HYBRID MOMENT FRAMES AND UNBONDED PT SHEAR WALLS

HYBRID MOMENT FRAMES AND UNBONDED PT SHEAR WALLS Opportunities Resulting From Recent ACI 318 Code Changes and the Work of ACI ITG s 1 and 5 Neil M. Hawkins, Professor Emeritus, University of Illinois PTI Convention, Portland, OR, May 4, 2009 Technical Session on Innovative PT Building/Parking Structure Design Copyright Post-Tensioning Institute all rights reserved. Page 1 of 35

USE OF PRESTRESSING IN SEISMIC FORCE RESISTING SYSTEMS (SFRS) Use of PT in SFRS Banned by ACI 318 for Many Years Research on Use of PT in SFRS Begun in 1982 by NIST Permitted by NEHRP Provisions Since 1988 Permitted by ASCE/SEI 7-05 and IBC 2006 Precast/PT ( Hybrid) Moment Frames Satisfying ITG T1.1 Permitted by ACI 318-02 Precast/PT Special Structural Walls Satisfying ITG 5.1 Permitted by ACI 318-08 Use of PT in SFRS Beams Permitted by ACI 318-08 Copyright Post-Tensioning Institute all rights reserved. Page 2 of 35

BARRIERS TO USE OF PT IN SFRS LIMITATION OF ACI 318 FROM 318-71 THROUGH 318-08 Sec. 21.1.1.8 of ACI 318-08 A reinforced concrete structural system not satisfying the requirements of this chapter (Chapter 21) shall be permitted if it is demonstrated by experimental evidence and analysis that the proposed system will have strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure satisfying this chapter. Requirement must be satisfied if systems involving PT or precast are to be assigned R and C d values comparable to those for reinforced concrete special structural walls and to therefore be economically competitive with reinforced concrete structural walls. Provisions of Chapter 21 are Prescriptive; Sec. 21.1.1.8 is a Performance Requirement. How to address that requirement? Copyright Post-Tensioning Institute all rights reserved. Page 3 of 35

BARRIERS TO USE OF PT in SFRS Interplay Between Code Jurisdictions Changing Pattern of Code Publication Dates Interplay of Loading and Material Codes NEHRP 2000 ACI 318-02 IBC 2003 NEHRP 2003 ACI 318-02 ASCE 7-05 IBC 2006 ASCE 7-05 Sup. 1 ACI 318-05 Copyright Post-Tensioning Institute all rights reserved. Page 4 of 35

Beam End Moment (kip-in) 1200 900 600 300 0-300 -600-900 Benefits of Use of PT in SFRS 2. Self-Centering Capability -1200-4 -3-2 -1 0 1 2 3 4 ColumnDrift (%) 120 80 40 0-40 -80-120 Copyright Post-Tensioning Institute all rights reserved. Page 6 of 35 Beam End Moment (kn-m)

ACI 318-08 PROVISIONS FOR LONGITUDINAL PT IN FLEXURAL MEMBERS OF SPECIAL MOMENT FRAMES Provisions Based on 1988 NEHRP Provisions Provisions Justified by Performance of Structures with PT Frame Beams in 1971 San Fernando and 1994 Northridge Earthquakes Provisions Justified by Tests Conducted at U of Washington. Copyright Post-Tensioning Institute all rights reserved. Page 7 of 35

ACI 318-08 PROVISIONS FOR HYBRID SPECIAL MOMENT FRAMES AND SPECIAL STRUCTURAL WALLS Hybrid Special Moment Frame. Acceptance Based on ITG T1.1-01 ( 374.1-05) Provisions Hybrid Special Coupled or Uncoupled Shear Walls. Acceptance Based on ITG 5.1-07 Provisions Provisions Justified Primarily by Tests Conducted at UCSD, U. of Washington, and Lehigh University Copyright Post-Tensioning Institute all rights reserved. Page 9 of 35

Conceptual Prototype Building for Design Studies for PRESS Building Seismic Frame Double Tee Flooring System Jointed Wall System Gravity Frame 25 ft. 100 ft. (30.5 m) 200 ft. (61 m) Copyright Post-Tensioning Institute all rights reserved. Page 10 of 35

PCI-NSF 60% Scale UCSD PRESSS Building Unbonded Post-Tensioned Vertically Coupled Precast Structural Walls Some Floors Precast Frames and Other Floors Hybrid Frames Copyright Post-Tensioning Institute all rights reserved. Page 11 of 35

CODIFICATION DOCUMENTS FOR HYBRID FRAMES Performance Requirements For Hybrid Frame = Reinforced Frame Per ASCE 7-05. Referenced in 21.8.4 of ACI 318-08 Prescriptive Requirements Design Procedures for Precast Industry Standard Frame Satisfying ACI T1.1-01 - Referenced in ACI 318R-08 Copyright Post-Tensioning Institute all rights reserved. Page 12 of 35

CODIFICATION DOCUMENTS FOR HYBRID WALLS Performance Requirements For Hybrid Wall = Reinforced Wall Per ASCE 7-05 Referenced in 21.10.3 of ACI 318-08 Prescriptive Requirements Design Procedure for Industry Standard Wall Satisfying ACI ITG 5.1 Requirements Copyright Post-Tensioning Institute all rights reserved. Page 13 of 35

HYBRID FRAME CHRONOLOGY Development Sponsored Primarily By Charles Pankow Builders 1986-96 Exploratory Test Program by NIST. 1994-95 Acceptance Criteria for Ductile Connections of Precast Frames Issued by ICBO for Evaluation Report of Connection Developed by Englekirk. 1995-98 Development of T1.1-99 by ITG 1 of ACI. 1996-01 Development of T1.2-XX by ITG 1 of ACI. 1996-97 ICBO Evaluation Report Obtained by Pankow. 1999 Validation Testing of an Interior, Exterior, and Corner Column Connections for Third and Mission Building per ACI T1.1-99 at University of Washington. Copyright Post-Tensioning Institute all rights reserved. Page 14 of 35

Hybrid Frame Concept Grout Pad THANK YOU and Precast Column Precast Beam Any Questions? Mild Steel Reinforcing Bars Unbonded PT Tendons Courtesy of The Nakaki Bashaw Group, Inc. Copyright Post-Tensioning Institute all rights reserved. Page 15 of 35

ACI T1.1- HYBRID FRAME ACCEPTANCE CRITERIA Scope: Design to be based on strong column/weak beam concepts; tests on modules must show dependable and predictable stiffness, strength, drift capacity and energy dissipation as required by acceptance criteria. Design Procedure: Prior to testing a design procedure must be developed for the type of hybrid frame for which acceptance is sought, and that procedure used to design test modules. Test Modules: A minimum of one test module for each characteristic configuration of intersecting members of hybrid frame; modules to be not less than one-third scale. Test Method: Modules subjected to sequence of increasing displacement controlled drifts with three fully reversed drift cycles for each limiting drift. Detailed data to be recorded in terms of module performance at key drift ratios. Testing to be continued until drift ratio exceeds 0.035. Copyright Post-Tensioning Institute all rights reserved. Page 16 of 35

ACI T1.1 -HYBRID FRAMES ACCEPTANCE CRITERIA 1. At Nominal Lateral Resistance E N Stiffness Greater than Code Req d. 2. For Specified Column Overstrength Factor λ : E max λe N For third complete cycle at drift ratio limiting drift ratio of 0.035 or more: 3. Peak Force 0.75E max for same loading direction 4. Relative Energy Dissipation Ratio 1/8. 5. Secant Stiffness at drift ratios + 0.0035 to -0.0035 0.05 times initial stiffness. Copyright Post-Tensioning Institute all rights reserved. Page 17 of 35

SPECIAL HYBRID FRAME VALIDATION TESTING FOR THIRD AND MISSION BUILDING S.Day, MSCE Thesis, University of Washington, 1999 Three Half-Scale Test Modules 1. Moment Transfer Parallel to Edge 2. Moment Transfer Normal to Edge 3. Corner Column to Bi-directional Beams Connection Copyright Post-Tensioning Institute all rights reserved. Page 18 of 35

ACI T1.2- PRECAST INDUSTRY STANDARD FOR HYBRID FRAME DESIGN Scope: Design for hybrid frames shown to satisfy T1.1-01. Special inspection required. Drawings must show details for debonding of special reinforcement, details for anchoring special reinforcement and PT and details for floor-slab frame connections Materials: Special reinforcement stress-strain properties equivalent to A706 steel. PT steel to conform to A416 and to remain elastic at E N. Interface grout to contain > 0.1% fibers. Framing System Req ts: Max total and story drifts at E N 0.024. Single bay beams combined with single or multiple story columns. Top and bottom special reinforcement of equal areas and strengths and debonded for specified length in beam at interface. PT concentric, sufficient to close any gap at beam-column interface after E N achieved, and providing clamping force at interface sufficient to resist shear due to factored gravity loads. Design Req ts for Beams, Interfaces, and Joints: Restrictions on interface grout thickness and formulas for calculating PT force, and probable flexural and shear strengths at interfaces and joints. Copyright Post-Tensioning Institute all rights reserved. Page 20 of 35

ACCEPTANCE CRITERIA FOR SPECIAL HYBRID COUPLED OR UNCOUPLED SHEAR WALLS - ACI ITG 5.1-07 Scope: Limited to new special unbonded post-tensioned precast concrete shear walls. Walls can be coupled or uncoupled. Walls must have h w / l w ratios 0.5 so that rocking, rather than shear, action controls. No height limitation on wall. Tests must establish predictable stiffness, strength, drift capacity, and relative energy dissipation as required by acceptance criteria. Based on the test results the structure as a whole must be demonstrated by analysis to be able to retain its integrity through the specified peak displacements. Copyright Post-Tensioning Institute all rights reserved. Page 21 of 35

ACCEPTANCE CRITERIA FOR SPECIAL COUPLED OR UNCOUPLED HYBRID WALLS - ACI ITG 5.1-07 Design Procedure: Prior to testing, design procedure required for prototype walls having the generic form for which acceptance is sought. ( ACI ITG 5.2) Development of procedure usually requires preliminary testing and analysis program. (PRESSS Building Tests). Procedure to be used to proportion test modules. Procedure must account for material non-linearity, cracking, deformations of members and connections, and reversal of loadings. Procedure must define methods for calculating initial stiffness, lateral strength, and ductility and specify detailing to ensure that ductility and strength, etc. Procedures must cover shear strength, sliding shear strength, boundary element reinforcement, and limiting strains for materials of coupling elements. Procedures must define methods to prevent local buckling of reinforcement, splice failure, excessive opening of joints, excessive slip on joints, and shear failure of panels. Copyright Post-Tensioning Institute all rights reserved. Page 22 of 35

ACCEPTANCE CRITERIA FOR SPECIAL HYBRID WALLS ACI ITG 5.1 Test Modules: No fewer than two modules. At least one module for each characteristic configuration of walls and for each limiting engineering design criterion. (shear, axial load, and M/V ratio at critical section for flexure). For intersecting walls, response for two orthogonal directions to be tested unless analyses shows testing is not needed. Where coupling elements are to be used, those elements must be included in test modules. Coupling devices must be of the same type, material and manufacture as in prototype. Module scale large enough to represent full complexities of behavior and not less than 1/3rd. Not less than half scale tests on coupling elements. Modules full-scale unless preliminary program has tested multiple modules of not less than 1/10 th scale. Walls at least two panels high unless wall is to be single panel. No. of panels = No. needed to validate engineering design criteria at connection between wall and foundation. Geometry and reinforcing details used to connect walls to foundation to replicate those of prototype structure. Where axial load due to gravity exceeds 0.05A g f c modules shall be subject to that load throughout test. Copyright Post-Tensioning Institute all rights reserved. Page 23 of 35

ACCEPTANCE CRITERIA FOR SPECIAL COUPLED OR UNCOUPLED HYBRID WALLS - ACI ITG 5.1 Test Protocol: Modules subjected to specified sequence of increasing displacement controlled cycles. Equal drifts applied at each floor level for coupled walls. Any axial load to be applied prior to lateral loads. 1) For uncoupled walls testing to continue until applied drift ratio in percent exceeds: 0.9 0.8[ h w / l w ] + 0.5 3.0 - (1) 2) For coupled walls, same criteria applies where h w /l w is the least value for each individual wall. 3) Three fully reversed cycles shall be applied at each limiting displacement. Copyright Post-Tensioning Institute all rights reserved. Page 24 of 35

ACCEPTANCE CRITERIA FOR SPECIAL UNBONDED POST-TENSIONED PC WALLS ACI ITG 5.1 Acceptance Criteria 1. Calculated Probable Lateral Resistance = E pr 0.9E pr E max 1.2E pr For third complete cycle at drift ratio limiting drift ratio: 2. Peak Force 0.8E max. No Rupture of Reinforcement 3. Secant Stiffness at drift ratios + 0.10 to -0.10 0.10 times initial stiffness. 4. Relative Energy Dissipation Ratio 1/8. 5. Force in Unbonded Tendon < 0.90 f py at half limiting drift 6. Maximum Foundation and Joint Shear-Slip < 0.06 in. Copyright Post-Tensioning Institute all rights reserved. Page 25 of 35

ITG 5.2- Prescriptive Precast Industry Standard for Design of Coupled or Uncoupled Shear Walls Materials Section 3 3.2 Ducts 3.3 Reinforcement : Special; Post-tensioned; Pretensioned 3.4 Concrete: f c for panels 4,000 psi; no strength limit. 3.5 Interface grout. Must contain at least 0.1% fibers 3.6 Coupling devices: Material to have deformation at failure at least 50% calculated deformation at design displacement 3.7 Corrosion protection required for prestressing tendons Copyright Post-Tensioning Institute all rights reserved. Page 26 of 35

ITG 5.2- PRESCRIPTIVE PRECAST INDUSTRY STANDARD FOR DESIGN OF HYBRID WALLS Structural System Requirements for Walls Sec.4 4.1 General: Load Path, Integrity at Limiting Drift Angle 4.2 Strength: Load Factors, Ω o Factor, Higher Modes ( Eberhard & Sozen, 1993). 4.3 Drift: Computation, Design Drift Angle Limits, Drift Angle Capacity Limits Design Story Drift Angle < 2/3rds value from Eq. (5-1) of ACI ITG 5.1 Drift Angle Capacity > Value from Eq. (5-1) of ACI ITG 5.1 4.4 Wall Characteristics: Thickness Constant, h w /l w 0.5, Each Wall Single Precast Panel or Vertically Stacked Panels with no Vertical Connections, Non- Linear Action Locations Limited to Wall-Foundation Interface. Post-tensioning Tendons Concentric in Wall with f prs < f py at 2/3rds drift angle from Eq. (5-1). 4.5 Distribution of Walls within Structures: Irregularity Considerations Per ASCE 7 4.6 Wall System-Gravity Load Frame Interactions Copyright Post-Tensioning Institute all rights reserved. Page 27 of 35

ITG 5.2 Prescriptive Precast Industry Document Uncoupled Wall System Design Concepts Response Concept Design Concept Sources of Experimental Data 1. Rahman and Restrepo: CE, Canterbury, NZ, August 20000 2. Perez, Pessiki, Sause and Lu, ACI SP-211 Lehigh University, PA 2003 3. DSDM Test-San Diego, 2008 Copyright Post-Tensioning Institute all rights reserved. Page 28 of 35

ITG 5.2 Prescriptive Precast Industry Standard Design Requirements for Uncoupled Wall Systems Section 5 5.2 Materials Panel reinforcement requirements per ACI 318. 5.3 Prestress Minimum force> Tensile strength of ED reinforcement. Maximum f se such that ED reinforcement yields before f prs >0.95 f py... 5.4 ED reinforcement to provide > 25% of wall base flexural strength@ M n Location, Anchorage, Debonding 5.5 ϕ times Interface Shear Strength V Ni ; μ = 0.5. = ϕ μc > base shear for M pr 5.6 Flexural Strength Boundary element requirements, M pr requirements Copyright Post-Tensioning Institute all rights reserved. Page 29 of 35

ITG 5.2 Prescriptive Precast Industry Standard Design Requirements for Uncoupled Wall Systems Calculation of M N and M pr. NA depth constant as drift increases from design drift to maximum; maximum drift >1.5 times design drift. At M pr max. strain in ED reinf. < 0.9 ε su and for prestress f prs < f py at 1.5 times design drift. Strains calc. assuming rocking of wall on its base 5.7 Lowermost panel detailing strengthen bottom corners, provide bottom horizontal reinforcement. 5.8 Interface grout thickness<1.5 in; strength f c and 2.5xstrength< f cc Copyright Post-Tensioning Institute all rights reserved. Page 30 of 35

ITG 5.2 Prescriptive Precast Industry Standard Design Requirements for Coupled Wall Systems Coupling Devices Yield Coupling Device Strain 0.7ε scu Coupling Device Fracture Design Concept Response Concept Sources of Experimental Data 1. Priestley et al. Preliminary Results of PRESSS Building Test, PCI Journal, 1999 2. Thomas and Sritharan, Evaluation of PRESSS Design Guidelines for Jointed Wall Systems Iowa State, 2004 Copyright Post-Tensioning Institute all rights reserved. Page 33 of 35

ITG 5.2 Prescriptive Precast Industry Standard Design Requirements for Coupled Wall Systems 6.3 No. of Coupling Devices per vertical Joint = 0.3M n /(n-1) F con l w ; F con = connector force at design displacement and n = no. of walls. Prestress Required area determined from forces at base at design displacement for wall providing larger(est) resistance; Required f se determined from conditions for trailing wall. That values used for all walls Shear Strength V N requirement at maximum probable flexural strength similar to that for uncoupled walls Flexural Strength Design moment M dw for wall providing greater(est) resistance : M dw = M n /n +(λ p n con F con l w /n); λ p = 0.9 for 2-wall system and 1.05 for three or greater wall system and n con = no. of coupling devices per vertical joint of walls. Copyright Post-Tensioning Institute all rights reserved. Page 34 of 35