CFT Column-to-Cap Beam Connections for Accelerated Bridge Construction in Seismic Regions Lisa Marie Berg A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Civil Engineering University of Washington 2014 Program Authorized to Offer Degree: Civil and Environmental Engineering ©Copyright 2014 Lisa M. Berg 1 University of Washington Abstract CFT Column-to-Cap Beam Connections for Accelerated Bridge Construction in Seismic Regions Lisa M. Berg Chair of the Supervisory Committee: Associate Professor Dawn E. Lehman Dept. of Civil & Environmental Engineering Due to their increased mechanical properties and ease of construction, Concrete filled steel tubes (CFTs) offer an efficient and economical alternative compared to traditional reinforced concrete columns. Despite their inherent advantages, the implementation of CFT construction within the US is limited due to unverified design expressions and lack of standard connection details. As an effort to facilitate the use of CFTs, a research program has been undertaken by the University of Washington to develop practical connection details for the use of CFTs in bridge construction in seismic regions. The focus of this research is the development of CFT column-to-cap beam connections. Three categories of the CFT column to cap beam connection have been experimentally evaluated; an embedded connection in which the steel tube is embedded in the cap beam concrete, a welded reinforced concrete connection in which headed reinforcing bars are welded to the inside of the steel tube and extended into the cap beam, and a reinforced concrete connection in which a traditional reinforced concrete cage is placed within the CFT column and extended into the cap beam. Experimental results show that these connections can achieve strength and ductility objectives with limited damage to superstructure components. 2 TABLE OF CONTENTS Chapter 1 Introduction…………………………………………………………....…15 1.1 Concrete filled tubes ....................................................................................... 14 1.2 Research objective .......................................................................................... 15 1.3 Report overview .............................................................................................. 15 Chapter 2 Review of Design Provisions……………………………………….....….18 2.1 AISC Design Provisions ................................................................................. 17 2.1.1 Material and Geometric Limitations ................................................... 17 2.1.2 Effective Stiffness ............................................................................... 18 2.1.3 Compressive Strength ......................................................................... 18 2.1.4 Flexural Strength ................................................................................. 19 2.1.5 Combined Flexure and Axial Compression ........................................ 19 2.2 AASHTO Design Provisions .......................................................................... 21 2.2.1 Stability Limitations............................................................................ 21 2.2.2 Effective Stiffness ............................................................................... 21 2.2.3 Compressive Strength ......................................................................... 22 2.2.4 Flexural Strength ................................................................................. 22 2.2.5 Combined Flexure and Axial Compression ........................................ 23 2.3 ACI Design Provisions ................................................................................... 23 2.3.1 Stability Limitations............................................................................ 23 2.3.2 Effective Stiffness ............................................................................... 24 2.3.3 Combined Strength ............................................................................. 24 2.4 Comparison of Design Provisions .................................................................. 25 3 2.4.1 Comparison of Stability Limits ........................................................... 26 2.4.2 Comparison of Effective Stiffness ...................................................... 26 Chapter 3 Literature Review…………………………………………………………29 3.1 Exposed Base Plate Connection ...................................................................... 28 3.2 Embedded Fixed-Base Connection ................................................................. 30 3.3 Reinforced Concrete-Filled Steel Tube Connection Tests.............................. 32 3.4 Embedded CFT Connections .......................................................................... 33 3.4.1 Kingsley (2005) Footing Connection Tests ........................................ 37 3.4.2 Williams (2006) Footing Connection Tests ........................................ 38 3.4.3 Lee (2011) Footing Connection Tests ................................................. 40 3.4.4 O’Neill (2011) Footing Connection Test ............................................ 41 3.5 Design Expressions from University of Washington Tests ............................ 42 3.5.1 Required Embedment Depth ............................................................... 42 3.5.2 Minimum Footing Depth .................................................................... 43 3.5.3 Shear Reinforcing Requirements ........................................................ 43 Chapter 4 Experimental Test Program…………………………………………...….45 4.1 Specimen Test Matrix ..................................................................................... 44 4.1.1 Embedded CFT Connections .............................................................. 45 4.1.2 Welded Reinforced Concrete Connections ......................................... 46 4.1.3 Reinforced Concrete Connection ........................................................ 47 4.2 Specimen Layout ............................................................................................ 47 4.3 Specimen Design and Construction ................................................................ 49 4.3.1 Caltrans Seismic Design Criteria ........................................................ 49 4 4.3.2 Embedded CFT Connections .............................................................. 51 4.3.3 Welded RC Connections ..................................................................... 57 4.3.4 RC Connection .................................................................................... 62 4.4 Specimen Materials ......................................................................................... 65 4.5 Experimental Test Setup ................................................................................. 67 4.6 Load Protocol .................................................................................................. 71 4.7 Specimen Instrumentation .............................................................................. 73 4.7.1 Global Instrumentation ....................................................................... 74 4.7.2 Column Instrumentation ..................................................................... 75 4.7.3 Connection Instrumentation ................................................................ 77 Chapter 5 Experimental Observations……………………………………………….85 5.1 Introduction ..................................................................................................... 84 5.2 Overview of Damage States............................................................................ 86 5.3 Specimen EMB80 ........................................................................................... 90 5.3.1 EMB 80 Damage State Summary ....................................................... 91 5.3.2 Low Drift Cycles (0.0% - 2.0% Drift) ................................................ 93 5.3.3 Moderate to High Drift Cycles (2.0% - 7.0% Drift) ........................... 94 5.3.4 Specimen Failure (7.0%+ Drift) ......................................................... 96 5.4 Specimen WRC ............................................................................................. 100 5.4.1 WRC Damage State Summary .......................................................... 101 5.4.2 Low Drift Cycles (0.0% - 2.0% Drift) .............................................. 103 5.4.3 Moderate to High Drift Cycles (2.0% - 8.0% Drift) ......................... 104 5.4.4 Specimen Failure (8.0% + Drift) ...................................................... 111 5 5.5 Specimen WRCUB ....................................................................................... 116 5.5.1 WRCUB Performance State Summary ............................................. 117 5.5.2 Low Drift Cycles (0.0% - 2.0% Drift) .............................................. 119 5.5.3 Moderate to High Drift Cycles (2.0% - 11.0% Drift) ....................... 119 5.5.4 Specimen Failure (11.0%+ Drift) ..................................................... 124 5.6 Specimen RC ................................................................................................ 127 5.6.1 RC Performance State Summary ...................................................... 128 5.6.2 Low Drift Cycles (0.0% - 2.0% Drift) .............................................. 130 5.6.3 Moderate to High Drift Cycles (2.0% - 9.0% Drift) ......................... 131 5.6.4 Specimen Failure (9.0%+ Drift) ....................................................... 135 5.7 Specimen EMB96 ......................................................................................... 137 5.7.1 EMB96 Performance State Summary ............................................... 138 5.7.2 Low Drift Cycles (0.0% - 2.0% Drift) .............................................. 140 5.7.3 Moderate to High Drift Cycles (2.0% - 7.0% Drift) ......................... 141 5.7.4 Specimen Failure (7.0%+ Drift) ....................................................... 144 Chapter 6 Data Interpretation and Analysis………………………………………..149 6.1 Force-drift response ...................................................................................... 148 6.2 Moment-drift response .................................................................................. 150 6.3 Longitudinal Strain Profiles .......................................................................... 153 6.3.1 Longitudinal Tube Strains................................................................. 154 6.3.2 Longitudinal Reinforcing Bar Strains ............................................... 156 6.4 Curvature Profiles ......................................................................................... 159 6.5 Column Rotation Profiles ............................................................................. 165 6 6.5.1 Embedded CFT Specimen Rotation Profiles .................................... 166 6.5.2 RC Connection Rotation Profiles...................................................... 168 6.6 Moment-Rotation .......................................................................................... 171 6.7 Column Displacement Profiles ..................................................................... 172 6.8 Connection Rotation and Column Rotation .................................................. 175 6.8.1 Embedded Specimen Rotation Contributions ................................... 175 6.8.2 WRC Specimen Rotation Contributions ........................................... 177 6.8.3 RC Specimen Rotation Contributions ............................................... 178 6.9 Column Buckled Shape Profile ..................................................................... 178 6.9.1 Buckled Profile of Specimen EMB80............................................... 179 6.9.2 Buckled Profile of Specimen EMB96............................................... 181 6.10 Fragility Curves ............................................................................................ 186 6.11 Assessment of Performance .......................................................................... 188 6.11.1 Comparison of EMB80 with Previous Work .................................... 190 6.11.2 Comparison of Specimens EMB80 and EMB96 .............................. 193 6.11.3 Comparison of Specimens WRC AND WRCUB ............................. 193 6.11.4 Comparison of Specimens RC and WRC ......................................... 193 6.12 Design Recommendations ............................................................................ 194 Chapter 7 Conclusions……………………………………………...………………197 7.1 Summary of Research ................................................................................... 196 7.2 Research Results and Conclusions................................................................ 197 7.3 Recommendations for Future Research ........................................................ 198 7 LIST OF FIGURES Figure 2.1: Plastic Stress Distribution Method ................................................................. 20 Figure 2.2: Strain Compatibility Method .......................................................................... 21 Figure 2.3: ACI Model for Strength Prediction ................................................................ 25 Figure 2.4: CFT Column redesigned from Laguna de Santa Rosa RC Column ............... 26 Figure 3.1: CFT-Footing Connection Configuration by Kawaguchi and Morino. ........... 28 Figure 4.1: Proposed CFT column-to-cap beam connections ........................................... 45 Figure 4.2: Embedded CFT force transfer mechanism ..................................................... 45 Figure 4.3: Basic Specimen Layout .................................................................................. 48 Figure 4.4: Specimen EMB80 Reinforcing Details .......................................................... 53 Figure 4.5: Specimen EMB96 Reinforcing Details .......................................................... 54 Figure 4.6: Embedded CFT connection construction sequence ........................................ 55 Figure 4.7: Cap Beam Reinforcing and Formwork .......................................................... 55 Figure 4.8: EMB80 tube and annular ring ........................................................................ 56 Figure 4.9: Specimens WRC, WRCUB, and RC Reinforcing Details ............................. 59 Figure 4.10: Welded RC Connections Construction Sequence ........................................ 60 Figure 4.11: Specimen WRC annular ring and reinforcing bars ...................................... 61 Figure 4.12: RC Connection Construction Sequence ....................................................... 63 Figure 4.13: Reinforcing cage of Specimen RC ............................................................... 64 Figure 4.14: Experimental Test Rig .................................................................................. 67 Figure 4.15: Williams Rods Layout .................................................................................. 68 Figure 4.16: Axial Load Apparatus .................................................................................. 69 Figure 4.17: Experimental test rig with Specimen EMB80 .............................................. 70 8 Figure 4.18: Target load protocol ..................................................................................... 72 Figure 4.19: Global Instrumentation Schematic ............................................................... 74 Figure 4.20: Column Instrumentation Schematic ............................................................. 76 Figure 4.21: EMB80 Strain Gauge Schematic .................................................................. 78 Figure 4.22: Strain gauge schematic for (a) Specimens WRC, WRCUB, and (b) RC ..... 80 Figure 4.23: EMB80 Optotrak Target Layout .................................................................. 83 Figure 5.1: Specimen drift ................................................................................................ 85 Figure 5.2: Idealized Drift History.................................................................................... 86 Figure 5.3: EMB80 Connection Detail ............................................................................. 90 Figure 5.4: EMB80 Moment-Drift Response ................................................................... 91 Figure 5.5: EMB80 Cracking on South side of Column at 0.41% drift ............................ 93 Figure 5.6: EMB80 Radial cracking at 1.45% drif ........................................................... 94 Figure 5.7: EMB80 hairline cracking on the west side of the cap beam at 2.25% drift ... 94 Figure 5.8: EMB80 South tube buckling at (a) 3.19% drift, and (b) 4.43% drift ............. 95 Figure 5.9: EMB80 South tube buckling at 7.0% drift ..................................................... 96 Figure 5.10: EMB80 tear initiation on North side of column at 7.0% drift ...................... 97 Figure 5.11: EMB80 tear initiation on South side of column at 7.0% drift ...................... 97 Figure 5.12: EMB80 tear propagation on North side of column at 8.54% drift ............... 98 Figure 5.13: EMB80 tear propagation on South side of column at 8.30% drift ............... 98 Figure 5.14: WRC Connection Detail ............................................................................. 100 Figure 5.15: WRC Moment-Drift Response ................................................................... 101 Figure 5.16: WRC soffit cracking at 0.3% drift.............................................................. 103 9