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?

32 – 50 MPa Signup and view all the answers

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

Tpb Signup and view all the answers

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

15 to 30 percent Signup and view all the answers

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

Friction along the member during prestressing Signup and view all the answers

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

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

What is a potential cause of immediate losses during prestressing?

Friction caused by duct alignment issues Signup and view all the answers

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

Time-dependent stress relaxation Signup and view all the answers

What is the primary purpose of prestressing in concrete structures?

To enhance compression strength and decrease deflection Signup and view all the answers

What effect does prestressing have on cracking in concrete?

It reduces cracking and enhances stiffness Signup and view all the answers

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

Positioning tendons to provide upward load Signup and view all the answers

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

They effectively become part of the concrete unit Signup and view all the answers

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

Application of pre-compression Signup and view all the answers

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

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

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

It provides greater resistance to tensile cracking. Signup and view all the answers

Which statement accurately describes tendons in prestressed concrete?

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

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

The strength that 95% of samples must exceed Signup and view all the answers

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

The concrete is pre-stressed in compression. Signup and view all the answers

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

Improved load resistance of the beam Signup and view all the answers

What effect does prestressing have on deflection within concrete structures?

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

What is typically the first step during the prestressing process?

The tendons are jacked to high stresses. Signup and view all the answers

Which feature is characteristic of a strand in prestressed concrete?

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

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

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

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

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

Which of the following statements is true regarding bonded tendons?

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

What is the primary purpose of using unbonded tendons?

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

In which application are unbonded tendons primarily permitted?

Slabs on ground. Signup and view all the answers

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

Strands are tensioned and jacked together as a group. Signup and view all the answers

What does 'stress compatibility' imply regarding bonded tendons?

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

Which of the following characterizes the mono-strand system?

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

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

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

What are the five main types of slab systems mentioned?

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

Which statement is true regarding two way edge supported slabs?

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

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

Extra moments arise at the change of section. Signup and view all the answers

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

The width should be about 6 times the depth. Signup and view all the answers

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

Concrete creep and shrinkage Signup and view all the answers

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

Higher strength concrete and high tensile strands Signup and view all the answers

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

Table 6.10.3.2 Signup and view all the answers

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

One way slab 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 (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

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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.