Prestressed Concrete Design AS 3600 Quiz

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 (f_pb) is actually the breaking stress, also termed as T_pb 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

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.