Sharif Digital Repository

The use of braced frames has become more popular in seismic design of buildings because of the simplicity in design and construction. The first event that occurs in the earthquake for a concentrically brace frame (CBF), is the buckling of the strut. The inelastic behavior of CBF systems is dominated by brace buckling, yielding and the post buckling behavior. The Inelastic performance of the brace is nonsymmetrical, because of the difference in the tensile and compressive strength of the brace and the deterioration in the resistance after buckling. so we have too complexity in modeling the inelastic performance of brace members. Also the subject of the near fault ground motion and design of... 

This study has been carried out for investigating the need for an appropriate and fast seismic analysis method to be applied in performance based design (PBD), and show the application of Endurance Time (ET) method and it’s advantages in PBD. Necessary theories were generated in hazard analysis, bi-directional seismic analysis, introduction of dynamic push-over curve and application. As a secondary goal, potential of this method in more complicated nonlinear domains is assessed. Application of ET and its advantages in PBD is shown with an example of SMF, NDCBF, CBF and BRBF building after modeling nonlinear interactional behavior of beams and columns, and also modeling buckling and... 

In this thesis, the nonlinear seismic behavior of typical Iranian building is evaluated. The evaluation results indicated that these building, which were designed, based on first edition of 2800 standard and before, due to ductility and stiffness deficiency, don’t satisfy the 3th edition of 2800 standard and FEMA356 requirements, then retrofit of the most old structures is recommended. Selected samples include ordinary moment resistant concrete frames. The effectiveness of three retrofit schemes, concentric steel bracing (CBF), Eccentric steel bracing (EBF) and buckling restrained bracing, are studied and compared. The results of investigation indicated that although CBF retrofit scheme is... 

The seismic performance of a low-rise nonductile reinforced concrete (RC) building rehabilitated using eccentric steel bracing is investigated. At first three-story building was Modeled using sap 2000 computer program then analyzed and getting the results. The effectiveness of the eccentric steel bracing in rehabilitating the building was examined. The effect of distributing the steel bracing over the height of the RC frame on the seismic performance of the rehabilitated building was studied. The behavior of the nonductile RC frame members is represented by a beam–column element capable of modeling the strength deterioration and the effect of the axial force on the yield moment and the... 

This thesis investigates the performance of concentrically braced frames through numerical simulation approaches. During a moderate to severe earthquake, the bracing members and their connections are expected to undergo significant inelastic deformations into the post-buckling range. Recently, provisions for special concentrically braced frames (SCBFs) were developed to exhibit stable and ductile behavior in the event of a major earthquake. Special Concentrically Braced Frames (SCBFs) are distinguished from Ordinary Braced Frames in that they possess improved post-buckling capacity. Performance based evaluation of SCBF system as well as ordinary concentrically braced frame (OCBF) system... 

General tall buildings consist of different types of structural systems, such as moment resisting frames, with and without shear walls, braces and outriggers. These types of structure have been widely analyzed by many researchers. However, only a few publications are available on the comparison of these structural systems with different stories. In this investigation five types of common structural systems including special moment resisting frame(SMRF), pin structure plus brace, pin structure plus shear wall, SMRF plus brace and SMRF plus shear wall are studied and by doing linear dynamic analysis, a criterion for both lateral displacement and steel tonnage of structures was presented.... 

The Overstrength, Ductility and the Response Modification Factor (R) for 70 steel X-braced , Inverted-V-Braced and Offcenter Bracing System frames were evaluated by performing pushover analysis of model structure with various stories (3,6,9,12&15 story) and span length (4,6&8 meter). Some result were compared with those from nonlinear dynamic analysis. According to the analysis result the R factors obtained from nonlinear dynamic analysis are 15% more than obtained from pushover analysis and R factors computed from pushover analysis were generally smaller than the values given in the design codes. R factors in Irregular structure in height are about 90% of R values in regular structure in... 

In this thesis, in order to evaluate the vulnerability of concrete structures equipped with Yielding Element (YE) against earthquakes, two single story/ span concrete frames with different bay-to-height ratios (B/H<1 and B/H>1) are considered. Since the nature of earthquakes is to release energy due to tectonic movements in the earth and the nature of damages in structures depends on the rate of absorbed energy, investigating the behavior of structures are based on the energy concept which is considered as the most important way to design structures against earthquake load. In order to provide seismic resistance against earthquakes and reduce the damages in main elements, it is required to... 

One of the most common failure modes of structures in earthquakes is soft story failure which causes by discontinuity of lateral force resisting elements like braces. so the ground floor acts as a soft floor. In this case columns are imposed to large deformation and plastic hinges are formed at top and bottom of the columns. This case is named as story mechanism (severe drift of the story). In most of this case building collapsed or going to collapse. To resolve this problem, different solutions are presented. One of the strategies under section (2-10) 2800 code is increase of designed load in columns of the soft floor. This measure cause to Significant increases in the sections of the... 

Observations made after previous earthquakes have shown the crucial role of irregularities in initiating damage in buildings with irregularity, either in Plan or in Elevation. Since in existing Building Codes, there is no provision for the distribution of the Equivalent Static Loads for such buildings, designers have no option but to resort to Dynamic Analysis. In this work, the seismic behaviour of a group of two-dimensional steel building structures with irregularity in elevation, namely Split-Level buildings, was studied. The studied structures consist of a series of 5- and 15-storey frames having two parts with splitting distance of 150 cm. Both, moment-resisting and braced frames were... 

In this Thesis Project the Behavior of Multi-story Steel Frames Equipped with Yielding elements has been investigated and analyzed. These elements act against severe earthquakes like fuse and cause dissipation in main part of input energy of structure. The position of these elements are at the cross point of braces. The most advantages of yielding elements in steel frames are ease to use and ability to replace after earthquake The main idea in this research is the effect of number of stories in buildings which are equipped with yielding elements.3,7 and 10-story steel frames with 3 bays and 15,20 and 25-story steel frames with 5 bays are the models for this investigation. At the first part... 

One of the most efficient methods in stiffening tall buildings to withstand lateral loads such as wind loading is to link the shear resisting core to the exterior columns using outrigger trusses. Many tall buildings such as Taipei 101 have benefited from outriggers. In this system, the outriggers force the perimeter frame to take part in resisting the bending moment of the lateral load acting on the structure by developing axial restraining forces in the windward and the leeward columns of the perimeter frame and forming a resisting moment.Most of the present analytical methods have studied the performance of outrigger braced structures neglecting shear deformation of the shear wall core and... 

Lateral forces on structures are transferred to braces as tensile and compressive loads. These loads enter the beams and columns through the gusset plates. The gusset connections to the beams and columns must be such that they be able to pass the loads through themselves. So far, five analytical models have been proposed for this connection and only one of them is recommended by AISC. These models estimate the existing forces and moments on the connection. The analytical models are: model 1 (KISS), model 2 (2A-AISC), model 3 (Thornton), model 4 (Ricker) and model 5 (Richard). Among these models, only model 3 is recommended by AISC because the others are more conservative. It should be noted... 

Considerable research work has been dedicated to the development of structural control systems to improve the performance of structures and reduce the casualties resulting from earthquakes. It is generally accepted that semi-active control systems result in better structural performance compared to what can be achieved using passive systems. However, it has been observed that the majority of the performance improvement obtained from semi-active control systems stems from the reduction of brace flexibility effects, rather than the control algorithm used. In this study, it is attempted to utilize this fact to improve the performance of passive control systems, or develop simple semi-active... 

In order to improve the performance of structures against earthquake loads, engineers believe that both minimizing the rigidity and maximizing the ductility; can lead to minimization of the energy input of the structure. However, the minimized energy shall be depreciated as much as possible in different stories so that the damage to the main and Irreplaceable structural members remains as low as possible.
Steel with high strength and ductility is relatively an ideal material for this purpose; however, the ductility level of this material is much higher than that is normally in use. In order to have a better utilization of the steel ductility, RBS (Ribbed Bracing System) was invented in... 

Offshore platforms are one of the most important structures in oil-rich countries, so that their economic benefit makes it very expensive to stop their usage. Therefore, it is essential that their performance is not disrupted by earthquake or the tidal waves made by severe storms. In the conventional bracing systems, it is assumed that the braces buckle under compressive forces; but Buckling Restrained Brace (BRB) does not buckle under pressure. In this study performance of BRB in jacket platform, with a case study of Resalat platform, has been investigated under earthquake load and powerful wave. The modeling of jacket equipped with this brace has been done in Abaqus software, and results... 

In this thesis, finite element (FE) models were developed to study hysteresis behavior of pipe braces subjected to cyclic loading. Thus steel braces previously tested by other investigators are used. The results of the analysis are in good agreement with the experiments. Besides, the FE analysis could also simulate both the overall and local buckling. As a result, a sharp growth in plastic strains is followed by geometric changes at the mid-span due to local buckling. The sharp growth of plastic strains is entitled as strain rises. In fact, the local buckling triggers rupture of braces at the mid-span. The local buckling initiation is identified by the sharp growth in plastic strains.... 

Controlling the structures under seismic excitations is a great challenge in the earthquake engineering. In this regard, bracing systems have been presented and applied to provide additional rigidity for the structures. However, conventional bracing systems are buckled under compressive forces. In this research ribbed bracing system (RBS) is proposed to deal with the buckling problem of conventional bracings. Full capacity of RBS is used under cyclic loading. Ribbed bracing system is constructed by adding a supplement part with a ribbed mechanism to the conventional bracing systems. As the supplement part is locked when the bracing is under tensile forces, the bracing system can absorb... 

By comparing the control systems, researcherstypically believe that the active systems of control have better performance than passive ones. However, the high costs and the required technology for their erection and maintenance have limited the use of active control systems. In order to improve the control system performance at lower costs, semi-active control systems were also considered. Their performance is based on creating the desired control force by changing the instrument characteristics.Moreover, recent researches have shown that the improved performance of these instruments in structures compared to passive control systems, are typically due to reducing the flexibility of the... 

In this study, an effective method is developed for design of passive control systems using viscous fluid dampers, considering the flexibility of braces used for installation of dampers. Using control design spectra, this approach provides a way to easily determine the optimum properties of the control system without complicated calculations or iterations.In the fist part of this study, it is attempted to find the optimum damping ratio of system by using the input energy spectra of ground motions. First, the relations among the dissipated energy in viscous dampers the strucutre and control system properties and structural response is evaluated.A theorically exact relation is established...