10.7 - Composite Slabs

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Transcript of 10.7 - Composite Slabs

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Previous | Next | Contents ESDEP WG 10 COMPOSITE CONSTRUCTION

Lecture 10.7: Composite SlabsOBJECTIVE/SCOPE To describe the design of one-way spanning composite slabs, formed using profiled steel sheeting and a concrete topping, including consideration of ultimate and serviceability limit state design according to Eurocode 4: Part 1 [1] for building structures. PREREQUISITES Lecture 9.1: Thin-walled Members and Sheeting Lecture 10.1: Composite Construction - General Lecture 10.6.1: Shear Connection I RELATED LECTURES All other lectures in Group 10. RELATED WORKED EXAMPLES Worked Example 10.4: Design of a Composite Slab SUMMARY Descriptions of composite slabs, typical profiled sheeting and means of ensuring composite behaviour are given. Design criteria are identified in terms of loads, design resistance and serviceability limits. Analysis of continuous slabs is based on elastic or plastic theories. The resistances of critical cross-sections are calculated considering all possible modes of failure. The design for the ultimate limit state design consists of checking that slab resistance is sufficient to withstand maximum predicted forces; design for the serviceability limit state is performed to limit concrete cracking and slab deflections, taking into account creep and shrinkage of the concrete. The above methods are illustrated by Worked Example 10.4.

1. INTRODUCTION1.1 DefinitionA composite slab consists of a cold-formed profiled steel sheet covered with a concrete slab containing reinforcement (Figure 1). Such slabs are generally used in frame structures, with steel floor beams, as discussed previously in Lecture 10.1. They can also be used in combination with other materials.

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In this type of construction the profiled sheet has several functions: it provides a working platform for construction. it acts as formwork for the concrete slab. it constitutes bottom reinforcement for the slab. The present lecture is mainly concerned with composite slabs when the steel-concrete bond has been formed, i.e. after hardening of the concrete. Design for the construction stage, when the profiled steel sheet supports the weight of wet concrete, is only considered briefly.

1.2 Types of Profiled SheetThere are many types of profiled sheet used for the construction of composite slabs (Figure 2). These types vary in form, rib depth, rib spacing, sheet size, style of lateral over-lapping; in the methods of stiffening the flat elements of the profile; and in the methods of mechanical connection which ensure bond between the steel sheet and concrete slab.

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The thickness of the sheets can vary from 0,75 mm to 1,5 mm but in normal practice it lies between 0,75 and 1,0 mm. The height of the profiled sheets can vary from 38 mm to 80 mm. Whatever the particular requirements for a steel framed building, it is probable that they can be met by using a profiled sheet from this range, as the typical criteria for sound insulation, fire protection, maximum span and maximum load can easily be met.

1.3 Steel-Concrete ConnectionThe bond between the concrete slab and the profiled sheet must be capable of transmitting longitudinal shear at the steel-concrete interface. This connection can be made in one or more of the following ways, as shown in Figure 3 (which has been taken from Fig. 7.1 of Eurocode 4 [1]): by the re-entrant shape of the ribs creating bond by friction (Figure 3a,b). by embossments on the flanges or ribs of the sheet (Figure 3c). by anchorages situated at the ends of the slab, consisting of stud connectors welded through the sheet (Figure 3d), shot-fired shear connectors (Figure 3e), or by deformation of the ribs (Figure 3f).

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2. DESIGN PRINCIPLES2.1 Design SituationsWhen designing composite slabs two distinct structural states must be checked: firstly, the temporary state of execution, when only the sheeting resists the applied loads; secondly, the permanent state, after the concrete is bonded to the steel giving composite action. Relevant limit states and load cases are considered for both design situations. a) Profiled sheeting as shuttering Verifications at the ultimate limit state and the serviceability limit state are required, with respect to the safety and serviceability of the profiled sheeting acting as formwork for the wet concrete. The effects of any temporary props used during execution, must be taken into account in this design situation.www.fgg.uni-lj.si/kmk/esdep/master/wg10/l0700.htm#SEC_1 5/21

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b) Composite slabs Verifications at the ultimate limit state and the serviceability limit state are required, with respect to the safety and the serviceability of the composite slab after composite behaviour has commenced and any props have been removed.

2.2 ActionsThe loads and other actions to be considered, for the ultimate and serviceability limit state, are given in the relevant Eurocodes. For the situation where the profiled sheeting acts as formwork, the following loads should be considered in the calculations, taking into account any propping effects: self-weight of the profiled sheeting. weight of the wet concrete. execution loads. temporary storage load, if applicable. The execution loads represent the weight of the operatives, any loads due to placing the concrete, and also take into account any impact or vibration likely to occur during execution. In accordance with Eurocode 4 [1], a representative value of execution loads (including any excess of concrete) can be taken to be 1,5kN/m2, distributed on an area 3m x 3m (or the span of the sheeting, if less) and 0,75kN/m2 on the remaining formwork surface. For the situation where the steel and the concrete act compositely, the loads acting on the slab should comply with Eurocode 1 [2]. self-weight of the slab (profiled sheeting and concrete) weight of floor finishes imposed loads For the serviceability limit state, long duration values of the loads are required for the calculation of deformations taking into account creep and shrinkage of the concrete.

2.3 Material PropertiesProfiled sheeting Steel used for the fabrication of profiled sheeting has a minimum nominal yield strength of 220N/mm2. In general, however, composite slabs are fabricated from profiled steel sheeting manufactured from galvanised steel of grades 280 to 350, according to European Standard pr EN 10147 [3]. The respective nominal values of yield strength for these steels are: Steel grade 280 : fyb = 280 N/mm2 Steel grade 350 : fyb = 350 N/mm2 The characteristic yield strength fyap , is equal to the nominal yield strength of the basic material fyb quoted above for calculating ultimate resistance. Concrete Concrete used for composite slabs can be made with normal or lightweight aggregate. The most commonly used grades of concrete (grading according to Eurocode 2 [4]) are given in Table 1, which also gives the following properties: characteristic cylinder 28 days compressive strength, fck; mean tensile strength, fctm, which is associated with the shear strength tRd ; and the secant modulus of elasticity, Ecm. Reinforcement All reinforcing steels used in composite slabs should conform to the requirements of Eurocode 2 [4]. The types concerned are essentially ribbed bars and ribbed wires, including welded mesh, fabricated from steels of classes of ductility A or B. Class A is recommended for negative moment reinforcement and fire resistance reinforcement. Characteristic values for the most commonly used reinforcing steels are given in Table 2.

2.4 Deflection Limitsa) Deflection during execution (construction stage) During execution, deflection of the profiled sheeting under loads due to self-weight and wet concrete, must not exceed a limiting value.www.fgg.uni-lj.si/kmk/esdep/master/wg10/l0700.htm#SEC_1 6/21

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For example, Eurocode 4 [1], sets this limit at l/180 or 20 mm, where l is the span of the sheeting between supports. In the case of propped profiled sheeting, props are considered as supports. In situations where greater deflection can be tolerated, calculation for the ultimate limit state should take into account the weight of additional concrete due to the deflection (the "ponding" effect). b) Deflection in the composite state (permanent state) Deflections in the composite state must be limited, in order that the slab may fulfil its intended function and that any other elements in contact with it (false ceilings, pipework, screens, partitions) will not be damaged. Deflection limits should, therefore, be considered relative to the use of the slab, the execution procedure and architectural aspects (aesthetics). The values recommended by Eurocode 3 [5], for floors and roofs in buildings, are as follows: d max l/250 (l is the span of the composite slab) d 2 l/300 where d max is the total deflection of the floor or roof, including any pre-camber and any variation of the deflection due to the permanent loads immediately after loading, and including d 2. d 2 is the variation of the deflection due to variable loading acting on the slab plus any time-dependent deformations due to the permanent loads. If the composite slab supports brittle elements (cement floor finishes, non-flexible partitions, etc), d 2 must be limited to l/350.

2.5 Verification