413 Orthotropic Plates

Plate equations are applicable to steel plate used as a deck. Between reinforcements and supports, a constant-thickness deck, loaded within the elastic range, acts as an isotropic elastic plate. But when a deck is attached to reinforcing ribs or is continuous over relatively closely spaced supports its properties change in those directions. The plate becomes anis-tropic. And if the ribs and floorbeams are perpendicular to each other, the plate is orthog-onal-anistropic, or orthotropic for short.

An orthotropic-plate deck, such as the type used in bridges. resembles a plane-grid framework (Art. 4.11). But because the plate is part of the grid. an orthotropic-plate structure is even more complicated to analyze. In a bridge, the steel deck plate, protected against traffic and weathering by a wearing surface, serves as the top flange of transverse floorbeams and longitudinal girders and is reinforced longitudinally by ribs (Fig. 4.29). The combination of deck with beams and girders permits design of bridges with attractive long, shallow spans.

Ribs, usually of constant dimensions and closely spaced, are generally continuous at floorbeams. The transverse beams, however, may be simply supported at girders. The beams may be uniformly spaced at distances ranging from about 4 to 20 ft. Rib spacing ranges from 12 to 24 in.

Ribs may be either open (Fig. 4.30a) or closed (Fig. 4.30b). Open ribs are easier to fabricate and field splice. The underside of the deck is readily accessible for inspection and

FIGURE 4.29 Orthotopic plate.

maintenance. Closed ribs, however, offer greater resistance to torsion. Load distribution consequently is more favorable. Also, less steel and less welding are required than for open-rib decks.

Because of the difference in torsional rigidity and load distribution with open and closed ribs, different equations are used for analyzing the two types of decks. But the general procedure is the same for both.

Stresses in an orthotropic plate are assumed to result from bending of four types of members:

Member I comprises the plate supported by the ribs (Fig. 4.31a). Loads between the ribs cause the plate to bend.

Member II consists of plate and longitudinal ribs. The ribs span between and are continuous at floorbeams (Fig. 4.31b). Orthotropic analysis furnishes distribution of loads to ribs and stresses in the member.

Member III consists of the reinforced plate and the transverse floorbeams spanning between girders (Fig. 4.31c). Orthotropic analysis gives stresses in beams and plate.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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