Abstract: Spring strips for spring mattresses, of the type having an intrinsical convexity, has a one piece construction capable of being manufacturing by a process of moulding, and includes end portions for coupling to a mattress frame by means of a plurality of longitudinal ribs in the manner of beams having longitudinally variable width edges with a maximum at the center line of the spring strip and a minimum near said end portions. The ribs are stiffened by a plurality of longitudinal and transversal diaphragms forming a grating of junctions and bars which constitute the area for the load support. For this purpose, a flexible plastic material is used.