نشریه سازه و فولاد
سال هفدهم شماره 42 (زمستان 1402)

In regular thick column base connections, because of severe pinching effect in the hysteresis diagram during loading and unloading, the stiffness and strength of the connection and the amount of energy consumption are reduced. In order to prevent pinching and improve the seismic performance of the column-base connections, it is suggested to connect the anchor rods vertically under the plates by wedeled connections without any eccentricity between anchor rods and the flange axis of the column. In the proposed column-base connections, because of the non-replaceability or repairability of the connection components, according to the design codes, all connection components, including the base plate, welded connections, anchor rods, and concrete, must remain elastic and the source of ductility should be inside the plastic hinge in the end section of the column. In this case, the anchor rods have the ability to carry tensile and compressive forces at any moment simultaneously. Because of the unknown behavior of welded connections under tensile loads, T-shaped connections were used to evaluate the strength and stiffness of special fillet welds, full penetration groove welds, cylindrical groove welds and conical groove welds. Digital images Correlation method was used to measure the strain field of welded connections and identify the type of failure of the anchor rods. As a result, the welded connections had the ability to withstand the maximum tensile capacity of the anchor rods and remained elastic. During the loading of the anchor rods, no failures occurred in the elastic region of the rods due to the reduction of resistance in the Heat Affected Zone (HAZ). As a result, there is no uncertainty in the behavior of welded connections under tensile load as a force-control component in the proposed connections. Fillet weld and full penetration groove welds were more superior in terms of cost than other welds.

Concrete-filled tube (CFT) columns are a favorable and economical option for designing mid-height and tall buildings. However, using internal continuity plates in conventional moment frames will weaken the panel zone since infill concrete is present inside the steel tube column. Due to perforation of the continuity plate, stress distribution in this plate changes, and its strength and stiffness decrease. The proposed method in this study is aimed to strengthen the continuity plates with shear stud so as to transfer a part of the high force of continuity plates to the concrete. In this regard, two different full-scale specimens of rigid connections of I-shaped beam to CFT column were tested under cyclic loading. In the other part, using the finite element method in Abaqus software, the experimental specimens were modeled and analyzed. As a result of the stiffening of the continuity plates, the connection's bending capacity and cumulative dissipated energy increased by 7% and 4%, respectively. Additionally, the stiffening moved the highest rupture index by approximately 20% of the beam flange width from the outer edge of the flanges of the beam to the longitudinal axis of the beam. The proposed connection was approved for use in seismic zones' intermediate moment frames.

In recent years, special truss moment frames (STMFs) have been widely used as a relatively new steel frame system for seismic hazard zones. These frames dissipate seismic energy through special ductile segments embedded near the middle of the span of the truss beam. In this study, the seismic behavior of STMFs with Truss columns was investigated by nonlinear static analysis (pushover), cyclic static analysis, and time history analysis and compared with the common STMFs reported in the literature. To achieve this goal, two 2D models with the same characteristics were converted into a nonlinear model based on seismic design criteria, design and with the help of the concentrated plasticity method. the results showed that STMFs with Truss columns have a better seismic performance than STMFs with filled-web columns. Compared to ordinary STMFs, ductility and energy dissipation were enhanced, hinges were more distributed at higher performance levels, and drifts were more evenly and uniformly distributed on stories for STMFs with Truss columns. In addition, STMFs were more affordable due to their lower weight and can be used to install pipes through Web columns.

Near field ground motions with forward directivity and fling step effects, can impose high seismic energy to the building. In this paper, the seismic performance of symmetrical steel of 5-, 10- and 20-story buildings with dual lateral load resisting system, in both torsionally-stiff (TS) and torsionally-flexible (TF) archetype buildings has been investigated. For this purpose, the buildings are subjected to nonlinear time history analysis, which are performed with applying two horizontal components of three sets of far field (FF), non-pulse (NP) and with forward directivity pulse (FD) records. The results are presented in the form of peak floor displacement, inter story drift ratio, story shear as well as ductility demands of braces, beams and columns for three categories of earthquake records. The results show that in torsionally-stiff and torsionally-flexible buildings of 5- and 10-story under the FD records, the maximum occures in the lower stories and the first mode is predominant. Also, for the 20-story torsionally-stiff building, the maximum demands occurs under the FD records in the lower stories, which the maximum difference between FD with NP and FF earthquake records, for the story drift ratios are 115% and 79%, respectively, and for the ductility of the columns, they are 894% and 798%, respectively. Also, contribution of higher modes of FF records for 20-storey buildings are greater than the FD and NP records; therefore, the maximum demands in the upper stories occurs under the FF records.

Using shape memory alloys as a novel tool due to their self-centering property to reduce damage and control seismic behavior of structures can be effective. Considering that the occurrence of aftershocks can always cause cumulative damage in structures, it is important to investigate the performance of the above systems under aftershocks. The present study evaluates a four-story steel frame in the state without braces and with convergent braces equipped with shape memory alloy dampers. To investigate the effect of aftershocks on the behavior of the two frames and perform incremental dynamic analysis, the OpenSees software was used. For this purpose, 29 records related to the main earthquake and the actual recorded aftershock were used. To investigate the response of the structures, first, the main earthquake was scaled to bring the structure to the collapse level, and then the aftershock was applied to the frames considering three levels of drift structure (0.01, 0.025, and 0.04) under the main earthquake. The results show that the braced structure equipped with a damper, compared to the structure without braces, experiences 13% more acceleration, 30% less residual drift, and 21% less relative drift after experiencing 0.04 drift caused by the main earthquake. Additionally, in both structures, more destructive effects are observed when they are under aftershock

The direct connection of column splices using partial joint penetration groove welds with a penetration percentage between 40% - 60% of the thickness of the column section was common until the Northridge and Kobe earthquakes. With the poor performance of beam-to-column welding connections after the Northridge earthquake and column splice welding connections after the Kobe earthquake, new design details and executive instructions were added to the regulations and the use of partial joint penetration (PJP) welding in moment frame structures was prohibited and designers were required to use complete joint penetration (CJP) welding (AISC 341-10). With experimental and numerical studies conducted on PJP welding with a higher penetration percentage in I-shaped sections, the possibility of using PJP welds with a penetration percentage of 85% was confirmed in the regulations (AISC 341-16). All previous studies were conducted on the behavior of PJP groove welds in I-shaped sections. in this study, a 3D finite element numerical model was simulated in Abaqus software in order to investigate the behavior of the column splice and the failure and spread of damage in the Heat Affected Zone (HAZ). By ensuring the proper performance of the finite element model, 33 box-shaped column splices have been modeled, and the effects of parameters such as penetration weld percentage, column thickness and cross-sectional dimensions on the column splice behavior has been evaluated.