How the Lake Natoma Crossing columns were built

The Lake Natoma Crossing is supported by 36 concrete columns in nine rows of four. Though some columns extend as deep as 80 feet into the bedrock below the bridge, crews didn't enter the water to build them.

Instead, they worked from cranes and a temporary platform called a trestle. They first sank cylindrical steel casings, 8 feet wide and up to 80 feet long, into the water, anchoring them in the bedrock. Then they pumped out the water from the casings and drilled out the river bottom and bedrock.

Once the shaft, or well, was drilled, workers fit a reinforcing steel cage into the casing and filled it with concrete, embedding the steel. When the concrete had hardened, or cured, workers fit another steel cage above the ground, overlapping the first one. This cage is tapered at the top to define the lines of the column.

The columns were shaped by a red plastic form that was fastened around the steel cage and filled with concrete. After 24 hours, it was removed and the columns were wrapped in plastic to cure for seven days. The plastic kept the concrete from drying too fast. 

After the columns cured, workers built a second form, this one of wood, to shape the decorative tops, or capitals. These forms also were filled with concrete and cured for seven days. Last, seismic isolation bearings were placed on the capitals.

Artwork by Kristine Bybee