Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS 1.25 specified by the seismic provisions of AISC (1992) and therefore, hardening of cyclic stress-strain curve is expected. The performance of structural joints under recent strong earthquakes pointed out the significant importance of weld design and execution. Therefore, beam stubs were connected to end plates by means of fillet welds, executed with special care by licensed welders. The welding technology was Flux Cored Arc Welding (FCAW) with Metal Active Gas (MAG) shielding and no preheating. A filler metal was selected, characterized by a nominal yield stress fy equal to 420 MPa and a nominal ultimate strength fu of about 520 MPa, respectively. This deliberate strength overmatch by the filler metal aimed at shifting the expected failure planes to the base metal adjacent to the weld. The test values of the yield and ultimate tensile strength for the weld metal samples extracted from virgin specimens comply well with the nominal ones as reported in Table 3.4. 3.4.2 Fracture mechanics-based characterization A series of laboratory tests was carried out to characterize the end plate material in its different microstructural states. As a matter of fact owing to the filler metal and the uneven temperature distribution, a welded joint is a compound of three different metallurgical regions: the fusion zone, the heat affected zone (HAZ), and the unaffected base metal. The HAZ is the area adjacent to the fusion zone, where the material has undergone a thermal cycle that alters the microstructure of the base material, though the temperature is too low to determine fusion. The microstructural characterization was performed by means of the optical microscope in order to determine the microstructural state in the different zones. Then, Vickers hardness measurements were carried out on different regions of the specimens highlighted by etching. Such results are reported in Table 3.5 A further important characteristic in seismic design is the material toughness, in particular in the HAZ. In order to meet this requirement filler material was selected with nominal toughness, as obtained through a Charpy V-Notch impact energy test (ASTM, 1988) greater than 70 J at -20 °C and 50 J at -40 °C. A series of Charpy impact tests was then carried out to characterize the notch toughness values both of base metal and of weld metal in two different directions. Columns 3 and 4 of Table 5 collect sample values extracted along (L) and orthogonally (T) the rolling-mill direction as shown schematically in Figure 3.5. More specifically, one may observe that large differences exist between the relevant impact energy values. This trend may be explained recalling that along the rolling-mill direction, the material microstructure is an aligned multi-layered structure with ductile behaviour made up of ferrite and perlite. 51
3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS 52 Specimen Material JA1-2** JB1-3** JA1-2B** JB1-3B** TC-2* TC-2** TC-3* TC-3** * Virgin ** Failed CVN J @ 20 o C HV (daN) L T L Weld (W) 75 - 259 End Plate (EP) 294 - 180 W 91 - 234 EP 232 - 188 W 24 152 151 EP 75 244 151 W 27 103 143 EP 83 >300 139 W 67 - 288 EP 107 196 133 W 79 - 297 EP 105 198 159 W 78 - 241 EP 124 223 180 W 82 - 297 EP 246 205 130 Table 3.5. Fracture Properties of Specimens HAZ W-L EP-L EP-T W = Weld EP = End Plate L = Longitudinal T = Transversal Figure 3.5. Locations and sample for Charpy V- Notch Impact Energy Test Therefore, more energy dissipation capacity can be provided. Moreover, the impact energy tests show that both the base metal and the weld metal are endowed with satisfactory CVN levels at room temperature, viz. with values higher than the minimum ones established by design codes. As the above impact tests were instrumented, we could track the evolution of the applied load in addition to the total absorbed energy. This allows identification of the various deformation and fracture process stages. More specifically, the crack initiates after considerable plastic deformation of the specimen. After an initial unstable propagation, it
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Dipartimento di Ingegneria Meccanic
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UNIVERSITÀ DEGLI STUDI DI TRENTO D
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Ai miei genitori
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INDEX 1 INTRODUCTION...............
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INDEX iii 4.10.3 Results of the PsD
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4. SEISMIC RESPONSE OF PARTIAL-STRE
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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Analysis and modelling of the seismic behaviour of high ... - Ingegneria

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