Bs8110
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
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
Links: in ribs Provided the geometry satisfies 3.6.1.3 ribs reinforced with a single bar or ribs in waffle slabs do not require links unless shear or fire resistance requirements so dictate. However consideration should be given to the use of purpose made spacers occupying the full width of the rib to ensure correct cover to the bar. Where two or more bars are used in a rib, the use of link reinforcement in addition to normal spacers is recommended except in waffle slabs, to ensure correct cover to reinforcement. The spacing of the links can generally be of the order of 1 m to 1.5 m depending on the size of the main bars The cover of the link reinforcement should satisfy the durability requirement of Table 3.4 but need not satisfy the requirements for fire resistance in Table 3.5 provided the cover to the main bars does so. 3.7 Flat slabs NOTE See 1.3.2 for definitions specific to flat slabs. 3.7.1 General 3.7.1.1 Symbols For the purposes of 3.7 the following symbols apply. av Asv be Cx C y dh F fyv hc l l1 l2 lc lh lx ly Mt n u uo v vc V Vt Veff x µ distance from the edge of the loaded area to the perimeter considered. area of shear reinforcement. breadth of effective moment transfer strip (see Figure 3.13). plan dimensions of column (see Figure 3.13). depth of the head. total design ultimate load on the full width of panel between adjacent bay centre lines ( = 1.4Gk + 1.6Qk). characteristic strength of shear reinforcement. effective diameter of a column or column head. given in Table 3.12 should be taken as the full panel length in the direction of span. panel length parallel to span, measured from centres of columns. panel width, measured from centres of columns lh. dimensions of the column measured in the same direction as lh. effective dimension of a head. shorter span of flat slab panel. longer span of flat slab panel. design moment transferred between slab and column. design ultimate load per unit area (= 1.4gk + 1.6qk). effective length of the outer perimeter of the zone. effective length of the perimeter which touches a loaded area. design shear stress. design concrete shear stress. design ultimate value of the concentrated load. design shear transferred to column. design effective shear including allowance for moment transfer. dimension of a shear perimeter parallel to axis of bending. angle between the shear reinforcement and the plane of the slab. 48 © BSI 30 November 2005