نشریه آنالیز سازه - زلزله
سال سیزدهم شماره 1 (بهار 1395)

Energy dissipation systems have been broadly used in structures during the recent decades in order to reduce earthquake and wind forces as well as reduction of structural lateral drifts within the code limits. Viscous damper is considered as one of the energy dissipation systems which are classified as velocity-dependent dampers among passive control systems and have been paid attention and their further detailed properties taken into account by many researchers. Viscous damper consists of a piston with some orifices inside the cylinder which contains highly viscous fluid. Energy dissipation of this damper is through pushing viscous fluid out of the orifices.
These are two types of these dampers: linear and nonlinear from which linear type with velocity power of one is more practical. The structural damping force is usually set according to the procedure described in FEMA273 and are optimized by three controlled modes of displacement, velocity and acceleration.
This study examines the effect of adding viscous dampers on seismic behavior of steel moment frames. For this purpose, three steel moment frames of 1, 3 and 6-story all in 3 bays, with viscous dampers having power factor of 1, 0.8 and 0.6 are selected. These sample structures are subjected to the nonlinear time history analysis under the El Centro, Kobe and Northridge earthquakes, and their response including displacement, acceleration and base shear is compared in two cases of with and without viscous dampers.
Finally, regarding nonlinear time history analysis results based on the structural behavior in three modes controlled by displacement, velocity and acceleration, proper damping forces are specified as 94.7, 240.1 and 557.1 kN respectively for one, three and six story structures based on maximum acceleration and base share obtained from FEMA273.

Today, concrete is known as one of the most used building materials in the world. Silica fume and its products are now known as one of the best mineral additives in concrete. Silica fume is mainly used to improve the properties of hardened concrete so when using Silica fume concrete properties after hardening usually more significant. But although various experiments have been conducted on the subject. But this thesis is considering other products of silica impact on the hardened concrete properties have been investigated, including compressive strength, permeability and Thawing -freezing of hardened concrete. Three mixing ratio is tested in the laboratory: 1- without additives and 350 kg per cubic meter of cement, 2- with 10% Silica fume powder and one percentage lubricant, 3- with 10% of the micro gel. Based on lab work conducted in the research and the gathering of results can be explained by the properties of hardened concrete, the use of powder and gel of Silica fume increased compressive strength of 37.5 and 56 percent respectively. Water penetration rate, in the second ratio increase 50 percent and in the third ratio decreased 25 percent, compared to the first ratio. The results indicate the desirability of Silica fume gel long-term impact on the quality of hardened concrete including its durability. While the silica fume powder, could adversely affect the permeability and Freezing-Thawing of concrete.

Soft story has caused the failure of many buildings in previous earthquakes. Several factors such as greater height of columns, removing some columns, lateral bracings, shear walls or infill walls due to architectural design in a story especially ground floor are the main reason of this phenomenon. Most of the seismic codes have criteria for vertical irregularity. This is usually based on the ratio of stiffness between adjacent storeys, but there is no recommendation for calculating the storey stiffness. On the other hand a great number of previous studies on stiffness have focused on approximate methods and exact methods have been almost neglected. In this study, soft storey, weak storey and approximate methods in literature are reviewed and then six exact methods for calculating the storey stiffness by using structural analysis software are investigated. This study is done on 2D reinforced concrete frames. Based on analysis results, method 3 with no modification in the model of structure and the triangular distribution of lateral forces similar to seismic forces has acceptable accuracy and desirable efficiency for designing and controlling structures.

One of the cases of human intervention in environment is vast variations in making parts of the land surface impervious, followed by changes in the hydrologic cycle. Porous concrete is an especial type of highly pervious concrete, compatible with the environment, and could be used a method to increase surface infiltrate and reduce urban runoff. In the present experimental research, to improve the performance of this type of concrete, effect of adding lightweight pumice and perlite (0.6-1.2 mm), and adding 10% fine grains to the samples, was investigated. In this respect, the effective parameters for using porous concrete in urban runoff system, such as compression strength, hydraulic conductivity and porosity of the samples were measured. Analysis of experimental results was performed by using SAS 9.4 software at 95% confidence level. Results showed that adding fine grains to porous concrete increases compression strength and reduces hydraulic conductivity and porosity. Addition of lightweight pumice increases compression strength and reduces hydraulic conductivity and porosity of porous concrete. But, lightweight perlite reduced all three parameters of hydraulic conductivity, porosity and compression strength. In general, lightweight pumice, which has higher density and more pore space in its structure with respect to perlite, had better performance.

To reduce or eliminate residual deformations in the main members of the structures after an earthquake, researchers presented the idea of using self-centering systems. Several types of self-centering systems have been proposed.One of the strategies for created self centering in structures, using post-tensioned(PT) steel moment connections. Until now several types of PT steel connections is provided. In this paper, results of numerical analysis using finite element method in ABAQUS software for five full-scale model of interior PT steel connections with bolted top and seat angles has been studied. The results of numerical analysis in this research demonstrate the beams and columns remain essentially elastic while inelastic deformation of the top and seat angles provided energy dissipation and also using post-tensioning method, increase loading capacity significantly without residual deformation in main members of the connection. In a PT connection, maximum stresses in beam created at the flange of beam and using longer reinforcing plate, cause decrease stresses created in beam and reduce various damage index and risk of brittle fracture in tension angles.Cyclic behavior of numerical model of PT connection with combined axial load and drift demand shows that the axial load reduces bearing capacity of connection.

Development of concrete technology is undergoing for achieving high strength concrete. Metakaolin (MK) for obtaining high strength and durable concrete recently is used in industry. The relatively high micro particles and pozolan activity and reaction with calcium hydroxide of cement, causes lower voids, permeability and increasing durability. In the present research the history of pozolans and their materials specially MK and their effects on mechanical properties to achieve an optimize percentage of MK instead of cement were investigated. The tests are included as workability, absorbing water, relative density, electric resistance (that shows permeability and corrosion resistance) and compressive strength. The tests were carried out on 0%, 10%, 15% and 20% of MK in replacement of cement on 150x150 mm samples on 7 and 28 days after casting. The results show that 10% of MK has better workability. 15% of MK higher compressive strength and 20% higher electric resistance and lower absorbing water and 15% have a reduced relative density

The study of design codes has been of great importance in last many years because the evaluation of similarities and differences between them can cause better recognize the seismic parameters and also realize the diverse aspects and weaknesses and strengths. According to Addoption of the forth edition of 2800 standard and major changes to the third edition, evaluation and vulnerability assessment of designed and built structures according to previous standard and comparison with the new standard demands is so important. In this study the seismic performance of reinforced concrete moment frames based on the third and forth edition of 2800 standard have been evaluated. At the end, the safety of designed frames such as displacement and drift ratio have been comparised according to both editions. Also the reinforcement used in the structures has been studied acconomically.