Column-tree Connections Research Articles

Cyclic Behavior of the Column-Tree Moment Connection with Weakened Plates: A Numerical Approach

The use of column-tree connections is common in controlled shop environments due to their cost-effectiveness in achieving ductile welds. Field bolts are also easy to install and inspect. However, there is currently no prequalification available for these connections, their performance is not fully understood, and the cost of aftermath repairs is still a major concern for owners. In this research, analytical and numerical studies were performed to assess the cyclic behavior considering the effects of the bolted splice location, bolt slippage, and splice plate thickness. Fourteen numerical models using the finite element method in ANSYS software were analyzed to evaluate the nonlinear behavior of moment connection configurations in terms of the strength, stiffness, ductility, energy dissipation, and overall cyclic response. The results showed that appropriately proportioned bolted splice connections can meet the requirements for prequalified moment connections. The models complied with the criteria established in AISC 358 and achieved flexural resistance that was higher than 80% of the beam plastic moment at 4% of the interstory drift ratio. The weakened plates concentrated the inelastic action, which allowed us to prevent the brittle behavior and damage to the column, welding, and other components of the moment connection. Complex geometries or specially fabricated parts were not required, providing a cost-effective way to control seismic-related damage. Also, required repairs are based on the replacement of standard parts, reducing operational detentions in facilities. Finally, the moment connection studied is classified as partially restrained (PR) according to the requirements established in AISC 360.

Seismic performance of earthquake-resilient prefabricated corrugated web beam-column connection with buckling-restrained plates

This paper presents a developed prefabricated column-tree connection with buckling-restrained plates, which is suitable for conducting post-earthquake repairs. The core member of the buckling-restrained plates is made of low-yield point steel, which can yield repeatedly to concentrate damage and thus protect the main members of the beam and column under seismic loads. The structural behavior of the developed connection was numerically elucidated. The seismic design requirements, methods, and procedures of the developed connection were discussed in detail. A parametric study was conducted to investigate the effects of some parameters — e.g., the form of the bolt holes, the width, thickness, and length of the core member of buckling-restrained plates, and the gap between the restrained plate and beam flange — on the connection behavior. The hysteretic or skeleton curves, proportion of energy consumed in each component, and equivalent plastic strain contours were presented and compared. The results indicate that the developed connection has good ductility and excellent energy dissipation capacity. The damage can be concentrated on the core member of the buckling-restrained plates to protect the main members without a decrease in bearing capacity. The parameters of the core member of the buckling-restrained plates are recommended for design.

Numerical study on prequalification and cyclic performance of column-tree connections

Column-tree connections are commonly employed as a shop-welded and field bolted connection in the steel moment resisting frames to ease installation and inspection. The prequalification and cyclic performance of these connections are not completely clear yet. In the present study, using nonlinear finite element analysis by ABAQUS FEA software, the effects of bolted splice location and design method on the prequalification and cyclic behavior of the column-tree connection are investigated. The numerical models are validated based on experimental test results. The numerical analysis results show that the column-tree connection prequalification depends on the stub beam length and bolted splice design method; moreover, the bolted splice configuration influences the monotonic and cyclic behavior, strength, bolts pre-tension loss, stiffness, fracture tendency, ductility, and energy dissipation characteristics of the connection. Also, it is observed that based on the moment strength and rotational stiffness of the bolted splice, the column-tree connection is classified as a rigid or semi-rigid moment connection. This finding needs to be taken into account in the column-tree moment frame design.

Seismic Performance of Column-Tree Moment Connections with Slit Dampers

To fully develop the plastic deformation on the replaceable members of beam-column connections in steel frame, this paper proposes a kind of prefabricated earthquake-resilient column-tree moment connections with slit dampers. A numerical analysis was conducted to investigate the structural behavior of the proposed connection. The finite element methods were validated through comparing the numerical results with test results. The results of numerical analysis showed that the column-tree connections with slit dampers could effectively move the beam plastic hinge to the slit dampers with the main components basically being in the elastic phase. The slit width and length was the main influence parameters, and the slit numbers and the gap between the slits had almost no influence on the hysteresis behavior of column-tree connections. The design theory of the slit dampers were developed to ensure that the slit dampers yield or stability failure before the yield of the stub beam and the common beam, so as to achieve the purpose of earthquake-resilience.

Seismic fragility analysis of steel intermediate moment frames with column-tree connections

Steel intermediate moment frames (SIMFs) with column-tree connections have an advantage in that they can ensure quality control and decrease construction cost and time by the factory welding of stub beams to columns and field bolting at the beam splice. Furthermore, the amount of steel can be reduced by distributing the seismic lateral load to the entire frame, for which beam-to-column joints are fully restrained. Despite such merits, concerns about slip at the beam splice and concentrated deformation in the weak panel zone also exist. Therefore, the influence of beam splice slip and weak panel zone deformation on the seismic performance of the SIMF with column-tree connections was examined numerically by seismic fragility assessment. For comparison, the seismic fragility of a SIMF with conventional field-welded connections was also examined. The result showed that the beams and panel zones share energy absorption in the SIMF with column-tree connections, even though the panel zones are weaker than the beams, because the beam splice slip can reduce the concentration of deformation on the weak panel zone. The collapse probability of a SIMF with column-tree connections is expected to be reduced to a value as low as a SIMF with conventional connections if the quality of test data and modeling are improved.

Seismic analysis of steel moment frames with column-tree connections

Column-tree connections are commonly used for steel moment frame buildings in East Asian countries because they can reduce labor and construction time. However, a column-tree connection has an inherent issue of splice slip between the stub beams and the mid-portion of the beam. In this study, analytical modeling to account for slip behavior of the column-tree connection is proposed and examined by comparing sub-assemblage test results and then applied to a nonlinear dynamic analysis of steel intermediate moment frame (SIMF) buildings. In the nonlinear dynamic analyses, two example buildings that use welded unreinforced flange-welded web connections and column-tree type connections, respectively, were designed, and their responses were compared. Comparison of the analysis results to sub-assemblage test results indicate that a macro model based on the lumped plasticity assumption can adequately simulate the behavior observed in test results. The distribution of plastic deformations among the beam, column, and beam-to-column joint was similar to those of the test results. The nonlinear dynamic analyses indicated that column-tree connections with a splice slip have positive effects on frames with a weak panel zone. The splice slip increased the rotational capacity of the assembly and reduced the inelastic demand of the panel zone because early slips occur at the beam splice before the weak panel zone yields. Column-tree construction is therefore a viable method for constructing SIMF buildings in areas of low-to-medium seismicity.

Local behavior of column-tree connections with considering bolt-hole clearance and bolt pretension effects

In this paper, effects of bolt-hole clearance and bolt pretension level on seismic performance of column-tree moment resisting frames with reduced beam section have been investigated. In a typical constructional environment, manufacturing tolerances and field situations cause inappropriate positioning of bolts in their corresponding holes in bolted connections, so bolt-hole diameter increases by incorrect methods like acetylene gas cutting method. On the other hand, nonlinear local behavior of the connection elements and effects of dynamic friction during the process of applying the bolt pretension cause wrong evaluation of bolt pretension level. In order to investigate these problems, models of column-tree connections with various bolt-hole clearances and bolt pretension levels subjected to cyclic loading were developed and analyzed with 3D FEM and their hysteresis curves were obtained and evaluated. Results show that bolt-hole clearance can reduce the flexural capacity of column-tree connections especially along with reduction in the bolt pretension level while interstory drift angle increases during inelastic cycles.

Seismic performance evaluation of Korean column-tree steel moment connections

The purpose of this study was to evaluate the seismic performance of existing Korean column-tree connection type in steel moment frames. The column-tree connection has frequently been used in steel moment frames because it is well known to provide high quality and economy with shop-welding and field-bolting in Korea and Japan. However, there have been no studies on the design details and ductility of column-tree connections. Therefore, in this paper, the seismic performance of non-composite and composite column-tree specimens was evaluated to investigate their problems. Full-scale specimens were fabricated with non-composite/composite strong axis and weak axis column-tree connections, respectively. The testing results showed that, although non-composite column-tree specimens reached a 5% story drift ratio, tiny cracks occurred at beam-to-column connections. In the case of composite specimens, the expected brittle fracture at the bottom flange was not detected. However, severe crushing of concrete slab occurred, and the bottom flange underwent great stress. Therefore, composite action should be considered when composite structures are designed because bottom flanges may fracture.

Cyclic testing of weak-axis column-tree connections with formation of plastic hinge at beam splice

The purpose of this study was to evaluate the seismic performance of weak-axis column-tree type connections used in steel moment frames. These connections are composed of a shop-welded and fieldbolted steel structure and can improve welding quality. On this basis, column-tree type connections are widely used in steel moment resisting frames in Korea and Japan. In this study, splices designed with a semirigid concept regarding the seismic performance of column-tree connections were experimentally evaluated. The structures can absorb energy in an inelastic state rather than the elastic state of the structures by the capacity design method. For this reason, the plastic hinge might be located at the splice connection at the weak-axis column-tree connection by reducing the splice plate thickness. The main variable was the distance from the edge of the column flange to the beam splice. CTY series specimens having column-tree connections with splice length of 600 mm and 900 mm were designed, respectively. For comparison with two specimens with the main variable, a base specimen with a weak-axis column-tree connection was fabricated and tested. The test results of three full-scale test specimens showed that the CTY series specimens successfully developed ductile behavior without brittle fracture until 5% story drift ratio. Although the base specimen reached a 5% story drift ratio, brittle fracture was detected at the backing bar near the beam-to-column connection. Comparing the energy dissipation capacity for each specimen, the CTY series specimens dissipated more energy than the base specimen.

Seismic performance evaluation of weak axis column-tree moment connections with reduced beam section

The purpose of this study was to evaluate the seismic behavior of weak axis column-tree connections used in steel moment resisting frames. Column-tree connections are composed of a shop-welded and field-bolted steel structure. Column-tree connections are well known to provide high quality and economy with shop-welding and field-bolting. In addition, column-tree connections are easier to assemble than field-welded connections. For these reason, column-tree connections are frequently used in steel moment resisting frames in Korea and Japan. Unlike the US and Europe, where strong axis connections are used, weak axis column-tree connections with a beam connection to the column web are often used for convenience in Korea. In this study, a basic weak axis column-tree connection that has been used in Korea was tested to evaluate its seismic performance. Also, two weak axis column-tree connections with a reduced beam section and a tapered beam section at the stub beam were fabricated and tested. The two specimens with the reduced beam section and tapered beam section successfully developed ductile behavior without brittle fracture until 5% story drift ratio. Therefore, the two specimens met the qualification criteria demanded by the AISC Seismic Provisions for use in SMFs. Although a basic weak axis column-tree connection specimen successfully reached 5% story drift ratio, brittle fracturing was detected at the backing bars near the beam-to-column connection. When compared with the energy dissipation capacity at the stub beam for each specimen, the specimen with the reduced beam section showed stable and ductile behavior.

Behavior and Design Recommendations of Splice Connection in Weak-Axis Column-Tree Steel Moment Frames

The main objectives of this paper was to investigate the cyclic performance of splice connection in weak-axis column-tree connections with the formation of plastic hinge assumed at splice connection and provide some design recommendations. By reducing the cross-sectional area of the splice plates, the splice connection in this test are designed as a weaken component to utilize the ductility capacity and energy dissipation. The results showed that it could develop the plastic rotation capacity in the beam splice connection and portion in the link beam but not increase the energy dissipation capacity during the same story drift ratio.

Seismic Performance Evaluation on Cyclic Testing of Weak-Axis Column-Tree Connections

The objective of this paper is to research seismic performance of the weak-axis column-tree connections. For this purpose, two specimens CTY600 and CTY900 were fabricated and tested in this experiment. From the test results, the CTY specimen successfully achieved a 5% rad story drift ratio without brittle fracture and the total plastic rotation reached over 0.04rad.

Cyclic testing of steel column-tree moment connections with weakened beam splices

Traditionally, the beam splice assemblies of column-tree connections are designed according to the full-strength design principle. Actually, the beam splice is set up at a certain distance from the column face, thus the plastic moment capacity of the beam splice does not need to be greater than the plastic moment capacity of the beam. This paper experimentally presents the effects of the weakened beam splice on the seismic performance of column-tree steel moment connections. Cyclic tests of two column-tree connection specimens with weakened beam splices and one specimen following the full-strength design principle were conducted to verify their inelastic behavior and failure modes. The test results showed that the moment resisting capacity of the column-tree connection specimens with weakened beam splices did not obviously decrease. Also, their energy dissipation capacity was better than that of the specimen following the full-strength design principle.

Cyclic testing of steel column-tree moment connections with various beam splice lengths

The purpose of this study was to evaluate the cyclic behavior of steel column-tree moment connections used in steel moment resisting frames. These connections are composed of shop-welded stub beam-to-column connection and field bolted beam-to-beam splice. In this study, the effects of beam splice length on the seismic performance of column-tree connections were experimentally investigated. The change of the beam splice location alters the bending moment and shear force at the splice, and this may affect the seismic performance of column-tree connections. Three full-scale test specimens of column-tree connections with the splice lengths of 900 mm, 1,100 mm, and 1,300 mm were fabricated and tested. The splice lengths were roughly 1/6, 1/7, 1/8 of the beam span length of 7,500 mm, respectively. The test results showed that all the specimens successfully developed ductile behavior without brittle fracture until 5% radians story drift angle. The maximum moment resisting capacity of the specimens showed little differences. The specimen with the splice length of 1,300 mm showed better bolt slip resistance than the other specimens due to the smallest bending moment at the beam splice.

Cyclic testing of weak-axis steel moment connections

The seismic performance of six types of weak-axis steel moment connections was investigated through cyclic testing of six full-scale specimens. These weak-axis moment connections were the column-tree type, WUF-B type, FF-W type, WFP type, BFP-B type and DST type weak-axis connections. The testing results showed that each of these weak-axis connection types achieved excellent seismic performance, except the WFP and the WUF-B types. The WFP and WUF-B connections displayed poor seismic performance because a fracture appeared prematurely at the weld joint due to stress concentrations. The column-tree type connection showed the best seismic behavior such that the story drift ratio could reach 5%.

Ductile moment connections used in steel column-tree moment-resisting frames

This paper presents analytical and experimental studies on the cyclic behavior of beam-to-column moment connections used in steel column-tree moment-resisting frames. The column-tree system is joining the column-trees and link beams in the field while the column-trees are fabricated in the shop by welding stub beams to the column. The proposed ductile column-tree connections have two distinctively improved connection details which are no weld access hole detail and a widened flange of the stub beam. Nonlinear finite element analysis demonstrated the effectiveness of the improved connection details which significantly reduce the stress concentration and plastic strain demands at the beam flange groove weld. Cyclic testing of three full-scale specimens was conducted to verify the proposed connection details. All the specimens successfully developed ductile behavior with no brittle fracture by forming the plastic hinging of the beam away from the beam–column interface. The widened flange and no weld access hole details are effective in reducing the potential of brittle fracture.