VERTICAL POST TENSIONING the River House Project Carol Hayek, PhD, MBA Chief Technical Officer, CCL

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Outline • The Project • Shear walls design and reasons for vertical post-tensioning • PT wall solution • Calculation of PT losses • Detailing • Constructability www.cclint.com

River House Project All post-tensioned concrete 38 story building • 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 Project Team

Grand rapids, MI

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• Structural Engineer: URS Corporation • Concrete Contractor: Kent Companies • Post Tensioning Supplier: CCL USA

River House Project

Shear Walls Varying Geometry

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Line 7

Line 11

Floor Plan

Shear Wall Design

• • • •

Lateral Stability Modeling Finite element model was used Approx 500 load cases Wind Load Basic load 90mph Reinforced concrete shear walls High drift and excessive tension

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Shear Wall Design

Possible Solutions • 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 www.cclint.com

Reasons for PT

Advantages of PT • Adds axial compression to counteract tensile stresses • Use uncracked section • Less rebar quantity, less congestion… • Can handle variable wall geometry

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PT Walls Solution

PT Forces in Wall • 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

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PT Loss Calculation

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

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PT Loss Calculation

Friction Losses • Angular friction loss in 3D dimensions (x,y,z) • Wobble loss as a function of tendon length • PT force at point x Px = Pstres sin g e

Shift of PT from circular column to wall

− (µ ( α +β) + k x 2 + y 2 + z 2 )

x y

Loss factor

• Seating Loss www.cclint.com

z

PT Loss Calculation

Long Term Losses • 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

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PT Loss Calculation

Elastic Shortening Stress-Strain relationship with εsteel proportional to εconcrete – Elastic Shortening ES (same for unbonded and nongrouted 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 load L

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= Total length of tendon

PT Loss Calculation

Creep, Shrinkage and Relaxation – 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 concrete SH = 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 PT Losses

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PT Loss Calculation

Long Term Loss Values ∆DL

fcpa

ESPT

ESDL

CRPT

CRDL

SH

RE

in

psi

ksi

ksi

ksi

ksi

ksi

ksi

Total Long Term Losses ksi

SHEAR WALL ON GRIDLINE 11 A, B,C,D 0.28 710 F, G, K, L 0.28 710 E, H, J, M 0.28 710

2.8 2.8 2.8

11.0 8.9 7.5

6.6 6.6 6.6

18.6 15.1 12.7

2.7 2.7 2.7

3.3 3.6 3.7

45.0 39.6 35.9

SHEAR WALL ON GRIDLINE 7 N 0.15 162 A, B 0.17 162 A, B, C, D 0.17 237 A, B, C, D 0.37 168 A, B,C,D 0.37 669 F, G, K, L 0.37 669 E, H, J, M 0.37 669

0.6 0.6 0.7 0.9 2.6 2.6 2.6

9.4 8.7 3.2 14.5 9.3 8.1 7.4

1.5 1.5 1.6 2.2 6.2 6.2 6.2

16.0 14.8 5.4 24.6 15.9 13.8 12.6

2.7 2.7 2.7 2.7 2.7 2.7 2.7

3.8 3.9 4.5 3.2 3.5 3.7 3.7

34.0 32.2 17.9 48.1 40.2 37.0 35.2

Tendons

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PT Loss Calculation

Total Losses Tendons

Tendon Length

Loss due to friction kip

LT Loss due LT Loss due to to Axial DL Prestressing Deformation

Total Losses

Loss due to friction

LT Loss due LT Loss due to to Axial DL Prestressing Deformation

Total Losses

kip

kip

kip

%

%

%

%

SHEAR WALL ON GRIDLINE 11 60 6 A, B,C,D F, G, K, L 74 6 E, H, J, M 88 7

4 4 4

6 5 4

16 15 14

12% 14% 14%

8% 8% 8%

13% 11% 9%

33% 32% 31%

SHEAR WALL ON GRIDLINE 7 N 37 5 A, B 46 5 A, B, C, D 126 6 A, B, C, D 60 6 A, B,C,D 93 6 F, G, K, L 107 7 E, H, J, M 117 7

2 2 2 2 4 4 4

5 5 2 8 5 5 4

12 12 10 16 15 15 15

11% 10% 13% 12% 14% 14% 16%

4% 4% 5% 5% 7% 7% 7%

11% 10% 4% 17% 11% 10% 9%

27% 25% 21% 34% 32% 31% 32%

Average

13%

6%

11%

30%

Percentage Values are with respect to jacking force

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Losses due to axial load vary from 4% to 17%

PT Detailing

PT System • 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 deviations 2” duct diameter ~ 3.5 x strand area • Mutli-strand stressing equipment easy to handle • Grouting in one operation

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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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PT Detailing

Curving of Tendons • 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 required A = 25% q / (0.6 fy)

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PT Detailing

Anchors Detailing at Blockouts Typical sections Front View Transverse section

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Constructability

Detailed Method Statement • • • •

Installation procedure and tolerances Stressing procedure Grouting procedure Field records

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Constructability

Installation

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Constructability

Stressing • 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 www.cclint.com

Constructability

Grouting • 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

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Constructability

Field Records • • • •

Strand installation Stressing records Grout mix records Grouting records

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Constructability

Field Feedback • 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

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

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THANK YOU!

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