نشریه سازه و فولاد
سال هفتم شماره 13 (بهار و تابستان 1392)

Generally, there are two methods for confrontation with earthquake force. The first method is to increase the seismic capacity of structure, for example, using lateral resistant systems such as moment frames, braced frames, shear walls and etc. The second method is to reduce the seismic requirement of structure using seismic base isolator systems. This paper, studies the effect of increase of period (as a result of reduction of the effective stiffness of seismic isolator) of seismic base isolator system of LRB type on the seismic response of threedimensional steel structural models with concentrically braced frame system. For this purpose, three, six and ten story buildings have been modeled in both states with fixed base and isolated base, along with different period and effective stiffness of isolator system in SAP2000 Software and have been investigated under dynamic spectral analysis (supposing that UBC97 code spectrum based on CAD and CVD). These buildings located in Zone 3 (seismic zone coefficient equal to Z= 0.3) and on Seismic Source Type “B” and SD soil type. The results of analysis, including period, modal participating mass coefficient, stories shear and base share, have been investigated and compared.

Beam-to-column connections have been found to be of great significance in influencing structural behavior at ambient and elevated temperatures. When steel-framed structures are subjected to fire, the load bearing capacity is decreased and the behavior of the joints is of particular concern. Observations from full scale fire tests and damaged structures confirm that connections have a considerable effect on the stability time of structural components in fire. Due to the high cost of elevated temperature tests, adequate experimental data about a broad range of connections is not available. One type of such connections is the connections between I-shape beam and pipe shape columns in oil platform decks. Considering the high probability of fire in oil platforms, study of the behavior of these connections at elevated temperatures is of great importance. In the current study, a number of experimental tests have been conducted on this connection type to investigate their flexural bahaviour and in particular the bending stiffness of the connection in fire. Effects of the temperature rise on the rotational response of the specimens are studied and the results are presented as temperature stiffness curves.

Today's the most important purpose in structural designing is to control structural seismic demand against lateral forces in global surface and local surface. FEMA273 attributes the majority of deformation demands to behaviors surfaces whether there is no rule about local cumulative deformation demands of elements. On the other side cumulative deformation demands parameters have suggested criteria for damaging rate so it can exhibit a different vision of structures reflex about acquired reflex of maximum deformation demands. Comprehensive behavior of structures system depends on answer and manner of structural surfaces elements and influence of elements reflexes. Because of neglecting of scientist about brace changing in structures height so there is a consideration about steel structure seismic behavior with brace changing in height in the article. A number of moment steel frames braced by different bays, time history non-linear dynamic are analyzed by the 3 earthquake and consideration of max drift story demand in structures suitable height for chagrining the kind of braces was suggested. Changing the kind of braces in certain height of structures can reduce the structural local seismic demand.

Performance of U-shaped metallic-yielding dampers has been studied using finite element method in steel frames. Dampers were added to the 3, 5 and 10 story building frames together with inverted V-braces. Nonlinear time history analyses were applied to models under four different earthquake ground motions. Models were used considering not only dampers nonlinear properties but also steel frames components nonlinear properties such as beams and columns through assigning plastic hinges. The results show that by adding the U-shaped dampers to moment resisting frames, nonlinear behavior is transferred from frame members to the damper system. Therefore, structural damages are reduced or removed. According to numerical results obtained from analyses of the frame models, adding the U-shaped dampers decreases maximum base shear of structure and the stories maximum story drifts by cutting down about 28 percent and 48 percent respectively.

Nowadays, the earthquakes records have been severally used in order to analysis and design of structures. Nevertheless, there are no registered earthquake records for each region regarding its conditions except some regions which have appropriate recorded accelerograms. In other regions, the artificial accelerograms can be used in order to analysis. Transform wavelet is one of the powerful methods which has been used to production of artificial accelerograms according to target spectrum. In the present study, in order to investigate the mentioned method, some of the registered records in different areas, related to far filled earthquakes, has been coincident to target responded spectrum. Iran’s designing spectrum based on the 2800 constitution related to the ground of type III, has been used for spectrum adaptation. In order to investigate the effect of initial records and synthetic generated records, steel frames containing 5, 10 and 15 floors have been evaluated. The results indicate that the wavelet method has a good performance for spectrum adaptation, and mismatch error reduces to about 5% and it produces accelerograms with different characteristics compared with the primary records. Also, the amounts of constituent response in the produced artificial records have been increased considerably compared to primary records.

Steel deck composite slabs are widely used for composite floor systems. In this slab, steel deck remains permanently in place as an integral part of the floor system, and the hardened concrete slab acts compositely with the profiled steel deck. Composite slabs lead to a notable reduction in the structure weight, the construction cost, time and have enough strength. Steel decks perform two functions: they act as formwork while concreting is done and as tension reinforcement bars. The concrete, distributes loads and protect the steel deck from fire. In this study, two composite frames with and without steel deck were modeled using the finite element software, ABAQUS and nonlinear dynamic analysis was done and their seismic behavior such as period, lateral displacement, base shear, plastic energy dissipation of the frames and crack formation procedure in concrete slab were investigated. Results showed that using steel deck in composite slab, reduced period and maximum lateral displacement of the frame but base shear and plastic energy dissipation of frames increased. Also plastic hinge formation procedure of two frames was the same.

About application of Tuned mass dampers (TMDs) on linear structures extensive researches have been done, while studies about TMD application on nonlinear structures are limited and have been based on some simplified assumptions on structure behavior or TMD design procedure. Hence, in this paper it has been decided to study the modeling of TMD on nonlinear structures using the real behavior for structure, and also to assess its effectiveness in improving the seismic behavior of structures. To this end, 3,9 and 20 stories two dimensional steel frames equipped with TMD have been analyzed using OpenSees software under both near-field and far-field earthquakes. For different values of TMD mass ratio, the reduction in controlled structures response such as maximum drift, root-mean-square (RMS) of drift and hysteretic energy has been determined. Results show OpenSees software high ability in modeling the nonlinear structure-TMD system. Based on numerical simulations, it has been found that the effectiveness of TMD in reducing the damage of nonlinear steel frames depends on earthquake characteristics such as frequency content and peak ground acceleration (PGA) as well as TMD mass ratio. Also in this paper a method based on sensitivity analysis has been proposed to determine the parameters of TMD to achieve the better performance in utilizing TMDs on nonlinear structures.