Segmental Bridge Aesthetics

The opportunities for superior aesthetic treatment of segmental bridge box design and its effects are numerous, as explained and illustrated in this article.

Figure 1: High Slope Segmental Concrete Box

A major benefit of segmental design is that the precast elements achieve a higher quality of finish and greater strength than cast-in-place box girders so, from a finish standpoint, they are more attractive visually. Most importantly, however, is that they can span greater distances through the combination of post-tensioning and higher compressive strength of the segmental units, so they allow for a thinner, more graceful profile with fewer columns and greater openness on the ground level. The reduction of visual “clutter” is significant, as are the possibilities for easier circulation and improved  sight  lines  at  ground  level due to the wider spacing of the columns.

 

 

 

 

 

 

Figure 2: High Slope Segmental 
Concrete Box

Illustrating the Advantages

Figure 3: Curved Alignment

Figure 4: Highly Sculpted 
Segmental Bo

Looking at the actual section of the segmental box, there are significant advantages that become apparent. First, being precast, the form can be more complex with greater sculptural possibilities because there is a high degree of repetition in the use of the forms. Second, the section can vary internally or with the use of alternating sections to provide special structural capacity for unusual conditions. While the conventional box will have webs that are near vertical (Figures 1 and 2), you can slope the webs more acutely and achieve a “richer” visual shape, as illustrated with the section shown in Figure 1. With a constant radius, you can introduce curves into the alignment and use segmental boxes (Figure 3). Incorporating curves into the section, you can create a very powerful sculptural form (Figures 4 and 5).

Figure 5: Highly Sculpted 
Segmental Concrete Box

We explored this latter option on a project in Bangkok i.e. Rama VIII Bridge. Our research indicated that the curved shape was competitive with cast-in-place concrete because we were able to achieve longer spans, required no form work along the alignment (through the use of launching trusses), and could complete the work 1 year to 1 1/2 years faster than with the cast-in-place option. 

Figure 6: Side Platform Structure - 
High Sculpture With Outrigger Frames

As mentioned earlier, alternating sections can be inserted between the standard section to get either a wider roadway or, as shown in Figures 6 and 7, cantilevers to carry side platforms for a light rail system. Other elements of the structure can be shaped to reinforce the visual theme of a bridge/viaduct. In Figure 3, the columns are elliptical in shape with vertical “steps” at the ends and a stepped flare at the top to create an Art-Deco/Modern Gothic theme. The parapet can be shaped to add to the theme, as shown in Figures 3 and 8.


A shaped glass fiber reinforced concrete light-weight concrete/fiberglass panel is attached to the outside face of the structural parapet in Figure 8. In this case, the segmental box was shaped as a “stepped” triangle to give a clean aeronautical-look, which the knife-edged parapet panels emphasize with their similarity to the leading edge of an airplane’s wing. The column is elliptical in section with a step on each side and a chevron-pattern of reveals that highlight the “V” shape of the segmental box girder.

Figure 7: High Sculpture with Outriggers Segmental Concrete Box

Figure 8: Triangular Concrete Box Girder

Lastly, the modular elements can be formed to give a tremendous variety of alignment geometry and solve unusual structural requirements. A cable assisted “C”-bent was devised to carry the curving guideway across a narrow intersection in a study for a light rail system in southern California (see Figure 9). The C-bent allowed the elimination of two or more straddle bents, thereby taking less land, reducing the visual clutter of columns in the streetscape and, best of all, creating a unique and beautifully soaring structure that emphasizes the smooth curving, sculptural quality of the segmental box guideway. 

Conclusions

 

Figure 9: Cable-Assisted C-Bent

Segmental box technology raises engineering to an art form when the sculptural possibilities are incorporated into the section, columns and column spacing, and parapet shapes. Proper collaboration with the architects and planners on a project can turn a civil structure into a “civic sculpture” that enhances the urban or rural setting in which it is built and creates a lasting visual form that is as enduring and beautiful as the Roman Aqueducts or the masonry bridges of Europe and Asia.