Performance and Rehabilitation of Type L FAA Airport Traffic Control Tower at San Carlos, California, for Seismic Loading

This paper presents a seismic evaluation of the San Carlos airport traffic control tower (ATCT) in San Carlos, California, that describes the tower, presents the seismic evaluation of the tower, and defines a rehabilitation approach that upgrades the structure to applicable life-safety standards. The San Carlos ATCT is a Federal Aviation Administration Type L tower, whose primary structure below the control cab (tower shaft) is four inverted L-shaped reinforced concrete members that frame together at the top of the tower shaft. The cab structure is a steel-moment frame. The tower was analyzed with a finite-element model using a response spectrum linear analysis. The goal of this evaluation was to determine if life-safety performance could be demonstrated, and if not, to develop an upgrade approach to achieve this performance. The seismic hazard was based on the maximum considered earthquake defined in the National Earthquake Hazards Reduction Program (NEHRP) recommended provisions. The evaluation shows that the tower cannot achieve life-safety performance due to large deflections in the cab and the formation of a collapse mechanism. The connection base plates for the cab columns (corner window mullions) will form a collapse mechanism at very low seismic motions. Formation of the collapse mechanism is due to base-plate bending failure and subsequent hinging at the base of each column. The recommended rehabilitation approach is to upgrade the columns to reduce deflections to acceptable levels, and also to protect the vulnerable connections. Upgrading the columns consists of welding plates on the interior and exterior column faces and welding deep structural tubing members to the base of each corner column. The writers considered the evaluation of this particular control tower to be of interest to the technical community because of the unique failure mechanism and proposed upgrade approach.