Prestressed Concrete Design

M.K. Hurst

• Publisher: CRC Press
• Year: 2017
• Topic: Technology & Engineering
• eBook ISBN:
  9781351990882
• Subtopic: Architecture Methods & Materials
• Index: Technology & Engineering

Prestressed concrete is widely used in the construction industry in buildings, bridges, and other structures. The new edition of this book provides up-to-date guidance on the detailed design of prestressed concrete structures according to the provisions of the latest preliminary version of Eurocode 2: Design of Concrete Structures, DD ENV 1992-1-1: 1992. The emphasis throughout is on design - the problem of providing a structure to fulfil a given purpose - but fundamental concepts are also described in detail. All major topics are dealt with, including prestressed flat slabs, an important and growing application in the design of buildings. The text is illustrated throughout with worked examples and problems for further study. Examples are given of computer spreadsheets for typical design calculations. Prestressed Concrete Design will be a valuable guide to practising engineers, students and research workers.

1
Basic principles

1.1 Introduction

Prestressed concrete is the most recent of the major forms of construction to be introduced into structural engineering. Although several patents were taken out in the last century for various prestressing schemes, they were unsuccessful because low-strength steel was used, with the result that long-term effects of creep and shrinkage of the concrete reduced the prestress force so much that any advantage was lost. It was only in the early part of the twentieth century that the French engineer Eugène Freyssinet approached the problem in a systematic way and, using high-strength steel, first applied the technique of prestressing concrete successfully. Since then prestressed concrete has become a well-established method of construction, and the technology is available in most developed, and in many developing, countries. An account of some of the early developments in prestressed concrete is given in Walley (1984).

The idea of prestressing, or preloading, a structure is not new. Barrels were, and still are, made from separate wooden staves, kept in place by metal hoops. These are slightly smaller in diameter than the diameter of the barrel, and are forced into place over the staves, so tightening them together and forming a watertight barrel (Fig 1.1). Cartwheels were similarly prestressed by passing a heated iron tyre around the wooden rim of the wheel. On cooling, the tyre would contract and be held firmly in place on the rim (Fig. 1.2), thus strengthening the joints between the spokes and the rim by putting them into compression.

The technique of prestressing has several different applications within civil engineering, often being used to keep cables taut when subjected to compressive forces. However, by far the most common application is in prestressed concrete where a prestress force is applied to a concrete member, and this induces an axial compression that counteracts all, or part of, the tensile stresses set up in the member by applied loading.

Figure 1.1 Barrel staves compressed with hoops.

Figure 1.2 Cartwheel compressed by contracting tyre.

Within the field of building structures, most prestressed concrete applications are in the form of simply supported precast floor and roof beams (Fig. 1.3). These are usually factory-made, where the advantages of controlled mass production can be realized. Where large spans are required, in situ prestressed concrete beams are sometimes used, and in situ prestressed concrete flat slab construction is increasingly being employed. This last technique is often associated with that of the lift slab, whereby whole floor slabs are cast and tensioned at ground level, and then jacked up into their final position.

In the field of bridge engineering, the introduction of prestressed concrete has aided the construction of long-span concrete bridges. These often comprise precast units, lifted into position and then tensioned against the units already in place, the process being continued until the span is complete. For smaller bridges, the use of simply supported precast prestressed concrete beams has proved an economical form of construction, particularly where there is restricted access beneath the bridge for construction. The introduction of ranges of standard beam sections has simplified the design and construction of these bridges (Fig. 1.4).

Some further examples of the many applications of prestressed concrete are shown in Fig 1.5–1. 8.

Figure 1.3 Examples of precast beams.

Figure 1.4 Examples of standard bridge beams.

Figure 1.5 Externally prestressed bridge deck (courtesy VSL International).

Figure 1.6 Prestressed flat slab construction (courtesy VSL International).

Figure 1.7 Prestressed liquid retaining structures (courtesy VSL International).

Figure 1.8 Prestressed double-tee roof beam.

One of the main advantages of prestressed over reinforced concrete is that, for a given span and loading, a smaller prestressed concrete member is required. This saving of the dead load of the structure is particularly important in long-span structures such as bridges, where the dead load is a large proportion of the total load. As well as a saving in concrete material for members, there is also a saving in foundation costs, and this can be a significant factor in areas of poor foundation material.

Another important advantage of prestressed concrete is that by suitable prestressing the structure can be rendered crack-free, which has important implications for durability, especially for liquid-retaining structures.

A third advantage is that prestressing offers a means of controlling deflections. A prestress force eccentric to the centroid of a member will cause a vertical deflection, usually in the oppo...

Table of contents

• Cover Page
• Half title
• Prestressed Concrete Design
• copy
• dedication
• Contents
• Preface
• Symbols
• 1 Basic principles
• 2 Properties of materials
• 3 Limit state design
• 4 Loss of prestress force
• 5 Analysis of sections
• 6 Deflections
• 7 Shear
• 8 Prestressing systems and anchorages
• 9 Design of members
• 10 Composite construction
• 11 Indeterminate structures
• 12 Prestressed flat slabs
• 13 Design examples
• Solutions to problems
• Index