©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 1 COMPOSITE FLOORS - II

©Teaching Resource in Design of Steel StructuresIIT Madras, SERC Madras, Anna Univ., INSDAG

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COMPOSITE FLOORS - II


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INTRODUCTION• 3 to 4 m spans not require propping and spans in excess

of 4 m requires propping

• Range of yield strength of decking steel - 220 to 460 N/mm2

• Light - weight concrete is preferable – Reduces effect of ponding deflection – Increases fire resistance

• Profiled deck depth ranges from 40 to 85 mm and metal thickness 0.6 mm to 2.5 mm

• Overall depth of composite slab > 90 mm • Thickness of concrete, hc, > 50 mm


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DESIGN SITUATIONSProfiled steel sheeting as shuttering

– Ponding effect has to be considered

– Account should be taken of effect of props, if any

– If central deflection () of profiled deck is less than /325 or 20 mm, whichever is smaller, then ponding effect may be ignored


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Loads on profiled sheeting

– Construction loads - weight of operatives, concreting plant and any impact or vibration during construction

– In any area of 3 m by 3 m (or the span length, if less), in addition to weight of wet concrete, construction loads and weight of surplus concrete should be provided for by assuming a load of 1.5 kN/m2

– Over remaining area a load of 0.75 kN/m2 should be added to weight of wet concrete


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Effective span

• Continuous slab - designed as a series of simply - supported spans. Effective span can be taken as lesser of following:

– Distance between centres of supports – Clear span plus the effective depth of the slab

• If, profiled deck sheet is propped during construction then, effective span is calculated using formula (Width of prop is neglected)


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Effective span - 1

B – Width of top flanges of supporting steel beams

dap- Depth of sheeting

- Actual span of composite floor

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apdBe


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Profiled sheeting - Composite slab stage

• Total loading acting is considered in design

• Load factors of 1.35 for dead load and 1.5 for imposed load are employed

• Load combinations in buildings

– Alternate spans carrying total factored loading due to imposed and dead loads. Other spans carrying only factored loading due to dead load

– Any two adjacent spans carrying total factored load due to imposed and dead load and all other spans carrying only factored dead load


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ANALYSIS FOR INTERNAL FORCES AND MOMENTS

Profiled steel sheeting as shuttering • Design based on elastic distribution of bending

moment is conservative

• Mn is moment capacity of deck support section, then at failure only a portion of Mn at support can be realised because of available ductility. Thus,

• k is determined experimentally for the type of profile used

nkM46.0pM28

w


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Profiled steel sheeting as shuttering - 1

Loading

Typical cross section

Moment – curvature relationship for a metal deck floor

Curvature ( )

e

Plastic deformation

Moment

u

Mu


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Profiled steel sheeting as shuttering - 2

0.071w2 Mp

kMn

Bending moment at failure after redistribution

Bending moment at first yield at support section0.125w2

Bending moment variation on two span decking


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Profiled steel sheeting as composite slab

Following methods of analysis may be used:

• Linear analysis with or without redistribution

• Rigid-plastic global analysis

• Elastic-plastic analysis

• Linear methods of analysis - Suitable for serviceability limit states as well as for ultimate limit states

• Plastic methods - Used in ultimate Limit State


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SERVICEABILITY LIMIT STATES

Cracking of concrete • Profiled deck sheeting protects lower surface of slab

• Cracking will occur in top surface where slab is continuous over a supporting beam in hogging moment regions

• Crack width will be wider over supports if each span of slab is designed as simply supported, rather than continuous, and if spans are propped during construction

• To counter cracking, longitudinal reinforcement should be provided above internal supports. Minimum recommended amounts are as 0.2% of the area of concrete above the sheeting, for unpropped construction, and 0.4% if propping is used


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SERVICEABILITY LIMIT STATES - 1

Cracking of concrete • If the environment is corrosive, the slabs should be

designed as continuous, with cracking controlled by providing additional reinforcement and ensuring that concrete cover for reinforcement is suitably enhanced

Deflection • Deflection of profiled sheeting due to its own weight and

wet concrete slab should not exceed e /180 or 20 mm,


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Deflection • For the composite slab stage, maximum deflection below

the level of the supports should not exceed span/250, and the increase of deflection after construction (due to creep and to variable load) should not exceed span/300, or span/350 if the floor supports brittle finishes or partitions

• Deflections may not be excessive when span-to-depth ratios are kept within certain limits:

– 25 for simply supported slabs– 32 for spans with one end continuous– 35 for internal spans


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Fire resistance • Adequacy is checked by ensuring that deflection

does not exceed span/20 under fire tests

• Load carrying capacity is checked by considering only embedded steel

• In buildings in-plane resistance and negative reinforcements at points of continuity add to strength under fire condition


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STEPS IN THE DESIGN OF PROFILED DECKING

• List the decking sheet data (from manufacturer’s data)

• List the loading

• Design the profiled sheeting as shuttering

– Calculate the effective length of the span

– Compute factored moments and vertical shear

– Check adequacy for moment

– Check adequacy for vertical shear

– Check deflections


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STEPS IN THE DESIGN OF PROFILED DECKING - 1

• Design the composite slab

– Calculate the effective length of the span

– Compute factored moments and vertical shear

– Check adequacy for moment

– Check adequacy for vertical shear

– Check adequacy for longitudinal shear

• Check for serviceability, i.e. cracking above supports and deflections

Documents

©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 1 COMPOSITE FLOORS - II