 Large volumes of newer types of pipefitting installation in the shift from World War I wartime industries to peacetime construction caused considerable difficulties.Jurisdictional problems also developed with other national Unions, but the United Association retained jurisdiction over important, growing areas of work like construction of industrial plants, public utilities, petroleum facilities and residential buildings.In the first half of the century, the United Association moved to formalize apprenticeship training programs, including making a five-year apprenticeship mandatory in 1921, and in 1938 holding that all apprentices be members of the United Association and attend related training classes.