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Prestressed Concrete WisDOT Bridge Manual Chapter 19 – Prestressed Concrete Table of Contents 19.1 Introduction ...................................................................................................................... 3 19.1.1 Pretensioning ........................................................................................................... 3 19.1.2 Post-Tensioning ........................................................................................................ 3 19.2 Basic Principles ................................................................................................................ 4 19.3 Pretensioned Member Design .......................................................................................... 7 19.3.1 Design Strengths ...................................................................................................... 7 19.3.2 Loading Stages ......................................................................................................... 8 19.3.2.1 Prestress Transfer ............................................................................................ 8 19.3.2.2 Losses .............................................................................................................. 8 19.3.2.2.1 Elastic Shortening...................................................................................... 8 19.3.2.2.2 Time-Dependent Losses............................................................................ 9 19.3.2.2.3 Fabrication Losses .................................................................................... 9 19.3.2.3 Service Load ................................................................................................... 10 19.3.2.3.1 Prestressed I-Girder ................................................................................ 10 19.3.2.3.2 Prestressed Box Girder ........................................................................... 11 19.3.2.4 Factored Flexural Resistance .......................................................................... 11 19.3.2.5 Fatigue Limit State .......................................................................................... 12 19.3.3 Design Procedure ................................................................................................... 12 19.3.3.1 Prestressed I-Girder Member Spacing ............................................................ 12 19.3.3.2 Prestressed Box Girder Member Spacing ....................................................... 12 19.3.3.3 Dead Load ...................................................................................................... 13 19.3.3.4 Live Load ........................................................................................................ 13 19.3.3.5 Live Load Distribution ...................................................................................... 13 19.3.3.6 Dynamic Load Allowance ................................................................................ 14 19.3.3.7 Prestressed I-Girder Deck Design ................................................................... 14 19.3.3.8 Composite Section .......................................................................................... 14 19.3.3.9 Design Stress .................................................................................................. 15 19.3.3.10 Prestress Force ............................................................................................. 16 19.3.3.11 Service Limit State ........................................................................................ 17 19.3.3.12 Raised, Draped or Partially Debonded Strands ............................................. 18 19.3.3.12.1 Raised Strand Patterns.......................................................................... 19 19.3.3.12.2 Draped Strand Patterns ......................................................................... 19 July 2021 19-1 WisDOT Bridge Manual Chapter 19 – Prestressed Concrete 19.3.3.12.3 Partially Debonded Strand Patterns ....................................................... 21 19.3.3.13 Strength Limit State ....................................................................................... 22 19.3.3.13.1 Factored Flexural Resistance ................................................................ 22 19.3.3.13.2 Minimum Reinforcement ........................................................................ 25 19.3.3.14 Non-prestressed Reinforcement .................................................................... 26 19.3.3.15 Horizontal Shear Reinforcement ................................................................... 26 19.3.3.16 Web Shear Reinforcement ............................................................................ 28 19.3.3.17 Continuity Reinforcement .............................................................................. 31 19.3.3.18 Camber and Deflection ................................................................................. 33 19.3.3.18.1 Prestress Camber .................................................................................. 34 19.3.3.18.2 Dead Load Deflection ............................................................................ 37 19.3.3.18.3 Residual Camber ................................................................................... 38 19.3.4 Prestressed I-Girder Deck Forming ........................................................................ 38 19.3.4.1 Equal-Span Continuous Structures ................................................................. 39 19.3.4.2 Unequal Spans or Curve Combined With Tangent .......................................... 40 19.3.5 Construction Joints ................................................................................................. 40 19.3.6 Strand Types .......................................................................................................... 40 19.3.7 Construction Dimensional Tolerances .................................................................... 41 19.3.8 Prestressed I-Girder Sections ................................................................................. 41 19.3.8.1 Prestressed I-Girder Standard Strand Patterns ............................................... 45 19.3.9 Prestressed Box Girders Post-Tensioned Transversely .......................................... 45 19.3.9.1 Available Prestressed Box Girder Sections and Maximum Span Lengths ....... 45 19.3.9.2 Decks and Overlays ........................................................................................ 47 19.3.9.3 Grout between Prestressed Box Girders ......................................................... 47 19.4 Field Adjustments of Pretensioning Force ...................................................................... 48 19.5 References ..................................................................................................................... 50 19.6 Design Examples ........................................................................................................... 51 July 2021 19-2 WisDOT Bridge Manual Chapter 19 – Prestressed Concrete 19.1 Introduction This chapter provides information intended for prestressed I-girders. Prestressed box girders and general prestressed concrete guidelines are also included in this chapter. The definition of prestressed concrete as given by the ACI Committee on Prestressed Concrete is: "Concrete in which there has been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are counteracted to a desired degree. In reinforced concrete members the prestress is commonly introduced by tensioning the steel reinforcement.” This internal stress is induced into the member by either of the following prestressing methods. 19.1.1 Pretensioning In pretensioning, the tendons are first stressed to a given level and then the concrete is cast around them. The tendons may be composed of wires, bars or strands. The most common system of pretensioning is the long line system, by which a number of units are produced at once. First the tendons are stretched between anchorage blocks at opposite ends of the long stretching bed. Next the spacers or separators are placed at the desired member intervals, and then the concrete is placed within these intervals. When the concrete has attained a sufficient strength, the steel is released and its stress is transferred to the concrete via bond. 19.1.2 Post-Tensioning In post-tensioning, the concrete member is first cast with one or more post-tensioning ducts or tubes for future insertion of tendons. Once the concrete is sufficiently strong, the tendons are stressed by jacking against the concrete. When the desired prestress level is reached, the tendons are locked under stress by means of end anchorages or clamps. Subsequently, the duct is filled with grout to protect the steel from corrosion and give the added safeguard of bond. In contrast to pretensioning, which is usually incorporated in precasting (casting away from final position), post-tensioning lends itself to cast-in-place construction. July 2021 19-3 WisDOT Bridge Manual Chapter 19 – Prestressed Concrete 19.2 Basic Principles This section defines the internal stress that results from either prestressing method. First consider the simple beam shown in Figure 19.2-1. Figure 19.2-1 Simple Span Prestressed Concrete Beam The horizontal component, P, of the tendon force, F, is assumed constant at any section along the length of the
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