©Teaching Resource in Design of Steel StructuresIIT Madras, SERC Madras, Anna Univ., INSDAG
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COMPOSITE FLOORS - II
©Teaching Resource in Design of Steel StructuresIIT Madras, SERC Madras, Anna Univ., INSDAG
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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
2
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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SERVICEABILITY LIMIT STATES - 2
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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SERVICEABILITY LIMIT STATES - 3
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