Abstract
Verification on load-bearing performance for a steel welded joint, which is usually used for a beam-column joint in a steel rigid frame, is of importance to secure structural safety of steel building structures. Many researches using experimental, numerical and theoretical approaches on that have been conducted since the Kobe Earthquake (1995) in Japan, when many welded joints were brittlely fractured. The verification methods used at actual structural design have been established, because the seismic performance was clarified by their valuable research results. On the other hand, there are few past researches on the fire resistant performance, hence that cannot be verified at fire resistant design for the steel structures. Main purpose of this study is to clarify the load-bearing capacity of the welded joints at high temperature and accumulate the experimental date using for the fire resistant verification.
The tensile tests under a condition on constant temperature and monotonic loading were conducted. After heating specimens by an electric furnace, tensile force was gradually applied for them. The specimens including a full penetration (FW specimen) or partial penetration (PW specimen) welded joint were fabricated. The base metal and weld material are JIS SN490B and JIS YGW11, respectively. For comparison of the experimental data of the welded specimens, the specimen without the welded joint (P specimen) was used for the experiments. All specimens were fabricated into a tapered shape. The welded joint was arranged at the minimum sectional area of the tapered shape specimen. The test temperatures from ambient temperature to 800 °C were used. For the FW specimen, the tensile tests were additionally conducted up to 900 °C. From a series of the tensile tests at the high temperature, the following experimental results were obtained.
The maximum load of the specimen gradually decreased with the test temperature increase. At the ambient temperature, the specimens ductility fractured after the peak of tensile load. On the other hand, they did not fracture after the peak at the high temperature and exhibited residual strength. In this region, the tensile load gradually decreased with the deformation increase. The PW specimens fractured at the weld metal in the partial penetrate welded joints. The FW specimens over 800 °C fractured along fusion lines between the weld metal and the heat affected zone, otherwise they fractured at the base metal. However, the specimens fractured at the welded joints possessed the sufficient load bearing and deformation capacity at the high temperature. From specimen observations after the tests, large residual plastic deformations in the tapered shape specimen and necking at the fractured section were observed for all specimens. The reduction factor on the tensile strength of the specimen including the welded joint, which is fundamental information to verify the fire resistant performance of welded joint, was estimated by the obtained experimental results. From the reduction factor curves at the elevated temperature, it was confirmed that both welded joint and base metal possessed the similar reduction factors over 600 °C.
