Keri L. Ryan

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.

Fractional Order Filter Enhanced LQR for Seismic Protection of Civil Structures

This study presents fractional order filters to enhance the performance of the conventional linear quadratic regulator (LQR) method for optimal robust control of a simple civil structure. The introduced filters modify the state variables fed back to the constant gain controller. Four combinations of fractional order filter and LQR are considered and optimized based on a new performance criterion defined in the paper. Introducing fractional order filters is shown to considerably improve the results for both the artificially generated ground motions and previously recorded earthquake data.

Comparative Response Assessment of Minimally Compliant Low-Rise Base-Isolated and Conventional Steel Moment-Resisting Frame Buildings

In this study, the multihazard response of code-designed conventional and base-isolated steel frame buildings is evaluated using nonlinear response history analysis. The results of hazard and structural response analysis for 3-story moment-resisting frame buildings are presented in this paper. Three-dimensional models for both buildings are created, and seismic response is assessed for three scenario earthquakes. The response history analysis results indicate that the performance of the isolated building is superior to the conventional building in the design event. However, for the Maximum Considered Earthquake, the presence of outliers in the response data reduces confidence that the isolated building provides superior performance to its conventional counterpart. The potential causes of the outliers have been carefully evaluated.

Analysis and Design of Inter-Story Isolation Systems with Nonlinear Devices

Seismic isolation systems that mitigate seismic response are generally applied at the base of a building; however, architectural, functional, and cost considerations have motivated the application of isolation systems at inter-story locations. In this article, we systematically examine the effectiveness of inter-story isolation systems as a function of their location, and explore alternative approaches for selecting their properties. Single-story isolation systems are shown to be effective in mitigating force demands above the isolation system but less effective in mitigating forces below the isolation system. Finally, the practical aspects of designing an inter-story isolation system to accommodate light loads are discussed.

Moment-Rotation Behavior of Force-Based Plastic Hinge Elements

The moment-rotation behavior of force-based frame elements is expressed as a function of plastic hinge length and moment-curvature parameters for two types of plastic hinge integration under the representative loading condition of antisymmetric bending. For modified Gauss-Radau hinge integration, there is a unique relationship between the resulting moment-rotation hardening ratio and parameters defining the plastic hinge length and moment-curvature hardening ratio. For two-point Gauss-Radau hinge integration, the spread of yielding across the hinge regions leads to a multilinear moment-rotation response, for which a secant approximation of the hardening stiffness is directed to a target plastic rotation. An example application demonstrates that significantly unconservative assessments of lateral load-carrying capacity can be attained if modeling parameters for plastic hinge length and moment-curvature strain hardening are not calibrated to account for the discrepancy between moment-curvature and moment-rotation behavior of an element.

Evaluation of Approaches to Characterize Seismic Isolation Systems for Design

Current design codes generally use an equivalent linear approach for preliminary design of a seismic isolation system. The equivalent linear approach is based on effective parameters, rather than physical parameters of the system, and may not accurately account for the nonlinearity of the isolation system. This article evaluates an alternative normalized strength characterization against the equivalent linear characterization. Considerations for evaluation include: (1) ability to effectively account for variations in ground motion intensity; (2) ability to effectively describe the energy dissipation capacity of the isolation system; and (3) conducive to developing design equations that can be implemented within a code framework.

Distribution of Lateral Forces in Base-Isolated Buildings Considering Isolation System Nonlinearity

The ASCE 7 equivalent lateral force method for base-isolated buildings applies a triangular distribution of forces to the superstructure. This distribution attempts to approximately account for the observed effects of isolation system nonlinearity on the superstructure response, but a more rational approximation is needed. Using nonlinear regression analysis of median response data from nonlinear response history analysis of representative systems, improved equations are developed to estimate the lateral force distribution in the superstructure. The ASCE 7 distribution, a revision considered by a SEAONC committee, and the improved distribution developed here are evaluated. Only the improved equations are accurate over many system parameters.

Computational Simulation of a Full-Scale, Fixed-Base, and Isolated-Base Steel Moment Frame Building Tested at E-Defense

AbstractA full-scale earthquake simulation of a five-story steel moment frame building at E-Defense, the world’s largest single shake table facility in Miki, Japan, in August 2011 provided realistic data to improve and validate current modeling approaches for steel structures with composite floor systems. In this paper, computational models of the tested fixed-base specimen and the specimen isolated with triple pendulum bearings were developed and validated using the test data, and the influence of various modeling assumptions on the accuracy of the numerical prediction was investigated. The results demonstrate that the displacement of the isolation system can be predicted accurately without a well calibrated superstructure model. However, the response of the isolated structure, especially when subjected to 3-dimensional motions, is quite sensitive to the frame modeling assumptions and the superstructure damping model. The modeling assumptions that most accurately predict the response of the tested specim...

Comparative Life Cycle Analysis of Conventional and Base-Isolated Buildings

This paper presents a case study life cycle analysis of a conventional and base isolated steel braced frame office building. The case study focuses particularly on the effects of moat wall pounding and business interruption using the FEMA P-58 methodology. Analytical results suggest that the overall performance of the base isolated building is far superior to the conventional building but that expected financial losses in the isolated building become significant if structural pounding occurs. The cost-effectiveness of the isolation is found to be particularly sensitive to the ability of businesses to relocate quickly and effectively after a large earthquake.

Aspects of Isolation Device Behavior Observed from Full-Scale Testing of an Isolated Building at E-Defense

A 5-story steel moment frame building was tested at E-Defense in August of 2011 with three different support configurations: supported by a triple friction pendulum isolation system, supported by lead rubber bearings in combination with cross linear bearings, and in the fixed-base condition. Nonstructural components and contents were installed on the 4th and 5th floors. The isolated buildings were subjected to strong excitations with the goal to approach the displacement limit of the base-isolation devices. The triple friction pendulum system was subjected to a variety of large ground motions, but did not reach its displacement limit as the friction was observed to be larger than during initial bearing characterization. The lead-rubber isolators were subjected simultaneously to large displacements and some tension. Nonstructural component damage and content disruption due to strong vertical excitation was observed in both isolation systems and in the fixed-base configuration.