جستجوی مقالات مرتبط با کلیدواژه « reinforced concrete » در نشریات گروه « مهندسی زلزله »

By increasing the risk of explosive terrorist attacks and threats to all types of structures and buildings, a careful study of the behavior of structures under explosive loading is considered as an important matter. A simple method to protect important structures against explosive loads is to improve the security of the environment by using protective perimeter walls against explosive loading. In this research, the effect of reinforced concrete perimeter walls with and without canopy has been investigated. Different parameters such as the geometrical shape of the perimeter wall, the element removal and degradation rate, wall displacement, the scaled distance change and the explosive mass have been investigated. Finite element software (LS-DYNA (R11.1.0) has been used for modeling. For this purpose, the perimeter walls were designed in three different shapes and conditions. The results indicate that the existence of canopies for the perimeter walls under explosive loading has positive effects, so that the canopied perimeter walls with a canopy angle of 90 degrees compared to other types of wall, at different station distances and in the same conditions, reduce the displacement rate in the protected structure by about 11 percent. Furthermore, the location of perimeter walls and obtaining a safe distance in placing these walls is very effective in enhancing safety and reducing the vulnerability of the facilities and structures protected by these walls.

Owing to the destruction of high-rise buildings during recent earthquakes, the use of a structural system of reinforced concrete moment frame with viscoelastic damper causes a loss of seismic energy and consequently reduces lateral displacement. In this study a high-Rise, 20-Storey, three-dimensional reinforced concrete frame with 8 Models, of which 4 models with moment frame structural system and 4 models with moment frame + viscoelastic damping system in two directions of X, Y located in the high seismic zone (A=0.3g) on the soil type III is considered. The aim of the present study is to investigate the horizontal lateral displacement (absolute and relative) of the stories, the base shear of frames under gravity loads, and lateral loads of the earthquake.  In order to modeling the viscoelastic materials, the Kelvin-Voigt model and to determine damping ratio(ζ) of frames with viscoelastic damper, stiffness coefficient (Kv) and damping coefficient (Cv) of viscoelastic dampers, the modal strain energy method have been applied. For seismic analysis of models without and with dampers, dynamic analysis of nonlinear time history via direct integration and modal (FNA) methods have been utilized, respectively.  Geometric modeling of all frames was done using the SAP2000-V15 software. The results indicate that with increasing span length (model 2 compared to model 1 and model 4 compared to model 3) and increasing the height of stories (model 3 compared to model 1 and Model 4 compared to model 2), the maximum absolute lateral and relative displacement of stories, and base shear along with X, Y without and with viscoelastic damper have an increasing trend. Also, the maximum percentage increase of the mentioned models with damper compared to without damper along with X, Y has decreased

The recorded recent earthquake events show that near-field earthquakes have different characteristics than far-field earthquakes. The most important distinguishing feature of near-field movements is the production of pulses due to the effect of orientation and the effect of permanent displacement. Therefore, it is necessary to study such effects on structures. In this research, first, the proposed mathematical model is validated according to the results of experiments. In order to study the nonlinear seismic behavior of post-tensioned precast concrete walls (PT-PCW), nonlinear dynamic analysis has been performed on six-story structures under near-field accelerations. Also, the effect of wall pier confinement on the self-centered performance and energy dissipation has been evaluated by performing dynamic analysis. Each acceleration is scaled to two levels of design-based earthquake (DBE) and the maximum considered earthquake (MCE) and then is used in the analysis. The performed analyzes show the optimal performance of the post-tensioned precast concrete wall system in response to earthquakes on DBE, so that at the end of the seismic load, the system does not suffer any structural damage, and minor lateral displacement remains in the system. The results also show that increasing the height of the wall pier confinement to a value slightly higher than the minimum specified in the seismic design codes promotes seismic behavior for MCE and increases the reliability of the system against overturning. The study also found that the use of post-tensioned cables in the boundary areas of the wall as an energy dissipation factor does not have a significant effect on improving the energy absorption performance of the system and the small deformations created in these members cause energy dissipation through crushing of concrete.

Considering the increasing use of FRP bars, as reinforcement in concrete structures, the study of the dynamic behavior of these types of structures and its behavioral comparison with steel reinforced concrete structures seem to be necessary. In this regard, in the present study, buildings with floors 2, 5, and 10 in two-dimensional and three-dimensional states were considered in two types of steel reinforced and FRP reinforced were modeled and analyzed in ABAQUS software. By using the results of the analysis, behavioral comparisons between these structures were carried out with a focus on displacement. By considering in the same conditions, the results of the analysis reveal that displacement in FRP-reinforced structures is more than steel-reinforced structures. Also, in the 2-storey building model, the relation between steel structures and FRP structures is linear with 90% regression coefficient was observed while in 5-storey buildings and 10-storey building’s behavior are nonlinear, with a regression coefficient of 89% and 87%, respectively.

The present paper aims at investigating the effects of strengthening structural components of reinforced concrete (RC) frames using FRP composite on their seismic performance. For this purpose, in a case study, the seismic performance of a typical building with moment frames in both plan directions as the lateral force resisting system and retrofitted with FRP wraps to deal with the weaknesses arose from increase in the number of stories compared to the original structure for occupancy reasons has been evaluated. Strengthening of the columns has been performed by considering the axial force and the biaxial bending moments at target displacement, while that of the beams has been performed based on the bending moments at the target displacement. The seismic performance of the strengthened structure has been investigated by nonlinear analysis methods based on concentrated plastic hinges model following ASCE 41-17 standard procedures. The results show that the use of FRP composites in strengthening RC components significantly improves the seismic performance by reducing the number of plastic hinges as well as the plastic rotation demands at the target displacement of the retrofitted structure.

Existence of short columns in buildings and bridges is a serious challenge in earthquakes. This destructive phenomenon occurs due to the difference in length of the column at a certain level that is mainly because of architecture consideration, such as the placement of building on a slop or restriction of column with nonstructural walls and openings or difference in story level in structures because the existence of mezzanine floor. Short columns have brittle shear failure in comparison with tall columns. This kind of failure causes a reduction in the energy dissipation capacity of the column. Shear failure is the most critical failure mode in RC short columns due to the none-observance of seismic details or sufficient transverse reinforcements against seismic loads. As concrete tensile stresses reach concrete tensile strength and the diagonal cracks appear, the concrete cover is detached and starts to shed. Then the failure and openings of transverse reinforcements and as a result the buckling longitudinal reinforcements occur. The above process leads to the disintegration of the core concrete and the sudden fracture and embrittlement of the column. In externally bonded reinforcement by FRP composites, FRP materials are different from the materials of the RC (concrete and steel) parts. The use of FRP is limited to high temperatures and has a low resistance to fire. On the other hand, strengthening with FRP composite materials is economically expensive. Mostly, High Strength Steel (HSS) bars have been used in the design and construction of the RC structures and not in strengthening. Today, due to the growing population and increased demand for raw materials and energy, solutions have been taken to optimize standards and to save on consumables, production and cost reduction. Steel reinforcements are one of the most widely used building materials with a huge number of applications in a variety of structures. Due to the considerable cost of using steel in structures, the use of HSRs has been considered as one of the major options. The use of HSRs has economic justification because of reduced human resources, reduced consumption of materials, time and manufacturing efficiency, reduced environmental damage because of the optimal utilization of materials and reduced transportation costs. Because of the greater tensile strength of these bars than ordinary ones, it leads to a brittle failure in concrete prior to rebar flaking. It, therefore, limits their application in regions with high seismic hazard. In this paper, with the modeling of nine RC short columns, without increasing the stiffness, their shear strength has increased with the help of composite and high strength steel. Two techniques were used to strengthen the diameter of the short columns against seismic loads. These techniques include EBR with FRP composite materials and NSM with HSS. The results show that in general, near surface mounted with high strength steel is more effective on increasing the dissipated energy and the ductility factor and externally bonded retrofitting is more effective on the increase of the load-displacement sub-curve and the peak load capacity.