the invention relates to a vehicle , notably a motor vehicle , chassis frame side rail made up of two section pieces of u - shaped cross section , welded together by longitudinal connecting tabs juxtaposed contiguously and more or less perpendicularly extending the ends of the branches of the u , in which each section piece has , between each of the tabs and the branches of the u that it extends , a joining area that forms two separate longitudinal plastic hinges that deform if the side rail is deformed by longitudinal compression so as to keep the welded joint between the tabs intact .