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.

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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