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

2. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 2 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/mm 2 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, h c, > 50 mm

3. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 3 DESIGN SITUATIONS Profiled 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

4. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 4

5. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 5

6. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 6

7. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 7

8. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 8 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/m 2 –Over remaining area a load of 0.75 kN/m 2 should be added to weight of wet concrete

9. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 9 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)

10. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 10 Effective span - 1 B – Width of top flanges of supporting steel beams d ap - Depth of sheeting - Actual span of composite floor

11. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 11 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

12. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 12

13. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 13 ANALYSIS FOR INTERNAL FORCES AND MOMENTS Profiled steel sheeting as shuttering Design based on elastic distribution of bending moment is conservative M n is moment capacity of deck support section, then at failure only a portion of M n at support can be realised because of available ductility. Thus, k is determined experimentally for the type of profile used

14. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 14 Profiled steel sheeting as shuttering - 1 Loading Typical cross section Moment – curvature relationship for a metal deck floor Curvature (  ) ee Plastic deformation Moment uu MuMu

15. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 15 Profiled steel sheeting as shuttering - 2 0.071w 2 MpMp kM n Bending moment at failure after redistribution Bending moment at first yield at support section 0.125w 2 Bending moment variation on two span decking

16. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 16 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

17. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 17 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

18. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 18 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,

19. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 19 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

20. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 20 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

21. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 21 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

22. ©Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG 22 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