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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
propagates in a stable way converting a large part of the absorbed energy into
plastic work.
Both the material characterization and the specimen failure mechanisms pointed
out the need of fracture mechanics tests in order to define the fracture toughness
of the base material in the elastic-plastic regime. These tests, performed according
to the ASTM E 813-89 code (1989), permitted the determination of the critical value
J1c, viz. the change of the elastic-plastic work per unit crack extension at the onset
of stable crack extension. Moreover, the critical value of the crack tip opening
displacement (CTOD), in agreement with ASTM E 1290-93 code (1993) was
measured. It provides a unique estimate of localized plastic strain. The samples
used to determine J1c embodied a mechanical notch that was sharpened with a
fatigue pre-crack induced by a constant amplitude sinusoidal loading. The multiple-
specimen technique was adopted being the specimens drawn from the end plate
normal to the weld bead. After fatigue pre-cracking, the specimens were subjected
to a displacement controlled three-point bending test up to the crack extension. A
value of J1c equal to 115 kJ/m 2 was obtained in accordance with literature data
(SAC Background Reports, 1997). In addition, the critical value of the crack tip
opening displacement (CTOD) was measured. In detail, the crack mouth opening
displacement was detected through a displacement gauge, by means of quasi-
static bending tests on a pre-cracked specimen. A value of 0.31 mm was obtained,
showing a good correlation with the corresponding J1c value. As a matter of fact,
the J1c value can be converted into the CTOD index through the following
relationship:
J = mσ CTOD
( 3.1 )
1c y
where m is the so-called constraint factor. It is worthwhile to emphasize that CTOD
values computed through the correlation in Equation (3.1) are approximate owing
to the inherent variation of toughness properties of materials in building
construction.
3.4.3 Testing equipment and measuring apparatus
All specimens were tested in a rigid reaction frame, illustrated in Figure 3.6. In
detail, ITS specimens were connected directly to the rigid counter-beam of the
frame while a column stub was attached to the counter-beam in order to test CTS
specimens as illustrated in Figure 3.6a. The loading arrangement relevant to CJ
specimens is depicted in Figure 3.6b. The column is kept horizontal and hinged at
both ends at the distance of 2000 mm while the horizontal loading is applied at the
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