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Plain & Reinforced Concrete-1 CE3601

Plain & Reinforced Concrete-1 CE3601. Lecture # 25 , 26 & 27 20 th April to 8 th May 2012 Analysis and Design of Slabs. Plain & Reinforced Concrete-1. Isolated One Way Slab. Plain & Reinforced Concrete-1. Continuous One Way Slab. Plain & Reinforced Concrete-1. Negative Moment.

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Plain & Reinforced Concrete-1 CE3601

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  1. Plain & Reinforced Concrete-1CE3601 Lecture # 25 , 26 & 2720th April to 8th May 2012 Analysis and Design of Slabs

  2. Plain & Reinforced Concrete-1 Isolated One Way Slab

  3. Plain & Reinforced Concrete-1 Continuous One Way Slab

  4. Plain & Reinforced Concrete-1 Negative Moment Positive Moment Continuous One Way Slab

  5. Plain & Reinforced Concrete-1 Minor Bending Continuous One Way Slab

  6. Plain & Reinforced Concrete-1 Example: Design a cantilever projecting out from a room slab extending 1.0m and to be used as balcony (LL = 300 kg/m2). A brick wall of 250 mm thickness including plaster of 1m height is provided at the end of cantilever. fc’ = 17.25 MPa fy = 300 MPa Slab thickness of room = 125 mm. Slab bottom steel 1in the direction of cantilever is # 13 @ 190 mm c/c.

  7. Plain & Reinforced Concrete-1 Solution: cantilever 125 mm 1m

  8. Plain & Reinforced Concrete-1 Solution: (contd…) Let we use the same thickness as of the room d Main steel in cantilever is at the top

  9. Plain & Reinforced Concrete-1 Solution: (contd…) Slab Load Self weight of slab 75 mm brick ballast/ screed 60 mm floor finishes Total dead load

  10. Plain & Reinforced Concrete-1 Solution: (contd…) Slab Load Live Load For a unit strip

  11. Plain & Reinforced Concrete-1 Solution: (contd…) 1.063m Per meter width

  12. Plain & Reinforced Concrete-1 Solution: (contd…) d # 13 @ 380 mm c/c already available in the form half the bent up bar from the room slab

  13. Plain & Reinforced Concrete-1 Solution: (contd…) Remaining steel required at the top Use Distribution steel

  14. Plain & Reinforced Concrete-1 #13 @ 380 c/c Solution: Slab bottom steel 1500 mm #10 @ 280 c/c #10 @ 380 c/c

  15. Plain & Reinforced Concrete-1 Two-Way Edge Supported Slabs

  16. Plain & Reinforced Concrete-1 Two-Way Slabs Slab resting on walls or sufficiently deep and rigid beams on all sides. Other options are column supported slab e.g. Flat slab, waffle slab. Two-way slabs have two way bending unlike one-way slab.

  17. Plain & Reinforced Concrete-1 Isolated Two Way Slab

  18. Plain & Reinforced Concrete-1 Positive Moment Negative Moment Continuous Two Way Slab

  19. Plain & Reinforced Concrete-1 Design Methods • ACI co-efficient method • Direct design method • Equivalent frame method • Finite element method Notes • In two-way slabs shorter direction strip carry greater %age of load. • Steel will be more in shorter direction. • Shorter direction steel will be placed near the outer edge to get more “d” means more lever arm to get more flexural capacity. Ly Lx

  20. Plain & Reinforced Concrete-1 ACI Co-efficient Method Unit width strip is taken in both directions. The strip is designed separately for +ve and –ve moment. • C = ACI co-efficientωu = Slab load “C” depends upon the end conditions of slab and the aspect ratio.Three tables are available for “C” • Dead load positive moment • Live load positive moment • -ve moment M+ coefficients are increased by 25 % and M- coefficients are reduced by 10 % to get the result more closer to accurate solution.

  21. Plain & Reinforced Concrete-1 Minimum Depth of 2-Way Slab for Deflection Control According to ACI-318-1963 hmin = (inner perimeter of slab panel)/180≥ 90 mm For fy = 300 MPa For fy = 420 MPa According to ACI-318-2008 Ln = clear span in short direction

  22. Plain & Reinforced Concrete-1 Example: Design the 4 marked slab panels of an ordinary house. Use US customary bars. fc’= 17.25 MPa fy = 300 MPa 1 2 4500 x 7000 6000 x 7000 Wall thickness = 228 mm 3 4 6000 x 6000 3500 x 6000

  23. Plain & Reinforced Concrete-1 Solution: Panel Edge Conditions Panel # 1Lx = 4.5m , Ly = 7.0mm = 0.64 > 0.5, 2-way slab Panel # 2Lx = 6.0m , Ly = 7.0mm = 0.86> 0.5, 2-way slab Panel # 3Lx = 3.5m , Ly = 6.0mm = 0.58 > 0.5, 2-way slab Panel # 4Lx = 6.0m , Ly = 6.0mm = 1 > 0.5, 2-way slab

  24. Plain & Reinforced Concrete-1 Solution: (contd…) Slab ThicknessGenerally same depth is preferred for one monolith slab. Calculate hmin for all the panels and select the largest value. Panel # 1 Panel # 2

  25. Plain & Reinforced Concrete-1 Solution: (contd…) Panel # 3 Panel # 4

  26. Plain & Reinforced Concrete-1 Solution: (contd…) Effective depth d2 d1 Long direction steel Short direction steel For short direction steel For longer direction steel

  27. Plain & Reinforced Concrete-1 Solution: (contd…) Slab Load Self weight of slab 75 mm brick ballast/ screed 60 mm floor finishes Total dead load

  28. Plain & Reinforced Concrete-1 Solution: (contd…) Slab Load Live Load

  29. Plain & Reinforced Concrete-1 Solution: (contd…) Minimum Steel For a unit strip

  30. Concluded

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