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This study focuses on mixing steel fiber in the concrete to improve the ductility and toughness of the columns. The purpose of the study is to evaluate the load capacity and deformation capacity associated with the amount of steel fiber and loading condition and to analyze the interplay between the steel fiber reinforced concrete and the welding built-up square tube in terms of structure and fire resistance performance. Reinforcement of concrete with steel fiber, when cross-section shape and boundary condition (load ratio) remained unchanged, improved fire resistance performance by 1.1–1.3 times. It is deemed that the area resisting thermal load increased and fire resistance performance was improved since the concrete reinforced with steel fiber restrained cracking. In addition, the fact that the cross-sections of the concrete were barely damaged indicates that load share capacity was greatly improved.

Welding process and throat depth are essential to reduce construction costs of steel tubes. However, the resulting welds should be checked to ensure construction safety especially when wet concrete is to be pumped into the tubes. In the past, tests (Mock-up Test) using actual size specimens were conducted to verify the feasibility of welding throat depth (WTD) during the concrete casting stage on site. However more than often, this is inapplicable owing to the high testing costs, difficulties in disposing of test specimens with wet concrete, and lack of suitable places to conduct tests. Accordingly, this study suggested a water pressure test method and carried out 6 tests on welded built-up square concrete-filled steel tubular (CFST) columns to evaluate the WTD safety. Recommendations are made for the WTD selection.

This study presents the development of an improved detail for a Concrete Filled Steel Tube (CFT) square column-to-beam partially restrained composite connection (PR-CC) and the evaluation of its hysteresis behavior under cyclic loading. The detail of the connection was designed to prevent brittle failure at the bottom of the connection due to the composite effect and to simplify its fabrication process. The suggested connection is a welded type of bottom beam flange connection and the existing PR-CC is a bolted type with a seat-angle. To evaluate their hysteresis behavior, specimens were fabricated at full scale and tested under cyclic loading. Results revealed that the suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 radian of the plastic rotational angle. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. The suggested type can be classified as a partially restrained connection overall under cyclic loading because it reached more than 0.03 rad of an inelastic rotation angle 80% capacity of the maximum moment capacity. This type was evaluated to exhibit equal or more ductility than the existing type. As a result, it is concluded that the welded bottom beam flange connection type can be used in practice for the CFT column–beam connection instead of the existing bolt connection using a seat-angle.

This paper studies the development of the through-type concrete-filled square steel tube column-to-beam connection, reinforced with an asymmetric lower diaphragm. This type of connection can be used in weak-earthquake regions such as Korea. A simple tension test was performed on the suggested lower diaphragms and the combined cross diaphragm in order to confirm their tensile behavior. Subsequently, four types of concrete-filled square steel tube column-to-beam connection (with combined cross diaphragm, nothing, horizontal T-bar and vertical plate as lower diaphragm) were fabricated in actual size and tested according to the ANSI/AISC SSPEC 2002 cyclic loading program. The horizontal T-bar and stud bolts in the vertical plates were designed to transmit the tensile stress from the bottom flange of the beam to the filled concrete. All the test specimens satisfied the 0.01 radian inelastic rotation capacity requirements for the composite ordinary moment frame (C-OMF) of the AISC seismic provisions. It was concluded that the suggested simplified lower diaphragms have sufficient strength, stiffness and plastic deformation capacity to be used in the field.

This paper presents a critical review of the performance of the currently available material models with regard to the mechanical behaviour of a concrete-filled steel tube (CFT) column at elevated temperatures. For this purpose, numerical heat transfer analysis and nonlinear thermal stress analysis were carried out by taking into account the existing material properties, such as the thermal and mechanical properties of concrete and steel. In addition, ten CFT column specimens with rectangular cross-sections of 300–350 mm in size were heated for two hours in accordance with KSF 2257 (equivalent to ASTM E119 and JIS A 1304). On comparing the numerical analysis and experimental results, it can be observed that we can validate and employ the existing material models such as EUROCODE, AIJ CODE, Lie’s material model, and Yin’s material model.