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Bidirectional quasi-static cyclic loading was applied to a subassembly of drywall partition walls integrated with cross-laminated timber rocking walls. Details aimed to reduce seismic damage to the...

Seismic isolation is known to effectively mitigate the effects of horizontal ground shaking in building structures. Recent research has suggested that vertical excitation causes high-intensity slab vibrations that lead to nonstructural component damage. A detailed case study of the application of three-dimensional (3D) isolation in a realistic multi-story frame building is presented. Specifically, the seismic responses of hypothetical special concentric braced frame buildings with horizontal and 3D isolation are compared to evaluate the effectiveness of 3D isolation to mitigate the vertical ground shaking. Overall, 3D isolation with a relatively short isolation period (0.5 s) is adequate to significantly mitigate the vertical acceleration in a flexible frame building, including the amplification at mid-slab relative to adjacent columns, without compromising the usual reductions in horizontal responses.

Prior studies, which have been limited to horizontal ground shaking, have generally concluded that soil–structure interaction (SSI) has a negligible influence on the seismic response of isolated bu...

This paper presents a new stochastic methodology for evaluating and quantifying the downtime of a structure in terms of a family of fragilities that represent the probability of exceeding p...

This paper presents the use of modern survey techniques, particularly light detection and ranging (LiDAR) scanning, to collect time-sensitive information before and after shake table experi...

Seismic base isolation systems are mainly used to reduce seismic demands and minimize earthquake effect on structure performance. In this study, the fundamental dynamic response of structures with ...

This study focuses on experimental investigation of a fail-safe, bi-linear, liquid spring magnetorheological damper system for a three-dimensional earthquake isolation system. The device combines the controllable magnetorheological damping, fail-safe viscous damping, and liquid spring features in a single unit serving as the vertical component of a building isolation system. The bi-linear liquid spring feature provides two different stiffnesses in compression and rebound modes. The higher stiffness in the rebound mode prevents a possible overturning of the structure during rocking mode. For practical application, the device is to be stacked together along with the traditional elastomeric bearings that are currently used to absorb the horizontal ground excitations. An experimental setup is designed to reflect the real-life loading conditions. The 1/4th-scale device is exposed to combined dynamic axial loading (reflecting vertical seismic excitation) and constant shear force that are up to 245 and 28 kN, respectively. The results demonstrate that the device performs successfully under the combined axial and shear loadings and compare well with the theoretical calculations.

Kinematic soil-structure interaction (SSI) effects cause foundation-level motions to deviate from free-field motions. Simplified procedures to account for kinematic SSI effects are available in design guidelines for regular buildings and have been calibrated for regular buildings with limited foundation size and depth of embedment. As a result, their application to structures with much larger foundation footprints and deeper embedment depths is questionable. Kinematic interaction effects can be used to reduce foundation-level ground motions relative to those specified in the free-field, and this can be cost-effective especially for structures with larger footprints and embedment depths. The objective of this study is to quantify the extent of deviations between the use of available simplified procedures and empirical data for a select group of structures with larger footprints and embedment depths. In this study, a dataset of earthquake motions recorded at the instrumented structures in Japan is used to evaluate kinematic SSI effects, which are quantified in terms of the transfer function and the ratio of response spectra between foundation motions and corresponding free-field motions. Transfer functions represent the ratio between the free-field and the foundation level of a structure motion in the frequency domain and can be obtained by dividing Power Spectral Density functions of foundation motion and corresponding free-field motion. Finally, these empirical functions are compared to the available simplified formulations such as the semi-empirical transfer function and the ASCE/SEI 41–17 recommended ratio of response spectra, and the results are presented in terms of residuals. The overall trends of residuals indicate that the current simplified procedures tend to overestimate the foundation motions in structures with large footprints and embedment depths, and the extent of overestimation is higher for the ratio of response spectra compared to transfer functions. Thus, in future studies, these simplified procedures should be revised for applications in structures with large footprints and embedment depths.

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 t...

Temperature profiles for bridge design were developed and introduced into current bridge design specifications after soffit cracking in prestressed concrete bridges was attributed to nonlin...

Seismic resilience and continued operation of bridges after earthquakes are important seismic design criteria. A new seismic protection concept for integral bridge piers is explored that uses sliding bearings to separate the superstructure from the piers. The influence of sliding bearings on the seismic response of a representative 3-span integral highway bridge is investigated. With sliding bearings, the pier column shear force was limited to the bearing design friction force. Furthermore, the abutment ductility demands were found to be insensitive to the friction forces in the sliding bearings because the bridge displacement demands were controlled by the equal displacement rule.

This paper reports on the evaluation of the response of unanchored building contents in two separate shaking experiments, with the goal of developing simplified seismic fragility functions to be used for content disruption. The content response was evaluated collectively, by defining qualitative categorical ratings of overall content disruption based on observation of specific behaviors, such as sliding, toppling, rolling, or falling. Next, these disruption ratings were correlated to demand parameters, such as peak floor acceleration and peak floor velocity. Fragility functions were developed by combining two alternative implementations of the bounding demand data method. Among the demand parameters evaluated, peak floor velocity is shown to be a consistent indicator of disruption when comparing results from the two independent data sets. Vertical acceleration is shown to also influence the demand intensities that trigger disruption.

Summary 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.

Chapter 17, “Seismic Design Requirements for Seismically Isolated Structures,” in the 2016 edition of the ASCE 7 Standard, Minimum Design Loads and Associated Criteria for Buildings and Oth...

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.

The commercial software platform SAP2000 has been used to design base isolated buildings supported by Triple PendulumTM isolators; however, limited full-scale test data are available to validate the use of the software for this application. Additionally, previous work has demonstrated that several pitfalls must be avoided when modeling these types of structures in SAP2000. The ability of SAP2000 to predict key engineering demand parameters used in the design of a base-isolated building is examined by using methods and assumptions employed in current design practice. The SAP2000 analysis results are compared to the data obtained from shake table tests of a full-scale base-isolated building and comparable predictions obtained from a more complex OpenSees analysis model of the same building. This study suggests that when methods and assumptions common to engineering practice are applied, SAP2000 can predict the responses with sufficient accuracy for design.

The data sets generated from E-Defense shake table experiments of a full-scale, five-story building tested with two innovative isolation systems and in the fixed-base configuration are available in the Network for Earthquake Engineering Simulation (NEES) Project Warehouse. The experiments included an integrated subassemblage of partition walls, suspended ceilings, and sprinkler piping on the fourth and fifth floors of the building; a full-story precast concrete cladding column cover; and enclosed areas staged with building contents. Totaling approximately 211 GB between three experiments, the data set includes isolator forces and displacements, floor accelerations, and story drifts, local accelerations, and relative displacements at strategic locations on the nonstructural components, as well as extensive videos and inspection photos. The project was a collaboration between U.S. researchers of the NEES Tools for Isolation and Protective Systems (TIPS) and NEES Nonstructural Grand Challenge projects and Japanese researchers from the National Research Institute of Earth Science and Disaster Prevention. Data set DOIs are 10.4231/D3X34MR7R, 10.4231/D3SB3WZ43, and 10.4231/D3NP1WJ3P.

This study presents a comprehensive design methodology for a magnetorheological-based damper device for a three-dimensional building isolation. The device acts as a suspension system itself by combining the liquid stiffness and controllable magnetorheological damping features in one unit. The bi-linear liquid stiffness feature enhances resistance to global rocking/overturning of the structural system by increasing the stiffness in the rebound mode compared to the compression mode. In the field, the system is combined with the conventional elastomeric bearings widely employed to mitigate the lateral seismic motions. During a seismic event, the system is subjected to dynamic vertical shaking and large lateral forces. The theoretical and simulation modeling to overcome this major challenge and achieve other system requirements are presented. In addition, a comprehensive optimization program is developed to achieve all design requirements. The modeling procedure is verified with experimental results. Also, the effectiveness of Displacement/Velocity-based control for a single degree-of-freedom system subjected to sinusoidal loading is evaluated.

Bearings used in the seismic isolation of buildings can be subjected to large horizontal deformations combined with high axial loads from overturning forces during strong shaking. Elastomeric bearing design requires an evaluation of the critical load capacity under this combined loading to ensure stability. To better understand and estimate the capacity of elastomeric bearings, a comprehensive experimental program was carried out to examine bearing stability using two quasi-static and one dynamic loading procedures. The first method to evaluate the bearing critical load follows previous experimental research, where a single elastomeric bearing is held at prespecified horizontal displacements while increasing the axial load until the critical load is achieved. In a second method implemented here, a similar setup is used to apply a constant axial load followed by increased horizontal displacement until the stability limit is observed. This second method is shown to be an accurate and more direct app...

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.

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.

In this study, the torsional response of buildings with peripheral steel-braced frame lateral systems is evaluated. A three-dimensional model of a three story braced frame with various levels of eccentricity is created and the effects of torsion on the seismic response is assessed for four hazard levels. The response history analysis results indicate that, unlike frame structures, the torsional amplifications in the inelastic systems exceed those of corresponding elastic systems and tend to increase with an increase in the level of inelasticity. The ability of two simplified procedures, elastic response spectrum analysis and pushover analysis, to capture the torsional amplifications in steel-braced frames is evaluated. Copyright © 2010 John Wiley & Sons, Ltd.

A telephone survey was conducted to determine design professionals' experiences and opinions of the design review process for seismic isolated buildings. Participants agreed that the review process is beneficial in concept, but the benefit can sometimes be obscured by poor implementation. Best practice recommendations include: (1) the review team should be organized and the review process should be planned at the onset of design; (2) all parties should mutually agree on the scope and level of detail of review; and (3) a process for conflict resolution should be pre-negotiated. Most of the discussion and recommendations generated by this study are applicable to design reviews in general. The specific code requirements for design review of isolated buildings were found to be in need of updating, but the concept of exempting certain structures from the review process, favored by some engineers, was not found to have widespread support.

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.

This study illustrates the application of frequency-dependent filters to improve the effectiveness of active control systems designed to mitigate the seismic response of large-scale civil structures. Band-pass pre-filters are introduced to optimal H2/LQG controllers to reduce the maximum singular value of input–output transfer matrices over a defined frequency range. A structured uncertainty model is proposed to evaluate robustness of stability and performance for a structure with nonlinear force deformation. The proposed perturbation model accurately characterizes variations in the stiffness matrix, thereby reducing over-conservatism in the estimated destabilizing perturbations. The aforementioned techniques are applied to a benchmark building. Numerical results indicate that introducing filters can enhance the performance of the system in almost all response measures, while preserving robustness of stability and performance. Copyright © 2010 John Wiley & Sons, Ltd.

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.

Although current code guidelines specify different seismic performance objectives for fixed-base and isolated buildings, the future of performance-based design will allow user-selected performance objectives, motivating the need for a consistent performance comparison of the two systems. Based on response history analysis to a suite of motions, constant ductility spectra are generated for fixed-base and isolated buildings. Both superstructure force (base shear) and deformation demands in base-isolated buildings are lower than in fixed-base buildings responding with identical deformation ductility. To compare the relative performance of many systems or to predict the best system to achieve a given performance objective, a response index is developed and used for rapid prototyping of response as a function of system characteristics. When evaluated for a life safety performance objective, the superstructure design base shear of an isolated building is competitive with that of a fixed-base building with identical ductility, and the isolated building generally has improved response. Isolated buildings can meet a moderate ductility immediate-occupancy objective at low design strengths, whereas comparable ductility fixed-base buildings fail to meet the objective.

In dynamic analysis, the energy dissipation in a base-isolated building is typically accounted for by allocating damping properties independently to the superstructure and to the isolation system. Rayleigh damping applied to the superstructure component alone is often used to represent the superstructure energy dissipation. At least one study has indicated that when used improperly, superstructure Rayleigh damping leads to excessive damping of the response of a base-isolated structure. As shown here, even when used as recommended for combining subsystems with disparate damping properties, Rayleigh damping results in undesirable suppression of the first mode response. To correct this behavior, stiffness-proportional damping can be used in lieu of Rayleigh damping. Stiffness-proportional damping is demonstrated to negligibly affect the first mode response, yet provide the expected energy dissipation in higher modes.

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.

A full-scale, 5-story steel moment frame building in base-isolated and fixed-base configurations was subjected to a number of ground motions using the E-Defense shake table. In these experiments, more than 84 m2 (900 ft2) of suspended ceiling with lay-in tiles, 90 m (300 linear ft) of partition walls with individual lengths varying from 1.5 to 9.8 m (5 to 32 ft), and three sprinkler branch lines were installed below the fifth and sixth (roof) floors of the building. Because the horizontal floor accelerations were generally constrained to relatively low values by the base isolation system, several damage states related to vertical floor system acceleration were observed. One key observation is that use of lateral bracing with compression posts did not improve the seismic response of suspended ceilings when subjected to strong vertical excitation. Acceleration amplification factors of the ceiling-partition-partition components relative to structural floor accelerations were computed. The code-presc...

The seismic response of a full-scale, 5-story steel moment frame building in base-isolated and fixed-base configurations with an integrated suspended ceiling-partition wall-sprinkler piping system that was shaken at E-Defense is critically assessed. Horizontal floor accelerations were constrained by the isolation systems to relatively low levels, which allowed observation of damage to the integrated system that was directly related to the vertical component of input acceleration. The floor slabs exhibited single mode vibration at their natural periods with widely varying effective damping. Peak vertical accelerations were amplified by an average factor ranging from 3 to 6 from the table to the middle of the floor slabs, at which amplification factors increased as slab vibration periods lengthened. Damage to the ceiling-partion-piping components initiated at slab accelerations of approximately 2 g and became extensive for slab accelerations exceeding 5 g. These metrics establish target vertical acc...

This paper focuses on a horizontal-vertical coupling effect observed in a full-scale shake table experiment of a 5-story moment frame building isolated with triple pendulum bearings. A significant increase or amplification of the horizontal floor accelerations was observed during three-dimensional (3D) shaking compared to XY (horizontal only) shaking with comparable input motions. The coupling phenomenon is explained by analytical and numerical simulation of a single degree-of-freedom (rigid mass) structure and multistory cantilever structure subjected to simplified sinusoidal motions, both isolated with triple pendulum bearings. The results of the simplified numerical study are extended to explain the horizontal-vertical coupling in the full-scale experiment, which are also validated by numerical modeling of the test structure. The vertical component of ground acceleration is shown to introduce a high-frequency component into the base shear that can excite higher modes of the base-isolated structure. The phenomenon is possible in any multistory structure isolated with friction bearings, and should be evaluated on a case-by-case basis.

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.

A comprehensive approach is developed to estimate relevant design quantities—lateral deformations and axial forces—in isolation systems composed of lead–rubber bearings. The approach, applicable to symmetric and asymmetric-plan systems, includes the effects of bidirectional excitation, rocking, and torsion; and is the culmination of previous work on this topic. The approach is based on nonlinear response history analysis of an isolated block using an advanced bearing model that incorporates the interaction between axial force and lateral response of the bearing, known as axial-load effects. The rocking response of the system and peak axial forces are shown to depend on the isolation period, the normalized strength—or yield strength normalized by peak ground velocity, the ratios of rocking frequency about each horizontal axis to vertical frequency, and the normalized stiffness eccentricity. In an attempt to develop results widely applicable to asymmetric-plan systems, eccentricity is introduced by varying the stiffnesses and strengths of individual bearings in an idealized, rectangular plan. This idealized system approach is shown to have limited success; when applied to actual asymmetric-plan systems the design equations to estimate response are accurate for lateral deformations but err by up to 25% for axial forces. Copyright © 2006 John Wiley & Sons, Ltd.

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.

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 \IT\dd\N 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.

A procedure based on rigorous non-linear analysis is presented that estimates the peak deformation among all isolators in an asymmetric building due to strong ground motion. The governing equations are reduced to a form such that the median normalized deformation due to an ensemble of ground motions with given corner period Td depends primarily on four global parameters of the isolation system: the isolation period Tb, the normalized strength η, the torsional-to-lateral frequency ratio Ωθ, and the normalized stiffness eccentricity eb/r. The median ratio of the deformations of the asymmetric and corresponding symmetric systems is shown to depend only weakly on Tb, η, and Ωθ, but increases with eb/r. The equation developed to estimate the largest ratio among all isolators depends only on the stiffness eccentricity and the distance from the center of mass to the outlying isolator. This equation, multiplied by an earlier equation for the deformation of the corresponding symmetric system, provides a design equation to estimate the deformations of asymmetric systems. This design equation conservatively estimates the peak deformation among all isolators, but is generally within 10% of the ‘exact’ value. Relative to the non-linear procedure presented, the peak isolator deformation is shown to be significantly underestimated by the U.S. building code procedures. Copyright © 2003 John Wiley & Sons, Ltd.

A 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...

An approximate method for linear analysis of asymmetric-plan, multistorey buildings is specialized for a single-storey, base-isolated structure. To find the mode shapes of the torsionally coupled system, the Rayleigh–Ritz procedure is applied using the torsionally uncoupled modes as Ritz vectors. This approach reduces to analysis of two single-storey systems, each with vibration properties and eccentricities (labelled ‘effective eccentricities’) similar to corresponding properties of the isolation system or the fixed-base structure. With certain assumptions, the vibration properties of the coupled system can be expressed explicitly in terms of these single-storey system properties. Three different methods are developed: the first is a direct application of the Rayleigh–Ritz procedure; the second and third use simplifications for the effective eccentricities, assuming a relatively stiff superstructure. The accuracy of these proposed methods and the rigid structure method in determining responses are assessed for a range of system parameters including eccentricity and structure flexibility. For a subset of systems with equal isolation and structural eccentricities, two of the methods are exact and the third is sufficiently accurate; all three are preferred to the rigid structure method. For systems with zero isolation eccentricity, however, all approximate methods considered are inconsistent and should be applied with caution, only to systems with small structural eccentricities or stiff structures. Copyright © 2001 John Wiley & Sons, Ltd.