Fire resistance and design assessment of Q355 circular hollow section joints

This paper presents an experimental investigation on fire resistance of circular hollow section (CHS) T-, Y- and K-joints. A total of 15 specimens subjected to brace axial load were heated inside an electric furnace, including 5 T-joints, 5 Y-joints and 5 K-joints. The temperature inside furnace follows ISO 834 standard fire curve. The principal parameters considered in this study include the dimensionless geometric parameters of joints β and γ, as well as the load ratio n. It is found that the failure mode of the axially loaded CHS joints under fire is chord plastification. Referring to the temperature-deformation curves of these CHS joints, it is suggested to adopt 1.5% ovalisation of the chord diameter as the failure criteria of the CHS joints under fire. The fire resistance of the CHS joints is seriously reduced with an increase in the load ratio n. By comparing the temperature development and the critical temperatures of the CHS joints obtained in the tests with the ones predicted by design codes, it is found that the measured temperature development curves of the CHS joints are significantly different from the calculated ones before the joint temperature reaches 550 °C. Further investigation is essential for temperature development analysis of CHS joints, especially in the aspects of the section factor A_m/V of CHS joint. The fire resistance performance of CHS T- and Y-joints is shown to be reasonably estimated with the resistance method provided in modern design codes, even though the estimation on CHS K-joints is relatively conservative.