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VERTICAL POST TENSIONINGthe River House Project

Carol Hayek, PhD, MBAChief Technical Officer, CCL

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• The Project• Shear walls design and reasons for

vertical post-tensioning• PT wall solution• Calculation of PT losses• Detailing• Constructability

Outline

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River House Project

Project Team• Structural Engineer: URS Corporation• Concrete Contractor: Kent Companies• Post Tensioning Supplier: CCL USAGrand rapids, MI

All post-tensioned concrete 38 storybuilding• Unbonded post-tensioned flat slab• Bonded post-tensioned transfer

girders with CCL-12 strand anchor system and multiple stressing stages

• Bonded post-tensioned vertical walls using CCL-4 strand anchor system

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Shear WallsLine

11

Line

7

Varying Geometry

River House ProjectFloor Plan

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Shear Wall Design

• Finite element model was used• Approx 500 load cases• Wind Load

Basic load 90mph• Reinforced concrete shear walls

High drift and excessive tension

Lateral Stability Modeling

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Shear Wall Design

• Options and limitations of reinforced concrete walls– Adding shear walls or increasing shear wall sizes

“not an option” due to:→ Architectural requirement→ High real estate value

– Adding rebar… already congested– Increasing concrete strength… f’c=8,000psi

• Alternative– Use of Post-Tensioning

Possible Solutions

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Reasons for PT

• Adds axial compression to counteract tensile stresses

• Use uncracked section• Less rebar quantity, less congestion…• Can handle variable wall geometry

Advantages of PT

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PT Walls Solution

• Design of walls using bonded system

• Optimal use of PT: only where needed

• Incremental PT forces varying from 500k to 1670k

• PT walls from ground to 9th floor

• RC walls for upper floors

PT Forces in Wall

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PT Loss Calculation

• Calculation of prestress losses to obtain the effective PT force per tendon– Friction Losses– Long Term Losses

Calculation of PT Losses

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PT Loss Calculation

• Angular friction loss in 3D dimensions (x,y,z)

• Wobble loss as a function of tendon length

• PT force at point x

• Seating Loss

)zyxk)((gsinstresx

222

ePP +++β+αµ−=

Loss factor

xy z

Shift of PT from circular column to wall

Friction Losses

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• Typical losses due to prestressing• Elastic Shortening of concrete• Creep of concrete• Shrinkage of concrete• Strand Relaxation

• Loss due to axial deformation caused by dead load weight on wall

Long Term LossesPT Loss Calculation

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Stress-Strain relationship with εsteel proportional to εconcrete

– Elastic Shortening ES (same for unbonded and non-grouted bonded tendons)

• Loss due to prestressing depends on average precompression

ESPT= Es εs with εs = Ke ( fcpa / Eci )

• Loss due to axial deformation

ESDL= Es εDL with εDL = (∆DL / L)

Es = Modulus of Elasticity of the PT steel

∆DL = Axial deformation due to dead loadL = Total length of tendon

Elastic ShorteningPT Loss Calculation

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– Creep CR• Loss due to prestressing CRPT= Kc Es ((fci – fsd) / Ec)

• Loss due to axial deformation in building CRDL= Kc Es (∆DL / L)

– Loss due to shrinkage of concreteSH = 8.2x10-6 Ksh Es (1-0.06 V/S )(100-RH)

– Loss due to relaxation of tendon RE = Kr*C –[J*(ESPT+CRPT+ESDL+CRDL+SH)]*C

Creep, Shrinkage and RelaxationPT Loss Calculation

PT Losses

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Long Term Loss ValuesPT Loss Calculation

Tendons ∆DL fcpa ESPT ESDL CRPT CRDL SH RE Total Long Term Losses

in psi ksi ksi ksi ksi ksi ksi ksi

SHEAR WALL ON GRIDLINE 11A, B,C,D 0.28 710 2.8 11.0 6.6 18.6 2.7 3.3 45.0F, G, K, L 0.28 710 2.8 8.9 6.6 15.1 2.7 3.6 39.6E, H, J, M 0.28 710 2.8 7.5 6.6 12.7 2.7 3.7 35.9

SHEAR WALL ON GRIDLINE 7N 0.15 162 0.6 9.4 1.5 16.0 2.7 3.8 34.0A, B 0.17 162 0.6 8.7 1.5 14.8 2.7 3.9 32.2A, B, C, D 0.17 237 0.7 3.2 1.6 5.4 2.7 4.5 17.9A, B, C, D 0.37 168 0.9 14.5 2.2 24.6 2.7 3.2 48.1A, B,C,D 0.37 669 2.6 9.3 6.2 15.9 2.7 3.5 40.2F, G, K, L 0.37 669 2.6 8.1 6.2 13.8 2.7 3.7 37.0E, H, J, M 0.37 669 2.6 7.4 6.2 12.6 2.7 3.7 35.2

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Percentage Values are with respect to jacking force Losses due to axial load vary from 4% to 17%

Total LossesPT Loss Calculation

Tendons Tendon Length

Loss due to friction

LT Loss due to

Prestressing

LT Loss due to Axial DL

Deformation

Total Losses

Loss due to friction

LT Loss due to

Prestressing

LT Loss due to Axial DL

Deformation Total Losses

kip kip kip kip % % % %

SHEAR WALL ON GRIDLINE 11A, B,C,D 60 6 4 6 16 12% 8% 13% 33%F, G, K, L 74 6 4 5 15 14% 8% 11% 32%E, H, J, M 88 7 4 4 14 14% 8% 9% 31%

SHEAR WALL ON GRIDLINE 7N 37 5 2 5 12 11% 4% 11% 27%A, B 46 5 2 5 12 10% 4% 10% 25%A, B, C, D 126 6 2 2 10 13% 5% 4% 21%A, B, C, D 60 6 2 8 16 12% 5% 17% 34%A, B,C,D 93 6 4 5 15 14% 7% 11% 32%F, G, K, L 107 7 4 5 15 14% 7% 10% 31%E, H, J, M 117 7 4 4 15 16% 7% 9% 32%

Average 13% 6% 11% 30%

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PT Detailing

• PT system that accommodates variable wall sections and geometry

• Multi-strand CCL anchors of 4x0.6” strand• Small size anchors and ducts to fit in walls and allow

profile deviations2” duct diameter ~ 3.5 x strand area

• Mutli-strand stressing equipment easy to handle • Grouting in one operation

PT System

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PT Detailing

• Every tendon is labeled• Tendons are staggered• Tendons are stopped incrementally• Anchors typically stopped at slab

soffit to avoid blockouts

Sample Sections and Elevations

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• Special consideration to sweep around openings– High Concentration of PT forces – Deviation forces need to be considered

• Pressure due to curvature– Deviation force (radial force) q = P/R per unit length

• Rebar needed– Anchoring of 25% is

requiredA = 25% q / (0.6 fy)

Curving of TendonsPT Detailing

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Anchors Detailing at BlockoutsPT Detailing

Typical sections

Transverse section

Front View

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Constructability

• Installation procedure and tolerances• Stressing procedure• Grouting procedure• Field records

Detailed Method Statement

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InstallationConstructability

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• Stressing to be done from top of wall• Anchors at bottom of wall used as accessible

dead ends• Anchors at top of wall used as stressing end• Anchors stopped at slab soffit to avoid

encasements• Multi-strand simultaneous stressing to control

intertwining of strands

Constructability

Stressing

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• Grout to be done by qualified personnel• Grouting for vertical tendons to start from

bottom• Grout vents placed at every floor• One-way flow of grouting should be maintained• Maintain grout pressure after ducts are filled

GroutingConstructability

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• Strand installation• Stressing records• Grout mix records• Grouting records

Field RecordsConstructability

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• Generally no problem• PT and rebar interference problems were held to

a minimum• Grouting went fine with vents being filled per

procedure requirement• Blockouts at dead end side were tight but

workable

Field FeedbackConstructability

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Conclusion• Vertical PT is a viable solution for lateral

stability• Vertical PT is a suited option for walls with

varying geometry• Understanding of PT losses is necessary• Thorough detailing is needed• Detailed construction method statements

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URS: Dave Stek, PE, SE(IL), LEED®APCalvin College: Leonard P. De Rooy, P.E. Kent Companies: Dave Turner, PE

Acknowledgments

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THANK YOU!