Post Tension Concrete Floor Systems

Post Tension Concrete Floor Systems

• Post-tension concrete floor systems are an elegant engineering system
• They improve service and strength behaviours of reinforced concrete
• Design of many engineering structures is cost effective
• They improve general performance of structures

Definition of Pre-stressing

• Preloading the structure before application of design loads
• Improve general performance
• The image shows the process with stages
• Unstressed beam, load deflection, Tendons stressed, Prestress forces, Prestress deflection (up)

Objectives of Pre-stressing

• Control or eliminate tensile stresses in the concrete
• Up to service load levels
• Control or eliminate deflection at specific load levels
• Allow for the use of high-strength steel and concrete

Prestressing - Basic Idea

• High tensile wire strands approximately 1870 MPa
• Higher strength concrete approximately 30-50 MPa
• Tendons cast within concrete
• Tendons are jacked to high stresses
• Reactions are pressed against ends of concrete member
• Stress is permanently transferred to cast-in end anchors

Terminology

• Strand: A group of high tensile wires woven together
• Tendon: Steel wire, strand or bar for pre-stressing
• Cable: Group of tendons collected together
• Pre-stressed: Prior stressing of both concrete and tendons
• Pre-tensioned: Tendons tensioned before concrete pouring
• Post-tensioned: Tendons tensioned after concrete has set. Commonly used in Australia.

Modern Anchorage Systems

• Mono-strand: Strands jacked individually
• Multi-strand: Strands jacked together
• Anchorage at slab edge, stressing pocket

Terminology (continued)

• Bonded tendons: Ducts filled with cement grout, effectively bonding to concrete like reinforcing steel
• Unbonded tendons: Tendons greased, enclosed in plastic sheaths; not grouted; allowed under some conditions

Load Balancing Concepts

• Suspension bridge cables balance vertical loads, not prestressed
• Prestressed structures use cables against slab to pre-balance slab load

Pre-stress Concepts - Summary

• Key concepts explained through diagrams
• Improved deflection and strength
• Cracking reduction, stiffness increase
• Pre-compression reduces cracking, and deflection
• Tendons provide upward load to carry self-weight

Some Basic Concepts

• Tendons grouted, part of concrete "free body", integrated whole
• External load application (self-weight plus other loads) considered
• C and T forces separate to form moment couple resisting external moment
• Basis for internal couple approach

Material Properties - Concrete

• Concrete strength ( f'_c): Uniaxial compression strength of test cylinders, 95% of test samples exceed value
• AS 3600 limits maximum compression fiber strain to 0.003
• Typical strength for prestressed concrete applications is around 32-50 MPa

Material Properties -Reinforcing Steel and Prestressing Tendons

• High tensile strands: strengths given in table
• Actual and idealized stress-strain curves shown.
• Characteristic breaking strand strength fp_b, and breaking load Tp_b

Introduction to Losses

• Prestressing force diminishes from instant steel is tensioned
• Losses occur during prestress transfer & through member life
• Range of losses is typically 15-30% of initial jacking force

Introduction to Losses (continued)

• Immediate losses: depend on method and equipment
• Deferred losses: gradual losses over the structure's life

Prestressing Losses (continued)

• Types of losses: elastic deformation, duct friction, anchorage slip, stress relaxation, shrinkage deformation, creep deformation.

Structural floor systems

• Horizontal spanning
• Transfer gravity loads to the vertical system
• Significant percent of structural cost
• Highly repetitive, efficiencies, cost savings
• Categorized as one and two way spanning

Reinforced and Prestressed Concrete

• Types of slabs: ribbed slab and beam, beam and (2 way) slab, band-beam and slab.
• Flat plate, - 2 way, Flat slab - two way, Residential, Smaller spans; Large areas: Hospital & Office, Common Office & Car-parks, Prestressed.

Sketching Concept Designs

• Concrete layout plan drawing
• Band beam and concept designs

Beam and Slab Depths

• Beam depth, slab depth
• Slab span for L/D ratios in band beam systems
• Concepts explained via diagrams.

Concrete layout plan drawing - Flat slab

• Dimensions and spacing of components explained via diagrams

Slab Systems

• Five types of slab systems (one-way supported by walls/beams, two-way support on 4 sides, flat plates without drop panels, two-way flat slabs with drop panels, one way slab and band beam)
• Analysis of one-way slab systems, mostly used in cores

One-way slab (supported on walls or narrow beams).

• Diagram and description of a one-way slab structure

Two-way slabs (supported on 4 sides)

• Diagram and description of a two-way slab structure

Slab Systems (continued)

• One-way slabs running across band beams.
• Moments arise across the band beam
• Band depth normally 100-200mm deeper than slabs

Prestressing key Considerations

• Higher strength concrete supports higher loads
• High-tensile strands up to 1870 MPa
• Prestressing loss due to concrete creep and shrinkage
• Strain example, and impact on strength bars

Prestressed Concrete-Improved Serviceability

• Uncracked sections, load balancing supports dead loads
• Reduced deflections in prestressed designs
• Sections approximate 70% depth of reinforced concrete
• Post-tensioning allows for longer spans

Determinate and Indeterminate Structures

• Statically determinate structures force and moments can be determined in static equilibrium.
• Hyperstatic/indeterminate structures have more members or support than needed for stability.
• Insufficient support results in instability

Statically Determinate Beams

• Types of beams: simply supported, single/double cantilever ends
• Post-tensioned beams often more economical for spans over 7-8 meters
• Span to depth ratio (L/D) for preliminary sizing.

Schematic Design Approaches

• Span to depth ratios (L/D) for preliminary sizing.
• Guidelines for various prestressed structural types (beams, transfers, heavy load, floors, continuous band beams, etc)

Practical Tendon Profiles

• Cable profiles conceptualized as parabolic, but follow "minimum practical radius" in real-life
• Smoother transition for ducts & tendons is important.

Prestressed Beams

• Shear carried by concrete, calculated for two conditions
• Regions (A, B, C) with varying moment/ shear ratios
• Importance of moment & shear in regions B, and the possibility of "flexure-shear" or "web-shear" cracking

Torsion In Beams

• Torsional cracking around beam perimeter
• Closed ties used as additional shear reinforcement
• Longitudinal forces resisted by longitudinal bars in corners

Explore the engineering marvel of post-tension concrete floor systems through this quiz. Understand the principles of pre-stressing and its objectives, and learn how these systems enhance the performance and cost-effectiveness of various structures. Test your knowledge on the important aspects of prestressing techniques and their applications.