Abstract: In a heavy structure e.g. a bridge which includes an element liable to low frequency vibration during erection, i.e. when the structure is unstable, another element which is an integral structure part after erection is used as the mass of a dynamic absorber for damping this vibration. In the example given, a launch rail (46) for erecting a bridge across a river is cantilevered out over the river, after which a support leg (50) is extended by means of an integral jack to engage the ground and form a stable structure. The support leg includes a lower part (54) which is suspended during erection from the upper part of the leg through a gas spring, with the gas spring and the mass of the lower part together constituting a dynamic absorber for the lauch rail during erection. After erection, structural loads are transmitted directly to the lower leg part via a trunnion.