Equipment Failure

The following narrative describes a coal bin failure:

One fall morning, workers at a preparation plant were ferrying mined coal over a series of conveyor belts toward the main coal bin. The coal bin served the dual purpose of storing and funneling coal into trucks for transportation. As normal daily operations ensued, some nearby workers witnessed the sudden catastrophic collapse of the coal bin. This collapse resulted in no deaths or injuries, but did produce significant damage to the bin and supporting structures. Furthermore, the plant operations were virtually shut down.

The coal bin was essentially a box frame structure composed of steel plates and rolled shapes. The bin was supported by four I-columns and is erected over a steel cone chute, which funnels coal as it is dropped into the bin. The cone was connected to the bin by a series of bolts along the cone top. The inspection revealed that the bottom cone chute collapsed from its attachment to the supporting bin. The southwest corner of the cone, which located the initial point of failure, contained several bolts which had sheared. Most notable of these bolts was the fact that significant corrosion and rust existed along the shear plane of the bolts, indicating that these bolts failed some time prior to the collapse. It was noted that the bolt heads could pass through several other holes along the top of the cone due to elongation. The elongation indicated that the bearing strength of the holes was exceeded. Evidence of fatigue was presented by the elongation of the holes and wearing of the bolt threads. The failure also resulted in impact and buckling damage to the column legs as the bin came down.

The physical evidence indicated that failure of the bolted connections between the cone chute and bin precipitated the collapse. Furthermore, some of the connections failed prior to the collapse. The connection between the bin and cone was made with A325-N bolts with a 5/8 inch diameter, 1-½ inch length, and countersunk heads. The design drawings specified that all holes to be countersunk and punched at a diameter of 13/16 inches. A friction plate was applied over the countersunk head to eliminate wearing. There were no torque specifications for any of the bolted connections, which indicates that the attachments constituted bearing connections.