نشریه سازه و فولاد
سال هجدهم شماره 43 (بهار 1403)

With the advancement of the construction industry, the use of high-strength steel has gradually increased. High-strength steels can withstand higher stresses compared to conventional steels, yet they exhibit less ductility. Currently, due to their high cost, they are primarily utilized in specialized industries. Connecting two steel pieces requires the use of welding or bolts. Fillet welding is the simplest and most commonly used type of welding. Therefore, investigating this type of weld in high-strength steel is of great importance. Fillet welds can also have different angles in the direction of applied force, which itself affects their strength. In this study, the influence of the welding angle on the axis of the applied force has been examined. First, an experimental model, considering the effect of defects in the weld, was modeled and validated using ABAQUS software. Subsequently, specimens were analyzed at various angles. The maximum strength of the weld and its ductility were investigated. Additionally, the recommended strength of the American steel code for welds at different angles was examined. The results showed a 14% average difference from the code. Unlike mild steels, an increase in the welding axis angle with the direction of the applied force increased the ductility of the fillet weld.

tuned mass dampers are a combination of mass, spring and dampers. tuned mass dampers are set on a specific frequency range, when the structure is subjected to external force in the desired frequency range, TMD reduce the responses of the structure by creating a force against the movement direction of the structure. The most important limitation of TMD is providing a sufficient mass ratio in order to optimally control the structure. The inerter element is an element with two terminals that can produce a force proportional to the acceleration difference created in its two terminals. One of the distinguishing features of this element compared to other structural control tools is the possibility of changing the mass matrix of the structure. Nowadays, extensive research is being done in order to combine the inerter element with structural control systems. Adding an inerter element to conventional tuned mass dampers significantly increases the efficiency of TMD and removes the limitation of TMD to provide a suitable mass ratio. The placement position of the inerter element in the control system is very important. If the inerter element is placed between two inappropriate levels in the system, it will disrupt the function of absorbing dynamic vibrations and cause the responses to intensify. In this research, the optimization of the parameters of the inerter element combination with tuned mass dampers is done using the particle optimization algorithm and with the ability to generalize to all structures

One of the practical and useful methods that is used in Eurocode 3 to analyze the bending behavior of connections is the component method. Based on this method, the moment-rotation curves of joints are estimated bilinearly based on the two parameters of bending strength and initial stiffness. Therefore, one of the most important parameters to evaluate the moment-rotation curve using the component method is to find the exact initial stiffness of the connection (Sj,ini ). An issue that is not mentioned in the Eurocode 3 and causes the obtained results to be insufficiently accurate is the issue of pre-tensioning bolts. The main goal of this research is to add the effect of prestressing bolts to the component method. For this purpose, the stiffness of the spring elements modeling the column flange in bending, the end plate in bending, and the bolt in tension, which are necessary to predict the rotational stiffness of connections with the end plate with prestressed bolts, should be investigated. For this purpose, first, relationships are presented to consider the effect of prestressing bolts in the component method. Then, several end plate connections that have been investigated by the researchers are evaluated in two cases without considering the effect of pre-tensioning of the bolts and considering this effect in order to verify the correctness of the presented relationships. The results show that the presented method for considering the prestressing of bolts is a suitable and efficient method, which makes the accuracy of the developed component method more accurate than the conventional component method. So that the rotational stiffness difference of the laboratory model with the component method without considering the effect of pre-tensioning of the bolts was reduced to -13.81% and by considering the effect of pre-tensioning of the bolts to -2.41%.

In the process of implementing steel structures with bolted connections, the bolts are temporarily tight by the workers and so-called tightened, in this article, called the initial stage. After performing the connections of several different parts in the form of primary fastening, in the following stages of construction, the bolts of connection are pre-tensioned so that the minimum pre-tensioning force of the regulations is created in them. Therefore, in the process of the construction of steel structures with pre-qualified connections of the steel moment end plate frame, the effects of the construction sequence are important, because during the construction, due to the different construction situations of the project implementation conditions, delays In the secondary fastening of the connections, the lack of completion of the lateral restraint system of the structure and on the other hand the possibility of earthquakes with different magnitudes can lead to local or general damage to the structure and ultimately lead to the imposition of exorbitant costs on the project. In this research, the acceleration records proposed by FEMA-P695 (ATC-63) were used with the nature far field, and then the vulnerability of the BUEEP connection was investigated using the fragility curves. Without implementing the secondary fastener in the structure, the connections have a semi-rigid behavior and the discussion of damping and reducing the seismic demand caused by it is more noticeable. This trend was more visible at lower accelerations. In general, in slight and medium performance levels, the effect of not using the secondary fastener is greater due to the slack of the connections and the need for lower drifts to reach this level of performance.

Tuned mass dampers (TMDs) are a combination of mass, spring, and damper that are added to a structure to reduce its response to lateral excitation, such as wind forces and earthquake excitation, especially in the displacement range. Due to the very rich frequency content of earthquake excitation, the use of TMDs in steel moment frames does not necessarily improve the structural behavior under all earthquake excitations. Exclamation The time-history method is a new nonlinear dynamic method for estimating the behavior of structures under earthquake excitation. In this analysis method, the structure is subjected to an artificial incremental excitation. The results of the nonlinear time history analysis show that the TMD reduces the average maximum roof displacement under selected ground motion records. The performance of the TMD also improves with increasing mass ratio and increasing building height. Adding a TMD to steel moment frames reduces the average maximum roof acceleration under selected ground motion records and also reduces the average maximum base shear of the building. The time-history method estimation of the maximum roof displacements of the structure at a hazard level equivalent to a 475-year return period earthquake is also 10% to 20% less than the nonlinear time-history method. Exclamation The time-history method estimation of the maximum roof acceleration of the structure at a hazard level equivalent to a 475-year return period earthquake is also 10% to 20% more than the nonlinear time-history method. Exclamation The time-history method estimation of the maximum base shear of the structure at the considered hazard level is also 5% to 15% more than the nonlinear time-history method.

The experience of past earthquakes and the weak seismic performance of some structures have highlighted the need for modern seismic systems. Therefore, further examination of lateral seismic systems, such as energy dissipation dampers, is essential for improving the seismic performance of existing systems. Energy dampers can generally be divided into two categories: velocity-based dampers, such as viscous dampers, and displacement-based dampers, such as yielding and friction dampers. Friction dampers are one of the most effective energy dissipating devices in structures, capable of dissipating seismic energy through friction without causing damage (yielding). This paper studies a new system combining a knee-braced frame (KBF) with a friction damper (KBFD). The damper used is a linear friction damper with a slotted bolted connection. The goal of incorporating the friction damper in this configuration is to help prevent out-of-plane buckling of the brace equipped with the damper and to maximize the capacity of damper to improve the seismic performance of the KBF. In this paper, the finite element model of the KBFD was first validated using experimental results. Then, six types of KBFD frames, with varying pre-tension forces in the friction damper considered as a variable parameter, were studied. These studies included performing pushover analysis to calculate the seismic parameters of the structure, such as ductility and behavior factors. The results of these analyses indicate an improvement in the seismic performance of the KBFD compared to the KBF, with both the ductility and behavior factors increasing. Based on the results, with increasing pre-tension force, the behavior factor also increases and is suggested to range between 7.9 and 9.0. Additionally, these frames were subjected to cyclic loading to further study their seismic behavior. The results of the frame analyses were reported as hysteresis curves, initial and secant stiffness, frame resistance, ductility, behavior factor, dissipated energy, and equivalent viscous damping. The results demonstrate that the friction damper effectively reduces the seismic demand on the braces and columns. Furthermore, the obtained hysteresis curves show a high level of input earthquake energy dissipation and appropriate seismic performance of the frames with the proposed configuration.