7 Flat Slabs

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Advanced Structural Concrete Design

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Flat Slab System • Column Heads • Division of Panels • Deflection • Crack Control • Shear • Arrangement of Reinforcement

CE 551 5510 0 - 20 2004 04

Flat Slab System • Column Heads • Division of Panels • Deflection • Crack Control • Shear • Arrangement of Reinforcement

CE 551 5510 0 - 20 2004 04

Class Objectives •Identify the types of slab construction, summarizing the advantages and disadvantages of each •Explain the methods of analysis of slabs •Determine the design moments of one-way and two way slabs •Design of flat slabs covering: division into strips, edge and corner columns, punching shear and provision of shear reinforcement

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Recommended Reading • James G. MacGregor, “REINFORCED CONCRETE:

Mechanics and Design”, 3rd Ed., Prentice-Hall, 1997, Ch. 13 and 14.

• Allen, A. H., “REINFORCED CONCRETE DESIGN TO BS8110 SIMPLY EXPLAINED”, E&FN Spon, London 1988, Ch. 14

•The next few slides are from the Reinforced Concrete Council

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Types of slabs

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Flat Slab

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Analysis of Slabs

course

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One-way spanning slab

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Simplified

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Two way action – A rectangular panel supported on all four side •

Curvature more severe along l x 

•

Moment is short span higher

•

Evaluation of moment is complex as behaviour is highly indeterminate

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Consider two strips AB and ED at mid-span. Deflection at central point C is the same. Deflection of udl beam =

5wl 4 384 EI 

∆=

kwl

4

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∆AB

= k nAB l y 4 

∆DE

= k nDE l x 4 

nAB and nDE are portions of total load intensity transferred to AB and DE respectively. Since ∆AB = ∆DE ,

n AB

=

4  x

wl 4  y

l

+l

4 x

n DE  =

4  y

wl 4  y

l

+l

4 x

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nDE > nAB Shorter span strip, DE, supports heavier portion of load and is subject to larger moment. Assumption – supports are unyielding If the supports of the single panel is flexible, e.g. beams, columns, etc. the distribution of moment is more complex. Degree of stiffness of yielding support determines the intensity of the steepness of the curvature contours in the lx and ly direction and redistribution of moments.

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Two-way spanning slab

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Mid Span Moments - corner supported

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Approximate moment in one direction in slabs supported on columns at corners

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Midspan moment - edge supported

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Approximate distribution of  moment in one direction in slabs with symmetrical supports on four sides

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Moment

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Restrained Corners

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Table of Values

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Table of Coefficients

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Adjusted

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Flat Slabs

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Flat Slabs - Analysis

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Flat Slab - Moments

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Widths

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Distribution

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Flat Slab – Moment Transfer

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U-Bars

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Flat Slab – Moment Transfer

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What does it depend on?

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Edge Beams

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Flat Slab - Shear

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Effective Shear Force

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Deflection

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Shear Reinforcement

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Due to the limited depth special reinforcement are required involving the use of shear heads and anchor bars and wires.

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