فهرست مطالب mohammad ghanizadeh

The present study aims to propose and evaluate a new hybrid system consisting of a desiccant cooling system and earth-to-air heat exchanger (EAHE). The thermal performance of the proposed system is compared with the usual desiccant cooling system and an evaporative cooler. The analogy method is used to model a desiccant wheel. To investigate the heat transfer in EAHE and soil around it, the EAHE and soil around it are divided into several equal parts. The heat transfer problem in the solution domain is solved using resistance-capacity models. The simulations are performed for Tehran city using outdoor realistic weather data. The results reveal that in the proposed hybrid cooling system, the average temperature of the entering air a room is 1.1 and 5.3 °C lower than that of the usual desiccant cooling system and the evaporative cooler, respectively. The EAHE parametric analysis shows that by increasing the pipe length from 30 m to 50 m, the average cooling capacity of the overall system increases by 3.44%. It was also found that increasing the pipe diameter does not affect the cooling capacity significantly. Increasing the volume flow rate value from 200 to 400 cfm increases the average cooling capacity by 94.58%.

This study focuses on an approach to improve the behavior of low-rise shear walls with a rectangular cross-section and a height-to-length ratio of unity. The significance of this study lies within the need for a better understanding of the complex shear mechanism, not to mention the shear-flexure interaction in low-rise shear walls which results in different failure patterns under seismic loading. Shear forces may cause diagonal tensile or compressive failures. Moreover, cracks at the foot of the wall during elastic deformation could cause the shear slip failure which could limit the ductility of the wall. To this end, validation studies were conducted on a shear wall specimen chosen from the literature (i.e., the specimen tested by the nuclear power engineering corporation of Japan (NUPEC) (1996).

2.1. Finite element modeling

Numerical analyses were initially carried out in a nonlinear finite element software specifically developed for concrete structures, namely ATENA. “3D nonlinear cementitious 2” material, a fracture-plastic model that combines models for fracture and plastic behavior was used. The classic orthotropic smeared crack formulation and crack band model are used in this model. In this model, the tensile softening branch is defined by two parameters, fracture energy, and the maximum crack width. Moreover, the post-peak branch of the compressive behavior is characterized by the ultimate strain. Bi-linear strain hardening stress-strain curve was used for reinforcements. Eight-node isoparametric brick elements and 2-node truss elements were used to define concrete and reinforcements, respectively. The “Newton-Raphson” solution method with an updated stiffness matrix in each iteration was used to solve the nonlinear system of equations. In order to investigate the behavior of low-rise concrete shear walls, a short shear wall with dimensions of 0.2 m (thickness), 3.5 m (height), and 3.5 m (length) with a rectangular cross-section resting on a 9-m length strip foundation proposed by the International Institute of Earthquake Engineering and Seismology was used. To investigate the influential parameters governing the behavior of the wall, the so-called wall was numerically modeled and was subjected to lateral increasing loads; details of the wall were according to ref Iiees (2011).

3.1. Validation studies

Comparison of experimental and numerical results in terms of shear strength and displacement values showed that the values are in very good agreement with one another and therefore the methodology was verified
3.2. Effect of slots Results obtained from the analysis for the specimen (as shown in Fig. 2) shows that the failure pattern of the wall is mainly shear-dominant with a sudden decrease in shear strength value after the peak load. As for the effect of openings, it can be observed in Fig. 3, when compared to the wall without slots, the post-peak ductility of the wall increases noticeably which could act as a warning prior to its failure. Moreover, according to Fig. 4, it can be seen that the tensile cracks have propagated throughout the height of the wall which helps to avoid stress concentration and make efficient use of the tensile capacity of the concrete.

Results showed that walls with slots have an overall improved ductility in comparison to models without slot. Additionally, creating slot in the wall helps in the redistribution of internal forces, prevents stress concentration. Furthermore, the presence of slots changes the failure pattern of the wall from shear-dominant to shear-flexure dominant. Finally, for a wall with a slot ratio of 0.15, displacement corresponding to the peak load increases by 138% compared to its counterpart with no slots.

With respect to the height to width ratio, the concrete shear walls are divided into two categories of cantilever and short shear walls. Cantilever shear walls with the aspect ratio greater than 2 demonstrate flexural behavior while short shear walls with the aspect ratio smaller than to 2 exhibit shear behavior. In this study the behavior of short reinforced concrete shear walls are investigated using the nonlinear finite element software ATENA 3D. The failure mode and the ultimate and residual shear resistance of the short shear walls, as well as the affecting parameters such as the magnitude of the axial force, the compression strength of concrete and yield strength of reinforcements are studied. It is resulted that the magnitude of axial forces has significance effect on the ductile behavior of short shear walls.

As the need for seismic designing of the earthquake resistant structures increases, numerous laboratory and numerical studies are being done to estimate the nonlinear response of these structures. With respect to the height to width ratio, the concrete shear walls are divided into two categories of cantilever and short shear walls. Cantilever shear walls with the aspect ratio greater than 2 demonstrate flexural behavior while short shear walls with the aspect ratio smaller than to 2 exhibit shear behavior. In this study the behavior of short reinforced concrete shear walls are investigated using the nonlinear finite element software ATENA 3D. The failure mode and the ultimate and residual shear resistance of the short shear walls, as well as the affecting parameters such as concentration of longitudinal and transverse bars at the edges of the wall, change of the horizontal and vertical bars spacing and the use of diagonal bars are studied. It is revealed that unlike the cantilever walls, the concentration of the bars at the edges have no influence on the increasing of ductility.