The Case of the 2000' Tall Collapsed Antenna Tower
Structural tower upgrades to the steel antenna tower were required to comply with FAA mandates about aviation lights and loads for high-definition TV antennae. The work crew was replacing the existing diagonals with larger diagonals and new struts in specific tower panel locations near the top of the tower. The work was nearly completed prior to the tower collapse.
The proposed design upgrade by the structural engineer was determined to be adequate to support all required loads. Wind was not a causal factor in the tower collapse. The workers near the top of the tower simultaneously removed a diagonal and a horizontal strut. These actions diminished the load carrying capacity of the leg to the extent that the leg was unable to support the loads. Ancillary contractors on this project, working at ground level, did not contribute to the tower collapse.
Simultaneous construction activities at specific sections of the tall tower, immediately before the incident, caused the tower collapse. Removing bolts at the intersection of diagonals, struts and redundant members were critical to the structural integrity of the tower. The legs lost the capacity to support the loads when the diagonals and struts were removed concurrently. The structural engineer did not adequately warn the contractor about the criticality of the combined interaction and the diminished load carrying capacity of the diagonal, strut and bolts to the stability of the entire tower.
The Lessons Learned:
During upgrades and replacement of tall tower diagonals, struts and bolts, the contractor must always use temporary bracing members or devices when necessary. Planned construction activities, planning and safe practices constitute interim substitute measures for distributing the critical load carrying capacity of upgraded tall tower sections and legs.