New Tacoma Narrows Bridge Caissons-PDF

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The New Tacoma Narrows Bridge is a suspension bridge built alongside the existing bridge in Tacoma, Washington. Its two towers are supported by large cellular concrete caissons, each measuring 130 by 80 feet and standing 216 feet tall. Construction was especially challenging because the caissons had to be positioned only 60 feet from the existing bridge piers, in currents that could reach seven knots.<br /><br />The caissons were built as floating structures, with steel cutting edges and temporary sealed bottoms. As construction continued, crews added concrete in stages while carefully maintaining freeboard and stability. Scale-model testing and computational fluid dynamics studies helped estimate current forces and design the temporary mooring system.<br /><br />Each caisson was held by 32 mooring lines connected to 500-ton plate anchors driven at least 50 feet into the seabed. These anchors provided a cost-effective, redundant alternative to conventional systems. Most passed proof tests, and the completed mooring arrangement kept the caissons clear of the existing bridge.<br /><br />The caissons were towed from the Port of Tacoma at a draft of about 47 feet, using large tractor tugs. Detailed planning, current monitoring, and staged connections to the moorings were essential to the operation. After landing pads were prepared on the seabed, crews used multiple survey systems to guide each caisson into position. Both landed within about a foot of their targets and well within the specified tolerances.<br /><br />Following touchdown, the temporary bottoms were removed, and the caissons were excavated to their final depth. The project demonstrated that traditional caisson construction methods could be successfully applied with modern modeling, anchoring, survey, and monitoring technology.

Keywords

New Tacoma Narrows Bridge

suspension bridge

Tacoma Washington

cellular concrete caissons

moorings

plate anchors

current monitoring

scale-model testing

computational fluid dynamics

bridge construction