نشریه سازه و فولاد
سال چهاردهم شماره 30 (زمستان 1399)

The damage distribution of the steel moment-resisting frame (SMRF) structures is greatly a function of the pulse period of earthquake. In addition of frequency content of earthquake, the deformation demand distribution pattern depends on the seismic intensity more. In this study, the effect of pulse period of near fault earthquakes on the damage index as inter-story drift demand distribution of SMRFs with special ductility is investigated for different performance levels, including Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) limit states. For this purpose, the seismic response parameters of three SMRFs with special ductility and 3, 9 and 15-story under the effect of 30 near fault earthquake records with different frequency characteristics are evaluated through the implementation of Incremental Dynamic Analysis (IDA). To study the effect of pulse period, the pulse-like near fault records are categorized into three groups of short, medium, and long-period records. Moreover, two numerical variables are defined to quantitatively describe the seismic demand distribution patterns over the height of the sample frames. Based on the results of this study, it is found that the concentration of the damage distribution in the SMRFs for highly nonlinear state of the seismic response, transfers to the lower stories. However, for elastic and near-elastic zones, the short-period pulse-like excitations transmit the seismic demands toward the upper and mid stories.

Due to the abundant advantages, such as ease of implementation, economic efficiency, and appropriate ductility T-shaped elements are widely used in many beam-column connections.  In this research, the through bolts were utilized to connect the Tee profile to the column flange. The direct transfer of stresses to the body of the column increases the length of the intermediate bolts under cyclic loads. Therefore, in an innovative effort, the external diaphragm of the column was used together with the through bolt. Finally, the joint performance of them was examined.
 In this study, based on the validated models in Abacus software, two samples were designed and tested in the laboratory. One of these samples contained a connection with through bolts and, another one had an external diaphragm. The overall performance of both joints was quite good and, they passed the acceptance criteria of rigid joints in the especial bending frames according to AISC and FEMA codes. Both laboratory samples achieved a relative rotation of 0.06 radians without declining resistance. The final failure in the sample without the external diaphragm occurred in the first cycle relative rotation of 0.07 radians and with the separation of the nut from the through bolts, while this issue in the sample with an external diaphragm is ductile and deformable in the flange and the web.

The maximum displacement at the buildingschr(chr('39')39chr('39')) isolated level is an important issue for designing an economical and practical isolation system. Due to various reasons such as uncertainties in the design, changes in estimated seismic characteristics, and changes in design codes, the considered gap size around the existing isolated structures may not be sufficient for having free movement during an earthquake. Therefore, it is necessary to reduce the isolated level displacements by using an efficient method to prevent the possibility of structural pounding. The purpose of this paper is to use a smart control system in the isolated level with the optimized fuzzy control algorithm to reduce based-level displacements by considering different gap sizes around the structure. In this research, the Magnetorheological damper with the optimized fuzzy logic algorithm is used at a based level of isolated structure with high damping isolators. The results show that the control system can improve the superstructurechr(chr('39')39chr('39'))s seismic performance and prevent poundings. Also, the dispersion and average of seismic responses of the superstructure in the probability density function of responses are reduced compared to structures without smart control systems.

In this study, to develop the Iranian loading protocol in the region, the detailed seismic, numerical, statistical, finite element and laboratory studies were conducted according to the region’s conditions. Finally, a loading protocol was proposed that provided a proper simulation of the region’s seismic demand. It also complied with the construction and design conditions of structures in the region and can apply to structural moment components, especially moment connections in the laboratory conditions. It is noteworthy that the region’s proposed loading protocol provided some criteria for evaluating the performance of moment connections in the target values level, qualifying conditions, and collapse threshold control of moment connections. To achieve the objectives of this study and to evaluate and match the proposed loading protocol’s performance conditions, two pairs of identical specimens of WUF-W and RBS welded moment connections were examined and evaluated in the laboratory subjected to the proposed loading protocol of the region and SAC loading protocol. It should be noted in the laboratory and numerical studies that the damage parameters and applied demand resulting from both loading protocols were evaluated and compared while matching and controlling the conditions and criteria presented in the proposed loading protocol of the region. It is noteworthy that the present study’s welded moment connections well satisfied the performance conditions and criteria of the proposed and SAC loading protocols.

The eccentrically braced frame has more ductility in comparison with concentrically braced frame and more lateral stiffness in comparison with moment resisting frame and involves the advantages of both moment frame and concentrically braced frame. Because of these reasons, this system has gained popularity in the last decades. I shaped and box-shaped beams are two usual link beams. In cases that lateral braces are hard to provide at both ends of the link beam, box link beam has some advantages comparing to the I-shaped beam because of its high torsional stiffness. Using transverse stiffeners in Box link beams, delays local buckling and leads to higher buckling stability and energy dissipation in links but increases the cost of construction and decreases the velocity of execution. In this research, low yield point steel, as a new material in seismic design of steel structures, has been used as web of link beam in order to enhance local buckling stability of links and eliminate intermediate stiffeners. Using low yield point steel in the web of link beam makes it possible to increase links web thickness and lowers the compactness ratio of the web as a result. Because of low yield stress, high ductility, and strain hardening, low yield point steel prevents buckling and fracture and increases energy dissipation of structure but using of low yield point steel as link web, leads to the about triple value of over-strength in comparison with usual steels that should be considered in the design of force-controlled members based on link beam capacity. According to this research, using Low yield point steel with a compactness ratio less than 16.5 and without intermediate stiffeners as link web, eliminate local shear buckling. Also using Low Yield Point steel in link beam may lead to a higher response modification factor.

Lateral torsional buckling is one of the important criterion for determining the flexural strength in steel beam design. probability of it increases when laterally unbraced length is long. On one hand, lateral bracing should have sufficient stiffness to provide lateral support for compression flange of beam. On the other hand, this support must has enough strength. Composite beams consist of reinforced concrete slab and steel beam. To join the reinforced concrete and steel beam, stud has been used to transfer the horizontal force from concrete to steel beams. In fact, it plays role of lateral bracing for top flange.  Bottom flange is also under pressure in the case of double curvature bending and there is probability of lateral torsional buckling. Common methods of beam lateral bracing are connecting two adjacent beams to each other and connectig bottom flange to concrete slab. In this study a new detail has been introduced for lateral bracing of the beam-slab composite system. In this study, bottom flange is connected to concrete slab by a pair of transverse web stiffeners and its rotation is prevented by a pair of transverse studs. Finite element analysis results show that this detail can prevent  bottom flange from lateral torsional buckling.