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The Bismarck, North Dakota Area Office investigated a fatal accident that involved the failure of a drape strand restraint device during the manufacture of prestressed concrete bridge beams. The process required the tensioning of six extremely high strength 0.60-inch-diameter wire ropes to a tensile load of approximately 47,000 (166,000 psi) pounds each. For load bearing purposes, the wire ropes are then embedded in concrete to create a slight arch along the longitudinal axis of the beam. The tensioning deck is comprised of two sections to simultaneously form two identical beams.
An employer was engaged in manufacturing prestressed concrete beams. To obtain the proper tension and arch, the wire ropes were strung through steel strand restraint devices near the base of the concrete beam. These wire ropes are identified as the "drape strands," which, when tensioned, impose a total upward force of approximately 13,400 pounds on each of the two outside restraint devices for each section. Within each section, the two outside restraint devices were supporting four wire ropes, while the two inside restraint devices were supporting only two wire ropes. The bottoms of the devices were positioned approximately 1.5 inches above the bed of the pouring deck using wood shims under each device. After the proper tension was placed on the cables, the shims were removed to allow the entire device and wire ropes to be embedded in concrete.
The removal of the wood shims was accomplished by using a sharpened piece of rebar, which was approximately 19 inches long by 3/8-inches in diameter. An employee struck the rebar with a hammer, splitting each shim, and then drove the broken shims out from beneath the device with the rebar. Employees were instructed not to lean over the pouring bed while removing the shims.
An employee was in the process of removing the shims when the two outside strand restraint devices (holding four cables) in one section failed. This, in turn, caused the two inside restraint devices (holding two cables) in the same section also to fail, but in a different manner. The two outside restraint devices failed at their base due to tensile and bending stresses. On the two inside devices the bolts holding the cables failed, possibly due to bending stresses. When the drape strand cables released, a cable struck an employee in the head, resulting in a fatality. Because of cold weather conditions at the time of the accident, cold brittle metal fracture also contributed to the restraint device (clevis) failure.
The non-pivoting design of the restraint devices also imposed bending (flexural) stresses in the base of the devices, which further contributed to the failure of the devices.
The following procedures are recommended to eliminate the potential hazard of restraint device failure and the unintended release of the drape strands or other wire cables under tension on the pouring bed:
Figures 2 and 3 show changes made to the process
Figure 3. Close-up View of Restraint Device.
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