In the bender/roller industry a compound curve is comprised of bending a steel member across both its strong and weak axes, or as some may describe it, as being curved in both plan and elevation. This type of bend can be seen in some architectual entrance canopies on rather larger buildings or in this application as a brick lintel for a specialty arched, bay window.
Some steel shapes are able to be curved in a compound fashion more easily than others. A steel angle is probably one of the harder shapes to roll compound. When rolled, its neutral axis forces the angle to naturally curve in an “Apex/Heel-Out” orientation rather than being truly compound. If this issue is not forseen by the bender/roller, then the member does not retain correct orientation throughout the piece and in most instances will not fit when hung in its final destination. A true compound bend allows the member to be bent in both plan and elevation while retaining its plumbness throughout the curve. So when steel angles are required with compound curves, most experienced rolling companies will revert to more simple shapes and simply fabricate the member through welded bars in order to retain better control and accuracy during the rolling/bending process.
Depending on the size of the angle iron specified by the architect/engineer, two similar or different width flat bars are rolled separately in order to achieve the horizontal and vertical legs of the angle. In one instance a 3/8 x 4 flat bar was used for the horizontal leg and a 3/8 x 6 flat bar was used for the vertical leg. When combined they make a 6 x 4 x 3/8 angle. Each flat bar is curved individually to the desired specifications of the compound curve. Once both of the pieces are formed, they are then fitted up in a jig for the flat bars to maintain their position in relationship to one another during welding and any further fabrication. Finally, hard-way, curved flat bars are welded on to each end as tabs, so the compound lintel could easily rest on the brick when fitted in its final position.