1,721,040 research outputs found
A simplified procedure for the seismic retrofit of bridges by seismic isolation: Part 1 - Assessment of suitability
No full textThe study presents a simplified procedure for the seismic retrofit of bridges by means of isolation, applicable to bridges with an isostatic or continuous deck layout, supported by conventional bearings that can be replaced by seismic isolators. The procedure consists of two steps: (1) the assessment of suitability of the bridge for seismic isolation; (2) the preliminary design of the isolation system. This contribution presents the first step of the procedure. The assessment moves from the geometry of the structure and the site characteristics, performing a sectional analysis of the base section of the piers, and comparing the acceleration related to the collapse mechanism with the limit acceleration that the piers alone could withstand. Then the procedure checks whether the piers can resist the seismic action once the inertial forces of the deck are mitigated by isolation. If the assessment is negative, measures must also be taken to strengthen the pier. The preliminary evaluation can be developed in a spreadsheet, without the need of complex finite element analysis. A parametric application of the procedure on a case study bridge with simply supported spans is finally presented
A simplified procedure for the seismic retrofit of bridges by seismic isolation: Part 2 - Predimensioning of the isolation system
Full text linkThis study presents a simplified procedure for the seismic retrofit of bridges by means of isolation system (IS), applicable to bridges with an isostatic or continuous deck layout, supported by conventional bearings that can be replaced by seismic isolators. The procedure consists of two steps: (1) the assessment of suitability of the bridges for seismic isolation; (2) the preliminary design of the isolation system. The first part of the procedure is presented in a companion paper. This contribution presents the second step of the procedure. A nonlinear static analysis of the existing bridge is performed, and its capacity curve is determined. This curve is then transformed into that of the equivalent SDOF system. Combining the information of the nonlinear static analysis and the bridge characteristics in the ADRS plane, the minimum characteristics of the isolation system to achieve a preset performance point are derived. Two scenarios are considered: in the first one it is sufficient to shift the period of the deck; in the second one, in addition to period shifting, it is necessary to introduce damping to control the displacement of the deck. Once the minimum characteristics of the isolators have been defined, the type and model can be identified through a search into databases of commercial devices. The application of the procedure to a case-study bridge and the validation of the method are finally shown
Modelling of a Novel Lead Damper and Application to an Existing RC Structure
No full textThe work presents a novel Prestressed LEad Damper with Straight Shaft (PS-LED), which incorporates several desirable characteristics, namely high stiffness and damping capability in a compact design, and low manufacturing cost, which make it suitable for social housing. The device provides a resistive force by the friction activated between a lead core and a shaft, showing a consistent rigid-plastic behavior without significant strength degradation; the shape of the hysteresis loops is essentially rectangular, resulting in an equivalent damping ratio of 0.55. A modelling procedure is suggested to represent in the OpenSees framework the non-linear behavior of the PS-LED for both static and dynamic analyses. In particular, for dynamic analyses, a finite element object of the PS-LED is formulated by means of a simple rheological model comprising a parallel of a non-linear spring and a viscous model. In order to show the effectiveness of the PS-LED device for the retrofit of reinforced concrete structures, a case-study is chosen as paradigmatic of a category of existing buildings in Italy designed according to outdated codes. The structure is retrofitted with the PS-LED in order to maintain an elastic behavior, with no plastic hinges, during the earthquake. Non-linear static analyses are performed on the upgraded building, showing a fair agreement between the design target and the actual performance. To provide a deeper insight, bidirectional non-linear dynamic analyses are further performed considering a suite of artificial bidirectional ground motions in accordance with the provisions of the Eurocode 8
Numerical Investigation of Curved Surface Sliders under Bidirectional Orbits
No full textThe study moves from the current debate about the suitability of the unidirectional tests regulated in the standards, and aims at investigating the effect of the displacement trajectory on the evaluation of seismic isolators.A 3D finite element thermal-mechanical model of a Curved Surface Sliders is formulated, and numerical analyses are performed considering unidirectional and bidirectional displacement-controlled orbits. The temperature rise at the sliding surface is calculated from the software and used to adjust at each iteration step the coefficient of friction of the bearing.The results of the analyses examining different orbits are compared in terms of maximum shear force, dissipated energy and temperature rise on the surface of the thermoplastic pad. The conclusions point to the fact that the unidirectional tests provide a conservative evaluation of the bearing performance, but are not suitable for determining the bearing properties that are needed for accurate nonlinear response history analyses. Furthermore, unidirectional tests tend to underestimate the temperature rise induced from bidirectional trajectories
A simplified design procedure for seismic upgrade of frame structures equipped with hysteretic dampers
Full text linkThis paper presents a simple and affordable design procedure for the seismic upgrade of frame structures equipped with hysteretic dampers. The proposed framework is aimed at leading the designer to proportion the damper device(s)) in order to achieve a desired structural performance level. According to the method, the structural system composed by frame and dampers is replaced by an equivalent Single Degree of Freedom (SDOF) system, characterized through its secant stiffness and equivalent viscous damping, both defined in relation to a “performance point” which is assigned on the basis of the allowable damage of the frame and on the first mode deformation of the main structure. The global stiffness and strength of the equivalent SDOF system are then distributed along the height of the frame according to a stiffness-proportionality criterion, and the properties of the damper units are calculated depending on the chosen layout. Two case-studies relevant to as many reinforced concrete frames are provided to demonstrate the effectiveness of the suggested procedure, obtaining a satisfactory agreement between the design target and numerical capacity curves. Non-linear dynamic analyses are further performed to assess the reliability of the methodology
EXPERIMENTAL AND NUMERICAL ASSESSMENT OF PRESTRESSED LEAD EXTRUSION DAMPERS
No full textThe study presents an experimental and numerical assessment of the Prestressed Lead Extrusion Damper (P-LED), an emerging energy dissipation device which provides energy dissipation through the plastic extrusion of lead, and achieves high specific output force by preloading of the working material. Tests are performed according to the European standard EN 15129. The damper shows a rigid-plastic behaviour with an essentially rectangular hysteresis loop, resulting in an effective damping of 0.55 regardless of the imposed deflection, and is able to sustain multiple cycles of deformation at the basic design earthquake displacement without degradation of the output, providing maintenance-free operation even in presence of repeated ground shakes. A 3D finite element model of the P-LED is formulated and used in a parametric study to investigate the influence of the device dimensions on the output force. The numerical data points are fitted by a simple model which can be used for designing the damper to a specified quasi-static force
Assessment of Non-Linear Analyses of RC Buildings Retrofitted with Hysteretic Dampers According to the Italian Building Code
No full textWhile the use of steel hysteretic dampers has spread in the last decade for both new and retrofitted constructions, the Italian Building Code (IBC), as well as the Eurocode 8, does not provide specific recommendations for the design and verification of structures equipped with this technology. Due to their strong non-linear behavior, the effectiveness of the design with these systems must be verified through non-linear analyses. Non-Linear Time-History analyses (NLTHAs) are the most reliable method, but they are computationally expensive. The aim of the study is to investigate the reliability of non-linear static procedures, allowed by the IBC as an alternative to NLTHAs, for the analysis of buildings equipped with hysteretic devices provided with high damping capability. A parametric study is conducted on two reinforced concrete residential buildings, typical of the Italian residential heritage, retrofitted with hysteretic braces characterized by different stiffness and ductility values. The retrofit design is verified using non-linear analyses, both static and dynamic, considering either natural or artificial accelerograms, as the IBC deems them as equivalent. Within this work, reference is made only to the IBC; however, given the significant similarity between the IBC and the European code, the outcomes are expected to have a broader impact and to be not limited to the Italian context. Therefore, although this work is a preliminary study, it is believed to offer some initial insights on the topic and serve as the foundation for a more in-depth study that could lead to a regulatory revision on the subject
Comparison of Linear and Nonlinear Procedures for the Analysis of the Seismic Performance of Straight Multi-Span RC Bridges
Full text linkThe assessment of the seismic performance of transportation infrastructures is of primary importance for the management of the aftermath of an earthquake. To perform such an assessment, various modeling approaches characterized by different levels of accuracy are available and modern seismic design codes provide recommendations about their use. Non-linear time-history analysis (NLTHA) is acknowledged as the most reliable method, but is difficult to implement and is computationally expensive. This paper aims to investigate the viability of less complex methods, but with low computational cost, for the assessment of straight, multi-span bridges and compare their performance against the results of NLTHA in order to quantify the expected accuracy. The study is developed considering three bridge archetypes with either simply-supported or continuous-deck layouts, representative of typical features of the Italian bridge stock. The bridges are analyzed first through nonlinear dynamic analyses, to define the benchmark solution; then linear dynamic analyses, such as Linear Time-History and Response Spectrum Analysis, nonlinear static analyses, such as MPA (Modal Pushover Analysis), and Equivalent Static Analysis are considered. A comparison among the examined procedures is eventually proposed, highlighting the strengths and weaknesses of each approach
Cyclic engagement of hysteretic steel dampers in braced buildings: a parametric investigation
Full text linkHysteretic steel dampers have been effectively used to improve the seismic performance of framed buildings by confining the dissipation of seismic energy into sacrifical, replaceable devices which are not part of the gravity framing system. The number of cycles sustained by the dampers during the earthquake is a primary design parameter, since it can be associated to low-cycle fatigue, with ensuing degradation of the mechanical properties and potential failure of the system. Current standards, like e.g. the European code EN 15129, indeed prescribe, for the initial qualification and the production control of hysteretic steel dampers, cyclic tests in which the devices are assessed over ten cycles with amplitude equal to the seismic design displacement d(bd). This paper presents a parametric study focused on the number of effective cycles of the damper during a design earthquake in order to assess the reliability of the testing procedure proposed by the standards. The study considers typical applications of hysteretic steel dampers in low and medium-rise steel and reinforced concrete framed buildings and different ductility requirements. The results point out that the cyclic engagement of the damper is primarily affected by the fundamental period of the braced building and the design spectrum, and that, depending on these parameters, the actual number of cycles can be substantially smaller or larger that recommended by the standards. A more refined criterion for establishing the number of cycles to be implemented in testing protocols is eventually formulated
Reliability analysis of two archetype RC buildings with hysteretic dampers
Full text linkThe manuscript deals with a study on the performance/reliability analysis of steel hysteretic dampers designed for the seismic retrofit of RC frame structures at the Basic Design Earthquake, BDE (characterized by a 10 % probability of being exceeded over the reference lifespan, VR) for higher hazard levels, represented by a Maximum Considered Earthquake, MCE, with either 5 %/VR or 2.5 %/VR exceedance probabilities. Indeed, seismic codes prescribe that anti-seismic devices designed to satisfy the performance requirements at the BDE must be able to resist earthquakes of higher intensity. Therefore, γ-factors amplifying the design displacement or force must be considered for the design of the device capacity. The objective of the study consists in evaluating the over-demand caused on the steel dampers incorporated in dissipative bracing systems when passing from the BDE to the two higher hazard levels, and comparing this over-demand to the values of the corresponding γ-factors proposed in current codes. Dampers with various ductility factors are considered to include the majority of current hysteretic devices. The amplification factors recommended in the codes for hysteretic dampers are shown to be not suitable for guaranteeing the damper reliability; moreover, a dependence of the reliability of the retrofitted structures upon the ductility of the damped braces is highlighted
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