Farzad Hejazi

Optimization of Earthquake Energy Dissipation System by Genetic Algorithm

Numerous recent studies have assessed the stability and safety of structures furnished with different types of structural control systems, such as viscous dampers. A challenging issue in this field is the optimization of structural control systems to protect structures against severe earthquake excitation. As the safety of a structure depends on many factors, including the failure of structural members and movement of each structural node in any direction, the optimization technique must consider many parameters simultaneously. However, the available literature on optimizing earthquake energy dissipation systems shows that most researchers have considered optimization processes using just one or a few parameters applicable only to simple SDOF or MDOF systems. This article reports on the development of a multiobjective optimization procedure for structural passive control systems based on genetic algorithm; this research focused on systems that would minimize the effects of earthquake based on realistic structural responses considering plastic hinge occurrence in structural elements and three-directional displacement in all structural nodes. The model was applied to an example of three-dimensional reinforced concrete framed building and its structural seismic responses were investigated. The results showed that the optimized control system effectively reduced the seismic response of structures, thus enhancing building safety during earthquake excitations.

Application of shape memory alloy bars in self-centring precast segmental columns as seismic resistance

The objective of this study is to investigate analytically the performance of self-centring precast segmental bridge columns with shape memory alloy (SMA) starter bars under nonlinear static and lateral seismic loading. For this purpose, a 3D finite element model for hybrid post-tensioned bridge column has been developed. The precast post-tensioned segmental bridge columns possessing a central tendon and adequate transverse confinement provided by the steel tube jacketing as self-centring bridge columns have an undesirable high lateral seismic demand due to their low energy dissipation. In order to eliminate this deficiency while keeping the residual displacement small, SMA starter bars are applied in this system. The effect of post-tensioning (PT) forces of the central strands and SMA bar size are investigated. The results indicate that in high seismicity zones, bridge columns with SMA bars at a higher level of PT forces have a superior performance against earthquake loading.

Analytical Model for Viscous Wall Dampers

By now, many civil engineering researchers have extensively studied the application of earthquake energy dissipation systems in seismic-resistant buildings. Earthquake energy dissipation systems play an important role in enhancing the sustainability of structures against seismic excitation. Frame buildings are strengthened by installing damper devices as supplemental structural members. This article presents the finite-element-based development of an analytical model for a viscous wall damper VWD device, an alternative to other earthquake energy dissipation systems, which can diminish the effect of earthquakes on structures and improve the seismic performance of multistory buildings subjected to ground motion. The constitutive law of VWDs has been formulated and integrated to develop a finite element model of VWD compatible with the reinforced concrete RC structure analytical model. Then, the finite element algorithm has been developed for inelastic analysis of RC buildings equipped with VWD devices capable of detecting damage to both structural members and damper connections under dynamic loading. Based on the developed system, the special finite element program was codified and verified by applying it to a real model of a RC building with supplementary VWD devices. Influence of VWDs on seismic performance of the RC building during earthquake excitation was evaluated. The proposed analytical model for VWD is verified by using experimental test data and analysis result proved that this energy dissipation system succeeds by substantially diminishing and dissipating a structures induced seismic responses. Also the parametric study indicated that the damping coefficient is very effective on performance of VWD.

A numerical study on seismic response of self-centring precast segmental columns at different post-tensioning forces

Precast bridge columns have shown increasing demand over the past few years due to the advantages of such columns when compared against conventional bridge columns, particularly due to the fact that precast bridge columns can be constructed off site and erected in a short period of time. The present study analytically investigates the behaviour of self-centring precast segmental bridge columns under nonlinear-static and pseudo-dynamic loading at different prestressing strand levels. Self-centring segmental columns are composed of prefabricated reinforced concrete segments which are connected by central post-tensioning (PT) strands. The present study develops a three dimensional (3D) nonlinear finite element model for hybrid post-tensioned precast segmental bridge columns. The model is subjected to constant axial loading and lateral reverse cyclic loading. The lateral force displacement results of the analysed columns show good agreement with the experimental response of the columns. Bonded post-tensioned segmental columns at 25%, 40% and 70% prestressing strand stress levels are analysed and compared with an emulative monolithic conventional column. The columns with a higher initial prestressing strand levels show greater initial stiffness and strength but show higher stiffness reduction at large drifts. In the time-history analysis, the column samples are subjected to different earthquake records to investigate the effect post-tensioning force levels on their lateral seismic response in low and higher seismicity zones. The results indicate that, for low seismicity zones, post-tensioned segmental columns with a higher initial stress level deflect lower lateral peak displacement. However, in higher seismicity zones, applying a high initial stress level should be avoided for precast segmental self-centring columns with low energy dissipation capacity.

Inelastic Seismic Response of RC Building with Control System

Conventional buildings are mainly designed based on elastic analysis of structures subjected to moderate earthquakes. In this case, the seismic forces are much smaller than the forces introduced by strong ground motions with the considered structural behavior going to nonlinear response during these severe earthquakes. Improving the earthquake resistance of reinforced concrete buildings using a variety of earthquake energy dissipation systems has received considerable attention in recent years by civil engineers. In the present study, a nonlinear computational scheme was developed to predict the complete nonlinear dynamic response of reinforced concrete framed buildings equipped with viscous damper device subjected to earthquake excitation. A finite element program code is developed based on the nonlinear analysis procedure of reinforced concrete buildings equipped with viscous damper devices and a two dimensional, five story models of RC buildings subjected to earthquake were analyzed. Result of nonlinear analysis of RC buildings which furnished by viscous dampers indicated that using of viscous dampers effectively reduced the damages occurring in the building and structural motion during severe earthquakes.

Effect of Soft Story on Structural Response of High Rise Buildings

Severe structural damage suffered by several modern buildings during recent earthquakes illustrates the importance of avoiding sudden changes in lateral stiffness and strength. Recent earthquakes that occurred have shown that a large number of existing reinforced concrete buildings are vulnerable to damage or even collapse during a strong earthquake. While damage and collapse due to soft story are most often observed in buildings, they can also be developed in other types of structures. The lower level containing the concrete columns behaved as a soft story in that the columns were unable to provide adequate shear resistance during the earthquake. Usually the most economical way of retrofitting such as a building is by adding proper bracing to soft stories. So, in this paper occurring of soft at the lower level of high rise buildings subjected to earthquake has been studied. Also has been tired to investigate on adding of bracing in various arrangements to structure in order to reduce soft story effect on seismic response of building. It is lead to assess the vulnerability level of existing multi-storied buildings so that they can be retrofitted to possess the minimum requirements. This will help in minimizing the impending damages and catastrophes.

Effect of an Opening on Reinforced Concrete Hollow Beam Web Under Torsional, Flexural, and Cyclic Loadings

HOLLOW SECTIONS HAVE BEEN INCREASINGLY APPLIED IN THE CONSTRUCTION OF BUILDINGS, BRIDGES, OFFSHORE STRUCTURES, AND TOWERS FOR PASSING ELECTRICAL AND MECHANICAL PIPES OR OTHER UTILITIES. ONE OF THE WEAKNESSES OF HOLLOW SECTIONS IS UNDER TORSION FORCE CAUSED BY EXTERNAL FORCE WHICH IS RARELY INVESTIGATED. IN PARTICULAR, THE BEHAVIOR OF HOLLOW SECTIONS WITH HIGH-STRENGTH CONCRETE (HSC) AND ULTRA-HIGH PERFORMANCE CONCRETE (UHPC) REMAINS POORLY STUDIED. THIS STUDY AIMS TO EXAMINE THE BEHAVIOR OF A REINFORCED CONCRETE HOLLOW BEAM WITH OPENING AND COMPARE IT WITH A HOLLOW BEAM WITHOUT OPENING. THE HOLLOW BEAM WITH AN OPENING IS MODELED USING THE FINITE ELEMENT METHOD AND ANALYZED UNDER TORSION, FLEXURAL, AND CYCLIC LOADING FOR TWO DIFFERENT HSC AND UHPC MATERIALS. THE EFFECT OF THE OPENING SECTION SIZE ON THE BEHAVIOR OF HOLLOW BEAM IS ALSO EVALUATED. RESULTS SHOWED THAT THE OPENINGS CREATED IN THE WEB OF HOLLOW BEAMS LED TO A DECREASE IN THE CAPACITY OF THE BEAMS ALTHOUGH THE HOLLOW BEAM WITH SMALL OPENING CAN CARRY ALMOST THE SAME LOAD AS THAT OF HOLLOW BEAM WITHOUT AN OPENING. THE RESULT ALSO INDICATED THAT THE CAPACITY OF UHPC BEAMS FOR TWISTING IS TWICE THAT OF HSC BEAMS.

Evaluation of response modification factor for steel structures with soft story retrofitted by viscous damper device

The soft story located in the basement of tall buildings for parking or shopping lot. Considering the limitation in architecture, these structures may be retrofitted by implementing earthquake energy dissipation such as viscous damper devices in soft story level. Nevertheless, an extensive review of related literature indicates that the effect of viscous damper on response modification factor is not considered. Therefore, this study proposed the response modification factors for steel structures furnished with viscous damper devices in soft story level and investigated the effect of implementing such devices on the response modification factor. In this research, steel structures with numerous stories were considered to evaluate the response modification factor, which was formulated based on three aspects, namely, strength, ductility, and redundancy factors. Quasi-static nonlinear analysis was performed using finite element software based on structural models equipped with damper devices installed in different locations of the parking level. No open bay is available in other floors to install the damper devices. Meanwhile, the overstrength, ductility, and response modification factors were established by conducting pushover analysis. Results of the study revealed that the response modification factors for structures equipped with damper devices are higher than those of structures without damper devices. Moreover, the value of response modification factor was affected by the number of stories and number of dampers. Considering the analytical results for different cases, the equation for determining the response modification factor for steel structures furnished by viscous damper devices in soft story level was proposed.

Dynamic Analysis of Variable Stiffness Bracing System in Structure

In this paper, the application of a variable stiffness bracing (VSB) system in structures subjected to earthquake excitation is presented. The considered variable stiffness system is includes of four curve leaf springs. The nonlinear geometry of leaf springs which are acting as bending component lead to nonlinear stiffness performance. The variable stiffness bracing system does not act much for small to intermediate vibration amplitudes but it’s operated to control unpredictably large story displacement. It means this retrofit’s technique avoid an increase force in structural component due to ordinary brace action. The single degree of freedom system (SDOF) is considered and dynamic analysis of aforementioned system, with Bare and normal braced frames are conducted and the results are compared. The efficiency of the proposed system is discussed and proved in light of numerical analysis.

Energy absorption evaluation of reinforced concrete beams under various loading rates based on particle swarm optimization technique

ABSTRACT This study proposes an energy absorption model for predicting the effect of loading rates, concrete compressive strength, shear span-to-depth ratio, and longitudinal and transverse reinforcement ratio of reinforced concrete (RC) beams using the particle swarm optimization (PSO) technique. This technique avoids the exhaustive traditional trial-and-error procedure for obtaining the coefficient of the proposed model. Fifty-six RC slender and deep beams are collected from the literature and used to build the proposed model. Three performance measures, namely, mean absolute error, mean absolute percentage error and root mean square error, are investigated in the proposed model to increase its accuracy. The design procedure and accuracy of the proposed model are illustrated and analysed via simulation tests in a MATLAB/Simulink environment. The results indicate the minimal effect of swarm size on the convergence of the PSO algorithm, as well as the ability of PSO to search for an optimum set of coefficients from within the solution space.