Unbonded PreStress Connections

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  • UNBONDED PRE-STRESSED

    CONNECTIONSProf. John F. Stanton

    University of Washington,Seattle, Washington, USA

    The ROSE School, Pavia.June 2009

  • REFERENCE DOCUMENT

    Design Guidelines for Precast Concrete Seismic Structural

    Systems

    John F. Stanton & Suzanne D. NakakiPRESSS Report No. 01/03-09

    Available from PCI (and on Rose School website)

  • UNBONDED PRESTRESSED CONCRETE FRAME:

    GOALS Zero residual drift (ZRD)

    Frame returns to vertical after earthquake

    Damped response

    Damping restricts peak drift

    Little damage even in a large earthquake

    Building can be re-occupied quickly

  • POSSIBLE CONFIGURATIONS:(Continuous beams)

  • POSSIBLE CONFIGURATIONS:(Continuous columns)

  • CONFIGURATIONS

    Continuous Columns Often used by precasters

    Continuous Beams Long Heavy Beams Columns must be accurately placed for bars to fit

  • REINFORCEMENT CONFIGURATIONS

    Unbonded prestressing only Simple to construct, but no hysteretic damping. Damping could be supplied by external devices or

    RC frames in parallel with UBPT frame.

    UB prestressing + rebar = Hybrid Frame Internal hysteresis (damping) supplied by rebar All-in-one package.

  • UBPT Prestressed-only Frame

    Shown with post-tensioned beams, multi-storey columns.Requires post-tensioning on site. Easy fit-up.

    Posttensioningtendon

    (unbonded) Posttensioninganchorage

  • UBPT Prestressed-only Frame

    Prestressingtendon

    Bondedregion

    Unbondedregion

    Shown with pretensioned beams, one-storey columns.Can prestress in plant. Columns must be accurately placed.

  • UBPT Prestressed-only Frame Components

    Unbonded PT tendon

    Grout

  • Hybrid Frame Components

    Unbonded PT tendon

    Bonded Rebar

    Rebar locally debonded

    Grout

  • Deformed Shape

  • Deformed Shape

  • Deformed Shape

  • Deformed Shape

  • Hybrid Frame Components(Courtesy Nakaki Bashaw Group)

    Pre-tensioned strands

    Beam rebar (not continuous)

    Sleeves for column bars (grouted)

    Grout bed at interface

    Column bar splices

    Only the pre-tensioned strands cross the interface

  • Hybrid Frame Components(Courtesy Nakaki Bashaw Group)

    Mild steel (A706) (grouted)

    Unbonded region

    Post Tensioned Tendon (ungrouted)

    Fiber Reinforced Grout

  • UBPT FRAME CONCEPT

    PT tendon provides strength

    PT provides elastic restoring force

    Damping is minimal (use 5% critical)

  • NON-LINEAR ELASTIC BEHAVIORLINEAR NONLINEAR

    ELASTIC

    INELASTIC

    Elastic: Load and unload along the same pathInelastic: Load and unload along different pathsLinear: Load/unload path is a straight lineNonlinear: Load/unload paths are not straight.

  • Hybrid Frame combines both features

    Force - Deflection

    Returns to verticalNonlinear elasticLow damping

    Residual driftNonlinear inelasticHigh damping

    PT only

    Rebar only

  • Hysteresis Loops - Combinations

    100/0

    25/75 0/100

    50/5075/25

  • DESIGN PROCEDURE

    Rational design procedure

    Design forces same as cast-in-place concrete SMRF

    Explicit evaluation of drift capacity

    Design of other parts of frame: per code

  • ASSUMPTIONS

    All deformation occurs at the interface, not in the beam.

    PT is unbonded the entire length of the frame, except for bonded region at each end.

    cgp is at mid beam-height.

  • LIMIT STATESWe need to define limit states at which the moment

    strength will be computed.

    Nominal moment strength: Corresponds to the design moment. Used to size the reinforcement at the interface

    Maximum probable strength Corresponds to the Maximum Credible

    earthquake Forms the basis for Capacity Design forces for

    adjacent elements.

  • NOMINAL STRENGTH LIMIT STATE

    Smaller of Onset of strain hardening in mild steel (if any). Yield of PT steel.

    Comparable to nominal strength of reinforced concrete frame

  • MAXIMUM PROBABLE STRENGTH LIMIT STATE

    Smaller of : Strain at peak strength of mild steel. Yield of PT steel.

    System must have at least 3.5% drift capacity at MPS limit state.

    UNBONDED PRE-STRESSED CONNECTIONS Prof. John F. StantonUniversity of Washington,Seattle, Washington, USAThe ROSE School, Pavia. June 2009REFERENCE DOCUMENTPOSSIBLE CONFIGURATIONS:(Continuous beams)POSSIBLE CONFIGURATIONS:(Continuous columns)CONFIGURATIONSREINFORCEMENT CONFIGURATIONSSlide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17UBPT FRAME CONCEPTNON-LINEAR ELASTIC BEHAVIORForce - DeflectionHysteresis Loops - CombinationsDESIGN PROCEDUREASSUMPTIONSLIMIT STATESNOMINAL STRENGTH LIMIT STATEMAXIMUM PROBABLE STRENGTH LIMIT STATE