ACTUALLY LOADED COMPRESSIVE EXPERIMENTS OF STEEL COLUMNS INCLUDING HIGH STRENGTH BOLTED CONNECTIONS AT ELEVATE TEMPERATURE

Abstract

　Full-scale fire resistance experiments for an axially loaded compressive steel column including high strength bolted connection were conducted under steady state condition ( the constant temperature and increasing load condition), to examine the load-bearing capacity and the possibility of shear fracture of high strength bolts after the column specimen exhibited the overall buckling. The specimen was fabricated from the JIS SS400 (wide flange shape and splice plate) and JIS F10T (high strength bolt). The column connection in the specimen was designed based on "AIJ Recommendation for Design of Connection in Steel Structures". The number of flange bolts arranged in the column connection was used as the experimental parameter. The three specimens without fire proofing materials and the specimen protected by that except for the connection part were prepared, respectively. The column specimen was heated above 700 °C by a gas furnace, and the axial force was applied to that under the condition when the specimen temperature is approximately constant. All of the specimens exhibited the stable residual strength after the buckling (the unprotected column specimen) or local buckling (the protected column specimen). It is well known that the high strength bolt possesses the lower strength than the mild steel at elevated temperature, however, the bolted connection in the specimen did not fracture. This is because the loaded axial force was transmitted through the metal touch between the column cross sections at the column connection, therefore the bolts were not sheared by that. On the other hand, the column connection was subjected to the bending moment owing to the overall buckling behavior, however, the bending moment reduced by the local buckling occurred in the center of column specimen and the fracture of column connection was avoided. In particular, the column connection designed by using the small numbers of the flange bolts, which possessed the lower bending strength than the required design strength at the ambient temperature, did not fracture and exhibited the stable residual strength after the overall buckling. It is possible to conclude that there is little possibility of bolt-fracture for the column connection at the fire based on the experimental results. The maximum axial force of the column specimen could be evaluated by the design value of overall buckling and the yield strength considering the effect of steel strain rate at the elevated temperatures.

　On the other hand, it is necessary to conduct the composite loading experiments at the elevated temperatures, regarding the column connection subjected to both axial force and bending moment, to examine the possibility of bolt-fracture owing to the bending moment. Furthermore, the evaluation of bending strength involving the fracture of bolted connection for the wide flange shape in the weak axis, which has not been clarified in the fire resistance field of steel structures, is required in the future research.