GCE Kannur

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
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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
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Support Conditions
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Action of Two-way Slabs
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Moments in two-way slabs
• Simply supported slabs
– Refer Table 27, Annexure D of IS 456-2000
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2
2
x x x
y y x
M w
M w




Cl. D-1.1, Annexure D of IS 456-2000
RankineGrashoff Formula

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RankineGrashoff Formula
• Equate the deflection of middle strips of slab
in two perpendicular directions.
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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
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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
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Conditions to use Table 26
• Loading on adjacent spans should be same
• Spans in each direction should be
approximately equal
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2
2
x x x
y y x
M w
M w




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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
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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
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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
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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
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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
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Corner Steel
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Negative Moments at Discontinuous
Edges
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STRUCTURAL DETAILING
TWO WAY SLABS
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SS SLAB
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RESTRAINED SLAB
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SIMPLY SUPPORTED SLABS
DESIGN EXAMPLES
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SS SLAB
RESTRAINED SLABS
DESIGN EXAMPLES
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17. 5 m
10m
8.75 m × 5 m
Beam
D
B C
A
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