Screwpiles: The Forgotten Lighthouses

Screwpile and Screw Cylinder Design 9 The early lighthouses used a solid five-inch-diameter wrought-iron piling, fitted with a two-foot-diameter screw. The iron caps and tension bars were thought to add enough strength that the foundation would hold up under the demanding marine conditions.

The holding power of the piling was directly proportional to the screw diameter. Mitchell’s tests conducted

during the 1838 construction of Maplin Sands Lighthouse

determined the holding power of a six-inch-diameter screw to be one ton, while a four-foot diameter screw could support 64 tons. The problem with screwpile and screw cylinder foundations in the Chesapeake Bay and North Carolina was not the screw holding power, but rather the damage to the pilings and supporting tension rods from ice floes. The photographs here show the foundation at Wade Point, North Carolina, in 1918 exhibiting a broken piling caused by ice damage.

Wade Point, North Carolina, showing leaning lighthouse structure, February 7, 1918 RG 26 LG 25 64A

This kind of damage was typical where relatively light, five-inch-diameter solid pilings were used. Later designs required as much as 16-inch diameter screw cylinders that were more resistant to ice damage.

Wade Point, North Carolina, foundation, March 26, 1918 RG 26 LG 25 65B

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