1,721,028 research outputs found
Performance-based seismic response of frame structures including residual deformations. Part I: Single-degree of freedom systems
No full textThe development of performance-based approaches for the design of structures to withstand earthquakes requires the capability to assess performance under different levels of seismic excitation. Although a great deal of work has been completed towards this goal, most of the efforts have focused on the maximum transient response or the cumulative energy dissipation characteristics as the main response indices. Considering the inevitable reality of residual deformations for systems responding in the material nonlinear range and the importance of these permanent deformations to performance characterisation, an alternative approach to quantifying performance is explored in this contribution. In this first of two companion papers, after discussion of the limitations of current performance assessment techniques to fully describe the post-earthquake state of a structure, the framework of a performance evaluation based on residual deformations including both structural and non-structural elements is first presented. A first insight into the parameters that influence residual deformations is presented through time-history analyses of hysteretic single degree-of-freedom systems. Hysteretic characteristics, post-yielding stiffness as affected by P-Δ effects, as well as maximum ductility are found to greatly influence residual deformations. It is also shown that without considering residual deformations, the performance of systems that are inherently self-centring cannot realistically be compared to other systems that sustain residual deformations
Performance-based seismic response of frame structures including residual deformations. Part II: Multi-degree of freedom systems
No full textThe role of residual deformations when evaluating the performance of multi-storey frame structures subjected to ground motion is investigated in this paper. The limitations of damage indices available in the literature, either based on ductility, energy dissipation or a combination of both, in capturing such a significant aspect of the seismic response of frame structures are discussed. The concept of residual deformations as a critical complementary indicator to cumulative damage, introduced in a companion paper (Part I) for single-degree-of-freedom (SDOF) systems, is herein extended to multi-degree-of-freedom (MDOF) frame systems. The seismic performance of multi-storey frame structures, either representative of new designed or existing structures, is investigated, focusing on the response in terms of residual deformations. Residual deformations are shown to be sensitive to the hysteretic rule adopted, to the system inelastic mechanism as well as to the seismic intensity. The influence of higher modes and P-Δ effects on the final residual deformations is addressed. A combination of maximum drift and residual drift in the format of a performance matrix is used to define the system's global performance levels and is then extended to a framework for an alternative performance-based seismic design and assessment approach
Effectiveness of simple approaches in mitigating residual deformations in buildings
No full textDevelopments in performance-based seismic design and assessment approaches have emphasized the importance of considering residual deformations. Recent investigations have also led to a proposed direct displacement-based design (DDBD) approach which includes an explicit consideration of the expected residual deformations as an integral part of the design process. Having estimated the expected residual deformations in a structure, engineers are faced with the problem of reducing them to meet the targeted performance levels under pre-defined seismic hazard levels. Previous studies have identified the post-yield stiffness as a primary factor influencing the magnitude of residual deformations in single degree of freedom and multiple degree of freedom structures. In this paper, a series of simple approaches to increase the post-yield stiffness of traditional framed and braced systems for the purpose of reducing residual deformations are investigated. These methods do not utilize recentring post-tensioned technology. This contribution addresses the feasibility of altering the lateral post-yield stiffness of structural systems by: (i) using different reinforcement materials with beneficial features in their stress-strain behaviour; (ii) re-designing the section geometry and properties of primary seismic-resisting elements; and (iii) introducing a secondary elastic frame to act in parallel with the primary system. The efficiency of each of these techniques is investigated through monotonic and cyclic moment-curvature and non-linear time-history analyses. Of these approaches the design and introduction of an elastic secondary system was found to be most effective and consistent in reducing residual deformations. A simplilfied design approach for achieving the desired increase of a system's post-yield stiffness is also presented. Copyright © 2007 John Wiley & Sons, Ltd
Development of probabilistic framework for performance-based seismic assessment of structures considering residual deformations
No full textRecently, the importance of considering residual (permanent) deformations in the performance assessment of structures has been recognized. Advanced structural systems with re-centering properties as those based on unbonded post-tensioning tendons are capable of controlling or completely eliminating residual deformations. However, for more traditional systems, which count for the vast majority of buildings, residual deformations are currently considered an unavoidable result of structural inelastic response under severe seismic shaking. In this article, a probabilistic framework for a performance-based seismic assessment of structures considering residual deformations is proposed. The development of a probabilistic formulation of a combined three-dimensional performance matrix, where maximum and residual deformations are combined to define the performance level corresponding to various damage states for a given seismic intensity levels, is first presented. Combined fragility curves expressing the probability of exceedence of performance levels defined by pairs of maximum-residual deformations are then derived using bivariate probability distributions. The significance of evaluating and accounting for residual deformations within a Performance-based Earthquake Engineering (PBEE) approach is further confirmed via numerical examples on the response of Single Degree of Freedom (SDOF) systems, with different hysteretic behavior, under a selected suite of earthquake records. Joined fragility curves corresponding to various performance levels, defined as a combination of maximum and residual response parameters, are derived while investigating the effects of hysteretic systems and strength ratios. It is observed that stiffness degrading Takeda systems result in lower residual deformations than elasto-plastic systems and show lower probability of exceeding a jointed maximum-residual performance level. For a chosen performance level, Takeda systems with higher strength ratios show better performance, particularly with lower intensity of excitations. Copyright © A.S. Elnashai & N.N. Ambraseys
The role of inelastic torsion in the determination of residual deformations
No full textRecent developments in performance-based seismic design and assessment approaches have emphasized the importance of properly assessing and limiting the residual (permanent) deformations typically sustained by a structure after a seismic event, even when designed according to current code provisions. The performance-based design framework for residual deformations, previously developed by the authors for 2-D regular structures, is further extended to the behavior of 3-D irregular (asymmetric in-plan) buildings. The seismic response of a set of single-story systems, comprising of seismic resisting frames, and modeled to represent alternative materials (concrete or steel), is investigated under uni-directional earthquake loading excitations. Different layouts in plan, leading to either torsionally unrestrained or restrained systems, are considered. The influence of varying torsional restraint is investigated to define how residual diaphragm rotations and center-of-mass displacements are affected by changing levels of stiffness and strength, or mass eccentricity. From these findings additions to the previously proposed estimation procedure are made, with a specific example used to validate the suggested changes. Finally, a general example is developed based on currently available methods of evaluating maximum torsion response. It is suggested that such approaches are likely to be insufficient as they do not explicitly define how seismic-resisting elements are influenced by inelastic torsional response
Full-scale Monitoring of a Tall, Slender Building with Coupling Viscoelastic Dampers
No full textTall, slender buildings are sensitive to dynamic vibrations caused by wind, and the design of the building may be governed by occupant motion perception. A method to control dynamic vibrations is to increase the damping of the structure, which may be accomplished by the addition of a supplemental damping system. A novel system, the Viscoelastic Coupling Damper (VCD), was implemented for the first time in a tall, slender building which was under construction during the completion of this thesis. This building was the subject of a year-long plus monitoring program where output-only system identification algorithms were applied to track the development of the dynamic properties through the construction of the building. Additionally, several large amplitude wind events occurred during the program allowing for the tracking of amplitude-dependent phenomenon. The role of the VCD system was evaluated, and numerical finite-element models were constructed with reference to the experimental resultsM.A.S
Fork Configuration Damper (FCDs) for Enhanced Dynamic Performance of High-rise Buildings
No full textThe dynamic behaviour of high-rise buildings has become a critical design consideration as buildings are built taller and more slender. Large wind vibrations cause an increase in the lateral wind loads, but more importantly, they can be perceived by building occupants creating levels of discomfort ranging from minor annoyance to severe motion sickness. The current techniques to address these issues include stiffening the lateral load resisting system, reducing the number of stories, or incorporating a vibration absorber at the top of the building. All of which have consequences on the overall project cost. The dynamic response of high-rise buildings is highly dependent on damping. Full-scale measurements of high-rise buildings have shown that the inherent damping decreases with height and recent in-situ measurements have shown that the majority of buildings over 250 meters have levels of damping less than 1% of critical. Studies have shown that small increases in the inherent damping can lead to vast improvement in dynamic response. A new damping system, the viscoelastic (VE) Fork Configuration Damper (FCD), has been developed at the University of Toronto to address these design challenges. The proposed FCDs are introduced in lieu of coupling beams in reinforced concrete (RC) coupled wall buildings and take advantage of the large shear deformations at these locations when the building is subjected to lateral loads. An experimental study was conducted on 5 small-scale VE dampers to characterize the VE material behaviour and 6 full-scale FCD samples in an RC coupled wall configuration (one designed for areas where low to moderate ductility is required and one with built-in ductile structural “fuse” for areas where high ductility is required). The VE material tests exhibited stable hysteretic behaviour under expected high-rise loading conditions and the full-scale tests validated the overall system performance based on the kinematic behaviour of coupled walls, wall anchorage and VE material behaviour. Analytical models were developed that capture the VE material behaviour and the FCD system performance well. An 85-storey high-rise building was studied analytically to validate the design approach and to highlight the improvements in building response resulting from the addition of FCDs.Ph
Flexure and Shear Base-mechanism for the Enhanced Resilience of RC Coupled High-rise Buildings
Get PDFThis thesis presents the development and analytical investigation of a flexure and shear yielding base-mechanism that improves the seismic performance of RC coupled wall high-rise buildings. Although current design practice achieves life safety and collapse prevention during major earthquake events, there is often extensive damage located in the plastic hinging regions including the wall bases and coupling beams. A system of buckling-restrained braces located below the core is implemented as an alternative seismic fuse. With this system, the first-mode and higher-mode responses of the structure are limited through a combination of flexural and shear yielding. Nonlinear time-history analyses have been carried out comparing the performance of a reference structure to this alternative design. Results demonstrate that the base–mechanism improves the overall performance of the structure by limiting damage and improving safety at all hazard levels. Connection details for the system and design steps are also proposed.M.A.S
Performance-based Design of RC Coupled Wall High-rise Buildings with Viscoelastic Coupling Dampers
Get PDFA new damping technology, the Viscoelastic Coupling Damper (VCD), has been developed at the University of Toronto for reinforced concrete (RC) coupled wall high-rise buildings. These dampers are introduced in place of coupling beams to provide distributed supplemental damping in all lateral modes of vibration. This thesis presents an analytical investigation of the application of VCDs in a high-rise case study building located in a region of high seismicity. A parametric study has been conducted to determine the optimal number and placement of the dampers to achieve enhanced seismic performance without compromising the wind response of the structure. Nonlinear time history analyses have been carried out in order to compare the seismic performance of a conventional coupled wall building to alternative designs incorporating VCDs. Results highlight the improved performance of VCDs over RC coupling beams at all levels of seismic hazard. A design procedure for seismic-critical buildings is proposed.MAS
Upgrade of Seismically Deficient Steel Frame Structures Built in Canada Between the 1960s and 1980s Using Passive Supplemental Damping
No full textA typical 1960s Type 2 Construction steel MRF hospital structure in Quebec,
representative of a prevalent construction philosophy of the time, was investigated and
modelled in OpenSees using an advanced strength degradation model. The structure
was then subjected to a nonlinear time-history analysis (NLTHA) for Montreal (MTL)
and Vancouver (VAN) ground motions and was found to be deficient under the design
hazard levels. Retrofits were proposed for the two orthogonal frames at both sites
using a performance-based approach. An experimental program determined that the
connections had less ductility than expected and began deteriorating around 2.0%
interstorey drift. The OpenSees model was updated according to the experimental
connection behaviour and the predicted NLTHA performance of the structure
worsened. The proposed retrofit designs for both orthogonal frames in both MTL and
VAN were updated with the new connection behaviour and final retrofit designs were
proposed.MAS
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