Abstract
Light-framed residential structures are generally considered to provide good life safety performance during an earthquake, but they can be vulnerable to costly damage during even moderate seismic events. This is the result of damage in this type of structures being triggered at very low levels of lateral deformation such as interstory drift ratios of approximately 0.2%. Of the estimated $40 billion of direct economic losses due to the 1994 Northridge Earthquake, over half is attributed to damage in wood-frame residential structures. In this NEESR project, a low-cost sliding isolation system made entirely of readily available “off-the-shelf” components is proposed and evaluated, which can greatly reduce interstory drift demands on low-rise structures, resulting in what is termed, “Nearly-Damage-Free DBE” construction. Analytical studies were conducted to determine slider friction properties, supplemental damping and restoring systems that are optimal for light-frame structures. These analyses showed that systems with about 20% sliding friction and small restoring forces are optimal, resulting in very small interstory drift demands while simultaneously reducing peak displacement and residual displacement demands in the isolation system under MCE level ground motions. Tests of the sliding isolators at Stanford University and the NEES @ Berkeley lab have been used to characterize the slider properties and to examine the dynamic response under high-intensity ground motions. Dynamic sliding tests were performed with and without restoring elements, including flat and concave-shaped isolation bearings when subjected to harmonic cyclic reversals and earthquake ground motions with peak ground velocities up to 1 m/s (40 in/s). Results demonstrate that the sliding displacement history of a structure can be well predicted for even large relative displacement ground motions, and that minimal post-sliding restoring stiffness can re-center a sliding structure after an earthquake. The impact of variable loads at supports with pressure dependent sliding units is discussed. Data from these component tests and analyses are being used to design full-scale shake table tests, planned to be conducted in 2014 at NEES @ UCSD.