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This paper evaluates factors that may influence slab vibration and/or induce a horizontal–vertical (H-V) coupled response of buildings with mass irregularities. Responses are investigated through computational simulation of a hypothetical three-story building with base isolation and conventionally configured. Induced mass eccentricities are observed to influence, in an unpredictable way, the vertical slab acceleration and the H-V coupled response, which is identified through modal analysis and floor spectral analysis. The implications of vertical slab vibrations and H-V coupling behavior on the design forces for nonstructural components and systems (NCSs) are evaluated, and modifications are proposed for consideration. [ABSTRACT FROM AUTHOR]

This paper focuses on slab vibration and a horizontal-vertical coupling effect observed in a full-scale 5-story moment frame test bed building in 2 configurations: isolated with a hybrid combination of lead-rubber bearings and cross-linear (rolling) bearings, and fixed at the base. Median peak slab vibrations were amplified-relative to the peak vertical shake table accelerations-by factors ranging from 2 at the second floor to 7 at the roof, and horizontal floor accelerations were significantly amplified during 3D (combined horizontal and vertical) motions compared with 2D (horizontal only) motions of comparable input intensity. The experimentally observed slab accelerations and the horizontal-vertical coupling effect were simulated through a 3D model of the specimen using standard software and modeling assumptions. The floor system was modeled with frame elements for beams/girders and shell elements for floor slabs; the insertion point method with end joint offsets was used to represent the floor system composite behavior, and floor mass was finely distributed through element discretization. The coupling behavior was partially attributed to the asymmetry of the building that was intensified by asymmetrically configured supplemental mass at the roof. Horizontal-vertical coupled modes were identified through modal analysis and verified with evaluation of floor spectral peaks. [ABSTRACT FROM AUTHOR]

Recent studies have indicated uncertainty about the performance limit states of seismically isolated buildings in very large earthquakes, especially if the isolator displacement demands exceed the seismic gap and induce pounding. Previous research has shown the benefit of providing phased supplemental damping that does not affect the isolation system response in a design event. A phased passive control device, or gap damper, was designed, fabricated, and experimentally evaluated during shake table testing of a quarter scale base-isolated three-story steel frame building. Identical input motions were applied to system configurations without a gap damper and with a gap damper, to directly assess the influence of the gap damper on displacement and acceleration demands. The gap damper was observed to reduce displacement demands by up to 15% relative to the isolated system without the gap damper. Superstructure floor accelerations increased substantially because of damper activation, but were limited to a peak of about 1.18 g. The gap damper reduces displacement most effectively if the ground motion contains one or more of the following characteristics: the spectral displacement increases with increasing period near the effective period of the isolation system, the motion is dominated by a single large pulse rather than multiple cycles at a consistent intensity, and the motion has a dominant component aligned with a major axis of the structure. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Studies have shown the effectiveness of providing supplemental energy dissipation in base-isolated structures to reduce displacements at the isolation level. A previous analytical study demonstrated the benefits of providing this energy dissipation at a specified gap larger than the design displacement. The gap before engagement allows the base isolation system to meet performance criteria in varying levels of ground excitation. Use of this 'gap damper' device eliminates undesirable effects often exhibited with large amounts of supplemental damping at lower intensity motions. Using results from an analytical study, the primary purpose of this research was to develop devices for practical implementation. Development of the devices demanded simplicity, feasibility, economy, and reliability to be an effective option in building design and construction. Multiple designs were proposed, and a final design was chosen based on selection criteria and finite element analyses. The device was designed and tested in Auburn University's Structural Research Lab. Experimental results were compared with theoretical models to verify behavior and make necessary adjustments for a shake table experiment. The design parameters were selected to accommodate re-use of the device for the shake table test. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY Base isolation systems generally perform well under design-level ground motions to reduce both interstory drift and acceleration demands. During a maximum considered earthquake, however, large displacements in the base level may cause pounding between the structure and perimeter moat wall, which can lead to very high acceleration in the superstructure. A phased passive control device, or 'gap damper', has been conceived to control base isolator displacement during extreme events while having no effect on the isolation system performance for earthquakes up to design level. It is by introducing an appropriate initial gap that the device triggers additional energy dissipation during large earthquakes to limit displacements. Various combinations of hysteretic and viscous damping mechanisms are utilized to provide desired additional energy dissipation. A numerical study that assesses the ability of various gap damper models to reduce the base displacement by at least 25% while limiting the acceleration increase at the roof level that results from the sudden engagement of a damping device is devised. The energy dissipation level provided by the damper is optimized to provide the best possible performance. For base isolation systems with effective periods of isolation in the 2.5-3.0 s range, gap damper models incorporating a viscous dashpot are very effective in controlling displacement, whereas gap dampers restricted to a hysteretic damping mechanism are ineffective. The gap damper is less effective for systems with longer periods of isolation (3.5-4.0 s) because the lower target acceleration in this range is more difficult to meet. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An earlier procedure that estimates the peak deformation in base-isolated buildings is extended to include overturning and thereby estimate the peak axial forces in individual isolators. Such tools can be used as part of a design procedure to predict and subsequently eliminate bearing tension by modifying the design. The procedure is based on nonlinear response history analysis of an isolated block using an advanced bearing model that incorporates the relation between axial load and bearing response, known as axial-load effects. Rocking of the structure and bearing axial-load effects are found to have little influence on the peak lateral bearing deformation; median response spectra are within 10% of those when rocking is neglected entirely. Furthermore, bearing axial-load effects can usually be neglected in determining the maximum and minimum bearing axial forces; cases are identified where the error in neglecting such effects exceed 10%. Because the structure has been modeled as rigid, the limitations of the procedure should be assessed for superstructure designs that allow significant structural deformation. [ABSTRACT FROM AUTHOR]

Existing models for isolation bearings neglect certain aspects of their response behavior. For instance, rubber bearings have been observed to decrease in stiffness with increasing axial load, and soften in the vertical direction at large lateral deformations. The yield strength of lead–rubber bearings has also been observed to vary with axial load, such that a lightly loaded bearing may not achieve its theoretical strength. Models that include these axial-load effects in lead–rubber bearings are developed by extending an existing linear two-spring model to include nonlinear behavior. The nonlinearity includes an empirical equation for the experimentally observed variation of yield strength. For numerical implementation, the bearing forces are found by solving the nonlinear equilibrium and kinematic equations using Newton’s method, and the instantaneous bearing stiffness matrix is formed from the differentials of these equations. The response behavior of the models is confirmed by comparison with experimental data. [ABSTRACT FROM AUTHOR]

A procedure based on rigorous nonlinear analysis that estimates the deformation and force of an isolator due to strong ground motion is presented. The procedure offers an alternative to the iterative equivalent-linear methods used by current U.S. building codes. The governing equation is reduced to a form such that the median normalized deformation of the system due to an ensemble of ground motions with a given corner period T[sub d] is found to depend on only two parameters: the natural period, defined from the postyield stiffness, and the normalized strength, or strength normalized by peak ground velocity. The dispersion of normalized deformation for an ensemble of ground motions is shown to be small, implying that the median normalized deformation is a meaningful estimate of response. The simple trends shown by the median normalized deformation led to the development of suitable design equations for isolator deformation. These design equations reflect a 13% increase when the excitation includes two lateral components of ground motion instead of just one component. For comparison, deformations estimated by the equivalent-linear method are unconservative by up to 50% compared to those found from the more accurate nonlinear spectrum, and building codes include at most a 4.4% increase for a second component. [ABSTRACT FROM AUTHOR]