ce5510-advance structural concrete design-flat slab

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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

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Class Objectives

Identify the types of slab construction, summarizing the advantages and disadvantages of each

Explain the methods of analysis of slabsDetermine 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 lx 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.

4Deflection of udl beam = 5wl 384EI

= kwl 4

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4AB = k nAB ly

4DE = k nDE lx

nAB and nDE are portions of total load intensity transferred to AB and DE respectively.

Since AB = DE ,

4 4wl wl == nDE 4

y 4

nAB l 4 +x

l 4 l + ly x y 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 theintensity of the steepness of the curvature contours in thelx 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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DESIGN AND

DETAILINGOF FLAT SLAB

ESE SOEDARSONO HS27 FEBRUARY 2002

CONTENT

Introduction Benefits Design Considerations Design Methodology Analysis of Flat Slab Detailing

INTRODUCTION

What is a flat slab? a reinforced concrete slab supported directly

by concrete columns without the use of beams

INTRODUCTION

Flat slab Flat slab with drop panels

Flat slab with column head Flat slab with drop panel and column head

INTRODUCTION

Uses of column heads : increase shear strength of slab reduce the moment in the slab by reducing

the clear or effective span

Flat slab with column head

INTRODUCTION

Uses of drop panels : increase shear strength of slab increase negative moment capacity of slab stiffen the slab and hence reduce deflection

BENEFITS

BENEFITS

Flexibility in room layout Saving in building height Shorter construction time Ease of installation of M&E services Prefabricated welded mesh Buildable score

Benefits . . .Benefits . . .

FLEXIBILITY IN ROOM LAYOUT

allows Architect to introduce partition walls anywhere required

allows owner to change the size of room layout

allows choice of omitting false ceiling and finish soffit of slab with skim coating

Benefits . . .Benefits . . .

SAVING IN BUILDING HEIGHT

Lower storey height will reduce building weight due to lower partitions and cladding to faade

approx. saves 10% in vertical members reduce foundation load

SlabSlab

BeamBeam

3.4m3.4m 2.8m2.8m

ConventionalConventional

SlabSlab

3.2m3.2m2.8m2.8m

Beam-FreeBeam-Free

Benefits . . .Benefits . . .

SHORTER CONSTRUCTION TIME

flat plate design willfacilitate the use ofbig table formwork toincrease productivity

Benefits . . .Benefits . . .

SINGLE SOFFIT LEVEL

Living RoomLiving Room Toilet

Toilet ShowerShowerKitchenKitchenYardYard

30307575

2602603030 3030

3030BalconyBalcony

155

155

Flat Plate SlabFlat Plate SlabSingle Level CeilingSingle Level Ceiling

Simplified the table formwork needed Simplified the table formwork needed

Benefits . . .Benefits . . .

EASE OF INSTALLATIONOF M&E SERVICES

all M & E services can be mounted directly on the underside of the slab instead of bending them to avoid the beams

avoids hacking through beams

Benefits . . .Benefits . . .

PRE-FABRICATED WELDED MESH

Prefabricated in standard sizes

Minimised installation time

Better quality control

Prefabricated in standard sizes

Minimised installation time

Better quality control

Benefits . . .Benefits . . .

BUILDABLE SCORE

allows standardized structural members and prefabricated sections to be integrated into the design for ease of construction

this process will make the structure more buildable, reduce the number of site workers and increase the productivity at site

more tendency to achieve a higher Buildable score

DESIGN CONSIDERATIONS

Design Considerations. . . .Design Considerations. . . .

WALL AND COLUMN POSITION Locate position of wall to maximise the structural stiffness for

lateral loads

Facilitates the rigidity to be located to the centre of building

Typical floor plan of Compass the Elizabeth

Design Considerations. . . .Design Considerations. . . .

OPTIMISATION OF STRUCTURAL LAYOUT PLAN

the sizes of vertical and structural structural members can be optimised to keep the volume of concrete for the entire superstructure inclusive of walls and lift cores to be in the region of 0.4 to 0.5 m3 per square metre

this figure is considered to be economical and comparable to an optimum design in conventional of beam and slab systems

Design Considerations. . . .Design Considerations. . . .

DEFLECTION CHECK

necessary to include checking of the slab deflection for all load cases both for short and long term basis

In general, under full service load, < L/250 or 40 mm whichever is smaller

Limit set to prevent unsightly occurrence of cracks on non-structural walls and floor finishes

Design Considerations. . . .Design Considerations. . . .

CRACK CONTROL

advisable to perform crack width calculations based on spacing of reinforcement as detailed and the moment envelope obtained from structural analysis

good detailing of reinforcement will restrict the crack width to within acceptable

tolerances as specified in the codes and reduce future maintenance cost of the building

Design Considerations. . . .Design Considerations. . . .

FLOOR OPENINGS

No opening should encroach upon a column head or drop Sufficient