Composite Slab

5/24/2018 Composite Slab

1/10

Chapter 6: Composite Slabs with Profiled Steel Sheeting

Summary:

Composite floors are frequently used in multi-storey building construction. A composite slab comprises steel decking, reinforcement and cast in situ concrete.

When the concrete has hardened, it behaves as a composite steel-concrete

structural element.

Modern profiled steel may be designed to act as both permanent formwork duringconcreting and tension reinforcement after the concrete has hardened.

Design of composite slabs requires consideration of the performance of the steelsheeting as shuttering during construction and as reinforcement to the hardened

concrete slab.

Loading, analysis for internal forces and moments, and section verification areexplained.

The shear connection between the steel sheeting and concrete is of particularimportance. This is usually determined by tests.

Design methods - the semi-empirical m-k method and the partial interactionmethod are explained.

Objectives:

The student should:

Appreciate the advantages of composite floors Recognise that the design of composite slabs requires consideration of the

construction and in service conditions

Be aware of the analysis methods available for determining internal forces andmoments

Know how to perform design checks at the serviceability and ultimate limit states Understand the basis of the semi-empirical and partial interaction design

approaches.

References:

Eurocode 4: Design of composite steel and concrete structures Part1.1 Generalrules and rules for buildings, EN 1994-1-1:Draft No. 2

Porter, M L and Ekberg, C E: Design recommendations for steel deck floor slabs,Journal of the Structural Division, American Society of Civil Engineers, Vol. 102,

No. ST11, November 1976, pp. 2121- 2136.

Wright, H D, Evans, H R and Harding, P W: The use of profiled steel sheeting infloor construction, Journal Constructional Steel Research, 1987, pp.279-295

Design Manual for Composite Slabs, ECCS Technical Committee 7 WorkingGroup 7.6, Report 87, 1995 Ritchie, J K and Chien, E Y L: Composite floor systems, Constructional Steel

Design An International Guide, Ed. Dowling, P J et al. Elsevier Applied

Science, 1992, pp.471-479


2/10

6 - 1

Contents:

1 General

2 Aspects of use of composite structures

2.1 Architectural

2.2 Economics

2.3 Functionality2.4 Service and building flexibility

2.5 Assembly

2.6 Comparison with other methods

3 Construction methods

3.1 Construction elements

3.2 Slabs

3.2.1 Reinforced concrete slabs

3.2.2 Pre-stressed concrete slabs

3.2.3 Profiled steel sheeting

3.3 Beams

3.4 Columns3.5 Joints


3/10

6 - 2

Chapter 6: Composite Slabs with Profiled Steel Sheeting

Introduction

A composite slab is a slab in which profiled steel sheets are used initially as

permanent shuttering and subsequently combine structurally with the hardenedconcrete and act as tensile reinforcement in the finished floor.

The profiled steel sheet must therefore be capable of transmitting horizontal shear at

the interface between the sheet and the concrete; pure bond between steel sheeting

and concrete is not considered effective for composite action. Composite behaviour

between profiled sheeting and concrete can be ensured by several means (see Fig.1):

a) mechanical interlock provided by deformations in the profile (indentations orembossments);

b) frictional interlock for profiles shapedin a re-entrant form;c) end anchorage provided by welded studs or another type of local connection

between the concrete and the steel sheet, only in combination with (a) or (b);

d) end anchorage by deformation of the ribs at the end of the sheeting, only incombination with (b).

Figure 1 Typical forms of interlock in composite slabs

Design situations

The following situations need to be considered:

a) Profiled steel sheeting as shuttering (Fig. 2): Verification is required for thebehaviour of the profiled steel sheeting while it is acting as formwork for the wet

concrete. Account shall be taken of the effect of props, if any (Fig. 3).

b) Composite slab: Verification is required for the floor slab after compositebehaviour has commenced and any props have been removed.


4/10

6 - 3

Figure 2 Construction stage Figure 3 Use of temporary props

Profiled steel sheeting as shuttering

A particular advantage of using a steel deck is its ability to act as a working platform

during construction (see Fig. 2). In calculations for the steel deck as shuttering, the

designer therefore needs to consider the following loads: the weight of concrete and the steel deck; construction loads including local heaping of concrete during construction; these are

to be obtained by reference to Part 1.6 of Eurocode 1.

storage load, if any; ponding effect (increased depth of concrete due to deflection of the sheeting).Ponding may be ignored if the central deflection of the sheeting under its own weight

plus that of the wet concrete, calculated for serviceability, is less than 1/10 of the slab

depth. If this limit is exceeded, this effect should be allowed for. It may be assumed

that the nominal thickness of the concrete is increased over the whole span by 0,7.

The recommended limit on deflection s of the sheeting under its own weight plus the

weight of wet concrete, excluding the construction load, is L/180 where L is the

effective span between supports (props being supports in this context).

Profiled steel sheeting as shuttering is treated in Part 6 of BS5950 and in Part 1.3 of

Eurocode 3. The design approach of both codes is now outlined.

Figure 4 Plate buckling


5/10

6 - 4

The resistance of profiled steel sheeting as shuttering is influenced by its resistance to

local plate buckling. Fig. 4 shows an element in compression. As buckling occurs the

greater flexibility of the middle region of the plate results in load being shed to

supported edges. The plate may therefore be taken as stressed uniformly over an

effective breadth be, see Fig. 5. The moment resistance is determined based on the

Figure 5 Profiled steel sheeting in positive (sagging) bending

effective section. Ribs are rolled into the parts of the sheeting expected to be in

compression to act as stiffeners. These reduce the slenderness b/t of such elements

and therefore increase their resistance to local buckling.

Typical sheeting will be able to span about 3 m as shuttering, particularly if laid as a

continuous element over two spans (see Fig. 6). With both spans loaded, this results in

the maximum elastic moment being at the internal support. Tests show that collapse

occurs after some redistribution of moment has occurred from the internal support

into the span region. When the limiting moment of resistance is reached in sagging

bending, the rotation at the internal support results in the resistance there being

somewhat below its maximum value. The ultimate load can be calculated by rigid-

plastic structural analysis, provided that this reduction is accounted for in the

calculation.

It is often found that in un-proppped construction, it is the construction condition that

governs design.

Verification of composite slabs for ultimate limit states

Part 5 of BS5950 and Part 1.1 of Eurocode 4 treat design of the slab as a composite

element. The design approaches are similar, except that the Eurocode gives an

additional method for determining resistance to longitudinal shear.

The following possible failure modes should be considered:

Flexure Longitudinal shear


6/10

6 - 5

Figure 6 Behaviour of continuous sheeting

Vertical shear Punching shearIn design checks for the ultimate limit state, it may be assumed that the whole of the

loading acts on the composite slab, provided this assumption is also made in design

for longitudinal shear.

A continuous slab is usually designed as a series of simply supported spans with

nominal reinforcement provided over intermediate supports.

Flexure

Plastic theory is used as the basis for calculating the bending resistance. Fig. 7 shows

the stress blocks to be assumed if the neutral axis is above the sheeting. The design

yield strength of the steel sheeting is denotedfyp,d.

Figure 7 Sagging bending if the neutral axis is above the steel sheeting


7/10

6 - 6

Longitudinal shear

This is the most likely failure mode at the composite stage, due to breakdown of

interaction between the sheeting and the concrete (see Fig. 8).

Figure 8 Failure by loss of interaction

In Eurocode 4 the design resistance against longitudinal shear may be determined by

the m-k method or by a partial connection method. The latter should only be used

though for composite slabs with a ductile longitudinal shear behaviour.

The longitudinal shear behaviour may be considered as ductile if the failure load

exceeds the load causing a recorded end slip of 0,1 mm by more than 10%.

The m-k method

It should be shown that the maximum design vertical shear VEdfor a width of slab b

does not exceed the design shear resistance Vl,Rddetermined from the following

expression:

+= k

Lb

AmdbV

s

p

Vs

p

Rdl,

(1)

where:

b, dp are in mm;Ap is the nominal cross-section of the sheeting in mm

2;

m, k are design values for the empirical factors in N/mm2

obtained from slab tests;

Ls is the shear span in mm;

Vs is the partial safety factor for the ultimate limit state.

The recommended value for Vs is 1,25.


8/10

6 - 7

Testing of composite floor slabs

Tests to determine the factors m and k are described in Annex B to Part 1.1 of

Eurocode 4. The recommended test set-up should be as shown in Fig. 9. The variables

to be investigated include the thickness and the type of steel sheeting, the steel grade,

the coating of the steel sheet, the density and grade of concrete, the slab thickness and

the shear span lengthLs.

Figure 9 Test set-up

Determination of design values for m and k

If the behaviour is ductile, the experimental shear force Vt should be taken as 0,5

times the value of the failure load Wt. If the behaviour is brittle this value shall be

reduced, using a factor 0,8.

If two groups of three tests are used and the deviation of any individual test result in a

group from the mean of the group does not exceed 10%, the design relationship may be

determined as follows:

From each group the characteristic value is deemed to be the one obtained by taking

the minimum value of the group reduced by 10%. The design relationship is formed

by the straight line through these characteristic values for groups A and B (see Fig.

10).

Design shear span

For design,Lsshould be taken as:

L/4 for a uniform load applied to the entire span length; the distance between the applied load and the nearest support for two equal and

symmetrically placed loads;

for other loading arrangements, including a combination of distributed andasymmetrical point loads, an assessment should be made based upon test results or

by the following approximate calculation. The shear span should be taken as the

maximum moment divided by the greater support reaction.


9/10

6 - 8

Note: b, dpand Lsare in mm,Apis in mm

2

, Vtis in N.

Figure 10 Evaluation of test results

The partial connection method

The breakdown of interaction before the attainment of the bending resistance (see

above) implies that the slab has only partial shear connection. An alternative to the m

k method is therefore to calculate a reduced design bending resistance MRdbased on a

limiting shear strength u,Rdat the steel-concrete interface. It then has to be shown that

at any cross-section the design bending momentMEddoes not exceedMRd.

The design resistanceMRdcan be determined from stress blocks (see Fig. 7) but with

Ncf replaced by:

Nc = u,RdbLx Ncf (2)

whereLx is the distance of the cross-section being considered to the nearest support.

The limiting shear strength can be determined from the tests described earlier.

Vertical shear and punching shear

These failure modes are treated by reference to Eurocode 2 in a manner similar to

reinforced concrete slabs (see Fig.11).

Deflection

The usual approach is to limit the span to depth ratio to the values limits given in

Eurocode 2 for reinforced concrete slabs. Eurocode 4 does give a calculation method

as an alternative.


10/10

6 - 9

Figure 11 Critical perimeter for punching shear

Documents

Composite Slab