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Collapse Performance Assessment of Steel-Framed Buildings Under Fires

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Collapse Performance Assessment of Steel-Framed Buildings Under Fires Department of Civil and Environmental Engineering Stanford University Report No. The John A. Blume Earthquake Engineering Center was established to promote research and education in earthquake engineering. Through its activities our understanding of earthquakes and their effects on mankind’s facilities and structures is improving. The Center conducts research, provides instruction, publishes reports and articles, conducts seminar and conferences, and provides financial support for students. The Center is named for Dr. John A. Blume, a well-known consulting engineer and Stanford alumnus. Address: The John A. Blume Earthquake Engineering Center Department of Civil and Environmental Engineering Stanford University Stanford CA 94305-4020 (650) 723-4150 (650) 725-9755 (fax) [email protected] http://blume.stanford.edu ©2007 The John A. Blume Earthquake Engineering Center THIS PAGE LEFT BLANK ii ABSTRACT ABSTRACT The main objective of this research is to investigate the collapse performance of steel-framed buildings under fires and to contribute to the development of methods and tools for performance-based structural fire engineering. This research approach employs detailed finite element simulations to assess the strength of individual members (beams and columns) and indeterminate structural sub-assemblies (beams, columns, connections and floor diaphragms). One specific focus of the investigation is to assess the accuracy of beam and column strength design equations of the American Institute of Steel Construction (AISC) Specification for Structural Steel Buildings. The simulation results show these design equations to be up to 60 % unconservative for columns and 80-100 % unconservative for laterally unbraced beams. Alternative equations are proposed that more accurately capture the effects of strength and stiffness degradation at elevated temperatures. About eight hundred simulations are performed to verify the proposed equations, accompanied by studies on members with different steel strengths and section sizes. The assessment technique for individual members is then extended to examine fire effects for indeterminate gravity frame systems, including forces induced by restraint to thermal expansion and nonlinear force redistribution due to yielding and large deformations. Structural sub-assemblies are devised to examine indeterminate effects of gravity-framing in a 10-story building, which is representative of design and detailing practice in the United States. Three types of sub-assemblies are considered, including an interior gravity column, a composite floor beam, and an exterior column-beam assembly. The sub-assembly models include the restraining effects of floor framing that surrounds (both horizontally and vertically) the localized compartment fire. The sub-assembly simulations support the following observations and conclusions: (1) the rotational end restraint provided by the columns above and below the fire story have a significant stabilizing effect on gravity columns in the fire zone (providing up to a 40 % increase in strength above the pin-ended condition at 400 °C), (2) vertical restraint of the heated column, by floor framing above the fire story, does not significantly impact the strength limit state of the columns in the fire zone (3) short of designing the building system with special redundant load paths, thermal iv ABSTRACT insulation is essential to avoid progressive collapse of highly-stressed gravity columns during building fires (4) thermal insulation requirements for beams can be reduced while preserving collapse resistance through enhanced connection details that are insulated, employ slotted holes to permit thermal elongation, and incorporate thermally protected reinforcing bars in the slab. These studies and conclusions are limited to evaluation of collapse safety and do not address aspects related to post-fire repairs and loss assessment. Uncertainty in the collapse behavior under fires is evaluated considering variability in the gravity loading and structural response parameters. Using the statistical information to quantify the random variables, the collapse probability of the column, beam and beam- column sub-assemblies is assessed by the mean-value first-order second-moment (FOSM) method. The collapse probability is conditioned with respect to the scaled intensity of fire compartment gas temperature, which is treated as independent variable. These studies indicate that the variability in the high-temperature steel yield strength is the most significant factor in the uncertainty assessment. The studies further show that for the design fire temperature, the probability of column failure ranges from 4 % to 38 % (β = 0.3-1.8) for designs based on the AISC strength provisions (with φ = 0.9). These probabilities reduce to 0.5 % to 3 % (β = 1.9-2.6) based on the proposed equations (with φ = 0.9). v ACKNOWLEDGEMENTS S This work was funded by the Fulbright graduate student fellowship and the John A. Blume Earthquake Engineering Center. This report was originally published as the Ph.D. dissertation of the first author. The authors would like to thank Professors Sarah Billington, Helmut Krawinkler, Jack Baker and Eduardo Miranda, for their advice on this research. The authors gratefully acknowledge Dr. Liang Yu and Professor Karl H. Frank at the University of Texas at Austin provided the essential test data of high strength bolts under elevated temperatures. Professor Emeritus Brady R. Williamson provided priceless research papers and reports. Dr. Barbara Lane and Dr. Susan Lamont helped the heat transfer simulation with their expertise. Scott Hamilton worked on risk assessment and framework of structural fire engineering with Professor Deierlein. Professor Paulo Vila Real at University of Aveiro in Portugal kindly provided the most recent draft of Eurocode. Professor Richard Liew at the National University of Singapore also provided his research papers and proceedings of past fire workshops. Dr. Ryoichi Kanno at Nippon Steel kindly arranged the use of the test data of steel at elevated temperatures performed by the Japan Iron and Steel Federation. Corus (British Steel) Swinden Laboratories provided their test data on steel beams at elevated temperatures. Karen Greig, Head Librarian at Engineering Library at Stanford University, obtained papers regarding structural fire engineering. vi TABLE OF CONTENTS Chapter 1 Introduction 1 1.1 Overview 1 1.1.1 Background and Focus of This Research 1 1.1.2 Performance-Based Fire Engineering 2 1.1.3 Role of Structural Fire Engineering 3 1.1.4 Behavior of Steel Structures Exposed to Fire 4 1.1.5 Domains for Limit-state Evaluation 5 1.1.6 Disaster of the World Trade Center 6 1.1.7 Uncertainties in Structural Fire Engineering 6 1.2 Objectives 7 1.3 Scope 8 1.4 Organization 9 Chapter 2 Overview of Steel Structures Exposed to Fire 11 2.1 Past Fire Disasters 11 2.1.1 Fires on Steel Structures 11 2.1.2 Broadgate Phase 8 13 2.1.3 One Meridian Plaza 15 2.1.4 World Trade Center Building 7 16 2.1.5 Windsor Building 20 2.1.6 Cardington Fire Test 23 2.1.7 Summary of Past Fire Disaster Review 25 2.2 Mechanical Properties of Steel under Elevated Temperatures 25 2.2.1 Experimental Results 25 2.2.1.1 Experiments by Harmathy and Stanzak 26 2.2.1.2 Experiment by Skinner 28 2.2.1.3 Experiments by DeFalco 29 2.2.1.4 Experiments by Fujimoto et al. 32 2.2.1.5 Experiments by Kirby and Preston 33 vii TABLE OF CONTENTS 2.2.1.6 Comparison of the Experiments 35 2.2.2 Equations of Stress-strain Curves 38 2.2.2.1 Eurocode Stress-strain Curves 38 2.2.2.2 AS4100 Stress-strain Curves 40 2.2.2.3 AIJ Stress-strain Curves 41 2.2.2.4 AISC Stress-strain Curves 44 2.2.2.5 Comparison of the Equations of Stress-strain Curves 45 2.2.3 Experiments by JISF 49 Chapter 3 Analysis of Individual Members 51 3.1 Summary 51 3.2 Introduction 51 3.3 Basis of Member Strength Evaluations 53 3.3.1 Steel Properties under Elevated Temperatures 55 3.4 Finite Element Simulation Model 57 3.5 Column Strength Assessment 61 3.5.1 AISC Column Strength Equations 62 3.5.2 EC3 Column Strength Equations 62 3.5.3 Assessment of Column Strengths 64 3.5.4 Proposed Column Strength Equations 67 3.5.5 Column Test Data 68 3.5.6 Influence of Yield Strength and Section Geometry 69 3.6 Beam Strength Assessment 70 3.6.1 AISC Beam Strength Equations 70 3.6.2 EC3 Beam Strength Equations 72 3.6.3 Proposed Beam Strength Equations 73 3.6.4 Assessment of Beam Strengths 74 3.7 Beam-Column Strength Assessment 78 3.7.1 AISC Beam-Column Strength Equations 79 3.7.2 Proposed Beam-Column Strength Equations 80 3.7.3 EC3 Beam-Column Strength Equations 80 3.7.4 Assessment of Beam-Column Strengths 80 3.8 Summary and Conclusions 83 viii TABLE OF CONTENTS 3.9 Limitations and Future Research 84 Chapter 4 Analysis of Gravity Frames 87 4.1 General 87 4.1.1 Overview 87 4.1.2 Benchmark Office-type Building Design 88 4.1.3 Failure Mechanisms and Sub-assembly Analysis Models 90 4.1.4 Time-temperature Relationships in Localized Fire 92 4.1.5 Organization of Chapter 4 93 4.2 Evaluation of Interior Column Sub-assembly 93 4.2.1 Summary 93 4.2.2 Introduction 93 4.2.3 Analysis Model 96 4.2.3.1 Modeling of System 96 4.2.3.2 Modeling of Column 98 4.2.3.3 Modeling of Constraint Springs 101 4.2.4 Evaluation of Critical Temperatures 106 4.2.5 Comparison between Design Equations and Sub-assembly Simulations 111 4.2.6 Improvement of Structural Robustness 112 4.2.7 Conclusions 114 4.3 Evaluation of Beam Sub-assembly 115 4.3.1 Summary 115 4.3.2 Introduction 115 4.3.3 Analysis Model
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