Early in the morning of October 12, 1996, four workmen in Cedar Hill, Texas, were hoisting a gin pole up the southeast face of the KXTX-TV transmission tower, with the intention of installing a new antenna atop the 1500-foot guyed tower. The gin pole fell to the ground, trailed by the collapsing transmission tower. The National Weather Service reported 14 mph winds at Red Bird Airport, Dallas, about 8 miles to the northeast, that day.
With few eyewitnesses to question, Exponent’s investigation relied heavily on engineering analysis and computer simulations of the failure. Elements of Exponent’s study included site inspections, laboratory testing of material collected from the collapse debris, and hoist testing.
During the jumping operation, in which the gin pole is hoisted along the tower, the gin pole’s center of gravity is usually raised above the jump plate. As the center of gravity of the gin pole is elevated above the jump plate, the gin pole becomes more sensitive to wind loads and is more prone to sway. At a given elevation above the jump plate, if the gin pole is subjected to a wind load below a critical magnitude, the pole will swing like a pendulum and return to a vertical position. However, at the same elevation, if the gin pole is subjected to a wind load above the critical magnitude, the pole’s center of gravity will swing past the tower leg.
Based on the evidence provided, the workmen did not discern the impending danger until an unexpected increase in wind speed caused the gin pole to sway too far. The rotation shifted the load to the northeast sling and caused the northeast arm of the jump plate to yield. The southwest arm of the jump plate then yielded, and the gin pole ripped away from the jump plate due to the sudden deformations.
The gin pole fell and struck a guy wire, which in turn caused shear failures in two sections of the tower. This triggered a series of failures in the tower sections that eventually brought the majority of the tower to the ground. In short, the failure of the gin pole/jump track and plate assembly, and the subsequent collapse of the transmission tower, were caused by an abrupt increase in wind speed, which overwhelmed the unsuspecting workmen.