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Questions and Answers
What is the maximum strain in the extreme compression fibre according to AS 3600 8.1.3?
What is the maximum strain in the extreme compression fibre according to AS 3600 8.1.3?
What is the typical range for the concrete compressive strength (f'c) used in prestressed concrete applications?
What is the typical range for the concrete compressive strength (f'c) used in prestressed concrete applications?
Which of the following statements about concrete's tensile strength (f'ct) is correct?
Which of the following statements about concrete's tensile strength (f'ct) is correct?
What is the typical range of overall losses in prestressing force?
What is the typical range of overall losses in prestressing force?
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What does the term Tpb refer to in the context of prestressing tendons?
What does the term Tpb refer to in the context of prestressing tendons?
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Which of the following is categorized as an immediate loss in prestressing?
Which of the following is categorized as an immediate loss in prestressing?
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What happens to the prestressing force in a tendon after it is first tensioned?
What happens to the prestressing force in a tendon after it is first tensioned?
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Deferred losses in prestressing are characterized by which of the following?
Deferred losses in prestressing are characterized by which of the following?
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Which factor does NOT contribute to immediate losses in prestressing?
Which factor does NOT contribute to immediate losses in prestressing?
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Which method is primarily used to improve the estimation of prestressing losses?
Which method is primarily used to improve the estimation of prestressing losses?
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What is the primary purpose of prestressing cables in a concrete structure?
What is the primary purpose of prestressing cables in a concrete structure?
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What does pre-compression primarily achieve in a prestressed concrete beam?
What does pre-compression primarily achieve in a prestressed concrete beam?
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What occurs when external loads are applied to a prestressed beam?
What occurs when external loads are applied to a prestressed beam?
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How do tendons contribute to prestressed concrete structures?
How do tendons contribute to prestressed concrete structures?
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Which of the following accurately describes the effect of draping tendons?
Which of the following accurately describes the effect of draping tendons?
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What is the characteristic value of concrete strength indicated by $f'c$?
What is the characteristic value of concrete strength indicated by $f'c$?
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What happens to the concrete free body once the tendons are grouted?
What happens to the concrete free body once the tendons are grouted?
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What is a potential effect of cracking in concrete?
What is a potential effect of cracking in concrete?
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What is the main difference between pre-tensioned and post-tensioned tendons?
What is the main difference between pre-tensioned and post-tensioned tendons?
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What occurs during the bonding of tendons within a concrete structure?
What occurs during the bonding of tendons within a concrete structure?
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Which system is primarily used in building structures in Australia?
Which system is primarily used in building structures in Australia?
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What is a characteristic of unbonded tendons?
What is a characteristic of unbonded tendons?
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What does the term 'multi-strand' refer to in anchorage systems?
What does the term 'multi-strand' refer to in anchorage systems?
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What is one limitation of unbonded tendons according to the regulatory standards?
What is one limitation of unbonded tendons according to the regulatory standards?
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What property of bonded tendons is essential for their ultimate load-bearing capability?
What property of bonded tendons is essential for their ultimate load-bearing capability?
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What type of ducts contain post-tensioned tendons?
What type of ducts contain post-tensioned tendons?
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What is a characteristic feature of a one way slab system?
What is a characteristic feature of a one way slab system?
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Which type of slab system is characterized by being supported on four sides?
Which type of slab system is characterized by being supported on four sides?
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What is the typical depth range of bands in one way slab systems compared to the slabs themselves?
What is the typical depth range of bands in one way slab systems compared to the slabs themselves?
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What type of prestressing strands are used in slab systems for high tensile strength?
What type of prestressing strands are used in slab systems for high tensile strength?
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What happens to a portion of the prestress in slab systems due to material behavior?
What happens to a portion of the prestress in slab systems due to material behavior?
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What is the significance of the span to depth (L/D) ratio in schematic design?
What is the significance of the span to depth (L/D) ratio in schematic design?
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Why must cables in continuous beams follow a minimum practical radius?
Why must cables in continuous beams follow a minimum practical radius?
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What distinguishes two way flat slabs from two way slabs?
What distinguishes two way flat slabs from two way slabs?
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In which region of a prestressed beam is shear cracking most prevalent?
In which region of a prestressed beam is shear cracking most prevalent?
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What should be considered along with the design of a one way slab across band beams?
What should be considered along with the design of a one way slab across band beams?
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Which of the following is NOT a type of slab system mentioned?
Which of the following is NOT a type of slab system mentioned?
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What type of cracking is characterized by both moment and shear influences?
What type of cracking is characterized by both moment and shear influences?
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What problem arises from idealized parabolic cable profiles in practical application?
What problem arises from idealized parabolic cable profiles in practical application?
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What cracking pattern results from torsion in beams?
What cracking pattern results from torsion in beams?
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How does the addition of depth near supports benefit structural performance?
How does the addition of depth near supports benefit structural performance?
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Which duct type is easier to profile in beam and slab designs?
Which duct type is easier to profile in beam and slab designs?
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Study Notes
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 fpb, and breaking load Tpb
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
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Description
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.
