Post-Tensioned Concrete Floors

327 Pages·1995·16.789 MB·English

by  Martin Williams, Sami Khan

Post-tensioned Concrete Floors
Harbour Exchange, London
Post-tensioned Concrete Floors
Sami Khan
Director, Bunyan Meyer and Partners
Martin Williams
Lecturer, University of Oxford, Department of Engineering Science
and Fellow of New College, Oxford
~
,U TT E R W O R T H
E! N E M A N N
Butterworth-Heinemann Ltd
Linacre House, Jordan Hill, Oxford OX2 8DP
-~A member of the Reed Elsevier plc group
OXFORD LONDON BOSTON
MUNICH NEW DELHI SINGAPORE SYDNEY
TOKYO TORONTO WELLINGTON
First published 1995
(cid:14)9 Butterworth-Heinemann Ltd 1995
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British Library Cataloguing in Publication Data
Kahn, Sami
Post-tensioned Concrete Floors
I. Title II. Williams, Martin
693.542
ISBN 0 7506 1681 4
Library of Congress Cataloguing in Publication Data
Kahn, Sami.
Post-tensioned concrete floors / Sami Kahn, Martin Williams.
p. cm.
Includes bibliographical references and index.
ISBN 0 7506 1681 4 (pbk.)
1. Floors, Concrete. 2. Post-tensioned prestressed concrete.
I. Williams, Martin. II. Title.
TH2529.C6K48 1995
690'.16 - dc20 94-36854
CIP
Typeset by Vision Typesetting, Manchester
Printed and bound in Great Britain by
Hartnolls Limited, Bodmin, Cornwall
CONTENTS
INTRODUCTION page ix
NOTATIONS xi
1 THE BASIC PRINCIPLES 1
1.1 Introduction 1
1.2 Prestressing in principle 2
1.3 Stress reversal 5
1.4 Tendons 6
1.5 Prestress losses 7
1.6 Initial and final stresses 8
1.7 Pre-tensioning and post-tensioning 9
1.8 Reinforced and post-tensioned concrete floors 10
1.9 Bonded and unbonded post-tensioning 14
1.10 Stressing stages 17
1.11 Construction tolerances 18
1.12 Fire resistance 18
1.13 Holes through completed floors 18
1.14 Post-tensioning in refurbishment 19
1.15 Some misconceptions about post-tensioned floors 20
2 MATERIALS AND EQUIPMENT 24
2.1 Formwork 24
2.2 Dense concrete 26
2.3 Lightweight concrete 35
2.4 Post-tensioning tendons 39
2.5 Prestressing hardware 47
2.6 Equipment 55
3 SLAB CONFIGURATION 61
3.1 General 61
3.2 Structural elements of a floor 64
3.3 Panel configuration 70
3.4 Span to depth ratio 74
vi CONTENTS
4 PLANNING A STRUCTURE 79
4.1 Design objectives and buildability 79
4.2 Restraint from vertical elements 82
4.3 Dispersion of the prestressing force 85
4.4 Column moments 87
4.5 Movements in a concrete floor 90
4.6 Crack prevention 91
4.7 Tendon profile 93
4.8 Access at the live end 94
4.9 Transfer beams 96
4.10 Durability 97
4.11 Fire protection 99
4.12 Minimum and maximum prestress 102
4.13 Additional considerations for structures in seismic zones 103
Example 4.1 107
5 TENDON PROFILES AND EQUIVALENT LOADS 108
5.1 General 108
5.2 Equivalent load 109
5.3 Secondary moments 112
5.4 Concordance 114
5.5 Tendon profile elements 114
5.6 Composite profiles 122
5.7 Tendon deviation in plan 129
5.8 Clash of beam and slab tendons 130
6 FLEXURE IN THE SERVICEABILITY STATE 132
6.1 The design process 132
6.2 Options in a design 134
6.3 Computer programs 136
6.4 Partial prestressing 137
6.5 Permissible stresses in concrete 138
6.6 Permissible stresses in strand 141
6.7 Analysis 142
6.8 Simply supported span 144
6.9 Continuous spans 147
Example 6.1 149
Example 6.2 151
Example 6.3 153
Example 6.4 156
7 PRESTRESS LOSSES 160
7.1 General 160
7.2 Friction losses 163
7.3 Anchorage draw-in 164
CONTENTS vii
7.4 Elastic shortening 168
7.5 Shrinkage of concrete 170
7.6 Creep of concrete 171
7.7 Relaxation of tendons 172
7.8 Tendon elongation 173
7.9 Tendon force from elongation 173
Example 7.1 174
8 ULTIMATE FLEXURAL STRENGTH 177
8.1 Failure mechanisms 177
8.2 Level of prestress 180
8.3 Applied loads 181
8.4 Procedure for calculating the strength 182
8.5 Ultimate stresses 185
8.6 Strain compatibility 189
8.7 Anchorage zone 191
Example 8.1 194
Example 8.2 197
9 DEFLECTION AND VIBRATION 198
9.1 Deflections 198
9.2 Vibration 206
Example 9.1 213
Example 9.2 216
Example 9.3 219
10 SHEAR 221
10.1 Shear strength of concrete 222
10.2 Beams and one-way slabs 224
10.3 Two-way slabs 230
10.4 Alternatives to conventional shear reinforcement 240
Example 10.1 244
Example 10.2 245
11 SLABS ON GRADE 249
11.1 The design process 250
11.2 Factors affecting the design 251
11.3 Traditional RC floors 257
11.4 Post-tensioned ground floors 259
11.5 Elastic analysis 261
11.6 Construction 267
Example 11.1 269
12 DETAILING 271
12.1 Drawings and symbols 271
viii CONTENTS
12.2 Minimum reinforcement 274
12.3 Tendon spacing and position 276
12.4 Deflection and cladding 277
12.5 Movement joints 277
12.6 Detailing for seismic resistance 281
13 SITE ACTIVITIES AND DEMOLITION 284
13.1 Storage of materials 286
13.2 Installation 287
13.3 Concreting 288
13.4 Stressing 289
13.5 Grouting 294
13.6 Finishing operations 295
13.7 Demolition 295
13.8 Cutting holes 303
REFERENCES 306
INDEX 309
INTRODUCTION
This book deals with the design of concrete building structures incorporating
post-tensioned floors. Post-tensioning is the most versatile form of prestressing, a
technique which enables engineers to make the most effective use of the material
properties of concrete, and so to design structural elements which are strong,
slender and efficient. Design in post-tensioned concrete is not difficult and, if
done properly, can contribute significantly to the economy and the aesthetic
qualities of a building. As a result, post-tensioned floors have found widespread
use in office buildings and car park structures, and are also frequently employed
in warehouses and public buildings. However, in spite of this, most prestressed
concrete texts devote comparatively little attention to floors, concentrating
instead on beam elements. This book therefore aims to answer the need for a
comprehensive treatment of post-tensioned floor design.
The first four chapters of the book give a detailed, non-mathematical account
of the principles of prestressing, the materials and equipment used, and the
planning of buildings incorporating post-tensioned floors. The following chapters
outline the detailed design process, including numerous worked examples, and
the book concludes with chapters describing site procedures for construction,
demolition and alteration. While the reader is assumed to have a grasp of the
basics of reinforced concrete design, no prior knowledge of prestressing is
required. The book is thus suitable for use by architects, contract managers and
quantity surveyors who may wish to gain an understanding of the principles
without going into the mathematical aspects of the design process, as well as
structural engineers requiring detailed design guidance. It is also intended for use
as an educational text by students following civil engineering, architecture and
building courses.
The title of the book reflects the fact that its emphasis is on the behaviour and
design of the floors themselves. Thus, while the effect of post-tensioned floors on
other structural elements such as columns and walls is considered, detailed
guidance on the design of these elements is not given; such information can be
obtained from any one of the many excellent reinforced concrete design texts
already available. Neither does this book deal with the prestressing of building
elements other than floors, such as foundations, moment-resisting columns or
vertical hangers. These elements are comparatively rare, or are not usually
prestressed. If guidance on design of such elements is required, reference should
be made to specialist literature.
In any book on post-tensioning comparisons with reinforced concrete are