Prepared by Construction Marketing * Bethlehem Steel Corporation * Bethlehem, PA 18016

Truss Bridge Report Page 3

Steel Truss Competitive for Sixth Street Bridge at Huntington, WV

Background and bid results

The existing Sixth St. Bridge, located in Huntington, West Virginia, had reached the limits of its useful life and a replacement structure was in order. Preliminary studies were made to determine the most cost-effective structure for the site. Being a major river crossing with an estimated cost well in excess of ten million, two alternates were chosen for final design: one steel and one concrete. The steel alternate chosen was a variable depth through Warren truss with a center span of 720 feet and anchor spans of 450 feet. The concrete alternate was a symmetrical precast cable-stayed bridge with a main span of 740 feet and end spans of 370 feet. Both alternates had conventional approach spans for their respective materials, i.e. plate girders or prestressed beams. The two alternates were designed by different consulting firms, each recognized as leaders in the field of long span bridges, under the direction of the West Virginia D.O.T. In June 1991, the bids were opened and the steel truss was found to be the most economical alternate by a substantial margin. Five bids were received for the steel alternate and two for the concrete alternate. The low steel bid of $23,728,600 was approximately 11% less than the low bid for the concrete alternate of $26,468,212. The low bidder was the C.J. Mahan Construction Co., of Grove City, OH.

Description of sixth street bridge truss spans

The truss will carry two 12 foot traffic lanes in each direction, as well as a 5 foot sidewalk for an overall width of 71’-10". Continuous through Warren trusses centered at 75 feet carry the floor system, which is composed of an 8" thick conventionally-reinforced concrete deck, simply-supported composite stringers and plate girder floorbeams. The 720 feet center span is flanked by 450 feet anchor spans with truss heights varying from 90 feet over the main river piers to 45 feet at the end portal. Panel point spacing is 45 feet. Truss members consist of box-shaped chord members, welded "H"-shaped verticals, and welded diagonals of both box and "H" shapes. Bracing members are fabricated from rolled sections. The superstructure is supported by main river piers and anchor piers consisting of prestressed concrete caps, conventionally-reinforced concrete stems and footings.

Approximately 570 tons of HP14x73 steel bearing piles were driven to support the concrete substructure elements. Roughly 5,500 tons of fabricated structural steel were required for the truss spans, while an additional 700 tons were needed for three (3) welded plate girder approach spans. Bethlehem furnished 1,102 tons of AASHTO M270 Grade 50 plates, 666 tons of Grade 36 plates and 382 tons of Grade 36 shapes for this project.

Studies Provide Cost-Effective Design

The normal pool of the Ohio River at Huntington is more than 35 feet above competent rock. In an effort to minimize the high cost of deep water foundations, the anchor spans provided were longer than those customarily found on three-span trusses so that the anchor piers would be located outside of the normal river flow. Not only was the need for two cofferdams eliminated (at an estimated savings of $750,000), but also the forces generated by the resulting span ratios were found to be better balanced, resulting in a more efficient superstructure. Further savings in deep water foundations were generated by reducing the actual size of the footings required for the two main river piers. This was accomplished by providing hammerhead piers with post-tensioned caps, instead of the conventional wall type pier.

A study of the floor system was conducted to determine the most economical arrangement of supporting members. By providing simple span stringers located between the floorbeams, which are located at the panel points, an 18% savings in weight of steel was realized. It was found that the increase in moments generated by the simple span stringers, in comparison to continuous stringers, was more than compensated for by being able to take advantage of the fully-braced, complete composite rolled beams. In addition, due to the constant panel-to-panel reaction generated by the simple spans, all interior floorbeams (35 total) were identical, except for the end-plate connections to the truss.

Traditionally, in order to relieve the internal stresses in the floor system caused by casting the deck, a series of stringer relief joints are strategically located throughout the length of the bridge. Historically these joints, which are similar to compression seal dams, are subject to leakage. Because they are located over floorbeams, they can cause considerable damage to primary elements of the truss over time. By staging the casting of the deck slab pours and the final tightening of the simple span stringers, the stringer relief joints were replaced with temporary deck block outs, and the final slab is continuous from one end of the truss to the other.

Bridges

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