This chapter corresponds to Section 9 of EN 1994-1-1, which has the following clauses:
• General Clause 9.1
• Detailing provisions Clause 9.2
• Actions and action effects Clause 9.3
• Analysis for internal forces and moments Clause 9.4
• Verification of profiled steel sheeting as shuttering for ultimate limit states Clause 9.5
• Verification of profiled steel sheeting as shuttering for serviceability limit
states Clause 9.6
• Verification of composite slabs for ultimate limit states Clause 9.7
• Verification of composite slabs for serviceability limit states Clause 9.8
9.1. General
Scope
Clause 9.1.1
The form of construction and the scope of Section 9 are defined in clause 9.1.1. The shape of the steel profile, with ribs running in one direction, and its action as tensile reinforcement for the finished floor, result in a system that effectively spans in one direction only. The slab can also act as the concrete flange of a composite beam spanning in any direction relative to that of the ribs. Provision is made for this in the clauses on design of beams in Sections 5, 6 and 7.
Clause 9.1.1(2)P
The ratio of the gap between webs to the web spacing, br/bs in clause 9.1.1(2)P, is an
important property of a composite slab. This notation is as in Fig. 9.2 and Fig. A.1 in Appendix A. If the troughs are too narrow, the shear strength of stud connectors placed within them is reduced (clause 6.6.4), and there may be insufficient resistance to vertical shear. If the web spacing is too wide, the ability of the slab to spread loads across several webs may be inadequate, especially if the thickness of the slab above the sheeting is minimized, to save weight.
Such a wide range of profiles is in use that it was necessary to permit the limit to br/bsto be
determined nationally. It should probably be a function of the thickness of the slab above the sheeting. As a guide, it should normally be less than about 0.6.
No account is taken of any contribution from the top flange of the sheeting to resistance to transverse bending.
The design methods for composite slabs given in Section 9 are based on test procedures described in clause B.3. Although the initial loading is cyclic, the test to failure is under static loading. Thus, if dynamic effects are expected, the detailed design for the particular project
Clause 9.1.1(3)P
Clause 9.1.1(4)P must ensure that the integrity of the composite action is maintained (clauses 9.1.1(3)P and9.1.1(4)P).
Clause 9.1.1(5) Guidance on the degree of lateral restraint provided to steel beams (clause 9.1.1(5)) is available in EN 1993-1-1 and elsewhere.106Inverted U-frame action relies also on flexural
restraint. This subject is covered in comments on clause 6.4.2.
Because of the wide range of profiles used, resistance to longitudinal shear has always been based on tests. Slabs made with some profiles have a brittle mode of failure, which is penalized in clause B.3.5(1).
Types of shear connection
Clause 9.1.2.1 As for other types of composite member, bond is not accepted in clause 9.1.2.1 as a reliable method of shear connection. Sheeting without local deformations of profile is permitted where the profile is such that some lateral pressure will arise from shrinkage of the concrete (Fig. 9.1(b)). Here, the distinction between ‘frictional interlock’ and ‘bond’ is, in effect, that the former is what remains after the 5000 cycles of loading specified in clause B.3.4.
The quality of mechanical interlock is sensitive to the height or depth of the small local deformations of the sheeting, so tight tolerances (clause B.3.3(2)) should be maintained on these during manufacture, with occasional checking on site.
Clause 9.1.2.2 connection, as defined in clause 9.1.2.2. They can be augmented by anchorages at the ends ofThese two standard forms of interlock are sometimes insufficient to provide full shear each sheet, as shown in Fig. 9.1, or design can be based on partial shear connection.
9.2. Detailing provisions
Clause 9.2.1.(1)P
Clause 9.2.1(2)P The limits to thickness given in clauses 9.2.1(1)P and 9.2.1(2)P are based on satisfactoryexperience of floors with these dimensions. No limits are given for the depth of the profiled sheeting. Its minimum depth will be governed by deflection. For a slab acting compositely with a beam, the minimum depths are increased (clause 9.2.1(2)P) to suit the detailing rules for stud connectors, such as the length of stud that extends above the sheeting and the concrete cover. A slab used as a diaphragm is treated similarly.
Clause 9.2.1(4)
Where a slab spans onto a hogging moment region of a composite beam, the minimum reinforcement transverse to its span is governed by the rules for the flange of the beam (e.g. Table 7.1), not by the lower amount given in clause 9.2.1(4).
Clause 9.2.3 The minimum bearing lengths (clause 9.2.3) are based on accepted good practice. The lengths for bearing onto steel or concrete are identical to those given in BS 5950: Part 4.107
9.3. Actions and action effects
Profiled sheeting
Clause 9.3.1(2)P Where props are used for profiled sheeting (clause 9.3.1(2)P), care should be taken to set
these at the correct level, taking account of any expected deflection of the surface that supports them. If verification relies on the redistribution of moments in the sheeting due to local buckling or yielding, this must be allowed for in the subsequent check on deflection of the completed floor; but this is, of course, less likely to be critical where propping is used.
Clause 9.3.2(1) For the loading on the profiled sheeting, clause 9.3.2(1) refers to clause 4.11 of EN 1991-1-6.108For working personnel and small site equipment, a note to clause 4.11.1(3)
proposes a characteristic distributed load of 1 kN/m2. Further guidance may be given in the
National Annex.
For the weight density of normal-weight concrete, Annex A of EN 1991-1-19recommends
24 kN/m3, increased by 1 kN/m3for ‘normal’ reinforcement and by another 1 kN/m3 for
unhardened concrete. In addition to self-weight, clause 4.11 of EN 1991-1-6 specifies an imposed load of 10% of the weight of the concrete, but not less than 0.75 kN/m2(which
usually governs), applied to a working area of 3 × 3 m, and 0.75 kN/m2outside this area.
This corresponds to a layer of normal-weight concrete about 35 mm thick, to allow for the mounding that occurs during delivery of fresh concrete. Guidance on the avoidance of overload during construction is available elsewhere.109
Partial factors for ultimate limit states are recommended in Table A1.2(B) of EN 1990, as 1.35 for permanent actions and 1.5 for variable actions. It would be reasonable to use 1.35 for the whole of the weight density of 26 kN/m3, explained above, even though the extra 1 kN/m3
for unhardened concrete is not strictly ‘permanent’.
Sometimes, to increase the speed of construction, the profiled sheeting is not propped. It then carries all these loads. This condition, or the check on the deflection of the finished floor, normally governs its design.
For the serviceability limit state, the deflection of the sheeting when the concrete hardens is important, for use when checking the total deflection of the floor in service. The construction load and the extra loading from mounding are not present at this time, so the deflection is from permanent load only, and the ψ factors for serviceability, given in Table A.1 of EN 1991-1-6, are not required.
Clause 9.3.2(2) Clause 9.3.2(1) refers to ‘ponding’, and clause 9.3.2(2) gives a condition for its effects to be
ignored. Where profiled sheeting is continuous over several supports, this check should be made using the most critical arrangement of imposed load.
Composite slab
Clause 9.3.3(2)
The resistances of composite slabs are determined by plastic theory or by empirical factors based on tests in which all of the loading is resisted by the composite section (clause
B.3.3(6)). This permits design checks for the ultimate limit state to be made under the whole
of the loading (clause 9.3.3(2)).
9.4. Analysis for internal forces and moments
Profiled steel sheeting
Clause 9.4.1(1) Clause 9.4.1(2)
Clause 9.4.1(1) refers to EN 1993-1-3,25 which gives no guidance on global analysis of
continuous members of light-gauge steel. Clause 9.4.1(2) rules out plastic redistribution where propping is used, but not where the sheeting extends over more than one span, as is usual. Subsequent flexure over a permanent support will be in the same direction (hogging) as during construction, whereas at the location of a prop it will be in the opposite direction.
Elastic global analysis can be used, because a safe lower bound to the ultimate resistance is obtained. Elastic moments calculated for uniform stiffness are normally greatest at internal supports, as shown in Fig. 9.1 for a two-span slab under distributed loading. The reduction in stiffness due to parts of the cross-section yielding in compression will be greatest in these regions, which will cause redistribution of moment from the supports to mid-span. In a technical note from 1984,110and in a note to clause 5.2 of BS 5950-4,107the redistribution is
given as between 5 and 15%. This suggests that redistribution exceeding about 10% should not be used in absence of supporting evidence from tests.
0.125wL2
0.070wL2
L L
Fig. 9.1. Bending moments for a two-span beam or slab for uniform loading; elastic theory without
Composite slab
Clause 9.4.2(3)
As the steel sheets are normally continuous over more than one span, and the concrete is cast over this length without joints, the composite slab is in reality continuous. If elastic global analysis is used based on the uncracked stiffness, the resulting moments at internal supports are high, as in the example in Fig. 9.1. To resist these moments may require heavy reinforcement. This can be avoided by designing the slab as a series of simply-supported spans (clause 9.4.2(5)), provided that crack-width control is not a problem. Other approaches that reduce the quantity of hogging reinforcement needed are the use of redistribution of moments (clause 9.4.2(3)), and of plastic analysis (clause 9.4.2(4)).
Clause 9.4.2(4) Numerical and experimental research on continuous slabs has been reported.111 With
typical relative values of moment resistance at internal supports and at mid-span, the maximum design loads calculated by elastic analysis with limited redistribution were found to be less than those obtained by treating each span as simply supported. This arises because the large resistance to sagging moment is not fully utilized.
If the slab is to be treated as continuous, plastic analysis is more advantageous. The studies showed that no check on rotation capacity need be made provided the conditions given in
clause 9.4.2(4) are satisfied.
Effective width for concentrated point and line loads
Clause 9.4.3 The ability of composite slabs to carry masonry walls or other heavy local loads is limited.The rules of clause 9.4.3 for the effective widths bm, bemand bevare important in practice.
They are based on a mixture of simplified analysis, test data and experience,107and are
further discussed, with a worked example, in Johnson.81The effective width depends on the
ratio between the longitudinal and transverse flexural stiffnesses of the slab. The nature of these slabs results in effective widths narrower than those given in BS 811017 for solid
reinforced concrete slabs.
Clause 9.4.3(5) The nominal transverse reinforcement given in clause 9.4.3(5) is not generous for a point load of 7.5 kN, and should not be assumed to apply for the ‘largely repetitive’ loads to which
clause 9.1.1(3)P refers.