Crane Accident Investigations

A rash of crane accidents has made headlines over the last few years.  Crane accidents not only destroy the cranes and their lifted load, they commonly cause injuries and fatalities.  Determining why a crane accident happened is fundamental to assigning liability, determining if a claim should be paid, and subrogating the claim, if appropriate.

Bureau of Labor Statistics data indicates that about 82 crane-related fatalities a year occurred in the period 1997 to 2006.  Another analysis indicated that 84% of crane-related fatalities involve the use of mobile cranes.  Tip-over is a common aspect of mobile crane mishaps.

Tip-over can be the result of many factors including exceeding the gross lift capacity, exceeding the crane capacity for the particular lift configuration, dynamic factors such as momentum induced loads, failure to use outriggers, failure of the outriggers, and failure of the crane support pad.  In all these cases, soil impressions can provide key clues in determining root cause.

When carefully inspected, soil impressions will be unique to the incident much like a fingerprint is unique.  Look for such characteristics as:

• Lateral or rotational skid marks
• Cratering with vertical and/or lateral displacement
• Cratering without surrounding displacement (punch-through)
• Symmetrical versus asymmetrical impressions

In addition, the general condition of the soil and that of any cribbing, if used, should be noted.

Lateral or rotational skid marks are often produced from the dynamic effects of the moving, hoisted load.  Many mobile cranes have a greater lift capacity over the front or rear of the crane truck than over the side.  When a load is being swung from the front or rear of the crane to the side, the lifted load can dynamically exceed the lift capacity.  As this occurs and tipping begins, the mass of the load will often induce the crane truck to rotate in reaction leaving tell-tale skid marks in the soil.

Cratering (deep impressions in the ground made by the wheels, tracks or outrigger pads of a mobile crane) with vertical and/or lateral displacement can be an indication of exceeding the capacity of the crane.  In this situation, as the crane lift exceeds capacity, tipping will occur placing a majority of the load on only a few points (i.e. front wheels, track toes/heels, front outriggers).  As the load increases on these points, they will often exceed the bearing capacity of the ground and produce deep impressions.  Upon careful inspection one will often find erupted soil and/or wave like characteristics around the crater indicating plastic deformation of the ground suggesting adequate bearing capacity for the lift.

In other instances, cratering may not be accompanied by vertical and/or lateral displacement.  In this case, the mobile crane wheel, track or pad will seem to have ‘punched-through’ producing a neat, clean hole with little surround soil displacement.  Punch-through can be indicative of inadequate native bearing capacity or an improperly prepared crane pad.

Symmetrical versus asymmetrical soil impressions associated with a crane tip-over are another identifying characteristic.  If three of four crane outrigger pads have deep impressions but the fourth pad made only superficial marks, one might suspect structural and/or mechanical failure of an outrigger.  In this case, the investigator should verify the structural and/or mechanical integrity of the crane outriggers.

General soil conditions can have a large effect on bearing capacity.  Excessive rain may cause an undercut of soil structure and cause premature failure when compared to typical soil moisture content and structural capacity.

Proper crane cribbing can prevent overload of native bearing soils but when incorrectly placed or inadequate structural capacity exists, cribbing can be a source of crane tip over accidents.

If the cause of a crane tip-over is suspected to be native soil or prepared crane pad capacity, borings and analysis by a qualified geotechnical engineer may be a necessary step in determining root cause, assigning liability, and subrogating the claim.

For more information, call 317-536-7000, or email at Info@KevinKennedyAssociates.com.