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Shallow Composite Floor DecksDesign information

Composite floor decking design is generallydictated by the construction stage condition,the load and span required for service, and thefire resistance required for the slab. Deck designis also influenced by the composite beamdesign.

Design Parameters

Fire rating dictates minimum slab depth. Concrete type also dictates minimum slab depth and

influences unpropped deck span. Deck span (unpropped) usually dictates general beam

spacing. Slab span (propped deck) dictates maximum beam spacing.

Two Stage Design

All Composite Floors must be considered in two stages. Wet Concrete and construction load

carried by deck alone. Cured concrete

carried by composite slab.

General design aimsDesigners generally aim to reduce temporary propping, so thespan and slab depth required governs the deck selection. Firerequirements usually dictate slab depth. For most applications,the imposed load on the slab will not limit the design.

Anti-crack meshFibreDeck can be used to replace anti crack mesh. Where meshis used, BS 5950: Part 4 recommends that it comprises 0.1% ofslab area. Eurocode 4 recommends that anti-crack mesh shouldbe made up from 0.2% of slab area for unpropped spans and0.4% of slab area for propped spans. The mesh shown in thequick reference tables complies with EC4 and the designprogram defaults to these values. You can still use the reducedBS mesh values by overriding this default in the design program.In slabs subject to line loads, the mesh should comprise 0.4% ofthe cross-sectional area of the concrete topping, propped andunpropped. These limits ensure adequate crack control invisually exposed applications (0.5 mm maximum crack width).The mesh reinforcement should be positioned at a maximum of30 mm from the top surface. Elsewhere, 0.1% reinforcementmay be used to distribute local loads on the slab (or 0.2% toEC4). Mesh laps are to be 300mm for A142 mesh and 400mmfor A193, A252 & A393.

Using forklift trucksIf you expect to use a forklift truck - or a similar concentratedloading - then you should use 0.5% minimum percentagereinforcement over the supports and 0.2% elsewhere to controlcracking. See SCI AD150 for further information.

Working on exposed floorsComposite floors are usually covered by finishes, flooring or acomputer floor, and because cracking is not visible, light topreinforcement is adequate - typically 0.1% of the gross crosssectional area. However where the composite slab is leftuncovered - such as for power- trowelled floor finishes -

cracking, particularly over the beams, may not be adequatelycontrolled by the light mesh provided. Although the crackingdoes not have any structural significance, you might see itsappearance - and the possibility of the crack edge breakdownunder traffic - as problems. To address this, refer to ConcreteSociety publication, 'Cracking In Composite Concrete/CorrugatedMetal Decking Floors Slabs' which provides valid mesh sizing anddetailing for specific crack width control.

If you are going to use forklifts you should also refer to SteelConstruction Institute advisory note 'AD 150, Composite Floors- Wheel Loads From Forklifts'.

Reduced mesh

If you are going to use EC4 mesh rules - as recommended bySteel Construction Institute and CMF - the full stipulated meshapplies to the slab 1.2m either side of every support. Elsewhere such as in the midspan area you may half the mesh area (to0.2% for propped and 0.1% for unpropped construction), aslong as there are no concentrated loads, openings or similar tobe considered. You must also check the reduced midspan meshfor adequacy under fire, for the rating required.

Bar reinforcementThe axis distance of bar reinforcement defines the distance fromthe bottom of the ribs to the centre of the bar, which has aminimum value of 25 mm, and a maximum value of the profileheight. Where used, bar reinforcement is placed at one bar perprofile trough.

Transverse reinforcementMetFloor composite floor decks contribute to transversereinforcement of the composite beam, as long as the decking iseither continuous across the top flange of the steel beam or alternatively - it is welded to the steel beam by stud shearconnectors. To find out more refer to BS5950:Part 3: Section3.1.Clause 5.6.4.

Choosing the concreteLightweight concrete (LWC) uses artificially produced aggregatesuch as expanded pulverised fuel ash pellets. This gives LWCconsiderable advantages in improved fire performance, reducedslab depth, longer unpropped spans and reduced dead load.

However, at the present time, LWC is not readily available insome parts of the country. Normal weight concrete uses anatural aggregate and is widely available. The strength of theconcrete must meet the requirements for strength of thecomposite slab and should not be less than 25N/mm2 for LWCor 30N/mm2 for NWC. Similarly, the maximum value of concretestrength should not be taken as greater than 40 for LWC or 50for NWC.

The modular ratio defines the ratio of the elastic modulus of

steel to concrete, as modified for creep in the concrete. In design to BS5950 and BS8110, the cube strength is used (in N/mm2). In design to EC3, the cylinder strength is used (in N/mm2). The concrete grade (C30/37) defines the (cylinder/cube strength) to EC3.

Concrete density

Without the precise information you should assume that wetdensity is used in the design of the profiled steel sheets and thatdry density in the design of the composite slab.

Fire Design

Fire insulationYou must take the fire insulation requirements of BS 5950: Part8 into account in the tables and design software.

Span/depth ratioSlab span to depth ratio is limited to a maximum of 30 forlightweight concrete and 35 for normal weight concrete.

Shear connectors in fire situationIf shear connectors are provided you can ignore any catenaryforces transferred from the slab to the support beams within thefire resistance periods quoted.

Fire Design Methods

There are two requirements here: Bending resistance in fire conditions Minimum slab depth for insulation purposes

You can calculate the capacity of the composite slab in fireusing the simple method or the fire engineering method:

Simple methodThe simple method is most economic and can be used forsimply supported decks or for decks continuous over one ormore internal supports. The capacity assessment in fire isbased on a single or double layer of standard mesh. Any barreinforcement is ignored.Fire engineering methodThe fire engineering method is for general application andshould be used for design to Eurocodes. The capacityassessment in fire is based on a single or double layer ofstandard mesh at the top and one bar in each concrete rib.For the shallow decks, the program assumes the bar ispositioned just below the top of the steel deck. For MetFloor

60 with a raised dovetail in the crest the bar will be placedbelow the dovetail.

The quick reference tables for shallow composite floorsgenerally use the simplified fire design method, which utilisesthe anticrack mesh as fire reinforcement. You can increase loadspan capability under fire by including bar reinforcement andusing the fire engineering design method.

Deflection limitsYou would normally agree deflection limits with the client. Inthe absence of precise information adopt the following limits:

Construction stageLe/130 (but not greater than 30mm)

Imposed load deflectionLe/350 (but not greater than 20mm)

Total load deflectionLe/250 (but not greater than 30mm)

According to BS5950 Part 4, ponding, resulting from thedeflection of the decking is only taken into account if theconstruction stage deflection exceeds Ds/10. Le is the effectivespan of the deck and Ds is the slab overall depth (excludingnon-structural screeds).

When the ponding of the concrete slab is not taken intoaccount, the deflection under construction load should notexceed the span/180 or 20mm overall whichever is the lesser.

Where ponding is taken into account the deflection should notexceed the span/130 or 30mm overall. The quick referencetables do take ponding into account, if deflection exceedsDs/10, or Le/180, and thus use span/130 or 30mm as adeflection limit. We recommend that the prop width should notbe less than 100mm otherwise the deck may mark slightly atprop lines.

VibrationCheck the dynamic sensitivity of the composite slab by referringto the Steel Construction Institute publication P076: Designguide on the vibration of floors. Calculate the natural frequencyusing the self-weight of the slab, ceiling and services, screedand 10% imposed loads, representing the permanent loads andthe floor.

Where there is no specific information you should ensure thatthe natural frequency of the composite slab is not greater than5Hz for normal office, industrial or domestic usage. Forapplications such as dance floors or those which supportsensitive machinery you may need to set the limit higher.

For design to the Eurocodes, the loads considered for thevibration check are increased using the psi-factor for imposedloads (typically 0.5). You can reduce the natural frequency limitto 4Hz, because of the higher load used in the calculation. Todetermine the vibration response of sensitive floors with greateraccuracy look at the calculation methods in the SCI / CMFpublication P354 Design of Floors for Vibration: A NewApproach. These figures enable designers to compare theresponse with the acceptance levels in BS 6472 and ISO 10137for building designs and in the NHS performance standard forhospitals, HTM 2045.

Shallow Composite Floor DecksDesign information

1.2m 1.2m

SupportBeam

SupportBeam

SupportBeam

1.2m 1.2m

Diagram showing ful