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POST-TENSIONING VS. PRESTRESSING This Seminar Is Intended To Post‐Tensioning vs. Prestressing PURPOSE STATEMENT POST-TENSIONING VS. PRESTRESSING This seminar is intended to • develop an understanding for the differences Miroslav F. VEJVODA, MBA, P.E. between precast-prestressed and post- tensioned concrete, and to Managing Director • provide basic understanding of the key steps Engineering & Professional Development in design and construction of post-tensioned prestressed concrete. ©Copyright Post-Tensioning Institute. All rights reserved ©Copyright Post-Tensioning Institute. All rights reserved precast.org/education precast.org/education 2 PRESENTATION LEARNING OBJECTIVES POST-TENSIONING INSTITUTE On the end of this seminar you should have a basic understanding of: • A nonprofit organization for the advancement of post-tensioned, prestressed concrete design • PT Basics: Prestressed Concrete and Post-Tensioning; Advantages and construction of PT; Modern PT Systems; Encapsulation and Durability; Tendon Protection Levels • Established in 1976 • Design Concepts: Load Balancing; Secondary Moments; Equivalent • Located in Farmington Hills, Michigan Frame; Basic Design Steps; Facts vs. Myths; Floor Systems; Shortening Restraint; Prestress Losses • Activities: • PT Installation and Construction: Installation Coordination; Tendon • Technical committees Arrangement; Construction Details • Standards, specifications, and technical documents • Stressing and Durability: Tendon Stressing and Safety; Grouting; • Certification for materials and field personnel Tendon Finishing; Field Inspection; Certification Programs • Marketing and promotional activities • Research projects ©Copyright Post-Tensioning Institute. All rights reserved ©Copyright Post-Tensioning Institute. All rights reserved precast.org/education 3 precast.org/education Structural Material PT BASICS Types • Prestressed Concrete and Post-Tensioning Reinforced Prestressed Structural Concrete Concrete Steel • Advantages of PT • Modern PT Systems • Encapsulation and Durability • Tendon Protection Levels Post-tensioning Pretensioning Bonded PT Unbonded PT precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 1 Post‐Tensioning vs. Prestressing PRESTRESSED CONCRETE PRE‐TENSIONING • What is Prestressing? Prestressing is a method of reinforcing concrete, counteracting applied loads by placing it in state of compression prior to the application of loads. • What is Prestressed Concrete? Concrete in which internal stresses (forces) are induced by means of prestressed reinforcement, typically, steel tendons. Two common prestressing methods are pre-tensioning and post-tensioning. 1. Tension Strands • Pre-Tensioning Method Steel tendons are stressed prior to concrete placement, usually at a precast plant remote from the construction site. 2. Cast Concrete – Bond strands to concrete • Post-Tensioning Method Steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site 3. Cut Strands – Transfer force to concrete precast.org/education POST‐TENSIONING CONCRETE MEMBER UNDER LOAD Section 1. Cast Concrete with Duct 2. Feed Strands through Duct 3. Tension Strands 4. Grout Duct (or other corrosion protection) precast.org/education REINFORCED CONCRETE UNDER FLEXURAL LOADING REINFORCED CONCRETE – SECTION precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 2 Post‐Tensioning vs. Prestressing NONPRESTRESSED REINFORCEMENT VS. POST-TENSIONING SIMPLE PRESTRESSED BEAM precast.org/education precast.org/education BEAM WITH HARPED TENDON BEAM WITH PARABOLIC TENDON precast.org/education precast.org/education AXIALLY- VS. ECCENTRICALLY-PLACED FEATURES PRESTRESSED REINFORCEMENT OF POST-TENSIONING 1. High Strength Steel 2. Load Balancing 3. Continuity 4. Precompression precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 3 Post‐Tensioning vs. Prestressing COMBINED STRESSES DUE TO PRESTRESS + EXTERNAL LOADING POST-TENSIONED CONCRETE ADVANTAGES • Long economical spans / shallow structural depth • Effective use of high strength materials • Wide flexibility in tendon location / uplift • PT force application in stages • Small deflections • Minimum amount of nonprestressed bonded reinforcement over supports of continuous PT members Sustainability precast.org/education precast.org/education BARE STRAND COIL UNBONDED VS. BONDED PT Photo Courtesy of G. Chacos precast.org/education precast.org/education 22 STRAND-WEDGE CONNECTION WEDGE PLATE WITH WEDGES precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 4 Post‐Tensioning vs. Prestressing MODERN UNBONDED TENDON SYSTEMS EXTRUSION PROCESS Photo Courtesy of GSI Post-Tension Plastic Hopper Extruder Grease Injector Cooling Trough Bare Strand Extruded Strand precast.org/education precast.org/education 26 TENDON CUTTING TO LENGTH FIXED-END ANCHOR Photo Courtesy of Suncoast Post-Tension Ltd. Photo Courtesy of G. Chacos Pull Installation Push Installation precast.org/education 27 precast.org/education 28 TENDON IDENTIFICATION SHIPPING OF TENDONS Photo Courtesy of VSL Photos Courtesy of G. Chacos Color Coding precast.org/education 29 precast.org/education 30 2018 Precast Show ‐ NPCA ‐ PTI 5 Post‐Tensioning vs. Prestressing ENCAPSULATED ENCAPSULATED TENDONS POST-TENSIONING SYSTEMS Photo Courtesy of General Technologies • For all structures designed by ACI 318 and 350 • Watertight encapsulation of the strand over the entire length of the tendon • Corrosion inhibiting PT Coating • Fully encapsulated anchorages • Properly grouted stressing pockets • Adequate concrete cover over all parts of the tendon, including the ends precast.org/education precast.org/education ENCAPSULATED TENDONS ENCAPSULATED TENDONS Photo Courtesy of General Technologies Photo Courtesy of General Technologies precast.org/education precast.org/education ENCAPSULATED TENDONS ENCAPSULATED TENDONS Photo Courtesy of Precision Hayes International Photo Courtesy of Precision Hayes International precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 6 Post‐Tensioning vs. Prestressing ENCAPSULATED TENDON ENCAPSULATED TENDONS COMPONENTS Photo Courtesy of Precision Hayes International Photo Courtesy of G. Chacos Positive Lock Ring Transition Tube Coated Anchor Encapsulation Cap Pocket Former precast.org/education precast.org/education STRESSING-END ANCHORAGES UNBONDED POST-TENSIONED SLABS precast.org/education precast.org/education UNBONDED TENDON EQUIPMENT ANCHORAGE COMPONENTS Transition Trumpet Duct Inlet/Outlet unbonded tendon Stressing Jack Wedges Confinement Steel Wedge Plate (Local Zone Reinf.) Bearing Plate Stressing Pump Gauge precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 7 Post‐Tensioning vs. Prestressing BONDED POST-TENSIONING ROUND AND FLAT PLASTIC DUCT Ducts for Post‐Tensioning precast.org/education precast.org/education ANCHORAGE AND PLASTIC DUCT BONDED TENDON ANCHORAGE Photo Courtesy of VSL Permanent Seal Plastic Duct Grout Cap Positive Mechanical Coupling of Duct precast.org/education precast.org/education 46 BONDED (GROUTED) TENDONS TWO-WAY SLAB WITH BONDED PT Photo Courtesy of VSL Photo Courtesy of VSL precast.org/education 47 precast.org/education 48 2018 Precast Show ‐ NPCA ‐ PTI 8 Post‐Tensioning vs. Prestressing MULTISTRAND EQUIPMENT TENDON PROTECTION LEVELS (PL)* Four levels of increasing protection: • Protection Level 1A (PL-1A): Galvanized duct; grout Class A; temporary grout cap • Protection Level 1B (PL-1B): PL-1A plus grout Class B; permanent grout cap Stressing Jack Strand Pusher • Protection Level 2 (PL-2): PL-1B plus galvanized or epoxy coated embedded part of anchorage; plastic duct; grout Class C or D; precast segmental duct couplers • Protection Level 3 (PL-3): PL-2 plus electrically isolated tendons or monitorable or inspectable tendons plus monitorable or inspectable at any time. Stressing Pump G r o u t P l a n t * Unified approach from the new PTI/ASBI M50.3-12 Specification for Grouted Post-Tensioning precast.org/education precast.org/education LOAD BALANCING DESIGN CONCEPTS Concept: Portion of dead load is balanced by counter- active forces in post-tensioning tendons • Load Balancing • Secondary Moments Counter active tendon forces: • Equivalent Frame • Axial compression + uplift loads • Basic Design Steps • Balance a portion of the load on the structure • Facts vs. Myths • Floor Systems “Stripping” post-tensioning tendons from the structure and • Shortening Restraint replacing the tendon’s influence as a series of • Prestress Losses equivalent loads precast.org/education precast.org/education LOAD BALANCING LOAD BALANCING 2 wlbal M1 = P • e = Primary Moment P 8e Load W P tan P tan CGC W P bal P Tendon e precast.org/education precast.org/education 2018 Precast Show ‐ NPCA ‐ PTI 9 Post‐Tensioning vs. Prestressing SECONDARY MOMENTS SECONDARY MOMENTS Mbal = M1 + M2 = Pe + Msec M2 = Secondary Moment Mbal = Balanced moment by post-tensioning Developed in post-tensioned concrete members due to equivalent loads prestressing forces Consequence of constraint by the supports to the free movement of the member • Only develops in indeterminate members • Simply supported beams have zero secondary moments Secondary reactions at supports due to prestressing M2 Significant: must be accounted for in the design of prestressed concrete indeterminate structures (load factor 1.0 in strength design per ACI 318-14, 5.3.11) Secondary Moments, M2, vary linearly between supports precast.org/education precast.org/education EQUIVALENT FRAME DESIGN STEPS FOR PT SYSTEMS 1 2 3 456
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