1502000

Akaski-Kaikyo, Japan, 1991 m

FIGURE 25.4 Firth of Forth Bridge and bridge model in U.K. (1980).

A structure designed in harmony with one other not only appeals to our hereditary sense of beauty, but also behaves a smooth load transfer path. Figure 25.4 shows the Firth of Forth Bridge built in Scotland, U.K., in 1890 and the bridge model. This bridge is a gain steel railway bridge with a main span of 521 m. The bridge designer Baker stated (Collins 2001) that

When a load is put on the central girder by a person setting on it, the men's arms and the anchor ropes come into the tension, and the men's bodies and the sticks come into compression. The chairs are representative of the circular granite piers. Imagine of chairs one third of a mile apart and the men's head as high as the cross of St. Paul's, their arms represented by the huge lattice steel girders and the sticks by the tubes 12 ft (3.6 m) in diameter at the base, and a very good notion of the structure is obtained.

25.2.4 Esthetics — Harmonizing Surroundings

A bridge is required not only to fulfill its function as a thoroughfare, but must use its structure and form to blend, harmonize, and enhance its surroundings. Although there are different views regarding esthetic practice in bridge engineering, the following guidelines presented by Svensson (1998) may be useful:

• "Choice of a clean and simple structural system,'' like a beam, a frame, an arch, or a suspended structure, the bridge must look trustworthy and stable.

• "Good proportion in all three dimensions'' between the structural members or between length and depth of bridges openings.

• "Good order of all the lines of edges of a structures,'' which determine the appearance. One should limit the number of directions that cause unrest, confusion, and worried feelings. For the transition from straight lines to curved lines, the curvature should steadily increase like a second-order parabola.

• The compatible integration of a structure into its environment, into the landscape of city. This is specially important with regard to the scale of the structure compared to the scale of the surroundings.

• The choice of the materials has considerable influence on the esthetic effects.

• Simplicity and restriction to the pure structural shape is important.

• Pleasing appearance can be enhanced by color.

• The space above the bridge should be shaped in such a way that the driver experiences the bridge and gets a comfortable feeling.

• A structure must be designed that the flow of forces is evident to the causal observer.

• Moderate esthetic lighting can enhance the appearance of a bridge at night.

Figure 25.5 shows a conceptual design of Ruck-a-Chucky Bridge crossing American River about 17km from the Auburn Dam in California. This horizontally curved cable-stayed bridge spanning 396 m was designed to anchor cables in the hillsides. Although the bridge was never built, the design fits the topography, the surrounding environment, and is a real well-conceived design (Lin 2001; Ruck-a-Chucky 2003). It was awarded by Progressive Architecture magazine in 1979. For more detailed discussion about esthetics, references are made to Leonhardt (1984, 2000), Billington (1983, 2000), and TRB (1991).

25.2.5 Bridge-Type Selection — Comprehensive Decision

The selection of bridge types is a complex task to achieve the owner's objectives. Table 25.2 shows a sample evaluation matrix, which maybe used to select the bridge types. For the items listed in Table 25.2, a priority factor may be assigned from 1 to 5 (1 = low, 2 = standard, 3 = high, 4 = very high, and 5 = extremely high). The quality rating can be on a scale of 1 to 5 (1 = poor, 2 = fair, 3 = good, 4 = very good, and 5 = excellent). The weighted rating is obtained by multiplying the priority factor with the quality-rating

FIGURE 25.5 Conceptual design of Ruck-a-Chucky Bridge (courtesy of T.Y. Lin).
TABLE 25.2 Bridge-Type Evaluation Form

Bridge type

Item

Priority Quality

Weighted rating

(1)

(2) (3)

(2) X (3)

Structural Traffic

Constructibility Maintenance and inspection Construction schedule impact Esthetics Environmental Future expansion Cost

Total rating

Structural Traffic

Constructibility Maintenance and inspection Construction schedule impact Esthetics Environmental Future expansion Cost

Total rating

FIGURE 25.6 San Francisco-Oakland Bay Bridge — east span.

factor and summed for each bridge alternative. The bridge type with the highest total score shall be the best candidate.

The collapse of a portion of upper and lower decks of the eastern portion (Figure 25.6) of the San Francisco-Oakland Bay Bridge (SFOBB) in the 1989 Loma Prieta Earthquake, California, demonstrated the critical need for seismic safety on bridges in San Francisco Bay Area. California Department of Transportation (SFOBB 2003) determined that it is more cost-effective to replace the existing east span with a new bridge than it would be to seismically retrofit the existing structure (Figure 25.6). Figure 25.7 to Figure 25.10 show four structure alternatives for the new SFOBB east signature span across the shipping channel. The Engineering and Design Advisory Panel (SFOBB 2003) for the new SFOBB east span adopted the following guidelines:

• The bridge should integrate into the site and surrounding environment by reflecting the grand scale of San Francisco Bay, by harmonizing with the existing western span of the bridge, and by landing gracefully on the Oakland shore and Yerba Buena Island.

FIGURE 25.7 San Francisco-Oakland Bay Bridge east span — cable-stayed alternative 1.

FIGURE 25.8 San Francisco-Oakland Bay Bridge east span — cable-stayed alternative 2.

• The replacement bridge should, by contrast or similarity, complement the existing Bay Bridge suspension span. One bridge should not diminish the visual quality or importance of the other.

• The new bridge should be visually memorable and convey a sense of gateway to Oakland.

• Views from the bridge when traveling toward Oakland should consider Oakland's central business district and waterfront.

• The bridge should provide a measure of visual continuity for motorists.

• The girders, piers, and rails should generally appear slender and provide for views by motorists on the bridge.

• Guardrails and handrails should be designed for maximum transparency to maintain views while meeting safety criteria.

• Night lighting on the bridge is an important consideration.

The single-tower self-anchored asymmetrical suspension span (180 m + 385 m) with a steel tower and a steel orthotropic deck (Figure 25.10) was the final selection from the above four alternatives (Nader et al. 2001). The bridge is under construction and expected to open to traffic in 2007.

FIGURE 25.9 San Francisco-Oakland Bay Bridge east span — suspension alternative 1.
FIGURE 25.10 San Francisco-Oakland Bay Bridge east span — suspension alternative 2 (selected).

Figure 25.11 shows a comparison of the four design alternatives evaluated for Minato Oh-Hashi in Osaka, Japan. The truss frame design was selected.

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