Fractures observed at the welded beam-column connections in steel moment-resisting frames after recent earthquakes have led structural engineers to investigate the hysteretic behavior of such connections. Extensive research was subsequently performed on connections, and much is now known about their behavior. However, the same cannot be said for the effects of this behavior on overall system response, particularly if degradation occurs in connection strength or stiffness. Some analytical studies have been performed, but experimental data are virtually nonexistent for systems with degrading connections. It is for this reason that the study presented herein was designed and carried out.
This study contains both experimental and analytical portions. The experimental portion consists of a series of 32 shaking table tests performed on a scale model test specimen. Idealized mechanical connections capable of mimicking different types of hysteretic behavior seen in real connections were used. Hysteretic behaviors considered were ductile bilinear, brittle fracture,
ductile fracture, deformation softening (negative post-yield stiffness), and strength degradation. The ability of the connections to achieve the desired behavior was verified by experimental testing prior to their use in the shaking table test specimen. In some tests, a high-speed data acquisition system was used to capture highly transient phenomena associated with brittle fracture. Observed phenomena included propagation of elastic waves, changes in beam curvature, local moment redistribution, and excitation of member higher modes. These phenomena were found to have small impacts on the system response.
The data from the experimental portion of the study were used to develop a computer model of the structure for use in analytical studies that examined the effects of various degradation-related hysteretic parameters, earthquake excitations, and frame properties. Both experimental and analytical results show that the effects of connection hysteretic degradation on system behavior depend on several factors, including system location on the response spectrum, degradation type and severity, and earthquake excitation amplitude.
All types of hysteretic degradation causing substantial strength loss had adverse effects on the system behavior (including collapse) for short to intermediate length structure fundamental periods (relative to the predominant excitation period). Degradation did not have similarly adverse effects on structures with longer periods.
System-level effects of connection fracture were dependent on the severity of connection moment capacity reduction and the post-fracture tangent connection stiffness. Negative post-fracture and post-yield stiffnesses contributed significantly to large displacements and even collapse in some cases, exacerbating the effects of geometric nonlinearity.
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