the ribs). 3.6.1.2 Hollow or solid blocks and formers Hollow or solid blocks and formers may be of any suitable material but, when required to contribute to the structural strength of a slab, they should: a) be made of concrete or burnt clay; b) have a characteristic strength of at least 14 N/mm2, measured on the net section, when axially loaded in the direction of compressive stress in the slab; c) when made of fired brickearth, clay or shale, conform to BS 3921, BS EN 772-1, BS EN 772-3 and BS EN 772-7.

© BSI 30 November 2005

45

BS 8110-1:1997

Section 3

3.6.1.3 Spacing and size of ribs In-situ ribs should be spaced at centres not exceeding 1.5 m and their depth, excluding any topping, should not exceed four times their width. The minimum width of rib will be determined by considerations of cover, bar spacing and fire. 3.6.1.4 Non-structural side support Where the side of a slab is built into a wall or rests on a beam parallel to the ribs, that side should be strengthened by the formation of a rib of width equal to that of the bearing. 3.6.1.5 Thickness of topping used to contribute to structural strength The thickness after any necessary allowance has been made for wear, should be not less than those of Table 3.17. 3.6.1.6 Hollow block slabs where topping is not used to contribute to structural strength When a slab is constructed to b) of Table 3.17 the blocks should conform to 3.6.1.2. In addition the thickness of the block material above its void should be not less than 20 mm nor less than one-tenth of the dimension of the void measured transversely to the ribs. The overall thickness of the block and topping (if any) should be not less than one-fifth of the distance...
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