The required groundwork for excellent aesthetic opportunity can be obtained at conceptual design stage of bridge projects. Not only do overall prestressed bridge geometry and alignment are specified at conceptual design phase but also structural depth, span length, and dimensioning of both superstructure and substructure members are established.
These bridge members are specified depend on site analysis and decisions on the most suitable bridge to produce a bridge that is both functional and aesthetically appealing. At conceptual design stage, it is required to apply engineering judgment and understand how to optimize construction to balance economy, functionality, and prolonged aesthetic quality.
It is demonstrated through experience that, not only visually pleasing bridges can be constructed at moderate construction costs but also the construction costs may be cut if efficient design is employed, the Natchez Trace Parkway Arches project is a compelling example which is shown in Figure-1.
So important is the consistency in the design of members and shapes that result in achieving efficiency and appealing aesthetics. Consistency in form, line, and pattern could lead to a uniform appearance among members of a bridge.
When the same cross section is used for superstructure elements, it makes visual continuity that consequently lead to more aesthetically appealing appearance and rise efficiency in construction because of repetition in the form, the Chesapeake and Delaware Canal Bridge near St. Georges which is shown in Figure-2 is an example of this approach.
Aesthetics Consideration of Long Span Prestressed Bridges at Conceptual Design Phase
- Alignment of prestressed bridge
- Prestressed bridge span length
- Structural depth of prestressed bridge
- Span to depth ratio of prestressed bridge
Fig.1: The Natchez Trace Parkway Arches Bridge demonstrates efficient design resulting in economical Construction
Fig.2: The Chesapeake and Delaware Canal Bridge in Delaware uses repetition of form and construction
Alignment of prestressed bridge
Bridge alignment is the initial and most substantial influential factor on the entire approach to design and aesthetics of the bridge. By and large, apart from the cases where new roadways are constructed, existing roads considerably affect the alignment of bridges.
The bridge, which may be new structure, parallel structure, or replacement structure, needs to fit within the roadways network. There are several factors that influence the bridge alignment for instance, elevation and grade of suggested or constructed roads, the terrain that the structure travel through, and features such as railroads, roadways, water bodies, or proposed or existing site constraints which the bridge crosses.
The length of the structure, specifically visually dominant main span length, control structure type and construction technique which most appropriate. Various aspects of construction method for instance delivery of materials, whether erection is carried out traffic or navigational channel, or whether environmentally sensitive areas need another level of special care or not is influenced by bridge alignment and location.
Finally, the entire aspects of construction methods must be taken into consideration while bridge alignment is specified.
Prestressed bridge span length
Not only does the entire bridge length but also individual span length depends on the existing constraints, possible construction method, and bridge alignment.
The span length is specified after the entire bridge and alignment is determined and site constraints are recognized in addition to determine unit configuration to form continuous structure of several spans and consequently decrease number of expansion joints.
Moreover, the purpose of span length determination is to specify optimum span length that can be used for as much of bridge length as possible. This lead to employ repetition which increase construction efficiency and thus the construction method will be more economical.
Furthermore, repetition of span length produces continuity in appearance and consequently aesthetically appealing bridge will be constructed.
Lastly, span length together with span depth is the most significant factor of creating aesthetic quality of the structure.
Structural depth of prestressed bridge
There are different parameters that affect structural depth determination. Generally, keeping the same depth for box girder and maintaining constant cross section are considerably decrease complication of casting and erection operation.
Combined with needed vertical clearance determination, the depth of the structure is addressed by assessing all spans and units to specify condition that controls and the minimum structural depth required.
However, in the case the length of the span is large it is more economical to apply different structural depth rather than using constant depth. large forced are close to piers whereas smaller forced are near to mid span, that is why deeper box girder is necessary at the former while smaller sections are required at the latter.
So, the depth of the section is changing along the span length to decrease both quantities and self weigh of the structure. This can be obtained with attractive, graceful, sweeping curves which improve the aesthetic appearance by making it more slender and elegant.
A compelling example of this is Four Bears Bridge as shown in Figure-3.
Fig.3: Four Bear Bridge in rural North Dakota
Span to depth ratio of prestressed bridge
Specifying excellent span to depth ratio is a principal parameter in the entire aesthetic appearance of the structure. During creating visually appealing structure, optimization of structural efficiency of the superstructure girder box cross section can be achieved through various available choices.
Normally, producing the best aesthetically appealing span length and superstructure depth is an iterative operation. It is claimed depend on experience that, excellent aesthetic bridge can be produced when utilized span to depth ratio is from 20 to 30.
Moreover, span to depth ratio of 15 on uniform spans is assumed to be pleasing aesthetically however, it is not suitable to use span to depth ratio smaller than 15. Furthermore, when span is increases, the forces close to the piers are commonly needs changing in structural height.
If clearance requirements permit the circular variations in the depth of the bridge is more visually appealing in comparison with linearly changing the depth of the bridge for instance Wabasha Freedom Bridge which is shown in Figure-4.
Fig.4: The Wabasha Freedom Bridge with span to depth ratio of 20