Figure 1: Superstructure Detail

The cable-stayed superstructure consists of two edge girders connected by a series of floorbeams that act compositely with a concrete deck slab (see Figure 1). The design features of these components are listed in Figure 2.

Edge Girders. The major force effects in the edge girders are bending moment and axial force. The inclined cables provide vertical support to the superstructure while the horizontal component of the cable tension induces compression. The compression is greatest at the tower and decreases to zero at the end of the side spans. At the middle of the main span, the superstructure may even be in tension.

Floorbeams. The floorbeams were designed as conventional, simply supported beams with an important additional aspect. Namely, torsion will be induced in the floorbeams during construction when the precast deck panels are installed on one side only. Stability under this condition was verified.

Concrete Deck Slab. The concrete deck slab is constructed in two parts, precast panels with cast-in-place concrete
closure strips. The slab is connected to the edge girders and floorbeams by shear studs.

Effective Width Of Composite Deck Slab

Each section of the superstructure will have different axial force distribution because of differing cable inclinations, cable vertical loads and axial force accumulation, plus the shear lag phenomenon. Based on our experience on the Chesapeake and Delaware Canal Bridge, we computed the effective width at each section of each loading case for bending moment and axial force separately.

Figure 2: Design Features

Influence Lines

Influence lines were prepared to determine axial force and bending moment at each section of edge girder due to live load. A 3-D computer model was analyzed linearly for influence lines. Influence lines were generated not only for the edge girder due to a unit load on the same side, but also due to a unit load on the opposite edge girder. The influence lines were also used to determine the loaded lengths of live load needed to calculate impact factors and effective slab width for bending moment and axial force.

Construction Load Cases

Designers of cable-stayed bridges face a major challenge: the design forces and moments can be governed by temporary construction phases as the superstructure is sequentially erected in cantilever out from the tower. As part of the design of the Owensboro Bridge superstructure, we established a detailed construction sequence and performed structural analysis for each of the construction stages, which numbered more than 100. The forces in the superstructure members at each construction stage were considered in the design.

Constructibility

The Owensboro Bridge superstructure was designed to be constructed of easily erected modules. Each module, consisting of 13.7-meter lengths of edge girders connected by three floorbeams, can be lifted by cranes that are available on the market and are of reasonable capacity. In a tightly orchestrated sequence, modules are erected, precast deck panels are placed and cast-in-place closure strips are poured.

Conclusion

The superstructure of the Owensboro Bridge was carefully designed to be competitive with the concrete alternative. It is simple, easy to fabricate, lightweight and can be erected with conventional equipment. It is designed so that each module, splice-to-splice, can be assembled off-site and erected in one piece at the site.