August 2006 Buckling and Fracture of Concentric Braces Under Inelastic Cyclic Loading By Benjamin V. Fell Graduate Research Assistant University of California at Davis Amit M. Kanvinde Assistant Professor University of California at Davis Gregory G. Deierlein Professor, Director of John A. Blume Earthquake Engineering Center Stanford University Andrew T. Myers Graduate Research Assistant Stanford University Xiangyang Fu Graduate Research Assistant University of California at Davis ____________________________________________________________________________ (A copy of this report can be downloaded for personal use for a nominal fee from www.steeltips.org) Title o Buckling and fracture of concentric braces under inelastic cyclic loading By Benjamin V. Fell, Amit M. Kanvinde, Gregory G. Deierlein, Andrew T. Myers, Xiangyang Fu Experimental findings and design implications from nineteen tests of large-scale concentric steel braces are presented. Rectangular hollow structural sections (HSS), round pipe, and W-shape cross sections of varying width-thickness and slenderness ratios are subjected to various loading histories. Ductile fracture during inelastic cyclic buckling is found to be driven by strain amplifi cation due to local buckling of the cross section. Cross section shape, width-thickness ratio, and brace slenderness ratio are the most important factors that control brace ductility. Pipe and wide-flange shapes are observed to be more resilient to fractur e as compared to HSS members that develop large local buckling induced strains at the corners. Parameters that have less significant effect on buckling and fracture response include concrete filling of HSS members and loading rates. The relatively low displacement ductility of the HSS members suggests the need to reduce the permissible section width- thickness requirements for seismic design. Tests with and without reinforcing at the slotted HSS and pipe to gusset plate connections demonstrate the effecti veness of the reinforcing plates to prevent premature net-section fracture. The maximum brace resistance is found to be bracketed between the calculated expected yield strength (R yFyAg) and expected ultimate strength (R tFuAg), using nominal values specified in design specifications. Micromechanics-based models to simulate ductile fracture are introduced that can generalize the findings of this research through detailed finite element analyses. __________________________________________________________________________________ Benjamin V. Fell, Graduate Research Assistant, University of California at Davis [email protected] ____________________________________________________________________________________________ Amit M. Kanvinde, Ph.D., Assistant Professor, University of California at Davis [email protected] ____________________________________________________________________________________________ Gregory G. Deierlein, Ph.D., P.E., Professor, Stanford University [email protected] ____________________________________________________________________________________________ Andrew T. Myers, Graduate Research Assistant, Stanford University [email protected] ____________________________________________________________________________________________ Xiangyang Fu, Graduate Research Assistant, University of California at Davis [email protected] ___________________________________________________________________________________________ This report is Copyright of the above author(s). All rights reserved. ____________________________________________________________________________________________ Disclaimer: The information presented in this publication has been prepared in accordance with recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, designer or architect. The publication of the material contained herein is not intended as a representation or warranty on the part of the Structural Steel Educational Council or of any other person named herein, that this information is suitable for any general or particular use or of freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use. Caution must be exercised when relying upon specifications and codes developed by others and incorporated by reference herein since such material may be modified or amended from time to time subsequent to the printing of this document. The Structural Steel Educational Council or the authors bears no responsibility for such material other than to refer to it and incorporate it by reference at the time of the initial publication of this document. “Buckling and fracture of concentric braces under inelastic cyclic loading”, Fell et al., 2006, All rights reserved. 2 ACKNOWLEDGMENTS The publication of this report was made possible in part by the support of the Structural Steel Educational Council (SSEC). The nineteen brace specimens were donated through the SSEC and their support is greatly appreciated. The authors also thank Jeff Eandi of Eandi Metal Works, Inc. for high-quality fabrication of the specimens. This research is supported by the National Science Foundation (NSF Grant CMS 0421492), the George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES), and the Structural Steel Educational Council (SSEC). The advice and guidance of Helmut Krawinkler (Stanford University), Stephen Mahin (University of California at Berkeley), Charles Roeder (University of Washington), Walterio López and Mark Saunders (Rutherford and Chekene) is greatly appreciated. In addition, the knowledgeable support of the UC Berkeley NEES lab personnel including Shakhzod Takhirov, Donald Patterson, Donald Clyde, David MacLam, and Jose Robles greatly assisted in the experimental aspect of this study. The authors also acknowledge support from the John A. Blume Earthquake Engineering Center at Stanford University and the University of California at Davis. Finally, the authors wish to thank SSEC members Walterio López, S.E., Michael Cochran, S.E., and Fred Breismeister, P.E. for their technical input and review of this report. “Buckling and fracture of concentric braces under inelastic cyclic loading”, Fell et al., 2006, All rights reserved. 3 __________________________________________________ TABLE OF CONTENTS ABSTRACT / Page 2 ACKNOWLEDGMENTS / Page 3 TABLE OF CONTENTS / Page 4 NOTATIONS / Page 5 CHAPTER 1. INTRODUCTION/ Page 7 CHAPTER 2. SUMMARY OF EXPERIMENTAL PROGRAM / Page 10 CHAPTER 3. INTRODUCTION TO CONTINUUM-BASED FRACTURE AND FATIGUE PREDICTIVE MODELS / Page 29 CHAPTER 4. DESIGN IMPLICATIONS / Page 34 CHAPTER 5. SUMMARY / Page 51 APPENDIX A. MATERIAL PROPERTIES / Page 53 APPENDIX B. EXPERIMENTAL HYSTERETIC PLOTS AND BACKBONE CURVES / Page 56 REFERENCES/Page 67 ABOUT THE AUTHOR(S) / Page 70 __________________________________________________ LIST OF PUBLISHED “STEEL TIPS” REPORTS / Page 72 “Buckling and fracture of concentric braces under inelastic cyclic loading”, Fell et al., 2006, All rights reserved. 4 _________________________________________________________________________ Notations _________________________________________________________________________ 2 Ag Gross cross sectional area, in 2 An Net cross sectional area, in Cd Deflection amplification factor D Diameter of PipeSTD section, in E Modulus of elasticity of steel, E = 29,000 ksi Fy Minimum specified yield stress of steel (AISC, 2001), ksi Fcr Critical buckling stress, ksi Fcr-Ry Critical buckling stress with R yFy amplification, ksi Fu Maximum specified ultimate stress of steel (AISC, 2001), ksi K Effective length factor Ke(c) Calculated elastic stiffness, k/in Ke(m) Experimentally measured elastic stiffness, k/in LB Brace length, in Mp Nominal plastic flexural strength, k-in P2% Maximum experimentally measured tensile force at 2% drift, kips Pc Compression backbone estimate, kips Pc,max Maximum experimentally measured compressive force, kips Pcr,exp Expected compression strength, kips Pmax Maximum experimentally measured tensile force, kips Pn Nominal axial strength of a compression member, kips Pt Tension backbone estimate, kips Pu,exp Expected ultimate strength, kips Py,exp Expected yield strength, kips R Average micovoid size R Seismic response modification coefficient R0 Initial average micovoid size Rt Ratio of expected ultimate strength to the minimum specified ultimate strength, F u Ry Ratio of expected yield strength to the minimum specified yield strength, F y T Triaxiality, T = σm/σe U Joint efficiency factor accounting for shear-lag Z Plastic section modulus, in 3 b Width of square HSS, in bf Width of flange, in t P P 2 p p e Equivalent plastic strain, e =∫ εij ⋅ ε ij dt 0 3 f’ c Specified compressive stress of concrete, ksi n Hardening coefficient r Governing radius of gyration, in. t Wall thickness of HSS or PipeSTD cross section, in “Buckling and fracture of concentric braces under inelastic cyclic loading”, Fell et al., 2006, All rights reserved. 5 tf Thickness of flange, in ∆0 Initial imperfection of an axial strut, in ∆a Brace axial deformation, in ∆n Critical buckling axial displacement, in Σθp Cumulative plastic drift, radians α Ry amplification factor β Angle between the horizontal plane and bracing member, degrees δ Lateral displacement of a buckling member,