STRUCTURE AND STEEL
Year:2014 | Volume:10 | Issue:15|Papers Count:10

Pole assignment method is one of the algorithms for calculating the control force in active control of structures. One of the problems of control systems is time delay due to data processing and apply control force to structure that will reduce the efficiency of the control algorithm. In this study, the effects of time delay on pole assignment method for the control of structures, as well as how to reduce its effectiveness will be evaluated. In order to reduce or remove the effect of time delay, delay term will be considered in the original equation in the beginning of the affair. It is carried out by using the Taylor series, and importing the time delay term in the matrixes of the system. Thus, the equations of the structure are simplified and will be solved such as simple equations without time delay. The obtained equations are studied by using pole assignment method and the optimal control force is calculated.

In order to study the true behavior of connections in structures without a rigid diaphragm, such as industrial sheds, that the beams carry moments in addition to axial forces, the combination of bending and axial force should be considered substantially. In this paper, two different end-plate moment connections were designed with one based on the thick plate behavior, and the one based on the thin plate behavior. Then, the behavior of both connections subjected to the combination of bending and axial forces were examined and the interaction diagram was drawn. It was seen that the axial force can result in a mode shift and a consequential reduction in the bending capacity of the connection. The interaction diagrams of both samples were similar to each other, proposing the hypothesis that the general shape of the interaction diagram is not a function of connection dimensions. Next, the connections were subjected to cyclic loading and as it was predicted, the connections with thin plate were failed in more cycles compared to the ones with thick plates. It was also observed that the ultimate moment capacity, yielding moment, initial stiffness, and energy dissipation of connection all increased in presence of compressive force, and they were all decreased when subjected to tensile force.

In semi rigid shear walls, the shear wall won’t be connected to the main columns of the structure frame. Instead, it will be connected to the secondary columns which are used for tension field in plate. In this paper, post buckling behavior and ultimate strength of semi rigid steel shear walls with different thicknesses and different yield stresses have been compared according to different analyzing methods, such as plate-frame interaction model (PFI), strip model and finite element method. The results show a good compatibility between these methods. Results show that using low yield point steel (LYP) in semi rigid shear walls is appropriate. The influence of plate thickness increasement on initial stiffness and ultimate strength is more than the yield stress increasement effect.

The beam-to-column connections in most steel frames are assumed to be pinned or rigid. All structural connections have some specified values of flexibility under the loading. In this paper, the effect of flexibility of connections in buckling analyses of a plane steel frame is investigated. Accordingly, end-fixity factor used for taking into account the pinned, rigid, semi-rigid, and khorjini connections in the analyses. Furthermore, new technique for analysis of steel frames with khorjini joints is proposed. Based on the presented approach, all analytic equations of semi-rigid frames are developed for analysis of khorjin frames. In order to verify the presented method, several numerical examples are solved. Moreover, the stiffness matrix of a beam-column element, which can take into account the second-order effects and flexibility of connections, is derived. This matrix is very general and can be applied to a frame member with any type of connection. The new formula is more accurate and efficient than the corresponding matrices obtained by other authors. By assembling the stiffness matrix of frame and calculating the minimum positive eigenvalue of this matrix, buckling load of the frame is obtained. To demonstrate the validity and efficiency of proposed approach, several numerical examples are solved. Results show that the response of khorjini frame is close to the one with the semi-rigid joints. The buckling loads of the steel frame with khorjini connections are close to the one with the semi-rigid joints.

In seismic vulnerability assessment of the structures, definition of a damage index is essential. The purpose of this paper is to define a new index called the index of "remaining useful life of the building" and determine its application in the study of seismic vulnerability asses of the structures. The remaining useful life index can apply for Prioritize of strengthening of a series of buildings. It can be also used in the insurance industry as an indicator to understand the status of buildings. In this study, three bracing steel structures with 4, 7 and 10 floors was selected and their remaining useful life was calculated and compared with the proposed useful life proposed by codes (50 years). The results show that this method can easily express the vulnerability assessment of the buildings.

Concentrically braced frames are among economical lateral resisting systems. However, due to brace buckling in severe earthquakes, these systems present a poor ductility and do not show appropriate seismic performance. One approach is to use buckling restrained Braces instead of ordinary braces to preclude buckling in compressive load. These braces consist of a ductile steel core, confined by a steel sleeve or a combined of steel sleeve and concrete filler, that tolerates the entire axial load of the brace and provides flexural, and buckling resistance. Another approach that is introduced recently is to add a Cast Steel Yielding Connector to the end of the braces. In addition to replaceability, this connector can absorb almost all the seismic energy and prevent compressive buckling that ensures tensile yielding in braces. Therefore, the yielding bracing system presents a stable hysteresis behavior. Although these braces are successfully fabricated and investigated experimentally, their seismic performance requires further research. This research investigates seismic performance of BRB frames and Yielding brace systems. For this reason, two groups of BRB and YBS frames are designed including 3, 5, 7, 10 and 12 story frames with chevron bracing configuration. Static nonlinear (pushover) and incremental dynamic analyses are carried out. Nonlinear program “OpenSEES” is employed for analyses. Finally ductility and over strength factor of frames are obtained. Also, seismic fragility curves of frames in immediate occupancy and global instability limit states are calculated and compared. According to the analysis results, BRB frames present higher ductility factor while YBS frames show larger values of over strength factor. Also, yielding brace system promotes the seismic performance of the buildings in immediate occupancy (IO) limit state. Utilization of these frames also can prevent developing soft story effect in short to medium buildings.

Y-shape bracing is one of the common bracing systems in providing appropriate stiffness against lateral loads which is used in design or seismic rehabilitation of structures. This type of bracing system has some architectural advantages in comparison to most of the concentric braced frames; on the other hand, these braced frames have relatively low lateral stiffness with the potential of out of plane buckling; so this paper aims at reducing these weak points by introducing an energy absorbing element. The proposed element consists of a box and a ring which is placed near the connection of one of bracing elements to the gusset plate. This ductile element is primarily studied in commercial finite element software. By changing in its dimensions and surveying the resulted behavior, appropriate dimensions and their related nonlinear behavior is extracted and applied in other studies. Using these results, the presented element is placed in some two-dimensional y-shape braced frames and the improvement in the behavior of the bracing system under near source earthquakes is investigated through nonlinear dynamic analysis. The results show the appropriate effect of the proposed ductile element in seismic behavior improvement of the y-shape bracing.

Convergent braces are usually used for stability of frames against lateral forces in seismic design. According to the seismic design philosophy, it's expected that convergent bracing systems could show a non-elastic and steady response under intense earthquakes. In such frames, braces are connected to beams and columns by a plate and non-elastic deformation occurs at the time of yield of the member by tensile and non-elastic buckling. The new experimental tests show that the seismic performance of convergent braces improves by connection plate design and permitting connection plate to yielding.In this research, the connection plate effects and its changes and also the inelastic finite element model and analysis methods are investigated and compared with experimental results. Important details in frames connection are including the effect of brace plate in rigidity of the column-beam connection, shape and thickness of connection plate and analysis of non-elastic deformations. The proposed results will lead to improve the shape and details of this type of connections.