A jib, used to extend the reach of a rough terrain crane boom, fell from its stowed position on the boom at a highway construction site, seriously injuring a worker below it. The subject crane was being used on-site with a vibratory hammer suspended from the boom to drive caissons for bridge construction. One of the attachment points of the jib consisted of a bracket with a hole that engaged a pin on the crane boom. This bracket was attached to a lattice tube on the jib with two U-bolts. The jib released at this attachment point and swung down after the U-bolts failed.
Our Approach
ESi inspected the subject crane and performed a laboratory-based inspection of the failed U-bolts. The U-bolts’ chemistry, hardness and microstructure conformed with drawing specifications as well as those measured and examined in exemplar U-bolts. Optical and scanning electron microscopic examination of the U-bolt fracture surfaces confirmed one of the bolts had a significantly long transverse fatigue crack emanating from its thread root. There was extensive evidence of fretting vibratory wear on the jib-boom connections. Witness marks consistent with the brackets having been re-positioned in the field were also observed. ESi participated in and video documented exemplar crane testing at the manufacturer in which strain gages were used to monitor vibratory forces during static and normal dynamic crane operation in properly assembled jib connection bracket U-bolts. Microscopic examinations before and after this testing revealed no cracking in the U-bolts. An ESi analysis of the bolted connection formed by the U-bolts also confirmed their design adequacy.
Services Utilized
Expertise Utilized
The Outcome
It was concluded that the crane had been misused leading to the U-bolt failures. The use of a vibratory hammer transmitted cyclic loading that caused the fatigue crack to form and propagate in one of the U-bolts leading to its failure. The remaining U-bolt alone could not support the jib weight, and it failed in overload. Unfortunately, when the jib swung down, counter to OSHA safety regulations, a worker was underneath and was struck and injured. The case went to trial , ESi testified on behalf of the defendant crane manufacturer, and the jury returned a verdict for the defense.
