Current Trends in PT Design and Construction in North America

Transcription

1 Current Trends in PT Design and Construction in North America March 10, 2015 San Diego, California Country Club Drive Farmington Hills, MI Telephone: (248) Fax: (248)

2 March 10, 2015 Current Trends in PT Design and Construction in North America Miroslav Vejvoda, MBA, P.E. Technical Director Post-Tensioning Institute March 10, 2015 Credits PTI seminars are developed in collaboration with several PTI committees. Special recognition to long time PT designers: Cary Kopczynski, Ken Bondy, Brian Allred, Don Kline, and others for their contributions. What is PTI? A non-profit organization for the advancement of prestressed post-tensioned concrete Established in 1976 Activities Technical committees (15) Specifications and technical documents Certification committees (8) Certification programs for materials fabrication and field personnel certification Marketing and promotional activities PTI JOURNAL Research projects and scholarships 3 Post-Tensioning Institute. All Rights Reserved 1

3 March 10, 2015 Overview General Structural systems Restraint to shortening Common design issues Quality and Durability March 10, 2015 General PT Applications & History PT Basics PT Systems & Protection Reasons to Consider PT Post-Tensioned slabs are typically 2 to 3 thinner than a rebar deck and use less reinforcement providing financial savings. Thinner slabs also benefit foundations, columns and seismic design The slab can be stressed when concrete reaches 3000 psi and the forms pulled after that The shoring and re-shoring times are reduced when using posttensioning Deflections and Long-term creep problems are significantly reduced by load balancing Deck moment of inertia approaches I (gross) as opposed to I (effective) resulting in reduced deflections Sustainability 6 Post-Tensioning Institute. All Rights Reserved 2

4 Post-Tensioned Concrete Advantages Photo Courtesy of Cary Kopczynski & Company 2201 Westlake, Seattle, WA Sustainability McKinney Place, Houston, Texas Courtesy of Walter P. Moore Espirito Santo Plaza, Miami, FL (Courtesy of LERA and Estroil Inc.) Post-Tensioning Institute. All Rights Reserved 3

5 How Much Post-Tensioning? Based on PTI tonnage statistics from 1972 and reasonable estimates before that About 5 billion square feet of building construction with unbonded post-tensioning About 50,000 post-tensioned buildings in the US Does not include bridges, earth applications, residential foundations 10 Post-Tensioning Applications 2013 North American Post-Tensioning Tonnage Earthwork 6% Bridges 10% S-O-G 45% Buildings 37% Misc. 2% 11 Landmarks in PT Buildings Introduction of strand systems (1962) Replaced button-head tendon system Development of ductile iron castings for singlestrand unbonded tendons (1963) Introduction of load-balancing design method (T.Y. Lin, 1963) Introduction of banded tendon layout for 2- way slab systems (late 1960s 1970s, Ned Burns 1985) Formation of Post-Tensioning Institute (1976) Improvements in corrosion resistance (1993) Encapsulated systems for 318 structures (2014) 12 Post-Tensioning Institute. All Rights Reserved 4

6 Testing at University of Texas ¼ Φ wires Bearing plate Shims Stressing washer Coldformed buttons Button-Headed (BBRV) Anchorage Innovations in post-tensioning Post-Tensioning Institute. All Rights Reserved 5

7 Structural Material Types Structural Steel Prestressed Concrete Reinforced Concrete Pre-Tensioned Post-Tensioned Bonded Strands Bonded Tendons Unbonded Tendons Internal External Prestressed Concrete What is Prestressing? Prestressing is a method of reinforcing concrete, actively resisting applied loads. Pre-Tensioning Steel tendons are stressed prior to concrete placement, usually at a precast plant remote from the construction site. Post-Tensioning Steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site Axially- vs. Eccentrically-Placed Prestressed Reinforcement Post-Tensioning Institute. All Rights Reserved 6

8 Combined Stresses Due to Prestress + External Loading Tendon Configurations Constant Harped Parabolic Load Balancing M 1 = P e = Primary Moment P w l 8e bal 2 Load W P tan P CGC W bal P tan P Tendon e Post-Tensioning Institute. All Rights Reserved 7

9 Where should the tendons be placed? Tension Moment Diagram No net eccentricity Tension No net eccentricity When moments are drawn on the tension side of a member, the diagram matches the general drape of the tendons. Post-Tensioning Losses Sources of Loss: Initial Losses Long Term Losses Friction, F Elastic shortening, ES Anchor set (wedge seating), A Concrete shrinkage, S Concrete creep, C Post-tensioning steel relaxation, R Types of Prestressing Tendons Bare Strand Monostrand Multistrand Post-Tensioning Institute. All Rights Reserved 8

10 Bonded vs. Unbonded PT PLASTIC TUBE P/T COATING STRAND (a) PLASTIC SHEATH FILLED WITH GREASE CONTAINING UNBONDED MONOSTRAND CORRUGATED METAL OR PLASTIC DUCT SPACE FILLED WITH GROUT STRANDS (b) CORRUGATED SHEATH WITH GROUTED STRANDS Wedge Close-up Extrusion Process Photo Courtesy of GSI Post-Tension Plastic Hopper Extruder Grease Injector Cooling Trough Extruded Strand Bare Strand 27 Post-Tensioning Institute. All Rights Reserved 9

11 Tendon Cutting to Length Photo Courtesy of Suncoast Post-Tension Ltd. 28 Fixed-End Anchor Pull Installation Photo Courtesy of G. Chacos Push Installation Field Installation 29 Tendon Identification Photo Courtesy of G. Chacos Identification Tag Color Coding 30 Post-Tensioning Institute. All Rights Reserved 10

12 Shipping of Tendons Photo Courtesy of VSL 31 Encapsulated Tendons When? Corrosive environments Decks exposed and/or near salt water Decks where de-icing salts will be used (Northern areas) PTI initiated change (PTI M , Addendum #3 December 2011): Fully encapsulated PT systems for all ACI 318 applications ACI refers to ACI ; this makes it a code requirement 32 Applicable Specifications ACI 423.7, ACI 301, and PTI M10.2 & M10.6 Tendon encapsulation for ACI 318 applications Protection of tendons during shipping, storage, and handling Watertight encapsulation of the strand over the entire length of the tendon, including anchorage PT coating; sheathing thickness; sheathing repair Translucent transition sleeves connected to anchors Sleeve and sheathing overlap Timely and proper tendon finishing; caps and pockets Adequate concrete cover Installer & Inspector qualifications (certification) Post-Tensioning Institute. All Rights Reserved 11

13 End Anchorages ENCAPSULATED ANCHOR STANDARD ANCHORS ENCAPSULATED ANCHOR WEDGES 34 Encapsulated Tendons Photo Courtesy of General Technologies, Inc. Encapsulated Tendons Photo Courtesy of General Technologies, Inc. Post-Tensioning Institute. All Rights Reserved 12

15 Bonded Tendon Photo Courtesy of VSL Permanent Grout Cap Seal Plastic Duct Positive Mechanical Coupling of Duct 40 Two-Way Slab with Bonded PT Photo Courtesy of VSL 41 March 10, 2015 Structural Systems Two-way Systems One-way Systems Preliminary Design Post-Tensioning Institute. All Rights Reserved 14

16 Two-Way Slab Construction Two-way framing action Spans typically 30 to 40 feet each direction Can have capitals, drop panels, beams & joist Efficient, least depth & material use Maximum tensile stress is limited to 6 f c Two-Way Slabs Photo Courtesy of Suncoast Post-Tension Ltd. 44 Two-Way Slabs Post-Tensioning Institute. All Rights Reserved 15

17 Two Way Slab with Caps Two-Way Slabs Banded Direction Tendons bundled together to create a pseudo beam A final effective force in the band is specified on the plans; similar to beams. Uniform Direction Tendons groups equally spaced perpendicular the bands A kip per foot force is specified on the plans; similar to one way slabs. Analysis method is the same for both directions. BANDED FLAT PLATE Post-Tensioning Institute. All Rights Reserved 16

19 Beam & (one-way) Slabs One-way framing action Beam spans up to 65 feet with 36 in. depth Slab spans up to 30 feet Effective frame action along beam lines Clean soffit for mechanical & electrical Maximum tensile stress is 12 f c Post-Tensioned Parking Garage Photo Courtesy of Suncoast Post-Tension Ltd. 54 Post-Tensioning Institute. All Rights Reserved 18

20 Long Span Beams Preliminary Design of Post- Tensioning Determine Minimum Prestressing One way Slabs/Parking Garages Aggressive environments use psi Normal environment use 125 psi Temperature tendons use 100 psi Two Way Slabs Minimum use 125 psi Load Balancing: 60% to 70% of DL for one- and two-way slabs 70% to 80% of DL for beams 100% for spandrel beams supporting ext. cladding 56 Variations to Estimations End bays may require more PT for the same span (higher moments, reduced drape, unrestrained condition) Heavy exterior skin loads that must be supported by the slab Parking slab PT quantity will increase due to increased clear cover requirements in aggressive regions Post-Tensioning Institute. All Rights Reserved 19

21 Preliminary Design Span/Depth Ratio Source: Design of Post-Tensioned Slabs With Unbonded Tendons; 3 rd Ed., PTI *These values apply for members with LL/Dl ratios < 1.0 Minimum Slab Thickness (Governed by Fire Rating, IBC 2003) Concrete Cover (Governed by Fire Rating; IBC 2003) Post-Tensioning Institute. All Rights Reserved 20

22 Restrained Vs Unrestrained A span is considered to be restrained when it can resist the horizontal forces developed during a fire Typically end bays are considered to be unrestrained unless calculations can show otherwise (ACI ) 61 Preliminary Design Tables PTI DC : Guide for Design of Post-Tensioned Buildings Preliminary Design Tables Two-way parking structures (flat plate or flat slab) Basic design parameters and assumptions: Loading: DL = self-weight + 5 lb/ft2 (0.2 kn/m2) (SDL); LL = 40 lb/ft2 (1.9 kn/m2) Slab minimum F/A = 125 psi (0.9 MPa) Three-span condition assumes a 62 ft (19 m) typical parking garage span Concrete: fc = 5000 psi (34 MPa); fci = 3000 psi (21 MPa) Assumed final effective force per 0.5 in. (12.7 mm) strand tendon = 27 kips (120 kn) Tendon CGS: Slab 1.5 in. (38 mm) top; 1 in. (25 mm) bottom interior span; 1.75 in. (44 mm) bottom exterior span Cover to nonprestressed reinforcement: Slab 1.25 in. (32 mm) top; and 0.75 in. (19 mm) bottom Column size: 14 x 28 in. (355 x 710 mm) Post-Tensioning Institute. All Rights Reserved 21

23 Preliminary Design Tables Two-way parking structures (flat plate or flat slab) Preliminary Design Tables Two-way parking structures (flat plate or flat slab) March 10, 2015 Restraint to Shortening Shear wall arrangement Pour strips / slip joints Post-Tensioning Institute. All Rights Reserved 22

24 Arrangement of Shear Walls 67 RTS Cracking Floor Shortening and Restraint Cracking Sources of Cracking Stiff lateral load resisting elements Short stiff columns Short floor-to-floor height Factors Contributing to Floor Shortening Elastic shortening due to precompression Creep shortening due to precompression Shrinkage Temperature drop Show Joint and Slip Details on Structural Drawings Inspect Slip Details During Construction 69 Post-Tensioning Institute. All Rights Reserved 23

25 Slab Shortening Components Category % Of Total Elastic 7 % Creep 9 % Shrinkage 56% Temperature 28% Total 100.0% Elastic and creep shortening contribute about 16% of the total slab shortening. The benefits of post-tensioning will offset the effect of these slightly higher movements. Shrinkage is the largest contributor to slab shortening with temperature being second. Note that shrinkage and temperature will be the same for both prestressed and nonprestressed structures. 70 Slab Shortening Components ASSUME 100 LONG SLAB AS FOLLOWS: POST-TENSIONING P/A PSI CONCRETE f STRESSING PSI TEMPERATURE CHANGE F DRYING 28 DAYS % ELASTIC SHORTENING PL = 190 (100) (12) =.07 AE 3,122,000 CREEP 2 (.07) =.14 THERMAL ( ) (30) (100) (12) =.20 DRYING SHRINKAGE LONG TERM (.00084) (100) (12) = 1.01 TOTAL Progress of Creep and Shrinkage with Time 100 % Shrinkage or Creep vs = volume to surface ratio Time in Days 72 Post-Tensioning Institute. All Rights Reserved 24

26 Restraint Cracking Photo Courtesy of Merrill Walstad 73 Slab Movement 74 Closure Strip CS s are used to reduce creep and shrinkage effects CS s must remain open for a specific amount of time (until creep and shrinkage has taken place) They are 3 to 4 ft wide temporary 3 to 4 ft. separations between two regions stressed separately CS Once creep and shrinkage takes place, CS is filled, tying the two slab regions together 75 Post-Tensioning Institute. All Rights Reserved 25

27 Closure Strip Slab is temporarily allowed to cure in smaller segments Can locate the lateral system in the middle of the individual pour Allows for internal stressing may be critical on subterranean projects Typically remain open for 30 to 60 days. Deck will still move for many months/years 76 Closure Strip Movement Joints 78 Post-Tensioning Institute. All Rights Reserved 26

29 March 10, 2015 Common Design Issue Facts about PT Common design issues Constructability ACI Facts About PT PT concrete is not crack free PT concrete is not water proof You can drill / make openings in PT slab If you drill into a tendon, it will not fly out of the building It is possible to upgrade / repair a PT structure PT structures are durable 83 Common design issues Structural modeling DO NOT use simplified analysis using coefficients (Direct Design Method) Use the Equivalent Frame Method, EFM Sec of ACI , excluding Sec and DO NOT use middle/column strip concept Apply total moment to entire bay section when using EFM Post-Tensioning Institute. All Rights Reserved 28

30 Common design issues Incomplete design Specify all PT requirements: Final effective force, or the number of tendons and corresponding final effective force Tendon profile Common design issues Not checking initial stress conditions Common design issues Over prestressing using sections that are too small for the amount of force being applied Post-Tensioning Institute. All Rights Reserved 29

31 Common design issues Not looking at beam/column joints and considering space needed for PT anchorages Common design issues Requiring 2 integrity tendons through the column (for two-way slabs; not for one-way slabs) Common design issues Not coordinating embed plates for curtain wall systems with the PT anchorage requirements Post-Tensioning Institute. All Rights Reserved 30

32 Common design issues Specifying the same force and high/low points in spans of significantly different lengths Common design issues End bays may require more PT for the same span Reduced tendon drape Restraint vs. unrestrained condition Heavy exterior skin loads that must be supported by the slab Parking slab PT quantity will increase due to increased clear cover requirements in aggressive regions Review Common design issues Added tendon anchorage area Review Post-Tensioning Institute. All Rights Reserved 31

33 Cantilever Tendons One-Way Slab Construction 95 Banded Tendon Anchorage ACI , Two horizontal bars at least #4 to be provided parallel to the slab edge; bars to extend at least 6 beyond the outermost anchorage edge Hairpins required when 6 or more anchorages are in a group (spaced < 12 apart) 96 Post-Tensioning Institute. All Rights Reserved 32

34 Common design issues Backup bars and bursting reinforcement Banded Tendon Anchorage Photo Courtesy of General Technologies, Inc. 98 Band Anchorage 99 Post-Tensioning Institute. All Rights Reserved 33

35 Transfer Girders Photo Courtesy of DYWIDAG Systems International Harped profile may be more efficient to resist concentrated loads PT forces can balance the dead loads Stage stressing to avoid overstressing the beams Multi-strand tendons when large forces are required 100 Podium Slab Congestion Near Supports Photo Courtesy of Walter P. Moore 102 Post-Tensioning Institute. All Rights Reserved 34

36 Bad Duct Layout Photo Courtesy of Magnusson Klemencic Associates 103 Bad Conduit Layout Photo Courtesy of Seneca Structural Engineering Inc. 104 Conduit Layout Photo Courtesy of Cary Kopczynski & Company 105 Post-Tensioning Institute. All Rights Reserved 35

37 Eccoduct Layout Photo Courtesy of Cary Kopczynski & Company 106 Eccoduct Layout Photo Courtesy of Cary Kopczynski & Company 107 P/T and Rebar Placement Banded direction reinforcement should be in the top most layer at the columns/walls Uniform direction reinforcement is directly below. Layout maximizes the profile of the tendons in each direction. Top bars should not have excessive droop away from the column Column/Wall should not extend into the future Reduces punching shear strength. If shear studs are used, top of the stud should align with the reinforcing at the column. Look for smooth tendon profiles No kinks. Post-Tensioning Institute. All Rights Reserved 36

40 Typical Re-Shoring Pour Strip For most two way slab applications the pour strip is not designed to be a cantilever The pour strip edges NEED to be fully re-shored to the foundation Without adequate re-shores pour strip deflections and flexural cracking can occur. Published in late 2014 Member based code No Chapter 18 on prestressing ACI Prestressing requirements in chapters as applicable Refers to referenced standard in for corrosion protection of unbonded tendons. New requirements for two-way slabs with bonded tendons and ; ACI Post-Tensioning Institute. All Rights Reserved 39

41 New in ACI Shrinkage and temperature reinforcement New in ACI Shrinkage and temperature reinforcement Elongations Measured elongation needs to be within 7% of calculated value (ACI , ); change in wording in , in an attempt to clarify requirement not achieved. For tendons outside the 7%, the EOR shall ascertain and correct the problem. Timing for elongation approval is critical. 120 Post-Tensioning Institute. All Rights Reserved 40

42 Elongation Discrepancy Causes of Improper Elongation Poor marking procedures Inaccurate measurements Inaccurate gauge reading Improper stressing procedure Math errors Excessive seating loss Equipment malfunction 121 March 10, 2015 Quality & Durability Finishing Inspection Certification Placement Inspection Requirements Check for damage to sheathing and encapsulation items. Record the repairs Verify number of tendons and CGS from structural drawings Verify that minimum number of tendons pass through column in both directions Remove/move excessive conduit, penetrations, etc., especially by the anchors and columns Look for tendons with extreme bends or odd configurations Conduit in the slab and penetrations too close to the anchorages 123 Post-Tensioning Institute. All Rights Reserved 41

43 Inspection of Finishing Tendon tails to be cut within one working day after approval of elongations (ACI 301). Tail cuts to be made between ½ in. to a distance specified by system supplier proper lock in of encapsulation cap. End caps to be installed within 8 hours of cutting of the tails (ACI 301). 124 Tendon Finishing Filling of Stressing Pockets (PTI FAQ 11) Protruding tendon end of proper length to accommodate end cap Surface preparation: free from PT coating, grease, form release agent, dirt, loose concrete, etc. Bonding agent High quality premixed cementitious chloride-free low-shrinkage repair grout Proper mixing and application 125 Certification Post-Tensioning Material Producers Certification of plants producing single strand unbonded tendons (PTI Certified Plants) Post-Tensioning Institute. All Rights Reserved 42

44 Field Personnel Slab-on-Ground Installer-Stressor Level 1 Unbonded PT Field Installation Level 2 Unbonded PT Ironworker Level 2 Unbonded PT Inspector Level 1 Bonded PT Field Installation Level 2 Bonded PT Field Specialist Certification Thank You! Questions? 128 Post-Tensioning Institute. All Rights Reserved 43