Isolated Buildings and the 1997 UBC Near-Source Factors

Computer simulations are employed to assess the effects of near-source ground motions on base-isolated buildings that meet the provisions of the 1997 Uniform Building Code. A six-story base-isolated building designed for N v = 1.6 exhibits essentially elastic structural behavior when subjected to six actual ground motions containing strong near-source effects. However, two simulated records, one intended to represent the most severe motions from the 1994 Northridge earthquake and the other a strong motion from a hypothetical M w 7.0 thrust earthquake produce larger responses well into the nonlinear range. In addition, a 113 cm ground displacement pulse of three-second duration, which is close to the period of the isolated buildings, causes story drifts of nearly 5% for the N v = 1.6 design and over 2% for a stronger N v = 2 design. Such drifts are effectively reduced when supplemental dampers are added alongside the isolators. The original N v = 1.6 design with supplemental damping in the amount of 20% of critical experiences only 1.3% drift for the same three-second ground displacement pulse.