POLITECNICO DI MILANO Scuola di Ingegneria Civile, Ambientale e Territoriale Corso di Laurea Magistrale in BUILDING AND ARCHITECTURAL ENGINEERING REAL SCALE CYCLIC TESTS FOR SEISMIC ASSESS OF CONCENTRICALLY BRACED FRAMES Supervisor: Chiar.mo Prof. Carlo Andrea CASTIGLIONI Correlator: Dott. Ing. Alper KANYILMAZ Master thesis of: Rico Umberto Matricola 852308 Academic Year 2014/2015 Index 1. Introduction ................................................................................................................................................ 4 1.1. EN 1993-1-1 design approach for concentrically braced frames ..................................................... 5 1.2. MEAKADO PROJECT ............................................................................................................................. 6 1.3. Thesis objectives .................................................................................................................................. 6 2. State of the Art ........................................................................................................................................... 8 2.1. Experimental researches about hysteretic behaviour of concentrically bracings ........................... 9 3. Design of test specimens ......................................................................................................................... 18 3.1. Introduction ....................................................................................................................................... 18 3.2. Experimental tests definition ............................................................................................................ 19 3.2.1. Steel characterization ................................................................................................................. 21 3.3. Design Procedure .............................................................................................................................. 22 3.3.1. Model of Pendulum Frame ........................................................................................................ 23 3.3.2. Bolts ............................................................................................................................................. 24 3.3.3. Main elements of the frame ...................................................................................................... 24 3.3.3.1. Beam-to-column joints ....................................................................................................... 26 3.3.3.2. Pinned base connection of the column ............................................................................. 28 3.4. Bracing system ................................................................................................................................... 30 3.4.1.1. Buckling check of the compressed element following the l’EC3 ..................................... 31 3.4.1.2. Tightening torque ............................................................................................................... 33 3.4.1.3. Connection joint to the main members ............................................................................ 35 3.4.1.4. Mid-plate connection ......................................................................................................... 38 3.4.2. Evaluation of the bearing capacity of the tension and compression diagonal ....................... 39 4. Experimental setup .................................................................................................................................. 40 4.1. Test bench ......................................................................................................................................... 40 4.1.1. Load application device .............................................................................................................. 44 4.1.2. Out-of-plane restraints .............................................................................................................. 45 4.2. Instrumentation ................................................................................................................................. 46 4.2.1. Linear displacement potentiometers ........................................................................................ 46 4.2.1.1. Operational principle .......................................................................................................... 46 4.2.1.2. Calibration ........................................................................................................................... 47 4.2.1.3. Pots layout .......................................................................................................................... 48 4.2.2. Uni-axial strain gauges ............................................................................................................... 54 4.2.2.1. Operational principle .......................................................................................................... 54 4.2.2.2. Technical characteristics of the strain gauges .................................................................. 54 4.2.2.3. Layout e strain gauges description .................................................................................... 55 4.3. Data acquisition system and load application ................................................................................. 59 5. Experimental results................................................................................................................................. 60 5.1. Loading protocol ................................................................................................................................ 60 5.2. Sign convention ................................................................................................................................. 63 5.3. Performance states ........................................................................................................................... 64 5.4. Test 1: X-shaped frame 2L60x8 ........................................................................................................ 65 5.4.1. Loading history ........................................................................................................................... 65 5.4.2. Global overview .......................................................................................................................... 66 5.4.3. Columns ...................................................................................................................................... 67 5.4.4. Bracings behaviour ..................................................................................................................... 68 5.5. Test 2: X-shaped frame No mid connection 2L60x8 ........................................................................ 70 5.5.1. Loading history ........................................................................................................................... 70 5.5.2. Global overview .......................................................................................................................... 71 5.5.3. Columns ...................................................................................................................................... 71 5.5.4. Bracings behaviour ..................................................................................................................... 72 5.6. Test 3: Single diagonal moment resisting frame 2L60x8 ................................................................ 74 5.6.1. Loading history ........................................................................................................................... 74 5.6.2. Global overview .......................................................................................................................... 75 5.6.3. Column and beams ..................................................................................................................... 76 5.6.4. Bracings behaviour ..................................................................................................................... 76 5.7. Test 5: X-shaped frame 2L70x7 ........................................................................................................ 78 5.7.1. Loading history ........................................................................................................................... 78 5.7.2. Global overview .......................................................................................................................... 79 5.7.3. Bracings behaviour ..................................................................................................................... 80 5.8. Test 7: Single diagonal frame 2L70x7 ............................................................................................... 81 5.8.1. Loading hystory .......................................................................................................................... 81 5.8.2. Global behaviour ........................................................................................................................ 82 5.8.3. Bracings behaviour ..................................................................................................................... 84 5.9. Test 15: No-bracings and no-gussets frame .................................................................................... 85 5.9.1. Loading history ........................................................................................................................... 85 5.9.2. Global overview .......................................................................................................................... 86 5.10. Test 16: No bracings frame ............................................................................................................. 88 5.10.1. Loading history ......................................................................................................................... 88 5.10.2. Global behaviour ...................................................................................................................... 89 5.11. Test 13: X-braced frame 2L60x8 ..................................................................................................... 92 5.11.1. Loading history ......................................................................................................................... 92 5.11.2. Global behaviour ...................................................................................................................... 93 5.11.3. Column and beams ................................................................................................................. 104 5.11.4. Bracings behaviour ................................................................................................................. 104 5.11.5. Energy dissipation .................................................................................................................. 105 5.12. Test 17: Pinned X-braced frame 2L60x8 ...................................................................................... 112 5.12.1. Loading history ....................................................................................................................... 112 5.12.2. Global overview ...................................................................................................................... 113 5.12.3. Column and beams ................................................................................................................. 114 5.12.4. Bracings behaviour ................................................................................................................. 115 5.13. Test 18: Pinned X-braced frame 2L70x7 ...................................................................................... 116 5.13.1. Loading history ....................................................................................................................... 116 5.13.2. Global overview ...................................................................................................................... 117 5.13.3. Columns .................................................................................................................................. 118 5.13.4. Bracings behaviour ................................................................................................................. 119 5.14. Test 19: Pinned single diagonal frame 2L70x7 ............................................................................ 120 5.14.1. Loading history ....................................................................................................................... 120 5.14.2. Global overview ...................................................................................................................... 121 5.14.3. Column and beams ................................................................................................................. 122 5.14.4. Bracings behaviour ................................................................................................................. 122 5.15. Test 23: Single diagonal pinned frame 2L60x8 ............................................................................ 124 5.15.1. Loading history ....................................................................................................................... 124 5.15.2. Global overview ...................................................................................................................... 125 5.15.3. Columns and beams ............................................................................................................... 126 5.15.4. Bracings behaviour ................................................................................................................. 126 6. Analysis of test results ............................................................................................................................ 128 6.1. Energy dissipation ........................................................................................................................... 128 6.1.1. Bolts slip and ovalization .......................................................................................................... 128 6.1.2. Bracings buckling ...................................................................................................................... 132 6.1.3. Idealization of the brace .......................................................................................................... 135 6.1.4. Strength and stiffness comparison .......................................................................................... 136 6.1.5. Overstrength from compression diagonal .............................................................................. 138 6.1.6. Pinned vs moment resisting frame .......................................................................................... 141 6.1.7. Gusset plate .............................................................................................................................. 142 6.1.8. Reserve ductility of the frame ................................................................................................. 144 7. Initial connection stiffness ..................................................................................................................... 147 8. Conclusions. Future research. ............................................................................................................... 155 APPENDIX A – Lab report about specimens monoaxial tensile tests ....................................................... 157 APPENDIX B – Complete bill of materials and design papers .................................................................. 159 Bill of materials ....................................................................................................................................... 159 Bill of fasteners ....................................................................................................................................... 160 Graphic charts ......................................................................................................................................... 161 APPENDIX C – BRACINGS BEARING CAPACITY .......................................................................................... 223 Bearing capacity brace-gusset plate connection .................................................................................. 223 Compression and tension brace resistance .......................................................................................... 227 Compression profile classification ..................................................................................................... 227 Tension diagonal strength .................................................................................................................. 227 Buckling and strength of compression diagonal ............................................................................... 228 Block tearing ........................................................................................................................................... 229 APPENDICE D – DESIGN OF PINNED FRAME ELEMENTS .......................................................................... 230 Hollow beam resistance and stability to compresion according to EC3 ................................................. 230 Hollow beam resistance and stability to compresion according to EC3 ................................................. 232 Weldings design and check ........................................................................................................................ 235 APPENDIX E – instrumentation layout ...................................................................................................... 237 Potentiometers layouts .......................................................................................................................... 237 APPENDIX F – bolt slip ................................................................................................................................ 243 References .................................................................................................................................................. 246 Acknowledgements .................................................................................................................................... 248 INDEX OF FIGURES Figure 1. Calculation framework for bracings configurations: a) Tension only b) Tension / Compression5 Figure 2. Design values scheme ..................................................................................................................... 6 Figure 3. Force-displacement cycles of a rod axially loaded to symmetrical loading history .................... 8 Figure 4. Force-displacement cycles of a braced frame ............................................................................... 8 Figure 5. Tests specimens ............................................................................................................................ 10 Figure 6. Maximum bearing capacity for each loading cycle. Single diagonal frame configuration. ....... 11 Figure 7. Maximum bearing capacity for each loading cycle. Cross-diagonal configuration frame. ....... 11 Figure 8. Cycle force-displacement for single diagonal specimens ........................................................... 12 Figure 9. Force-displacement cycle of cross configuration braced specimens ......................................... 12 Figure 10. Asymmetrical deformed configuration ...................................................................................... 13 Figure 11. Adimensional energy absorbed for each loading cycle ............................................................ 13 Figure 12. Comparison between numerical and real behaviour ............................................................... 14 Figure 13. Hysteretic response of a brace undergoing inelastic cycles ..................................................... 15 Figure 14. Construction detail of the tested specimen .............................................................................. 16 Figure 15.Generic scheme of the frame ..................................................................................................... 18 Figure 16. Frame configuration during the tests ........................................................................................ 19 Figure 17. Samples extracted from angular profiles of the braces for tensile tests ................................. 21 Figure 18. Conceptual scheme of a) MRF and b) PF (measures in meters) ............................................ 23 Figure 19. Braced pendulum frame ............................................................................................................. 23 Figure 20. Front view of MRF (measures in meters) .................................................................................. 25 Figura 21. Front view of PF (measures in meters) ...................................................................................... 25 Figure 22. Beam-column joint in MRF ......................................................................................................... 26 Figure 23. Construction details of beam-column connection (measures in mm) ................................... 26 Figure 24. Beam-column joint in PF ............................................................................................................ 27 Figura 25. Construction details of beam-column connection (measures in mm).................................... 27 Figure 26. Pinned base plate in MRF ........................................................................................................... 28 Figure 27. Detail of pinned base plate in MRF ............................................................................................ 28 Figure 28. Construction details of pinned base plate in PF ........................................................................ 29 Figure 29. Detail of pinned base plate in MRF ............................................................................................ 29 Figure 30. Front view of the PF configurations (measures in meters) ...................................................... 34 Figure 31. Front view of the MRF configurations (measures in meters) ................................................... 34 Figure 32. Angular gusset plate welded to the main members ................................................................. 35 Figure 33. Modular connection system in MRF .......................................................................................... 35 Figure 34. Modular connection system in PF .............................................................................................. 36 Figure 35. Construction details of corner connection modular systems in MRF (measures in mm)....... 37 Figure 36. Construction details of corner connection modular systems in PF (measures in mm) .......... 37 Figure 37. Steel mid-plate of the bracing system ....................................................................................... 38 Figure 38. Construction detail of the mid-connection (measures in mm) ................................................ 38 Figure 39. MRF Test setup (measures in meters) ....................................................................................... 41 Figure 40. PF Test setup (measures in meters) .......................................................................................... 42 Figure 41. View of the test bench ................................................................................................................ 43 Figure 42. Load transmission device of frames (measures in mm) ........................................................... 44 Figure 43. Fixing phase of the load cell to the MRF ................................................................................... 44 Figure 44. Sphere joint welded to the specimen ........................................................................................ 45 Figure 45. Sphere joint welded to the specimen ........................................................................................ 45 Figure 46. Electric scheme of a stick linear potentiometer ...................................................................... 46 Figure 47. Calibration of a potentiometer through digital micrometer .................................................... 47 Figure 48. Linear regression line of a pot .................................................................................................... 47 Figure 49. Pots for horizontal displacement of the frame ......................................................................... 48 Figure 50. Pots for horizontal displacements (real test bench) ................................................................. 49 Figure 51. Potentiometers for measuring braces buckling in X configuration frame ............................... 50 Figure 52. Pots for buckling measurement of the brace in one diagonal configuration .......................... 50 Figure 53. Pots for relative slide evaluation of the braces ......................................................................... 51 Figure 54. Scheme for rotation calculation of the beam-column joint ..................................................... 52 Figure 55. Potentiometers for rotation of beam-column joints ............................................................... 52 Figure 56. Pots for base plate slip ............................................................................................................... 53 Figure 57. Potentiometers for the measurement of base plates sliding ................................................... 54 Figure 58. Structure of an electrical sheet strain gauge ............................................................................ 55 Figure 59. Installed strain gauges: a) on continuous brace b)on right column c)on the bottom beam .. 55 Figure 60. Strain gauges layout.................................................................................................................... 56 Figure 61. Detail of the bracings strain gauge installation ......................................................................... 57 Figure 62. Installation details of the strain gauges on the main members a) column of MRF b) beam of MRF c) column of PF .................................................................................................................................... 58 Figure 63. Data acquisition systems ............................................................................................................ 59 Figure 64. Storey drift definition ................................................................................................................. 60 Figure 65. Loading protocol definition on control unit of actuator ........................................................... 62 Figure 66. Adopted sign convention: a) positive frame displacement b) negative frame displacement 63 Figure 67. 2L60x8 X-braced frame specimen on test bench ...................................................................... 65 Figure 68. Loading history ............................................................................................................................ 65 Figure 69. Initial step hysteresis loops- Coordinates for initial stiffness ................................................... 66 Figure 70. a) Frame hysteresis loop . b) Initial steps-global. ...................................................................... 67 Figure 71. Load trend on columns. .............................................................................................................. 67 Figure 72. Load on bracings over times. Data derived by strain gauges records. .................................... 68 Figure 73. Axial forces over drift applied during test. ................................................................................ 69 Figure 74. Loading history ............................................................................................................................ 70 Figure 75. Global test hysteresis loops........................................................................................................ 71 Figure 76. Load trend on columns ............................................................................................................... 72 Figure 77. Load on bracings over times. Data derived by strain gauges records. .................................... 72 Figure 78. Axial forces over drift applied during test. ................................................................................ 73 Figure 79. Specimen on test bench ............................................................................................................. 74 Figure 80. Loading history ............................................................................................................................ 74 Figure 81. Global test hysteresis loops........................................................................................................ 75 Figure 82. Load trend on columns ............................................................................................................... 76 Figure 83. Load on bracings over time and displacement applied. ........................................................... 77 Figure 84. Trend of load on brace and columns. Comparison. Force distribution scheme ..................... 77 Figure 85. Specimen on test bench ............................................................................................................. 78 Figure 86. Test 5: loading history ................................................................................................................ 78 Figure 87. Test 5: global test hysteresis loops. ........................................................................................... 79 Figure 88. Test 5: axial load over time and displacement imposed (experimental results) ..................... 80 Figure 89. Test 7: Specimen on test bench before test initiation .............................................................. 81 Figure 90 Test 7: loading history ................................................................................................................. 81 Figure 91. Test 7: Global test hysteresis loops ........................................................................................... 82 Figure 92. Test 7: plastic hinge during post-buckling phase (0.62% of drift) ............................................ 83 Figure 93. Test 7: plasticization of diagonal edge (0.62% of drift) ............................................................ 83 Figure 94. Test 7: collapse mechanism of brace (0.69% of drift)............................................................... 84 Figure 95. Test 7: diagonal axial load – displacement applied (experimental results) ............................. 84 Figure 96. Specimen on test bench ............................................................................................................. 85 Figura 97. Test 16: loading history .............................................................................................................. 85 Figure 98. Global test hysteresis loops ........................................................................................................ 86 Figure 99. Web angles yielding .................................................................................................................... 87 Figure 100. Test 16: specimen on test bench ............................................................................................. 88 Figure 101. Test 16: loading history ............................................................................................................ 88 Figure 102. Initial stiffness ........................................................................................................................... 89 Figure 103. Global test hysteresis loops ..................................................................................................... 90 Figure 104. Test 16: gusset plates yielding. T-stub section. ....................................................................... 90 Figure 105. Beam-column node rotation .................................................................................................... 91 Figure 106. Yielding mechanism of the gusset plates ................................................................................ 91 Figure 107. Test 13: loading history ............................................................................................................ 92 Figure 108. Initial stiffness (plot at first ±6 mm cycle) ............................................................................... 93 Figure 109. First slip occurred when frame under compression, force peak value 665 kN, drift -0,23% (plot at first compression peak of ±8 mm cycle) (2800 s time step) ......................................................... 94 Figure 110. Bearing capacity reduction (plot at first compression peak of ±8 mm cycle) (5550) ........... 94 Figure 111. Second slip (plot at first compression peak of ±12 mm cycle, drift -0,46 %, force peak value 675 kN) (6550 s time step) ........................................................................................................................... 95 Figure 112. First slip occurred when structure under tension (plot at first tension peak of ±13 mm cycle, drift +0.51%, force peak value 703 kN) (6950 s time step)............................................................. 95 Figure 113.Continuous brace buckling, +0.59 % (+15 mm) drift cycle, force peak value 750 kN (9850 s time step) ......................................................................................................................................... 96 Figure 114. 4. plastic hinge is formed at mid-span of the north-west continuous diagonal ................... 96 Figure 115. 5. Plastic hinge formed after first in-plane buckling on mid-span of NE diagonal ................ 96 Figure 116. Global behaviour. Stiffness deterioration. (13080 s time step) ............................................. 97 Figure 117. Plasticization of frame nodes ................................................................................................... 97 Figure 118. Plasticization, cracks opening and propagation at NW discontinuous diagonal ................... 98 Figure 119. Global behaviour. Brace axial load-drift ratio. (15150 s time step) ...................................... 98 Figure 120. Crack opening at plastic hinges yet presented at frame nodes ............................................. 99 Figure 121. Global behaviour (17150 s time step) ................................................................................... 100 Figure 122. Failure along the stitch plate bolts in mid-span of the north-east discontinuous diagonal. (second frame tension peak of ±30 mm). Gross-section separation at mid-span of NE discontinuous brace. Cracks, initiated after plastic hinge formation, propagate. .......................................................... 100 Figure 123. Global behaviour (18150 s time step) ................................................................................... 101 Figure 124.Failure at mid-span of the north-west continuous diagonal (first frame compression peak of ±38 mm cycle). ........................................................................................................................................... 101 Figure 125. Global behaviour (20200 s time step). .................................................................................. 102 Figure 126. Frame at test end; yielding at gusset. .................................................................................... 102 Figure 127. Main phases on a drawing ..................................................................................................... 102 Figure 128. Load trend on beams .............................................................................................................. 104 Figure 129. Axial load of bracings over time and displacement applied ................................................. 105 Figure 130. Calculation scheme hysteresis loop area .............................................................................. 105 Figure 131. Energy dissipation for each cycle ........................................................................................... 106 Figure 132. Scheme of glabal (a) and local (b) buckling mechanism ....................................................... 107 Figure 133. Curve of cumulative energy dissipated during test 13. ........................................................ 107 Figure 134. Compare cumulate the energy dissipation comparison. Test 13 vs Test 16 ...................... 108 Figure 135. Cumulative energy dissipation of positive displacement loops ........................................... 109 Figure 136. Calculation scheme ................................................................................................................. 109 Figure 137. Cumulative energy of negative drift cycle loops ................................................................... 110 Figure 138. Cumulate energy dissipation during positive drift cycle vs negative drift cycle loops ....... 111 Figure 139. Energy dissipation for each cycle (Test13 vs Test3) .............................................................. 111 Figure 140. Specimen on test bench ......................................................................................................... 112 Figure 141. Loading history ........................................................................................................................ 112 Figure 142. Initial stiffness ......................................................................................................................... 113 Figure 143. Global test hysteresis loops ................................................................................................... 114 Figure 144. Buckling mechanism of the system. Brace touched the beam.. .......................................... 114 Figure 145. Load trend on columns ........................................................................................................... 115 Figure 146. Pre-buckling phase axial loops in bracings ............................................................................ 115 Figure 147. Specimen on test bench ......................................................................................................... 116 Figure 148. Loading history ........................................................................................................................ 116 Figure 149. Initial step and global test hysteresis loops........................................................................... 118 Figure 150. Load trend on columns ........................................................................................................... 118 Figure 151. Bracing axial load over time and displacement applied ....................................................... 119 Figure 152. Detail of plastic hinge on continuous brace. ......................................................................... 119 Figure 153. Specimen on test bench ......................................................................................................... 120 Figure 154. Loading history ........................................................................................................................ 120 Figure 155. Hysteresis loops after buckling and at test end. ................................................................... 121 Figure 156. Trend of load on brace and columns. Comparison. Force distribution scheme. ................ 122 Figure 157. Bracings axial load over time and displacement applied ...................................................... 123 Figure 158. Buckling of diagonal (+0.19% drift applied)........................................................................... 123 Figure 159. Specimen on test bench ......................................................................................................... 124 Figure 160. Loading history ........................................................................................................................ 124 Figure 161. Global hysteresis loops pre-yielding and after yielding of the brace ................................... 125 Figure 162. Trend of load on brace and columns. Comparison. Force distribution scheme. ................ 126 Figure 163. Bracings axial load over time and displacement applied ...................................................... 127 Figure 164. Buckling of diagonal. (+0.19% drift applied) ......................................................................... 127 Figure 165. Test 5: base shear – drift ........................................................................................................ 128 Figure 166. Scheme of involved force at bolt connection ....................................................................... 129 Figure 167. Slip and its effects on internal surface of brace at connection to gusset ............................ 129 Figure 169. Bolt holes ovalization at brace joints ends ............................................................................ 131 Figure 169. Test 3-7-18-23. Evaluation of axial buckling load of brace through base shear (values about test 5) .......................................................................................................................................................... 132 Figure 170. Diagonal inflexion over axial load in bracings ....................................................................... 133 Figure 171. Test 3-7-19-23: bracing behaviour idealization .................................................................... 135 Figure 172. Test 7: calculation scheme of frame global stiffness ............................................................ 137 Figure 173. Hysteresis loops comparison ................................................................................................. 138 Figure 174. X-braced vs Single diagonal frame - initial stiffness .............................................................. 139 Figure 175. X-braced vs Single diagonal frame - global behaviour ......................................................... 140 Figure 176. Global behaviour semi-rigid vs pinned frame ...................................................................... 141 Figure 177. Global base shear-drift behaviour of the frame with and without gusset plate connections ..................................................................................................................................................................... 142 Figure 178. Geometry of beam-to-column connection ........................................................................... 143 Figure 179. Initial stiffness comparisons of X-braced semi-rigid and pinned frames ............................ 143 Figure 180. Initial stiffness comparisons of semi-rigid and pinned frames ............................................ 143 Figure 181. Peak base shear reached in the tests .................................................................................... 144 Figure 182. Ductility resources of the 2L60x8 braced frame ................................................................... 145 Figure 183 Ductility resources of the 2L70x7 braced frame ................................................................... 145 Figure 184. Base shear and Drift values at various instances (A, B, C, D, E) .......................................... 146 Figure 185. Specimen X60 during post buckling and after bracings’ complete failure ......................... 146 Figure 186. Semi-rigid beam-to-column connection ............................................................................... 147 Figure 187. Geometrical scheme for deriving parameters ...................................................................... 148 Figure 188. Angle and gusset cross-section for stiffness estimation ...................................................... 149