This paper deals with electro-slag weld (ESW) joint of built-up box section column with TMCP steel. The objective is to clarify the effect of thickness of column skin-plate on toughness of ESW joint. Charpy impact test was carried out on ESW joints with various thickness of column skin-plate and inner diaphragm. Though charpy absorbed energy of heat affected zone (HAZ) decreases as the column skin-plate becomes thinner, the influence of thickness of column skin-plate is small when the column skin-plate is thin compared to inner diaphragm. The results of charpy impact test and heat transfer analysis showed the relationship between charpy absorbed energy and the cooling rate, and the relationship between the cooling rate and the thickness of the column skin-plate and inner diaphragm. From these relationships, the estimated value of charpy absorbed energy of HAZ was shown.
