This paper deals with the base anchor structures with high fatigue resistance for additional column members such as poles for lighting and information on elevated highway bridges. The lower part of the steel column and the bottom flange plate of an additional column member are generally strengthened with triangular rib plates in order to set tightly the additional column member on the highway bridge. However, unfavorable stress concentration is often observed around the welded zone at the tops of the triangular rib plates. These welded zones are also one of the locations where welding defect tend to occur because of the difficulty of welding. For these reasons, two new types of base anchor structures without triangular rib plates are proposed in this paper. These are the flare type and the increased thickness type of base anchor structures. It is concluded through the fatigue test that these new type base anchor structures can have high fatigue resistance compared with the ordinary base anchor structure using triangular rib plates. It is also observed from the fatigue test that the fatigue strength of the ordinary base anchor structure using triangular rib plates can be improved substantially by finishing smoothly the welding beads at the tops of the triangular rib plates.
