The North Halsted Bridge Project incorporates quite a variety of curved steel sections rolled in quite a variety of ways to achieve some rather spectacular effects particularly with its lighting.
Start with the curved box girders fabricated from steel plates rolled for the top and bottom sides of the boxes and steel plate cut out to form the sides of the boxes. 2ft-6in wide by 3ft-6in deep steel tie box girders are built up from these four plates joined by using bolted angle connections in each corner.
Welded together the resultant curved box girders form the two main arches of the bridge. Their path traces a parabolic curved with a vertical rise of 35ft and a span of 157ft.
Other curved steel sections for this bridge include 1-1/2in x 30.45in flats rolled against the weak axis to a 2ft-5in inside radius with 41 degrees of arc and with straight tangents at each end measuring 2-1/2in and 72-1/2in. What was unusual about bending these flats was that they were to be bent hot. The bend areas were preheated to 850 degrees F. because their radius is below the specified radius for cold-forming fracture-critical steel. Along with bending hot, another requirement was to “ease” or round the edges of plate in the bend area to further reduce the risk of cracking. The Chicago DOT, the engineer of record, the general contractor, the structural steel fabricator and the erector all signed off on the procedure.
The North Halsted Bridge has lights along the length of the top arch. Obtuse formed steel angle made from 1/4in bridge quality steel carries and protects this lighting system. The angles were painted several times in the field (final color blue), and have other architectural features added. A curved 4×4 structural angle could have performed the same function, but the rounded corner and sloped edges make for an attractive transition from the heavy girder to the lighting.
So . . . flat plate curved for curved box girders, flat plate curved hot for fracture critical areas, and formed obtuse angle curved for an elaborate lighting system all combined to achieve the spectacular appearance of the North Halsted Bridge in Chicago.