Trends Winter 2010 | Page 12

salvageable. The County wanted the design of the new structures to maintain the original character of the railroad trestles. The way to do that is through unique design details, said Hisham Sunna, PhD, PE, principal-in-charge for Ayres Associates. “The aesthetic details are what connect the new bridges to the history and the location,” he said. Designers came up with several key details to help make the pedestrian trail bridges look more like the old railroad trestles, starting with the pilings that support the structures. The pilings are unique in their shape, color, and placement. Designers chose 16-inch round piles rather than more typical piles like 24-inch square. The concrete itself was dyed to look older and darker – more like the old railroad bridge. “Because some of the characteristics of this structure are completely unique, it was challenging to find a fabricator that was willing and able to create them,” said Steven Laney, PE, Ayres Associates’ supervisor of structural engineering. An added benefit of the dyed concrete 12 | is that it would require less maintenance without the need for repainting, he said. The number and spacing of the bridge supports were also purposely designed to mimic the old railroad structure. While today’s typical trail bridge may have only two pilings in each bent, the Roberts and Dona Bay bridges have five to seven pilings that were placed similarly to the old railroad bridges. Perhaps the most striking feature is the look of railroad ties along the profiles of the bridges. “The contractor and the fabricator built a concrete form that would essentially stamp an indentation at regular intervals and recreate the look of the ties,” Laney said. The new bridges are better for the water quality and habitat in this delicate area. “The old trestles were creosote-soaked lumber, and removing them was the best thing that could’ve happened environmentally,” Boudreau said. Creosote is a wood preservative that may have harmful health and environmental impacts. TRENDS TRENDS | 13