In some European countries concrete has replaced steel and cast iron for many heavy machines with substantial technical and economic benefits. In general, the cost of a concrete machine element has been found to be from 50 to 70 percent lower than its metal equivalent, with a weight saving between 50 and 90 percent. Designers have found that the simplest way to use reinforced concrete for machine frames, is to attempt to duplicate the original steel shape. Prestressing has been used because of the greater advantages that follow if the concrete is prestressed in two directions. A slight excess of prestress is usually applied to take care of service losses due to shrinkage and creep, to avoid the danger of cracking and to allow both concrete and steel to be proof tested before operation of the machine begins. Tendons are left ungrounted to adjust the stress later or even to replace the tendons. Prestressing virtually removes any limitation on the size of an element. Any series of units can be cast separately and then prestressed together to convert them into a monolith. This factor is very valuable for combating vibration. It then becomes possible to "tune" the structure so that it will not vibrate at its resonant frequency.