Fatigue cracks were found in beam-column connections of steel piers in the Metropolitan Expressway, and examination of retrofitting measures for these cracks has been proceeded. Total of 566 out of 2200 columns are found to have cracks in the connections. Detailed investigations of the damaged piers including opening of damaged weld bead, investigation of fabrication sequence were carried out with the piers which had comparatively large cracks. Starting point of cracks were turned out to be characteristic internal flaw which contain un-welded cavity and metal-touch zone in most cases. A damage database was constructed based on these investigations and fatigue damage modes of the steel pier are examined based on it.