POST-TENSIONED SLABS IN BUILDINGS

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  • EUROCODES

    Workshop - 27-29 November 2006, Varese, Italy

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    POST-TENSIONED SLABS IN BUILDINGS

    Increasing popularity /versus/

    Lack of specific coverage by Euro codesby

    Guy A. TABETLEBANON

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    Workshop - 27-29 November 2006, Varese, Italy

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    Building the Future

    Post-tensioned Slabs in Buildings

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    PRINCIPLE

    OF POST-TENSIONED

    CONCRETE

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    Illustrative example

    P

    2 P cos

    For harped profile

    P

    Three-span post-tensioned beam

    Tendon

    P

    V

    PA

    BW

    Free-body diagram of a tendon section between its low point (A) and point of inflection (B)

    w =For parabolic shapeL2

    8P x sag

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    WVdVa

    P P

    Free-body diagram of tendon

    Free-body diagram of beam after removal of tendon

    Force system between tendon and beam

    P

    Va W Vd

    P

    Illustrative example

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    Benefits of Post-Tensioned slabs

    Longer Spans Flat Soffits Flexibility of Layout & Services Deflection & Crack Control Thinner Slabs Reduced Storey Height Lighter Structure Fast Construction Cost Saving

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    Who Benefits ?

    Clients More clearance & More Flexibility Less Columns, Larger Open Spaces

    Architects Easier Internal Planning More Elegant Structure

    Structural Engineers More solutions Better Structural Performance

    Contractors Quicker and Easier Construction

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    IN SLABStwo categories of PT systems are used :

    Un-bonded system&

    Bonded system

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    Single strand coated with corrosion inhibiting grease and encased in polyethylene sheathing

    PT force is transferred to the concrete by the anchors provided at the ends

    Un-bonded Tendons

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    Bond is achieved throughout the length of the tendon by a cementitious matrix called grout

    PT force along the tendon is a function of the deformation of the concrete

    Flat corrugated ductBonded system tendon detail

    for beams & slabs

    Strand position in a circular duct

    Bonded Tendons

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    Structural Forms

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    Structural Forms

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    Preferred Option Flat Slab

    Minimum Thickness Flat Soffit

    Structural Forms

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    LOAD BALANCING

    A comprehensive solution to Post-Tensioningby T.Y.Lin

    Simple - General Powerfulfor analyzing simple & complex PT structures

    Principles Remove the PT tendons from the structure & replace it with all

    the forces that it exerts on the structure when in place Analyze the structure under this given set of

    equivalent loads

    Design Concept

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    Load Balancing

    Three-span post-tensioned beam

    Tendon

    WVdVa

    P P

    Design Concept

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    Modeling of two way slab system

    Methods available : Equivalent Frame Method Finite Element Method .

    Analysis of the Structure

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    Analysis of the Structure

    Finite Element Model Models a whole Floor Actual Tendon Locations

    Plane Frame Model Models a single sub-frame

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    Most common modeling of gravity loading and post-tensioning:

    Slab systems are modeled as rows of intersecting two dimensional slab frames.

    A single story slab frame consists of a line of column supports bounded by its tributary

    Floor levels are treated each separately

    Equivalent Frame Method

    Analysis of the Structure

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    Typical slab frame

    Analysis of the Structure

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    STRUCTURE PERSPECTIVE

    Finite Element Model

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    ELEMENT MESH PLAN

    Finite Element Model

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    BANDED TENDON LAYOUT

    Finite Element Model

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    DISTRIBUTED TENDON LAYOUT

    Finite Element Model

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    Finite Element Model

    TOP FIBER STRESSES

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