Case Studies : Cooper River Bridge : Parallel Chord Continuous Truss
 The parallel lines of the constant-depth truss communicate the feeling of a visually cleaner, more open structure, as shown in Figure 1.2. and Figure 1.3. This is in contrast to the trusses of previous generations, many of which appear unsightly due to a jumble of bracing, lacing bars, and pin-connected joints, as shown in Figure 1.4. The elimination of unnecessary bracing, together with the fact that many members are of the same length, makes the parallel chord truss easy and economical to fabricate.
Lateral bracing in the form or X- and K-bracing, is needed in the plane of the floor system, and in the plane of the top chord. This bracing is used to insure integral functioning of the trusses and to provide a load path for transmitting horizontal loads to the foundation. Loads arising from wind and earthquake, which act perpendicular to the truss and lie in the plane of the top chord, are transferred to the supporting piers through strengthened web members. Dead load is minimized in the main span by combining lightweight concrete (115 pcf) in the main span floor system with normal weight concrete (150 pcf) in the anchor spans. The floor system is comprised of three essential parts. Floorbeams are the strong beams which span between trusses, normal to the direction of traffic, and are connected at the bottom chord panel points, or "joints" of the truss, as shown in Figure 5.2. Stringers are the smaller beams which rest on top of the floorbeams and run along the direction of traffic. Typically these beams are spaced between five and ten feet apart. For the Cooper River Bridge, the spacing is 7'-11". Both the stringers and floorbeams are steel. The reinforced concrete deck slab rests on top of the stringers. In this bridge, the slab is 7-1/2" thick.
The consultant's feeling about the parallel chord alternative (ref. 4) : The parallel chord truss alternative provides for an efficient utilization of materials, and would present a bold, contemporary crossing over the Cooper River. It would be a new type of structure for a large highway bridge in the United States, since none have been built here for that purpose. This type of structure is sometimes used in the U.S. for utility crossings, but many highway and railroad bridges of this type have been built throughout Europe since World War II, primarily because of aesthetics. This structure eliminates the busy, criss-cross appearance of most other truss types when viewed from different angles. There are no known problems which have developed on parallel chord truss highway bridges in Europe. No figures for this section. |
