• Journal of the Earthquake Engineering Society of Korea
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• v.16 no.6
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• pp.21-28
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• 2012

Welded Unreinforced Flange-Welded Web (WUF-W) connection is one of Special Moment Frame (SMF) specified in ANSI/AISC-358. From the experimental test of WUF-W connection specimens conducted by the previous study, fracture occurred in the beam flange before achieving total inter-story drift angle of 0.04radian required for Special Moment Frames (SMF) system even though the specimens satisfied the design and detailing requirement specified in ANSI/AISC-358. These results are estimated as problem of the access hole geometry. In this study, a full-scale WUF-W connection specimen was made with a modified access hole geometry, and tested with the same test setting and loading as the previous test. From test results, the deformation capacity of the tested WUF-W connection specimen exceeded 4%, which is required for connections in SMF system. Comparing with the WUF-W specimens of the previous study, the strain demand of the beam flange in the tested specimen was decreased and energy dissipation capacity of the specimen was improved.

• Journal of the Earthquake Engineering Society of Korea
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• v.28 no.6
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• pp.373-380
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• 2024

Steel moment frame connections are vital in moment frames designed to resist forces transferred from adjoining beams and columns. The welded unreinforced flange-welded web (WUF-W) connection is one of the pre-qualified connections used for steel special moment frames (SMFs). This study developed drift-based fragility functions for WUF-W connections based on test data of 35 WUF-W connection specimens from ten previous experimental studies. Four different damage states were defined to calculate the fragility: onset of yielding, local buckling, strength loss, and fracture. Fragility functions were derived assuming that lognormal distribution was validated using the Kolmogorov-Smirnov (K-S) test. It was confirmed that fragility functions for WUF-W connections were accurately proposed within a specified confidence interval. The fragility functions proposed in this study exhibit smaller standard deviations compared to FEMA P58, thereby reducing the likelihood of overestimating or underestimating damage at specific drift ratios. Furthermore, these functions remain within the confidence intervals across all damage states, contributing to improved accuracy in damage analysis and loss estimation in performance-based earthquake engineering.

• Journal of the Earthquake Engineering Society of Korea
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• v.21 no.5
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• pp.205-213
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• 2017

The WUF-W moment connection is a pre-qualified connection that can be used for special moment frames specified in current seismic design specifications. Since the stress distribution near the connection varies according to access hole configuration, the cyclic performance of WUF-W connections is strongly affected by the access hole configurations. To evaluate the connection performance according to various access hole configurations, it is expensive to conduct experiments with many connection specimens. Instead, finite element analyses (FEA) can be performed. Throughout the FEA, stress and strain distribution in the connection can be monitored at each loading step. The purpose of this study is to construct nonlinear 3-dimensional FE models for accurately predicting the cyclic behavior of WUF-W connections. For predicting connection fracture using FEA, an appropriate response index detecting the incidence of connection rupture is proposed.

• Journal of Korean Society of Steel Construction
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• v.29 no.4
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• pp.303-310
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• 2017

The welded unreinforced flange-welded web(WUF-W) connection is an all-welded moment connection, which is qualified for special moment frame(SMF) connections. However, previous studies reported that some WUF-W connections did not meet the drift requirement specified for SMF connections and such unexpected connection performance was resulted from weld access hole geometry. The objective of this study is to determine the permissible ranges of major access hole parameters such as access hole slope and the length of flat portion that lead to the satisfactory performance of WUF-W connections using Nonlinear finite element analyses are conducted. This study also proposes simple empirical equations to check the validity of access hole parameters to be selected in design process.

• Journal of the Earthquake Engineering Society of Korea
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• v.22 no.6
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• pp.353-360
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• 2018

This paper is to investigate the micro-behavior of the double-span beams with WUF-W seismic connection under combined axial tension and moment and to propose the rational rotational capacity of it for progressive collapse-resistant analysis and design addressing the stress and strain transfer mechanism. To this end, the behavior of the double-span beams under the column missing event is first investigated using the advanced nonlinear finite element analysis. The characteristics of fracture indices of double-span beams with WUF-W connection under combined axial tension and flexural moment are addressed and then proposed the rational rotational capacity as the basic datum for the progressive collapse-resistant design and analysis. The distribution of fracture indices related to stress and strain for the double-span beams is investigated based on a material and geometric nonlinear finite element analysis. Furthermore, the micro-behavior for earthquake and progressive collapse is explicitly different.

• Journal of Korean Society of Steel Construction
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• v.20 no.6
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• pp.829-838
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• 2008

In this study, cyclic tests of beam-column connections composed with members applicable to the domestic low-middle rise steel buildings were conducted to develop seismic connection details and its evaluation. Connection types and material properties of the steel were testing variables and the difference between the newly developed seismic rolled section (SHN490) and existing rolled section (SM490) was also investigated. Distributions of the yield strength and the ultimate strength of the SHN490 rolled section were relatively uniform comparing to those of the SM490 rolled section Brittle fracture in the weldments of the test specimens was not observed. Instead, fracture occurred at heat-affected zones or the stress-concentrated point near the weld access hole of the beam flanges. In the case of identical rolled-section specimens, the rotational capacity and dissipated energy of the WUF-W connection was larger than those of the WUF-B connection. In the case of identical connection types, the rotational capacity and dissipated energy of the SHN490 section connection was larger than those of the WUF-B section connection.

• Steel and Composite Structures
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• v.15 no.5
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• pp.507-518
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• 2013

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%.

• Journal of Korean Society of Steel Construction
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• v.20 no.4
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• pp.483-491
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• 2008

There has been much focus on the strong axis steel moment connections after the Northridge earthquake in 1994. However, research studieson the seismic behavior of weak axis moment connections could be hardly found despite the fact that these connection details have been frequently used as seismic details of MRF in Korea. Therefore, the objective of this research is to provide better knowledge on the seismic behavior of weak-axis steel moment connections, which can be widely applicable to many structures with similar characteristics. For this purpose, an experimental program was designed and performed with twotypes of weak-axis steel moment connections, namely the bracket type and WUF-B type, based on the survey of existing field data and literatures. Using the experimental results obtained from the quasi-static cyclic testing of these specimens, structural performances of the joints such as hysteretic curves, maximum strength capacities and the strain of reinforced bars were investigated. From the test results, the bracket-type connection was shown to have more than a 5% story drift capacity, compared with the WUF-B type connection's 4%. These specimens were also shown to have higher strength capacities than the nominal design strength. The bracket-type connection showed a slow strength degradation after maximum strength was researched. However,the WUF-B type connection showed a rapid strength degradation that caused brittle behavior.

• Journal of Korean Society of Steel Construction
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• v.20 no.5
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• pp.617-624
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• 2008

Well-designed steel moment connections will undergo local buckling before they exhaust their available rotation capacity, and inelastic post-buckling deformation plays a major role in defining the connection rotation capacity. An approximate analytical method to model strength degradation and failure of beam plastic hinges due to local buckling and estimation of the seismic rotation capacity of fully restrained beam-column connections in special steel moment-resisting frames under both monotonic and cyclic loading conditions is proposed in this study. This method is based on the plastic mechanism and a yield line plastic hinge (YLPH) model whose geometry is determined using the shapes of the buckled plastic hinges observed in experiments. The proposed YLPH model was developed for the improved WUF-W and RBS connections and validated in comparison with experimental data. The effects of the beam section geometric parameters on the rotation capacity were discussed in the companion paper (parametric studies).

• Journal of Korean Society of Steel Construction
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• v.20 no.5
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• pp.625-632
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• 2008

In the companion paper (Model Development), an analytical model estimating the available rotation capacity of fully restrained beam-column connections in special steel moment-resisting frames was proposed. In this paper, two limit states were considered as the connection rotation capacity criteria: (i) strength degradation failure when the strength falls below the nominal plastic strength due to the local buckling of the beam's cross-section and (ii) low-cycle fatigue fracture caused by plastic strain accumulation at the buckled flange after only a few cycles of high-amplitude deformation. A series of analyses are conducted using the proposed model with two limit states under monotonic and cyclic loadings. Beam section geometric parameters, such as flange and web slenderness ratios, varied over the practical ranges of H-shapedbeams to observe their effect on the rotation capacity and low-cycle fatigue life of pre-qualified WUF-W connections.