PRESTRESSED CONCRETE DESIGN

Size: px

Start display at page:

Download "PRESTRESSED CONCRETE DESIGN"

Cameron Parrish
6 years ago
Views:

1 PRESTRESSED CONCRETE DESIGN A01_Prestressed Concrete Design_FM.indd i 4/29/2013 4:00:08 PM

2 A01_Prestressed Concrete Design_FM.indd ii

3 PRESTRESSED CONCRETE DESIGN Praveen Nagarajan Department of Civil Engineering NIT Calicut, Kerala A01_Prestressed Concrete Design_FM.indd iii

4 Senior Editor Acquisitions: Sandhya Jayadev Editor Production: M. Balakrishnan Copyright 2013 Dorling Kindersley (India) Pvt. Ltd This book is sold subject to the condition that it shall not, by way of trade or otherwise, be lent, resold, hired out, or otherwise circulated without the publisher s prior written consent in any form of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser and without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise), without the prior written permission of both the copyright owner and the publisher of this book. ISBN First Impression Published by Dorling Kindersley (India) Pvt. Ltd, licensees of Pearson Education in South Asia. Head Office: 7th Floor, Knowledge Boulevard, A-8(A), Sector 62, Noida , UP, India. Registered Office: 11 Community Centre, Panchsheel Park, New Delhi , India. Compositor: White Lotus Infotech Pvt. Ltd., Puducherry. Printed in India. A01_Prestressed Concrete Design_FM.indd iv

5 To my parents, Mrs. Rani Nagarajan and Dr N. M. Nagarajan, and my Teachers at NIT Calicut and IIT Madras A01_Prestressed Concrete Design_FM.indd v

6 A01_Prestressed Concrete Design_FM.indd vi

7 Contents Preface xv About the Author xvii Chapter 1 Basic Principles Introduction Need for Prestressed Concrete Brief History of Prestressed Concrete Structural Behaviour of Prestressed Concrete Member Methods of Prestressing Pre-tensioning Post-tensioning Types of Prestressed Concrete Classifi cation as per IS: Other Classifi cations Comparison with Reinforced Concrete Applications of Prestressed Concrete Design Code 13 Chapter 2 Materials Introduction Prestressing Steel High Tensile Steel Need for High Tensile Steel Types of Prestressing Steel Modulus of Elasticity of Prestressing Steel Maximum Initial Prestress in Tendon 23 A01_Prestressed Concrete Design_FM.indd vii

8 viii Contents 2.3 Concrete Need for High Strength Concrete Compressive Strength of Concrete Tensile Strength of Concrete Modulus of Elasticity of Concrete Time-dependent Deformation of Concrete 28 Chapter 3 Limit State Design Introduction Limit States Characteristic and Design Load Characteristic and Design Strength of Material Characteristic and Design Stress Strain Curves Stress Strain Curves for Concrete Stress Strain Curves for Prestressing Steel Design Requirements as per LSM Limit State Design of Prestressed Concrete Members 39 Chapter 4 Losses in Prestress Introduction Immediate Losses Loss Due to Elastic Shortening of Concrete Loss Due to Friction Loss Due to Slip in Anchorage Time Dependent Losses Loss Due to Shrinkage of Concrete Loss Due to Creep of Concrete Loss Due to Relaxation of Steel Total Loss in Prestress 65 Problems 66 Chapter 5 Analysis of Sections Introduction Analysis at Serviceability Limit State 68 A01_Prestressed Concrete Design_FM.indd viii

9 Contents ix Combined Load Approach Internal Couple Approach Concept of Equivalent Loads Load Balancing Decompression Moment (M dec ) Cracking Moment M cr Additional Stress in Tendon Due to Bending Flexural Behaviour of Prestressed Concrete Member Load Defl ection Behaviour Stress in Tendon Modes of Failure in Flexure Types of Section Analysis at Ultimate Limit State Analysis of Rectangular Sections with Bonded Tendons Analysis of Post-tensioned Rectangular Beams Having Unbonded Tendons Analysis of Flanged Sections 120 Problems 127 Chapter 6 Shear and Torsion Introduction Design for Shear Effect of Prestress in Shear Strength Identifi cation of Zones for Shear Design Ultimate Shear Resistance Ultimate Shear Resistance of Concrete V uc Design of Shear Reinforcement Minimum Shear Reinforcement Maximum Ultimate Shear Force (V u, max ) Steps for Shear Design Design for Torsion Equilibrium Torsion and Compatibility Torsion Failure of Concrete Member Due to Torsion 146 A01_Prestressed Concrete Design_FM.indd ix

10 x Contents Design Methods for Torsion Design Provisions for Torsion as per IS: Detailing of Torsion Reinforcement Steps for Torsion Design 157 Problems 163 Chapter 7 Anchorage Zones Introduction Anchorage Zones in Pre-tensioned Members Bond Mechanisms Transmission Length Flexural Bond Length Development Length End Zone Reinforcement Anchorage Zones in Post-tensioned Members Bearing Stresses Behind Anchorage Bursting Forces in Anchorage Zones End Zone Reinforcement 176 Problems 184 Chapter 8 Deflections Introduction Deflections in Uncracked Beams Short Term Defl ection at Transfer st Long Term Defl ection at Service Condition D Is Deflection of Type 3 Members Deflection Limits 190 Problems 197 Chapter 9 Design of Members Introduction Governing Inequalities Minimum Section Modulus 202 A01_Prestressed Concrete Design_FM.indd x

11 Contents xi Minimum Section Modulus for the Top Fibre Z a Minimum Section Modulus for the Bottom Fibre Z b Design of Prestressing Force Magnel Diagram Cable Zone Selection of Cross-section Flexural Efficiency Factor Q General Guidelines for Beam Sections Requirements for Flexural Reinforcement Concrete Cover Spacing of Tendons Minimum Longitudinal Steel Minimum Side Face Reinforcement Design Procedure for Prestressed Concrete Members 222 Problems 222 Chapter 10 Composite Members Introduction Analysis at Serviceability Limit State Stresses in Precast Web at Transfer Stresses in Precast Web After Time Dependent Losses Stresses in Precast Web After Casting of In-situ Slab Stresses in Composite Section at Service Condition (Total Design Load Condition) Stresses Due to Differential Shrinkage Horizontal Shear Transfer Ultimate Moment of Resistance Design of Composite Members Minimum Section Modulus Required Prestressing Force and Allowable Cable Zone 245 Problems 245 A01_Prestressed Concrete Design_FM.indd xi

12 xii Contents Chapter 11 Indeterminate Structures Introduction Effects of Prestress in Indeterminate Structure Primary and Secondary Moments and Shears Section Moments and Concrete Moments Pressure Line Due to Prestressing Force Calculating the Effects of Prestress Using the Concept of Equivalent Loads Linear Transformation of Cable Profile Concordant Cable Analysis of Real Cables Calculation of Elastic Stresses in Concrete Behaviour at Ultimate Load 266 Problem 268 Chapter 12 Slabs Introduction General Design Procedure One-way Slabs Edge-supported Two-way Slabs 274 Problems 283 Chapter 13 Circular Prestressing Introduction Circumferential Prestressing Analysis at Transfer Analysis at Service Condition Design Prestressed Concrete Pipes Prestressed Concrete Circular Tanks Behaviour Analysis 292 A01_Prestressed Concrete Design_FM.indd xii

13 Contents xiii Design Detailing Requirements Ring Beams Analysis and Design 300 Problems 303 References 305 Index 307 A01_Prestressed Concrete Design_FM.indd xiii 4/29/2013 4:00:11 PM

14 A01_Prestressed Concrete Design_FM.indd xiv 4/29/2013 4:00:11 PM

15 Preface This book deals with the analysis and design of prestressed concrete members as per the IS: specification. It is intended to be an easy-to-read textbook for a first course in prestressed concrete design for senior undergraduate and postgraduate students of Civil Engineering. The book will also be useful for practising engineers and help them understand the basis behind the code recommendations. Spread across 13 chapters, the key features of the book are: The concepts of prestressed concrete design are explained in a simple manner. The contents of each chapter are enhanced with new and better figures for improved understanding. Detailed derivations are presented for almost all equations. The book follows a step-by-step approach for problem solving, with each step explained from first principles. Chapter 1 explains the behaviour of the prestressed concrete member and introduces the different methods of prestressing and types of prestressed concrete. The need for high tensile steel and high strength concrete for prestressed concrete structure is brought out in Chapter 2, which also describes briefly some of the important properties of these materials. The philosophy of limit state design is discussed in Chapter 3. The various sources and methods to evaluate the loss of prestressing force in both pre-tensioned and post-tensioned prestressed concrete members are dealt with in Chapter 4. Chapter 5 analyzes the behaviour of prestressed concrete member in flexure at both serviceability and ultimate limit state. It elaborates on the different methods to calculate elastic stress distribution in concrete, namely, the combined load approach, the internal couple approach and the equivalent load approach. The chapter also examines the procedures to find the ultimate moment of resistance of both rectangular and flanged sections. Chapters 6 and 7 examine the design for shear, torsion and anchorage zones. Methods to predict the short term and long term deflections of prestressed concrete beams are elucidated in Chapter 8. A01_Prestressed Concrete Design_FM.indd xv 4/29/2013 4:00:11 PM

16 xvi Preface Chapter 9 outlines the procedure to be followed for the design of statically determinate prestressed concrete beams (Type 1 and Type 2 members). The methods of analysis and design of composite members are revealed in Chapter 10. Chapter 11 gives a brief introduction to the analysis of indeterminate prestressed concrete members. The design of one-way and two-way slabs is delineated in Chapter 12. Finally, Chapter 13 covers the design of prestressed concrete pipes, watertanks and ring beams. I thank Ms. Sandhya Jayadev of Pearson Education, for her support and Mr. Beljith P., for typing the manuscript. Comments, feedback and suggestions for the improvement of the book are welcome. Praveen Nagarajan A01_Prestressed Concrete Design_FM.indd xvi 4/29/2013 4:00:11 PM

17 About the Author Dr Praveen Nagarajan is an alumnus of NIT Calicut and IIT Madras. An illustrious student of Civil Engineering throughout his college life, he worked briefly for L&T Ramboll, Chennai, before joining as faculty at NIT Calicut. Included among Dr Praveen s many areas of interest are reinforced and prestressed concrete, steel structures, bridge engineering, structural reliability and structural dynamics topics in which he has published numerous technical papers. A recipient of several awards for technical excellence such as the Valli Anantharamakrishnan Merit Prize (2003) from IIT Madras, E P Nicolaides Prize (2009) from the Institution of Engineers (India) and the Best Young Teacher award (for the academic year ) from NIT Calicut, he has guided more than 25 M.Tech. projects. Currently Assistant Professor at the Department of Civil Engineering, NIT Calicut, he is also guiding three research scholars to their doctoral degrees. A01_Prestressed Concrete Design_FM.indd xvii 4/29/2013 4:00:11 PM

18 A01_Prestressed Concrete Design_FM.indd xviii 4/29/2013 4:00:11 PM

ST7008 PRESTRESSED CONCRETE

ST7008 PRESTRESSED CONCRETE QUESTION BANK UNIT-I PRINCIPLES OF PRESTRESSING PART-A 1. Define modular ratio. 2. What is meant by creep coefficient? 3. Is the deflection control essential? Discuss. 4. Give