Two way slabs rajesh sir
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Dr. B. Rajeevan gave a lecture on designing two-way slabs. Two-way slabs are supported on all four sides with a ratio of longer to shorter span less than 2. They discussed defining two-way slabs, their support conditions, how loads are shared, and calculating moments using the Rankine-Grashoff formula or tables from the Indian code IS 456. Design includes choosing thickness based on span-to-depth ratio, determining reinforcement area from moments, checking deflection and minimum steel requirements, and providing corner reinforcement for restrained slabs. Examples demonstrated designing simply supported and restrained slabs.
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Two way slabs rajesh sir
- 1. Lecture Notes 15 July 2010 Dr. B. Rajeevan 1 Dr. B. Rajeevan Senior Lecturer Department of Civil Engineering Govt. College of Engineering Kannur E-mail: rajeevan@gcek.ac.in Mob: 9495 333 088 ContactTime: 4 pm – 5 pm DESIGN OF TWO WAY SLABS STRUCTURAL DESIGN 15 July 2010 Dr. B. Rajeevan 2 Definition • Slabs supported on all four sides • Ratio of longer span to shorter span < 2 • Slabs supported on all four sides but ly/lx >2 -- ? – Loads shared by supports along shorter span – Hence one way slab 15 July 2010 Dr. B. Rajeevan 3 Support Conditions 15 July 2010 Dr. B. Rajeevan 4 Action of Two-way Slabs 15 July 2010 Dr. B. Rajeevan 5 Moments in two-way slabs • Simply supported slabs – Refer Table 27, Annexure D of IS 456-2000 15 July 2010 Dr. B. Rajeevan 6 2 2 x x x y y x M w M w Cl. D-1.1, Annexure D of IS 456-2000 RankineGrashoff Formula
- 2. Lecture Notes 15 July 2010 Dr. B. Rajeevan 2 RankineGrashoff Formula • Equate the deflection of middle strips of slab in two perpendicular directions. 15 July 2010 Dr. B. Rajeevan 7 15 July 2010 Dr. B. Rajeevan 8 2 x xq L x y 2 y xq L Table 27, Annexure D of IS 456-2000 Notes • Use Table 27 to design 2-way slabs, for which provisions were not made to prevent lifting up of the corners. • Otherwise, refer Table 26 15 July 2010 Dr. B. Rajeevan 9 Restrained Slab • Slabs which are cast integral with RCC frames and which are not free to lift up at the corners. – Can be continuous or discontinuous at the edges – Refer Table 26 to design such 2-way slabs 15 July 2010 Dr. B. Rajeevan 10 Conditions to use Table 26 • Loading on adjacent spans should be same • Spans in each direction should be approximately equal 15 July 2010 Dr. B. Rajeevan 11 2 2 x x x y y x M w M w 15 July 2010 Dr. B. Rajeevan 12
- 3. Lecture Notes 15 July 2010 Dr. B. Rajeevan 3 Comments • Yield line analysis made by Taylor to derive the coefficients. – Moment redistribution not possible • Equivalent Frame Analysis – recommended by ACI (American Concrete Institute) – Moment redistribution possible 15 July 2010 Dr. B. Rajeevan 13 Design of 2-way SS slab – Table 27 • Assume slab thickness based on l/d ratio – Minimum 90 mm to 100 mm – Simply supported -28; Continuous - 32 • Calculate design load, wu = 1.5 (DL+LL) • Determine ly/lx and refer Table 27 – Calculate Mx and My • Determine area of steel. • Compute spacing of bars 15 July 2010 Dr. B. Rajeevan 14 Simply supported SLAB Design of SS slab – Table 27 • Spacing – 3d or 300 mm – Limit to 200 mm for slab thickness < 300 mm • Check for deflection – Use Ast1 and lx • Check for minimum steel – 0.12bD or 0.15bD • Structural Detail 15 July 2010 Dr. B. Rajeevan 15 Design of Restrained slab – Table 26 • Assume slab thickness based on l/d ratio – Minimum 90 mm to 100 mm – Simply supported -28; Continuous - 32 • Calculate design load, wu = 1.5 (DL+LL) • Identify the case from Table 26 (Case 1 to 9) • Determine ly/lx and refer Table 26 – Calculate Mx and My • Determine area of steel. • Compute spacing of bars 15 July 2010 Dr. B. Rajeevan 16 Restrained SLAB Design of Restrained slab – Table 26 • Provide corner steel – Area of corner steel = 0.75 times the area of max positive steel – Provide corner steel in a grid of size 0.2Lx • Spacing – 3d or 300 mm – Limit to 200 mm for slab thickness < 300 mm • Check for deflection – Use Ast1 and lx • Check for minimum steel – 0.12bD or 0.15bD • Structural Detail 15 July 2010 Dr. B. Rajeevan 17 Corner Steel 15 July 2010 Dr. B. Rajeevan 18
- 4. Lecture Notes 15 July 2010 Dr. B. Rajeevan 4 Negative Moments at Discontinuous Edges 15 July 2010 Dr. B. Rajeevan 19 STRUCTURAL DETAILING TWO WAY SLABS 15 July 2010 Dr. B. Rajeevan 20 15 July 2010 Dr. B. Rajeevan 21 SS SLAB 15 July 2010 Dr. B. Rajeevan 22 RESTRAINED SLAB 15 July 2010 Dr. B. Rajeevan 23 15 July 2010 Dr. B. Rajeevan 24
- 5. Lecture Notes 15 July 2010 Dr. B. Rajeevan 5 SIMPLY SUPPORTED SLABS DESIGN EXAMPLES 15 July 2010 Dr. B. Rajeevan 25 15 July 2010 Dr. B. Rajeevan 26 15 July 2010 Dr. B. Rajeevan 27 15 July 2010 Dr. B. Rajeevan 28 15 July 2010 Dr. B. Rajeevan 29 15 July 2010 Dr. B. Rajeevan 30
- 6. Lecture Notes 15 July 2010 Dr. B. Rajeevan 6 15 July 2010 Dr. B. Rajeevan 31 15 July 2010 Dr. B. Rajeevan 32 SS SLAB RESTRAINED SLABS DESIGN EXAMPLES 15 July 2010 Dr. B. Rajeevan 33 15 July 2010 Dr. B. Rajeevan 34 15 July 2010 Dr. B. Rajeevan 35 17. 5 m 10m 8.75 m × 5 m Beam D B C A 15 July 2010 Dr. B. Rajeevan 36
- 7. Lecture Notes 15 July 2010 Dr. B. Rajeevan 7 15 July 2010 Dr. B. Rajeevan 37 15 July 2010 Dr. B. Rajeevan 38 15 July 2010 Dr. B. Rajeevan 39 15 July 2010 Dr. B. Rajeevan 40 15 July 2010 Dr. B. Rajeevan 41 15 July 2010 Dr. B. Rajeevan 42
- 8. Lecture Notes 15 July 2010 Dr. B. Rajeevan 8 15 July 2010 Dr. B. Rajeevan 43