Cairns, John William (1976) The strength of lapped joints in reinforced concrete columns. PhD thesis. http://theses.gla.ac.uk/1472/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Glasgow Theses Service http://theses.gla.ac.uk/ [email protected] THE STRENGTHOF LAPPEDJOINTS IN REINFORCEDCONCRETE COLUMNS by J. Cairns, B. Sc. A THESIS PRESENTEDFOR THE DEGREEOF DOCTOROF PHILOSOPHY Y4 THE UNIVERSITY OF GLASGOW by JOHN CAIRNS, B. Sc. October, 1976 3 !ý I CONTENTS Page ACKNOWLEDGEIvIENTS 1 SUMMARY 2 NOTATION 4 Chapter 1 INTRODUCTION 7 Chapter 2 REVIEW OF LITERATURE 2.1 Bond s General 10 2.2 Anchorage Tests 12" 2.3 Tension Lapped Joints 17 2.4 Compression Lapped Joints 21 2.5 End Bearing 26 2.6 Current Codes of Practice 29 Chapter 3 THEORETICALSTUDY OF THE STRENGTHOF LAPPED JOINTS 3.1 Review of Theoretical Work on Bond Strength 32 3.2 Theoretical Studies of Ultimate Bond Strength 33 3.3 Theoretical Approach 42 3.4 End Bearing 48 3.5 Bond of Single Bars Surrounded by a Spiral 51 3.6 Lapped Joints 53 3.7 Variation of Steel Stress Through a Lapped Joint 59 3.8 Design of Experiments 65 Chapter 4 DESCRIPTION OF EXPERIMENTALWORK 4.1 General Description of Test Specimens 68 4.2 Materials 72 4.3 Fabrication of Test Specimens 78 4.4 Test Procedure 79 4.5 Instrumentation 80 4.6 Push-in Test Specimens 82 CONTENTScontd. page. Chapter 5 PRESENTATION OF TEST RESULTS 5.1 Joint Behaviour 85 5.2 Analysis of Test Results 92 Chapter 6 DISCUSSION OF TEST RESULTS 6.1 Introduction 104 6.2 General Behaviour of Lapped Joints 105 6.3 Effect of Bar Deformations 114 6.4 Contribution of End Bearing 114 6.5 Influence of Concrete Strength 118 6.6 Effect of Lap Length 121 6.7 Influence of Secondary Reinforcement 125 Chapter 7 COMPARISONWITH THEORETICALANALYSIS 7.1 Introduction 130 7.2 Push-in Tests 130 7.3 Lapped Joints 133 Chapter 8 COMPARISONWITH EXISTING DESIGN RULES 144 Chapter 9 CONCLUSIONSAND RECOMMENDATIONS 154 REFERENCES 157 Appendix A RESULTSOF TRIAXIAL COMPRESSIONTESTS 164 Appendix B STRAINS MEASUREDON REINFORCINGBARS 167 Appendix C SPECIMENCALCULATIONS FOR A TYPICAL COLUMN 168 THE STRENGTH OF LAPPED JOINTS IN REINFORCED CONCRETECOLUMNS by J. Cairns, B. $c. SUMMARY This thesis presents the results of a total of 51 tests on full scale columns in which the main reinforcement was lap jointed. In order to investigate the contribution of the bearing of the ends of the lapped bars to joint strength, several columns. were tested with end bearing of bars eliminated, or with bond of bars elim- inated within the lap length. Failure of lapped-joints was always preceded by extensive splitting of the concrete cover along the line of the lapped bars, and usually took place as the-links at the ends of the lap yielded, thus illustrating the bursting nature of. the bond action of ribbed reinforcing bars. At an early stage of the experimental investigation, it was realised that using the results of standard cube tests would not allow the ultimate strength of lapped joints to be calculated with sufficient accuracy. A method was evolved to calculate the stress- strain curve for the concrete in the column, and to extrapolate this relationship to obtain the ultimate strength of the concrete. The results obtained from the experimental investigation indicate that both the ultimate strength of a short lapped joint confined by links and the net contribution of end bearing to ultimate joint strength vary linearly with concrete compressive strength. Ultimate bond strength was also found to vary linearly with the resistance to splitting of a "push-in" test specimen, and -1- the quantity and positioning of links is shown to influence ultimate joint strength. Ultimate bond strength was also found to be influenced by the projected rib area/unit length of a bar. A theoretical analysis of joint strength, based on the Coulomb- Mohr equation of failure, is also presented. The analysis shows that joint strength may be regarded as having two components, one linearly related to the compressive strength of the concrete, the other linearly related to the available resistance to the bursting forces produced by bond and end bearing. Theoretically derived expressions are shown to be in good agreement with experimental results.. The results of the theoretical and experimental investigations are used to formulate design rules for compression lapped joints. A comparison with the requirements of several current codes of. P. practice is made, and t is shown that the requirements of B. S. C. 110: 1972 may be inadequate in certain cases. It is recommended that increased links be specified at lapped joints. -2- ACKNOWLEDGEMENTS The work described was conducted in the Department of Civil Engineering at the University of Glasgow., The author is grateful to Dr. P. D. Arthur, Professor H. B. Sutherland and to the late Professor W. T. Marshall for the facilities, advice and encour- agement which they provided, and to the laboratory staff for their interest and assistance.. In. particular, the help of Messrs. J. Thomson, J. Coleman and A. Galt is gratefully acknowledged. The author wishes to thank the Associated Portland Cement Co. for the Scholarship which enabled him to undertake the work. -3- NOTATION Where applicable, the standard symbols of B. B. C. P. 110: 1972(9) have been used. Ar area of one rib of a ribbed deformed bar projected on a plane perpendicular to the bar axis Asv area of one leg of link c concrete cover to reinforcement E secant modulus of elasticity of concrete measured at c500 a strain of 500 x 16-6- P ultimate force available to counteract bursting forces c produced by bond or end bearing of, reinforcing bar Fh bursting force produced in one direction by bond of ribbed deformed bar Ft bursting force produced in one direction by end bearing PP2 forces link by pair of lapped , exerted on one each of a bars fc ultimate compressive strength of concrete in test specimen fý concrete cylinder compressive strength fcf ultimate of concrete at height cj compressive strength gs 'Where strains measured, and through lapped joint respectively. fc ultimate tensile strength of concrete t fsc f upper and lower limits of ultimate stress developed u, I. in compression reinforcement calculated from theoretical analysis hr height of rib above surface of bar core at any point tb length of bar over which bond stresses develop it lap length -4- n no. of turns of wire spiral, or no. of links within lap s' clear spacing between pairs of lapped bars ties in a t spacing of computer model v velocity of ultrasonic pulse in concrete. vg vj velocity of ultrasonic pulse at gauge height, and , through lapped joint V crack width z extension a inclination of failure wedge and failure cone to bar axis angle of cones to bar axis 19 compression b unit cohesion of concrete E strain cc strain in concrete at ultimate compressive stress Eßt strain in concrete at ultimate tensile stress angle of internal friction of concrete ratio of unloaded to loaded area of concrete block angle V Poisson's ratio angle direct stress a'n normal stress on failure surface below -a rib or the end of a bar Cr. bearing stress Crr radial stress on bar. from failure wedge below rib O't stress on plane through middle of cone below end of bar 8- i a, ,d major and minor principal stresses (yd standard deviation shear stress shear strength of concrete 0 diameter of lapped bars 0c diameter of wire spiral in push-in tests -r ratio = F1/ F2 Throughout this thesis, all equations,, are given in S. I. units. w r .. 6_ ." CHAPTER I INTRODUCTION The design of lapped joints between reinforcing bars in compression has received little attention in comparison with the large number of investigations conducted on lapped joints between bars in tension, as it has been assumed that it can be based on the results of tests on tension lapped joints. However, this approach ignores the fact that the bearing of the ends of a reinforcing bar in a compression lapped joint may transfer a substantial proportion of the force in the bar. Compression lapped joints are generally necessary in columns at every floor in a multi-storey structure, and are required to develop the full design strength of the reinforcement, as they cannot be positioned where the stress in the reinforcement is low. Considerable savings might therefore be possible if it could be shown that lap-lengths as at present specified by building codes could be reduced with safety. Research in tension lapped joints has shown the importance of the tensile strength of the concrete forming the cover to the. re- inforcement in resisting the bursting forces produced by the bond action of ribbed reinforcing bars.. The tensile strength of concrete in uniaxial tension-is higher than the tensile strength of concrete in biaxial tension-compressiont the stress situation which develops round reinforcement in compression lapped joints. This will reduce bond strength in comparison with tension lapped joints.