Gordon P. Warn

Stability of Elastomeric and Lead-Rubber Seismic Isolation Bearings

Elastomeric and lead-rubber bearings are two commonly used types of seismic isolation devices. During seismic events, some of the bearings in an isolation system will be subjected to large axial compressive loads, caused by gravity plus overturning forces, accompanied by simultaneous large lateral displacements. However, the critical load capacity of elastomeric bearings has been shown to reduce with increasing lateral displacement. The design of isolation systems composed of these types of bearings therefore requires that stability at the maximum displacement be demonstrated. The current procedure to assess the stability uses a ratio of areas, referred to as the overlapping area method, to determine the critical load capacity at a given lateral displacement that must be greater than a combination of axial forces imposed on the bearing. Although the overlapping area method provides a simple means of calculating the critical load at a given lateral displacement, it lacks a rigorous theoretical basis and ha...

Identification of the controlling mechanism for predicting critical loads in elastomeric bearings

AbstractAssessing the stability of individual isolators is an important consideration for the design of seismic isolation systems composed of elastomeric bearings. A key component for the stability assessment is the prediction of the critical load capacity of the individual bearings in the laterally undeformed (service) configuration and at a given lateral displacement (seismic). The current procedure for estimating the critical load capacity of an elastomeric bearing at a given lateral displacement, with a bolted connection detail, uses a ratio of areas to reduce the critical load capacity from that in the laterally undeformed configuration, referred to as the reduced area method. Although the reduced area method provides a simple means for the estimate, it lacks a rigorous theoretical basis and is unable to capture the trends observed from experimental data. In this study, the capability of two analytical models for predicting critical loads and displacements in elastomeric bearings is evaluated by comp...

Mechanistic Model for Simulating Critical Behavior in Elastomeric Bearings

AbstractWhen an elastomeric bearing is subjected to simultaneous vertical compressive load and lateral displacement, the shear force can pass through a maximum beyond which the bearing exhibits negative tangential horizontal stiffness and a condition of unstable equilibrium. This behavior has been experimentally demonstrated and has important implications on the stability and earthquake response of elastomeric seismic isolation bearings. Yet, analytical bearing models used for numerical earthquake simulation assume a positive second-slope stiffness irrespective of vertical load and/or bearing lateral displacement and therefore are unable to simulate the experimentally observed bearing behavior. Semiempirical bearing models have been developed and some of these models have been shown to simulate the influence of vertical load and lateral displacement on the shear force response with reasonable accuracy, however these models rely on a number of experimentally calibrated parameters, making them impractical f...

Blast Resistance of Steel Plate Shear Walls Designed for Seismic Loading

Steel plate shear walls (SPSWs) have become an increasingly popular lateral force resisting system in buildings. Although originally conceived to resist earthquake forces, recent developments raised questions as to the ability of SPSWs to resist blast loading, whereby the plate would resist out-of-plane impulsive pressures. To investigate this, two 0.4-scale single story SPSW specimens, representing the first story of a four story prototype SPSW, were fabricated and subjected to explosive charges. The out-of-plane resistance of the infill plate was analyzed using nonlinear finite-element analysis (FEA) and yield line theory. Results of these analyses showed the out-of-plane resistance is governed by the large deformations and inelastic material behavior and that yield line theory significantly underestimated the out-of-plane resistance in comparison with the finite-element analysis for infill plates typical of SPSW construction. Based on these results a simplified plastic analysis procedure is proposed to estimate the out-of-plane resistance of SPSW infill plates that is shown to agree well with the results of the FEA. Results of the experimental investigation showed the SPSW had a limited capacity to resist out-of-plane blast loading and that the typical seismic detail for connecting the infill plate to the boundary frame might not be appropriate for blast applications.

Sustained-Load and Fatigue Performance of a Hybrid FRP-Concrete Bridge Deck System

The design and construction of bridge systems with long-term durability and low maintenance requirements is a significant challenge for bridge engineers. One possible solution to this challenge could be through the use of new materials, e.g., fiber-reinforced polymer (FRP) composites, with traditional materials that are arranged as an innovative hybrid structural system where the FRP serves as a load-carrying constituent and a protective cover for the concrete. This paper presents the results of an experimental investigation designed to evaluate the performance of a 3/4 scale hybrid FRP-concrete (HFRPC) bridge deck and composite connection under sustained and repeated (fatigue) loading. In addition, following the sustained-load and fatigue portions of the experimental study, destructive testing was performed to determine the first strength-based limit state of the hybrid deck. Results from the sustained-load and fatigue testing suggest that the HFRPC deck system might be a viable alternative to traditiona...

Exploring the Low Shape Factor Concept to Achieve Three-Dimensional Seismic Isolation

The current state-of-practice in the U.S., and elsewhere, for designing elastomeric seismic isolation bearings utilizes closely spaced intermediate steel shim plates with thin rubber layers that result in shape factors ranging from 15 to 30. Such high shape factors (HSF) produce a vertical stiffness several thousand times larger than the horizontal stiffness thereby providing isolation only in the horizontal plane (2D). While the large vertical stiffness has been thought to be desirable to minimize rocking in slender structures with an elevated center of mass it also results in a lower period of vibration in the vertical direction that can align with the dominant frequency content of the vertical component of earthquake ground shaking. A low shape factor (LSF) bearing concept to achieve three-dimensional (3D) isolation was explored in the past for the nuclear industry. Though this research demonstrated, through analysis and a prototype design, that the LSF concept could effectively provide isolation in both the horizontal and vertical directions, system level testing and implementation were never realized. This paper presents the results of an analytical, parametric, study aimed to further explore the low shape factor concept to achieve three-dimensional isolation. The results of this study suggest that 3D isolation might be achieved for low and mid-rise structures using the LSF concept if the bearing shape factors are less than four and supplemental vertical damping is included at the plane of base isolation.

Weak-Axis Behavior of Wide Flange Columns Subjected to Blast

Much of past research in the civilian area on the response of civil structures to explosive loading has focused on large detonations in the far field that result in relatively uniform pressure distribution over the structure and specific structural elements. A paucity of research has been conducted that investigates the effect of explosive loading in close proximity to key structural elements. The studies that have been conducted focused primarily on loading perpendicular to the strong axis of bending that result in global deformation, but no rupture or loss of material. Through experimental testing and finite-element simulation, the present study investigates the effect of blast loading on wide flange columns loaded perpendicular to the weak axis of bending. This loading scenario is critical for such columns because the near field shock wave can rupture the web, and in some cases, lead to material loss; both conditions can potentially jeopardize the axial load carrying capacity of the column as a result of increased demands on flanges and possible local buckling of the unrestrained flanges. Therefore, this critical scenario needs to be considered for developing blast resistant measures or assessing the remaining axial and bending capacity of the column. Finite-element simulation can be used for this purpose; the analyses conducted as part of this study replicate, with reasonable accuracy, the experimentally obtained localized deformation, ruptures, and loss of material as a result of blast load, although the finite-element simulation is less successful at replicating the global deformation of the column.

Introduction of a Tradeoff Index for Efficient Trade Space Exploration

The development of many-objective evolutionary algorithms has facilitated solving complex design optimization problems, that is, optimization problems with four or more competing objectives. The outcome of many-objective optimization is often a rich set of solutions, including the non-dominated solutions, with varying degrees of tradeoff amongst the objectives, herein referred to as the trade space. As the number of objectives increases, exploring the trade space and identifying acceptable solutions becomes less straightforward. Visual analytic techniques that transform a high-dimensional trade space into two-dimensional (2D) presentations have been developed to overcome the cognitive challenges associated with exploring high-dimensional trade spaces. Existing visual analytic techniques either identify acceptable solutions using algorithms that do not allow preferences to be formed and applied iteratively, or they rely on exhaustive sets of 2D representations to identify tradeoffs from which acceptable solutions are selected. In this paper, an index is introduced to quantify tradeoffs between any two objectives and integrated into a visual analytic technique. The tradeoff index enables efficient trade space exploration by quickly pinpointing those objectives that have tradeoffs for further exploration, thus reducing the number of 2D representations that must be generated and interpreted while allowing preferences to be formed and applied when selecting a solution. Furthermore, the proposed index is scalable to any number of objectives. Finally, to illustrate the utility of the proposed tradeoff index, a visual analytic technique that is based on this index is applied to a Pareto approximate solution set from a design optimization problem with ten objectives.Copyright

Dynamic Stability Testing of Isolation Systems Composed of Elastomeric Bearings and Implications for Design

Elastomeric and lead-rubber seismic isolation bearings have been widely used in the United States and around the world for the past thirty years. During earthquake ground shaking, these bearings will be subjected to vertical compressive loads due to gravity plus seismic forces, accompanied by simultaneous large lateral displacements. The design of isolation systems composed of these bearings, therefore, requires stability of the isolation system and individual bearings at the maximum displacement be demonstrated. The current codified procedure for assessing the stability of individual bearings uses a ratio of areas, referred to as the reduced area method, to determine the critical load capacity of the bearings at a given lateral displacement. This critical load capacity must be greater than a combination of vertical forces imposed on the bearing for stability to be demonstrated. While the reduced area method provides a simple means for estimating the critical load of a bearing at a given lateral displacement, it lacks a rigorous theoretical basis and has been shown to provide inconsistent predictions by comparison to data obtained from quasi-static testing of individual bearings. Dynamic stability testing of two isolation systems composed of four elastomeric bearings was performed at the University at Buffalo using the UB-NEES earthquake simulators to gain an improved understanding of the stability behavior of the individual bearings and global system under representative earthquake ground shaking. This paper presents: (1) a summary of the dynamic stability tests; (2) sample experimental results; and (3) a discussion of the implications for the assessment of stability in design. The new experimental results presented in this paper demonstrate: (1) instability in individual bearings does not necessarily lead to global instability; (2) the state of the global isolation system should be used to assess stability to economize the design; and (3) the reduced area method is not able to accurately predict the global stability of the isolation system.

Three-Dimensional Base Isolation Using Vertical Negative Stiffness Devices

ABSTRACT A three-dimensional (3-D) base isolation system to control both the horizontal and vertical components of ground motion is presented in this paper. The system is adopting a negative stiffness device (NSD) that can be considered as an adaptive passive protection system, which can apparently change the stiffness of the structure. This work is focused on studying through numerical simulations the mitigation performance of the NSD against strong earthquakes in the vertical direction. The base isolation arrangement consists of elastomeric bearings acting both in the horizontal and vertical direction and NSDs acting only in the vertical direction. So, a 3-D base isolation is achieved, where it is assumed that the NSDs affect the vertical stiffness of the system only. Numerical analyses show that the presence of NSDs reduces the vertical acceleration in the structure. Nevertheless, accordingly with the passive control theory, the relative displacements increase. Therefore, it seems advisable a supplemental damping to mitigate this effect. Thanks to the presence of rubber isolators, it is possible to employ their inherent damping without introducing specific dampers in the vertical direction.