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This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) 582 Composite deck slabs case this negative (hogging) moment is equal to that of the positive (sagging) moment of single-span decking. Many profiles with wide troughs are weaker under negative moment than positive, and the effect of the localized reaction at the internal support is to reduce further the negative moment resistance. In design to code requirements, therefore, continuous decking may appear to be weaker than simplysupported decking even though in reality it must be considerably stronger. In general the load capacity of continuous decking is obtained from tests. The reduced construction load on adjacent spans has the effect of reducing the design negative moment so that elastic design more closely approximates the results of tests under the full design load (see following section). 20.5.2 Bending resistance of stiffened profiles The compressive strength of a thin plate is presented in terms of an effective width of plate acting at its full yield strength. This effective width be is based on a semiempirical formula 1 considering the post-buckling strength of the plate, and is given by (20.1) where t is the plate thickness, b is the plate width, both in mm, and py is the design strength of the steel in N/mm2 857t Ê 187t ˆ bc = Á1 - ˜ but �b ÷ py Ë b÷ py¯ . Where a stiffener is introduced, then the dimension b is based on the flat plate width between the stiffener and the web (see Fig. 20.3). However, there is a limiting size of stiffener below which it does not contribute to the increased capacity of the unstiffened plate. In most practical cases the web of the profile is fully effective. The moment resistance of the section is evaluated using this effective width. In the calculations to BS 5950: Part 4 the design strength of the steel is taken as 0.93 times the specified yield strength. Under positive moment the top plate is in com- b be/2 be/2 Fig. 20.3 Behaviour of stiffened plate in compression assumed stress distribution stress distribution
This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) pression, whereas under negative moment the bottom plate is in compression. This suggests that the most efficient profiles are symmetric in shape. For serviceability calculations, the actual stress replaces the design stress in Equation (20.1). For most serviceability calculations, this stress may be taken as 65% of the design stress. The construction condition generally determines the permissible spans of the slabs; this is the case which has the greatest influence on the economy of the method of construction. Design based on elastic moments in continuous decking is very conservative because at failure there is a significant redistribution of moment from the most highly stressed areas at the supports to the mid-span area, initially as a result of elastic effects because the stiffness of the section changes with applied moment. Beyond the point at which yield takes place, plate collapse and some ‘plastic’ deformation occurs with increasing deformation. If the decking were able to deform in an ideal plastic manner, as in Fig. 20.4(a), then the ‘plastic’ failure load of an end-span would be approximately 8 w = ( M + 046 . KM ) L 2 p n where Mp, and Mn are the elastic moment resistances of the profile under positive and negative moment respectively. The factor K (less than unity) is introduced because only a proportion of Mn can be developed at the large support rotations corresponding to failure. The key to this is the post-elastic behaviour of the section illustrated in Fig. 20.4(b). From tests on different profile shapes it can be shown that the value of K has a lower bound over the normal range of sheet thicknesses and spans. Typically, the reduction in Mn (as reflected in the K value) corresponding to a support rotation of 2–3° to develop Mp is 50%.This method of design is covered in more detail in CIRIA Technical Note 116. 6 Design to BS 5950: Part 4: 1994 more closely approximates to this failure load, although it is still conservative by 10 to 15%. 20.5.3 Composite condition Basic design 583 The cross-sectional area of the decking Ap acts as conventional reinforcement at a lever arm determined by the centroid of the profile, as shown in Fig. 20.5.The tensile forces in the decking are developed as a result of the bond between the decking and the concrete, enhanced by some form of mechanical connection, such as by indenting the profile. If the shear-connectors used to develop composite action between the slab and the steel beam are also included in the design, then the full flexural capacity of the slab can usually be mobilized because of the end anchorage provided. If not, failure is usually by slip between the decking and the concrete, known as incomplete shear connection as indicated in Fig. 20.6. This means that the full bending resistance of the slab is not developed. Nevertheless, the degree of composite action is sufficiently
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