Prestressed Concrete Design AS 3600 Quiz
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Questions and Answers

What is the maximum strain limit in the extreme compression fibre according to AS 3600?

  • 0.002
  • 0.0025
  • 0.004
  • 0.003 (correct)

    • Why should characteristic tensile strength f'ct not be relied upon for strength calculations?

  • It is only applicable at high stress levels.
  • It does not consider time-dependent factors.
  • It varies significantly with temperature.
  • It is designed primarily for serviceability calculations. (correct)

    • What begins to diminish as soon as the steel tendon is first tensioned?

  • The recorded value in the jack gauge.
  • The tensile strength of the concrete.
  • The compressive strength of the member.
  • The prestressing force in the tendon. (correct)

    • What is the typical range for concrete compressive strength f'c in prestressed applications?

    <p>32 – 50 MPa</p> Signup and view all the answers

    Which term refers to the 'breaking load' of reinforcing steel or prestressing tendons?

    <p>Tpb</p> Signup and view all the answers

    What percentage range is commonly considered for overall losses in initial jacking force?

    <p>15 to 30 percent</p> Signup and view all the answers

    Which of the following is categorized as immediate losses in prestressing?

    <p>Friction along the member during prestressing</p> Signup and view all the answers

    Deferred losses in prestressing are characterized by which of the following?

    <p>They are gradual losses that occur over the life of the structure.</p> Signup and view all the answers

    What is a potential cause of immediate losses during prestressing?

    <p>Friction caused by duct alignment issues</p> Signup and view all the answers

    Which factor does NOT contribute to immediate losses in the prestressing process?

    <p>Time-dependent stress relaxation</p> Signup and view all the answers

    What is the primary purpose of prestressing in concrete structures?

    <p>To enhance compression strength and decrease deflection</p> Signup and view all the answers

    What effect does prestressing have on cracking in concrete?

    <p>It reduces cracking and enhances stiffness</p> Signup and view all the answers

    In the context of tendons in concrete, what does 'draping' refer to?

    <p>Positioning tendons to provide upward load</p> Signup and view all the answers

    What happens to the tendons once they are grouted in concrete?

    <p>They effectively become part of the concrete unit</p> Signup and view all the answers

    Which factor contributes most significantly to the reduction of deflection in prestressed concrete?

    <p>Application of pre-compression</p> Signup and view all the answers

    What effect does an external load have on the forces within a beam post-prestressing?

    <p>It causes separation of C and T forces into a moment couple</p> Signup and view all the answers

    What is a significant benefit of using high-strength steel and concrete in prestressing?

    <p>It provides greater resistance to tensile cracking.</p> Signup and view all the answers

    Which statement accurately describes tendons in prestressed concrete?

    <p>Tendons are high tensile wires intended to be pre-stressed.</p> Signup and view all the answers

    What is the characteristic value of concrete strength represented by f'c?

    <p>The strength that 95% of samples must exceed</p> Signup and view all the answers

    What happens to the concrete during the jacking process in prestressing?

    <p>The concrete is pre-stressed in compression.</p> Signup and view all the answers

    Which of the following is a result of the internal couple approach in prestressed concrete?

    <p>Improved load resistance of the beam</p> Signup and view all the answers

    What effect does prestressing have on deflection within concrete structures?

    <p>It controls or eliminates deflection at specific load levels.</p> Signup and view all the answers

    What is typically the first step during the prestressing process?

    <p>The tendons are jacked to high stresses.</p> Signup and view all the answers

    Which feature is characteristic of a strand in prestressed concrete?

    <p>It consists of multiple high tensile wires woven together.</p> Signup and view all the answers

    What is the significance of the compressive state in prestressed concrete?

    <p>It allows the concrete to support higher tensile loads without cracking.</p> Signup and view all the answers

    What is the main difference between pre-tensioned and post-tensioned tendons?

    <p>Pre-tensioned tendons are tensioned before pouring concrete, while post-tensioned tendons are tensioned after.</p> Signup and view all the answers

    Which of the following statements is true regarding bonded tendons?

    <p>They are filled with cement grout after stressing, bonding them to concrete.</p> Signup and view all the answers

    What is the primary purpose of using unbonded tendons?

    <p>For corrosion protection while allowing monitoring and replacement of strands.</p> Signup and view all the answers

    In which application are unbonded tendons primarily permitted?

    <p>Slabs on ground.</p> Signup and view all the answers

    Which of the following correctly describes a multi-strand anchorage system?

    <p>Strands are tensioned and jacked together as a group.</p> Signup and view all the answers

    What does 'stress compatibility' imply regarding bonded tendons?

    <p>They can develop additional stresses under bending, enhancing performance.</p> Signup and view all the answers

    Which of the following characterizes the mono-strand system?

    <p>Allows for individual adjustment of each strand after installation.</p> Signup and view all the answers

    What is a common characteristic of all pre-stressed systems?

    <p>They incorporate tendons that are initially stressed before application loads.</p> Signup and view all the answers

    What are the five main types of slab systems mentioned?

    <p>One way slab, two way slabs, two way flat plates, two way flat slabs, one way slab and band beam</p> Signup and view all the answers

    Which statement is true regarding two way edge supported slabs?

    <p>They are rarely prestressed and found as small areas in building cores.</p> Signup and view all the answers

    What is a key characteristic of one way slabs supported on band beams?

    <p>Extra moments arise at the change of section.</p> Signup and view all the answers

    What depth is typically required for bands to be considered as bands rather than beams?

    <p>The width should be about 6 times the depth.</p> Signup and view all the answers

    What factor leads to an amount of prestress being lost in concrete?

    <p>Concrete creep and shrinkage</p> Signup and view all the answers

    What material properties allow for higher load support in prestressed concrete?

    <p>Higher strength concrete and high tensile strands</p> Signup and view all the answers

    Moment coefficients can be referenced from which section of AS 3600?

    <p>Table 6.10.3.2</p> Signup and view all the answers

    Which type of slab is designed to run across band beams with specific considerations for additional moments?

    <p>One way slab</p> Signup and view all the answers

    Study Notes

    Post Tension Concrete Floor Systems

    • Post tension concrete floor systems are an elegant engineering system
    • Improve many service and strength performance behaviours of reinforced concrete
    • Achieve cost effective and practical solutions for many engineering structures

    Definition of Pre-stressing

    • Preloading a structure before application of design loads
    • Improves general performance
    • Key stages include:
      • Unstressed beam
      • Load deflection (down)
      • Tendons stressed
      • Prestress forces
      • Prestress deflection (up)
      • Total deflection (flat)

    Prestressing

    • Intentional creation of permanent stresses in a structure for improved behaviour and strength
    • T.Y. Lin (1955) defined prestressing

    Objectives of Pre-stressing

    • Control or eliminate tensile stresses in the concrete, at least up to service load levels
    • Control or eliminate deflection at some specific load level
    • Allows the use of high-strength steel and concrete

    Prestressing - The Basic Idea

    • high tensile wire strands ≈ 1870 MPa
    • higher strength concretes f' ≈ 30 – 50 MPa
    • Tendons are cast within the concrete, initially free within ducts
    • Ducts are grouted later to bond tendons
    • Tendons are jacked to high stresses before grouting
    • Reactions are transferred to cast-in end anchors, becoming permanently fixed
    • Duct profile varies according to the member's purpose
    • Result: tendons are permanently pre-stressed in tension, concrete in compression

    Terminology

    • Strand: a high tensile wire element woven together
    • Tendon: wire, strand or bar intended for prestressing
    • Cable: groups of tendons collected together in a duct or anchorage
    • Pre-stressed: prior stressing of concrete and tendons before service use
    • Pre-tensioned: tendons stressed before concrete pouring and bonded after concrete gain strength
    • Post-tensioned: tendons laid in ducts and stressed after concrete strength achieved (most common in Australia)

    Construction Systems - Modern Anchorage Systems

    • Mono-strand: strands are jacked individually
    • Multi-strand: strands are jacked together
    • After stressing and grouting, strands bond to the member concrete

    Material Properties – Concrete

    • Concrete strength (f'c): uniaxial compression strength of test cylinders
    • Exceeded by 95% of test samples
    • AS 3600 8.1.3 limits extreme compression fibre strain to 0.003
    • Concrete retains most strength at this limit, but starts to decrease
    • Typically f'c = 32 – 50 MPa for prestressed applications

    Material Properties – Concrete (Cont'd)

    • Concrete experiences time-dependent strain (creep and shrinkage)
    • Concrete has a characteristic tensile strength (f'ct) used in serviceability calculations
    • f'ct is not reliable as a strength measure

    Material Properties – Reinforcing Steel and Prestressing Tendons

    • High tensile wire strands with strength up to 1870 MPa
    • Characteristic breaking strength (fpb) is actually the breaking stress, also termed as Tpb for breaking load
    • Table 3.3.1 in AS 3600 provides tensile strengths of commonly used strands

    Introduction to Losses

    • Prestress force diminishes from the instant the tendon is tensioned
    • Losses include immediate and deferred (time-dependent) losses (15-30% of initial jacking force)
    • Examples of loss causes:
      • Elastic deformation
      • Duct friction
      • Anchorage slip
      • Stress relaxation
      • Shrinkage deformation
      • Creep deformation

    Structural Floor Systems – Basic Concepts

    • Horizontal spanning (transfer gravity loads to vertical system)
    • Highly repetitive, therefore, efficient, resulting in cost savings
    • Categorized as one and two-way spanning

    Reinforced and Prestressed Concrete

    • Types of slab systems: ribbed slab and beam, beam and two-way slab, band-beam and slab, flat plate, flat slab,
    • Different structural types (historic, industrial, residential, large areas, etc) suit different slab types

    Sketching Concept Designs – Band Beam

    • Concrete layout plan drawing for band beams
    • Includes columns and gridlines for reference
    • Shows sections and step-ins

    Beam and Slab Depths

    • Diagram showing beam depth and slab depth
    • Indicates slab span L/D ratios in band beam systems

    Concrete Layout Plan Drawing – Flat Slab

    • Layout plan drawing for flat slabs
    • Shows slab span for L/D ratios

    Slab Systems

    • Slab systems are categorized into: one-way slab, two-way slab, two-way flat plates, two-way flat slabs, one-way slab and band beam

    One-way Slab (supported on walls/narrow beams)

    • Description of one-way slab system supported on walls or narrow beams

    Two-way Slabs (supported on four sides/Waffle)

    • Description of two-way slab system supported on four sides

    Slab Systems (Cont'd)

    • One-way slab system spanning across band beams analyzed like continuous beams
    • Cable profiling to reach highest point at the band edge considering secondary reinforcement and extra moments
    • Bands normally 100-200mm deeper than slabs with width 6 times the depth

    Prestressing Key Considerations

    • Higher strength concrete and high anchor stresses
    • High tensile strands (up to 1870 MPa)
    • Prestressing losses due to concrete creep and shrinkage, significant in lower strength bars (example: 100 MPa loss for 500 x 10-6 shrinkage strain)

    Prestressed Concrete –Improved Serviceability

    • Sections uncracked and load balancing supports dead load
    • Much reduced deflections
    • Sections approximately 70% the depth of an equivalent reinforced concrete section
    • Post-tensioning enables longer spans

    Statically Determinate and Indeterminate Structures in Prestressed Design

    • Statically determinate structures have forces and bending moments determined by static equilibrium (e.g., simple beam)
    • Statically indeterminate structures have more members or reactive restraints than necessary for stability (e.g., introduction of a redundant support)
    • Insufficient support can create instability

    Statically Determinate Beams

    • Statically determinate beams include simply supported beams, single span beams with single/double cantilever ends, or single span cantilevers themselves
    • Post-tensioned beams economical than simply supported ones for spans >7-8m
    • Cantilever spans > 3m also more economical as post-tensioned
    • Span-to-depth (L/D) ratio adjustment for preliminary sizing: L/D ≈ 1.2 to 1.4 adjustment to reduce depth required for comparable reinforced beams (without sacrificing serviceability)

    Schematic Design Approaches

    • L/D ratios guidelines for preliminary sizing of members (beams, transfer members, heavy load, floors, continuous band beams, one-way slabs, two-way flat plates, Two-way flat slabs with drop panels, Water Tight Structural Slabs)
    • Notes on fire rating considerations for slab thicknesses

    Practical Tendon Profiles

    • Conceptualising cable profiles in continuous beams as parabolic, but in reality, cables have a minimum radius of curvature for smooth transition
    • Practical profiling adjustments for ducts and tendons, particularly in dealing with unbalanced live load cases

    Prestressed Beams – Ultimate Shear

    • Shear carried by concrete, calculated for moment cracking, flexure-shear cracks, or web-shear cracks
    • Region A: mostly moment cracking
    • Region B: moment and shear cracking
    • Region C: web shear cracking

    Torsion in Beams

    • Torsional forces cause diagonal cracking around the beam perimeter
    • Torsion reinforcement (closed ties/stirrups) supplemental to main shear reinforcement
    • Longitudinal forces resisted by longitudinal bars in corners

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    Post Tension Concrete Floor Systems CIVIL3811/9811 PDF

    Description

    Test your knowledge on the principles of prestressed concrete according to AS 3600. This quiz covers key concepts including strain limits, strength calculations, losses in prestressing, and the effects of prestressing on concrete structures. Ideal for students and professionals in civil engineering.

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