Abstract: A stiffening member, as well as a diaphragm of a building in which the stiffening member is used, is disclosed. The diaphragm includes a horizontal wide-flange beam, a corrugated deck, and a corrugated stiffening member. The stiffening member is an elongate, corrugated strip. The stiffening member's corrugations are formed by a series of U-shaped portions that alternate with, and are integrally connected to, a series of horizontal upper portions. Each U-shaped portion consists of three planar portions: two substantially vertical side portions and one horizontal floor portion. Each U-shaped portion is reinforced with two parallel raised ribs. In a second embodiment of the stiffening member, the U-shaped portion is reinforced with two parallel tab portions attached at the sides of the stiffening member. The stiffening member nestingly overlies the corrugated deck, and is attached to the deck and to the underlying horizontal beam with either welds or pins.