There are many buildings that lack seismic performance, and these buildings have to be retrofitted as soon as possible before the next great earthquake comes. In case of old steel structures, using a brace is effective to reduce the maximum story drifts induced by earthquakes and to protect non-structural members. In this paper, experiments on improvement of the ultimate strength of bolted angle connections using the additional fillet welding were carried out. The test results show that the additional fillet welding brings an increase in the ultimate strength. The effects of the additional welding depends on their position. From these test results, the effective length considering the effect of the welding position is proposed to evaluate the increase in the ultimate strength. Finally, a simple seismic retrofit design procedure using the additional fillet welding to the bolted angle connections is proposed.
