Towers in Motion (available in Structural Engineering Special Section only)

by Larry Light, P.E., Retired Struct. Engrg. Div. Mgr.; J. Ray McDermott Engrg., LLC, Houston, TX,
Cheng-Yo Chen, P.E., Struct. Engrg. Staff Consultant; J. Ray McDermott Engrg., LLC, Houston, TX,

Serial Information: Civil Engineering—ASCE, 2000, Vol. 70, Issue 8, Pg. A2-A7

Document Type: Feature article


Undersea towers are typically designed to rigidly resist the forces of water and wind. But one type of tower is designed to be flexible and, rather than rely on its platform strength to rigidly resist all of the wave force, to use its inertia to resist much of the force from storm waves. Called compliant towers, they were developed to counter the high stiffness requirements of conventional fixed platforms in water depths exceeding 1,000 ft (300 m). These towers are relatively slender bottom-founded platforms whose dynamic interaction with storm waves acts to impart less overall shear and overturning moment on the tower than would be the case if calculated statically. The inertia of a compliant tower and the water entrapped and enclosed by tower members resist wave-induced deflections and thus reduce the strength that must be built into the tower. The motions of a compliant tower help in another way, too: They affect the relative velocities between the tower members and the waves to produce damping that reduces harmonic vibrations. A tower can be made compliant by giving it freedom to sway about its base, by decreasing its cross section to reduce its stiffness, by combining a reduced cross section with increased foundation flexibility to make the structure flexible yet stable, or by building a hinge, or articulation point, in the lower part of the tower and providing long tubular members to act as springs connecting the upper and lower parts of the tower legs.

Subject Headings: High-rise buildings | Wave forces | Wind engineering | Wave velocity | Storms | Stiffening | Motion (dynamics)

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