A series of investigations were conducted to clarify the change in mechanical properties of SBHS subjected to heating and cooling assuming fire, and to examine whether the change can be estimated by hardness tests. When SBHS was heated above 600 ℃ and then cooled, the material strength decreased due to recrystallization and transformation. A clear correspondence was observed between the mechanical properties of SBHS subjected to heating and cooling and the Leeb and Vickers hardnesses. The yield stress and tensile strength could be estimated by the hardness tests with an error of approximately 5 % from the tensile test results.

In this study, the mechanical properties of the weld metal applied to the joints between the cold press formed steel tube and the through diaphragm were investigated by the welding test and the heat conduction analysis. The parameters of the welding test were the welding conditions such as the heat input, the type and shape of weld joint. The characteristic point was that the maximum heat input was set to 60 kJ/cm which was the upper limit for the welding robot as opposed to normal 30 kJ/cm. From the test and analysis results, it was clarified that the welding conditions affected the inter-pass temperature and the cooling time of weld metal. There was a correlation that the strength of weld metal increased with the increase the cooling speed.

The cast iron deck is a lightweight deck slab with excellent fatigue resistance. There is a problem that the deck is separated when negative bending acts on the joint. As a countermeasure, we proposed a structure that uses both friction joint by the splice plate to the upper surface of the deck and tension type joint of end ribs. In this study, we confirmed the behavior of this joint structure up to the final stage and clarified the limit state by the elemental experiment.