Chang-Hwan Lee

This study proposes an innovative and practical retrofitting method for reinforced concrete (RC) roof T-joints with substandard structural details. Retrofitting was performed by assembling steel plates with threaded rods on the roof T-joints, which have typically only been applied to exterior-roof beam–column joints (BCJs). The aim of the retrofitting method was to fully utilize the capability of the column to dissipate seismic energy while limiting the shear demand in the joint panel zone. Four roof T-joint sub-assemblage specimens were constructed and tested using a reverse cyclic loading test. One control specimen and three retrofitted specimens were used with variations in the initial prestressing force, epoxy adhesive, and haunch availability. The effects of the retrofitting variants on the test results, such as the crack patterns, failure mode, and hysteresis behavior, were analyzed. A significant increase in the maximum lateral strength by an average of approximately 138% was observed after applying the proposed retrofitting method. Moreover, when the haunch was removed, the specimen retrofitted with a high initial prestressing force dissipated the most cyclic energy, which was at least seven times that of the control specimen. This considerable seismic performance was achieved when the column was designed with appropriate deformability to develop flexural strength without being hindered by joint shear failure or the short-column effect. Thus, this study makes a notable contribution to the development of seismic retrofitting technology, particularly for BCJs at the roof levels of RC buildings.

Concrete-filled steel tube (CFT) columns are widely used as structural systems because of their high load-bearing capacity and material efficiency. However, under fire conditions, elevated temperatures degrade the mechanical properties of both steel and concrete. When combined with initial geometric imperfections, these factors significantly affect the load distribution and the fire resistance of the structure. Understanding how material properties and geometric factors change in CFT columns at elevated temperatures is essential for ensuring safe and efficient design. This study used the ASTM E119-88 fire curve to establish the relationship between the surface temperature of the structure and the fire resistance duration of the CFT column. Heat transfer and mechanical analyses of the structure were conducted using ABAQUS 2024 software. A comparison of simulation and experimental data showed that the numerical model was highly accurate. The study also addressed the effects of initial geometric imperfections, considering amplification factors of L/1000 and L/500, and compared the simulation results with the experimental data. The results demonstrated that initial geometric imperfections significantly influenced the fire resistance of the columns. Additionally, this study examined the material properties under high-temperature conditions as specified in the AISC 360-22 standard. The study compared the simulation results with the Eurocode standards and experimental data. The findings suggested that utilizing the material properties specified in the AISC 360-22 standard resulted in more conservative predictions of fire resistance for CFT columns, compared to the Eurocode standards. Furthermore, Appendix 4 of the AISC 360-22 standard was used to calculate the fire resistance rating of the CFT column. These calculations were compared with the simulation and experimental results to evaluate the reliability of using ABAQUS 2024 simulation software.

Headed cast-in specialty inserts are often developed and employed for their improved constructability and time-saving benefits over traditional cast-in and post-installed anchors. Their acceptance criteria (AC 446) were established by The International Code Council Evaluation Service. However, previous research on the behavioral characteristics of headed specialty inserts is rare. This study investigates the performance of a newly developed insert anchor for use as an earthquake-resistant anchor for fire protection systems and suspended lightweight piping. Firstly, three cyclic tension tests of the specialty insert with a threaded rod were conducted in accordance with AC 446. These tests resulted in all experimental subjects achieving the target strength without defects. Subsequently, monotonic tension and shear tests of the insert anchor in cracked concrete were performed five times each to assess the tensile and shear capacities of the new insert anchor. The results were compared with the nominal strength determined by the anchor design equation provided by the ACI 318 for cast-in headed anchors, finding that the latter adequately approximates the concrete breakout strength of the new insert anchor in tension but underestimates the concrete pryout strength in shear. Based on the measured breakout failure surface angles, a projected area of the failure cone of the corresponding anchor with a relatively large head size is proposed.