1,721,178 research outputs found
Robustness improvements for 2D reinforced concrete moment resisting frames: Parametric study by means of NLFE analyses
Full text linkThis study develops a 2D computational parametric analysis of ordinary reinforced concrete (RC) frames, located in seismic zone, with the aim to evaluate some design suggestions with respect to their effectiveness in increasing the structural robustness. Specifically, a five-storey and four-span 2D RC moment resisting frame is considered, designed in a highly seismic area according to both Italian and European codes. Subsequently, respecting the seismic design code provisions through a cyclic design procedure, some modifications are suggested regarding the layout of the longitudinal reinforcement bars of the beams to exploit the continuity, Vierendeel behavior and influence of the side face reinforcement bars in the beams. For the different modifications, nonlinear finite element pushdown analyses of the whole frames are performed by imposing a monotonically increasing vertical displacement at the point of the column removal in Atena-2D and considering the presence of the orthogonal framed systems. Furthermore, two different failure scenarios are examined. The numerical force-displacement capacity curves corresponding to the different proposed design suggestions are investigated and compared. The results have demonstrated the effectiveness in improving the structural robustness of the proposed solutions, especially, of the side face reinforcement bars with respect to both flexural and catenary behavior. The outcomes have also highlighted the compatibility between design criteria of both robustness and earthquake engineering
Optimal Response of Isolated Multi-span Continous Deck Bridges Subjected to Near Fault and Far Field Events
Full text linkThe present study analyses the optimal friction coefficient for the seismic isolation of composite bridges, equipped with single concave friction pendulum (FPS) devices. The bridge is modelled through a six-degree-of-freedom system while the FPS friction property is described through a model that accounts for the dependency on the velocity. By introducing a time scale and a length scale, a nondimensional analysis has been used to solve the equations of motion. In detail, the response is analysed independently on the peak ground acceleration-to-velocity ratio. Furthermore, two different sets of seismic events are considered: far field and near fault. Then, many bridge models are analysed by changing different parameters (i.e., pier period, deck period, mass of the deck and of the pier and friction coefficient). By minimizing the substructure response, an optimum value of the friction coefficient is computed as function of the ratio between the period of the deck and the period of the seismic input
Seismic reliability-based design of hardening and softening structures isolated by double concave sliding devices
No full textThis study proposes seismic reliability-based design relationships in terms of behavior factors and displacement demands for softening and hardening structures equipped with double concave sliding bearings (i.e., DFPS). An equivalent 3dof system is adopted, that is characterized by a hardening or softening post-yield slope, representative of the superstructure response, and by velocity-dependent laws to model the frictional responses of the two surfaces of the DFPS. The yielding characteristics of the superstructures are defined for increasing behavior factors, provided by the codes, in compliance with the seismic hazard of L’Aquila (Italian site) and with the life safety limit state as provided by NTC18. Considering several natural ground motions and different elastic and inelastic structural properties under the hypothesis of modelling the friction coefficients of the two surfaces of the DFPS as random variables, incremental dynamic analyses are developed to assess the seismic fragility. From the convolution integral of the fragility curves with the seismic hazard curves related to L’Aquila (Italian site), assuming a lifetime of 50 years, the corresponding reliability curves are computed. Precisely, seismic reliabilitybased linear and multi-linear regressions relating the displacement ductility demand to the behavior factors for the softening and hardening superstructures together with seismic reliability-based design (SRBD) curves for the two surfaces of the double concave sliding bearings are provided as the design relationships
Robustness analysis of reinforced concrete structures: design issues
No full textThe goal of this work is to evaluate the structural robustness of a reinforced concrete building, designed in a highly seismic area, in order to define some improvements of the design criteria. In particular, a five-storey and four-span 2D frame designed according to Italian and European code rules, also accounting for the seismic design, is presented. Some modifications are considered in terms of continuity of longitudinal bars in order to investigate effects on the structural robustness. Those applied modifications are then checked to respect the ultimate limit state and seismic capacity-design verifications. Then, using the finite element software Atena 2D, displacement-controlled pushdown analyses are performed. By studying the resulting force-displacement capacity curves, the capability of the structure to show a from a robustness view point is analysed
Comparison of Different Approaches to Derive Global Safety Factors for Non-linear Analyses of Slender RC Members
Full text linkThe present study relates to comparison between different approaches for definition of global safety factors for non-linear analysis of slender RC members with reference to new or existing structures. Firstly, a benchmark set of 40 experimental results on reinforced concrete columns is presented. After the description of the main features of the benchmark test sets the related non-linear numerical models have been realized using fiber-modelling as solution strategy. Then, appropriate assumptions concerning aleatoric and epistemic uncertainties have been performed with the aim to run probabilistic analysis of global resistance for each one of the 40 columns. The results of the probabilistic analysis are useful to define global safety factors in line to the global resistance method. Finally, the comparison between different approaches to derive global safety factors is presented and discussed
Seismic reliability of structures equipped with frictional isolators with improved lateral impact behavior
No full textA novel frictional device to achieve seismic isolation, the Lateral Impact Resilient Double Concave Friction Pendulum (LIR-DCFP), has been suggested as an alternative to mitigate the adverse effects of internal lateral impacts. These impacts between the inner slider and the restraining rims of the concave plates can produce the failure of the bearing and a dramatic increment in the ductility demand of the superstructure. The LIR-DCFP has an improved inner slider with an internal gap capable of limiting the magnitude of the impact and dissipating a critical amount of energy during the impact. This work presents a reliability-based comparison of the seismic performance of structures isolated through LIR-DCFP devices or classical Double Concave Friction Pendulum (DCFP) bearings. The response of the superstructure was represented using a simplified elastoplastic model considering hardening and softening post-yield behaviors. The isolation system was represented using models based on rigid body dynamics, including the lateral impact behavior. A parametric analysis was performed, including a wide range of structural properties, two types of frictional isolators, and considering the friction coefficients as relevant random variables. The aleatory uncertainties of the seismic input were included by selecting different sets of natural records matching the conditional spectra of a site in Riverside (California). Incremental Dynamic Analyses were conducted to characterize the statistics of the maximum ductility demand of the superstructure and determine the probabilities exceeding limit state thresholds, (i.e., the fragility curves). Finally, using the seismic hazard curves, the seismic reliability was determined. For increasing values of the internal gap, better seismic performance is achieved compared to the performance obtained employing DCFP devices. This better performance is improved if the superstructure is designed to remain in its elastic range if the internal impact is not produced. Reductions up to 19% in the probabilities exceeding limit state thresholds in terms of maximum ductility demand in a time frame of 50 years are possible by replacing classical DCFP bearings with LIR-DCFP isolators
Seismic performance of bridges isolated with FPS
Full text linkThe scope of the present study is focused on the evaluation of the seismic response of bridges isolated by single concave sliding pendulum isolators (FPS) for the different structural properties when the presence of the rigid abutment is considered or neglected (i.e., isolated viaducts). In this way, they have been defined two specific multi-degree-of-freedom (mdof) models to simulate the elastic behavior of the reinforced concrete pier in combination to the infinitely rigid presence of the deck and to the presence of the rigid abutment if considered. Both the numerical models also account for the non-linear velocity-dependent behavior of the FPS bearings. Considering the aleatory uncertainty in the seismic input by means of several natural records with different characteristics, a parametric analysis is developed for several structural properties. The relevant results expressed as the statistics in non-dimensional form with respect to the seismic intensity have permitted to study the differences between the two numerical models in relation to the effectiveness of the seismic isolation
Seismic Performance of Bridges Depending on the DCFP Device Properties
Full text linkThe present investigation examines how the properties of the double concave friction pendulum (DCFP) devices influence the seismic performance of isolated multi-span continuous deck bridges. The numerical simulations are carried out using an eight-degree-of-freedom model to reproduce the elastic behavior of the pier, associated to the assumption of both rigid abutment and rigid deck, and the non-linear velocity-dependent behavior of the two surfaces of the double concave friction pendulum isolators, under a set of natural records
A parametric analysis of the seismic performance of bridges as a function of the DCFP device properties
Full text linkThe present investigation examines how the properties of the double concave friction pendulum (DCFP) devices influence the seismic performance of isolated multi-span continuous deck bridges. The numerical simulations are carried out using an eight-degree-of-freedom model to reproduce the elastic behavior of the pier, associated to the assumption of both rigid abutment and rigid deck, and the non-linear velocity-dependent behavior of the two surfaces of the double concave friction pendulum isolators, under a set of natural records with different characteristics. The results in terms of the statistics related to the relevant response parameters are computed in non-dimensional form with respect to the seismic intensity considering different properties of both DCFP isolators and bridge
Evaluation of the Failure Probability of a 2D RC Frame Subjected to Column Loss
Full text linkThis study regards the evaluation of the failure probability of a symmetrical 2D reinforced concrete frame
composed of 4 spans and 5 floors, in case of an accidental event which causes the central base column loss. The frame is an internal one of a typical building designed in a highly seismic area, characterised by a high ductility class. The frame is modelled in the non-linear finite elements software Atena 2D, accounting for both geometrical and material nonlinearities.
The uncertainties relevant to the problem are included by sampling both material and action variables,
adopting the Latin Hypercube Sampling technique. To compute the failure probability associated to the accidental scenario, two sets of analyses are considered: the first set to compute the capacity of the structure against the column removal by means of displacement-controlled pushdown analysis; the second set to evaluate the demand in terms of external loads, properly combined within the accidental combination according to the codes. The external load is then amplified in order to include the dynamic effects characterising a scenario of a structural member loss. Finally, the probability of the demand exceeding the capacity is evaluated
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