Abstract
Some ultra-high-strength steel frames with hat-shaped section and jointed flange by spot welding were given 4 point bending deformation. The validity of 4 point bending test system was confirmed by some simulation. The test system could give the frames constant moment in the wide range that was characteristic of 4 points bending condition. Also, by static and dynamic 4 point bending test, in case of the frames with short flange in cross direction, a crack occured in heat affected zone around spot welding point on the flange of the frames had fracture. On the other hand, the frames with large flange did not have fracture in the heat affected zone.
