Sung-Mo Choi

Structural Capacities of Tension Side for CFT Square Column-To-Beam Connections with Combined-Cross Diaphragm

The purpose of this study is to suggest a new CFT column-to-beam connection type, which will have an axis direction that takes into consideration the asymmetry of connections. Through experimental study and finite element analysis, the study also compares the structural capacities of the new CFT column-to-beam connection type with those of existing connection types. Existing connection types for comparative study are those with an interior diaphragm and through diaphragm. The new connection type suggested uses a combined-cross diaphragm, featuring a column flange penetration in the major-axis direction and an interior diaphragm in the minor-axis direction. In order to analyze and compare the initial stiffness and strengths of the connections considered simple tension tests along with finite element analysis were performed on the specimens, while idealizing the tension-applied beam flange. Stress concentrations, according to the geometric configurations and load transfer mechanisms of diaphragms, were analyzed through finite element analysis. The results show that the existing connection types, in terms of initial stiffness and ultimate strength, do not precede the combined-cross diaphragm, and that the stress concentration on the connection can be relieved by improvement to the detail.

Fire resistance of concrete filled square steel tube columns subjected to eccentric axial load

The fire resistance of concrete filled square steel tube columns (CFT columns) without fire protection under a constant axial load has been previously examined. The purpose of this present study is to investigate the fire resistance of CFT columns when they are subjected to combined loads. This paper presents a nonlinear thermal stress analysis method for predicting the mechanical behavior and fire resistance of CFT columns under eccentric axial load (axial load and flexure moment). This method is based on the stress-strain characteristics of materials at high temperatures and on the mechanics of column deflection curves. From the results of the developed computational technique, it was demonstrated that as the yield stress and rigidity of the steel tube rapidly decreased for approximately 30 minutes, the decrease of flexure moment capacity increased significantly more than that of the axial load capacity. In addition, as the load eccentricity ratio increased, the fire resistance time drastically decreased. However, the time of maximum expansion under eccentric axial load was independent of the load eccentricity ratio.

Ductility Characteristics of Partially Restrained Beam-To-Column Composite Connections in Concrete Filled Square Tubes

This paper presents the development of an improved detail in partially restrained beam-to-column composite connections in concrete filled square tubes and the evaluation of its structural characteristics and behaviour under monotonic and cyclic loading. Studies for the bolted seat-angle connections of existing partially restrained composite connection (PR-CC) details have been conducted mostly on shallow beam-to-column connections. In case of deep beam-to-column connections, the fabrication becomes complicated because the sizes of the seat-angles are larger than the connections, and the number of bolts and welding length are increased. This study suggests a new detail of PR-CC which can be applied to concrete-filled tubular structures incorporating the effect of composite slabs and the performance for construction work. A welded bottom beam flange connection is proposed to enhance the capacity of the bottom of the connection and to improve ductility and fabrication. In addition, a reduced beam section (RBS) is adapted for the bottom beam flange to examine its effect on ductility. A seat-angle connection with penetrating bolts is also suggested and compared with the welded bottom beam flange connection. Both monotonic and cyclic loading tests are conducted on the five full scale specimens to compare and evaluate their ductility characteristics.

Structural behavior of tension joint with high-strength bolted split-tee

The strength and behavior of split-tee connections that are applicable to all tensile connections with high-strength bolts have been studied in many countries. The design code of each country (AISC, Eurocode 3) does not consider various field conditions, however, such as the painting of the connection surface, the length of the bolt shank, and the preload. Moreover, the previous studies (Bahaari) that used the link element on the preload of the bolt could not predict the correct behavior of high-strength bolts. In this study, to evaluate the tensile capability of split-tee connections, a total of 20 split-tee specimens with two bolts each were fabricated and tested. The experiment parameters that were used were the thickness of the T-flanges, painting, preload, and the length of the bolt shank. The joint stiffness of split-tee connections was compared with the stiffness equation of Faella et al., and finite-element method analysis with Ansys 8.0 was conducted. Further, to verify the reliability of the highstrength-bolt model, the load-to-bolt stress relation in the experiment test and the analysis results were compared.

Fire Resistance Performance of Welded Built-up Square CFT Columns with Reduced Intumescent Paint

Welded built-up square CFT columns are widely employed in construction field thanks to their structural efficiencies by avoiding stress concentration area and improving workability in fabrication and maximizing the composite effect enabled by bent ribs. Although welded built-up square CFT columns have structural advantages over other members and are widely used, they are classified as non-fireproof in Korea because the steel tubes are directly exposed to a fire. Thus, fire coating required for the columns by the law results in inefficient design. This study suggests welded built-up square CFT columns with reduced coating to enable improved fire-resistance performance for 2˜3 hours. The purpose of the study is to analyse their fire-resistance performance using the variable of axial Load ratio which is the major factor in the performance and suggest efficient functional design with reduced coating.

Structural Behavior of Two-Seam Cold Formed Square CFT Column to Beam Connections

The concrete-filled tube (CFT) column has the excellent structural performance. But it is difficult to connect with column and beam because of closed section. Its Solution, 2 members of ㄷchennel in which Internal diaphragm is installed were welded beforehand and the method of making Rectangular Steel Tube was proposed. According to upside and downside junction shape, Internal diaphragm suggested as symmetric specimen and asymmetric specimen. The upper and lower diaphragm of the Symmetric specimen used the same horizontal and The upper diaphragm of the Asymmetric specimen used the horizontal plate and the lower diaphragm used the vertically plate. In this research, 4 T-shape column to beam steps connections were tested with cyclic loading experiment in order to evaluate the structural capability of the offered connection. Symmetric specimens be a failure in 0.03rad from beam flange. And Asymmetric specimens be a failure in 0.05rad from column interface. The comparison results of All specimens shown similar to energy absorption capacity in 0.02rad.

Seismic Performance of Steel Braces Jointed with High Performance Fiber Reinforced Cementitious Composites and Steel Bars

Steel-braced frames have been widely used for retrofitting due to their effectiveness in terms of their light weight and short construction period. The steel-braced frame retrofitting method is difficult to apply, however, to the inner frames of buildings. Consequently, there has been a demand for a braced frame retrofitting method that can be broadly and easily applied to both the inner and outer frames of buildings. In this study, a retrofitting method using steel-braced frames with HPFRCC and steel bars is proposed. The pre-fabricated upper and lower segments of the brace member are connected on-site with HPFRCC, meaning that the proposed method is applicable to the inner frames of buildings as a seismic retrofitting method that has the advantage of simple and rapid installation. The tension force is transmitted only through the steel bars, which transmit the compression force to the HPFRCC of the joint that connects the three brace units. The results of cyclic tests of four specimens clearly showed that steel braces using HPFRCC and steel bars ensure stable structural performances and improve construction efficiency.