It appears that the frequency of crane accidents has increased in 2008.
Validated statistics are not available, but look at recent news reports and you'll see that both high-profile wrecks and crane-related fatalities are higher than they have been for many years.
The most basic cause of the majority of crane wrecks is gravity. This covers most crane failure events, other than power line contacts and a few others.
Analyzing data about crane loss causes reveals three basic types of accidents: tipping failures, structural failures and collisions. According to the Crane Institute of America (CIA), collisions that include contact with power lines are historically the number one cause of fatal crane accidents. Most often, the victim is not a crane operator, but a load handler or rigger on the ground. Prevention of this type of loss is deceptively simple, but required precautions are often overlooked. Some of the easiest controls are de-energizing the power lines or posting dedicated spotters.
Crane Tipping
Tipping failures have been shrinking as a percentage of crane losses. In the past, twenty to thirty years, cranes have been constructed of heavier, stiffer materials, especially in lattice booms. Load charts illustrate a crane's gross capacity based on the length of boom, the load radius or the boom angle, and the quadrant of the pick. On many older cranes, this reflected a nearly equal split between tipping and structural limits. Tipping limits on the load charts indicate that exceeding these capacities would tip the crane over, while structural limits indicate that a structural failure would occur.
Newer models are often constructed of lighter weight materials, especially the booms. Lighter weight booms usually yield a greater lifting capacity as a percentage of boom weight to gross capacity. Other reasons for lighter booms include lower cost of materials for construction and lower component weight. This last factor eases the task of transportation and assembly of cranes in the field.
More than thirty years ago, when a higher percentage of crane capacities were in the tipping part of the chart, many operators subscribed to a "seat of the pants" style of operation. With skill and care, operators could feel a crane "getting light," and back off the load prior to tipping the crane. While this type of operation was never sanctioned, it was not uncommon.
Mobile crane tipping occurs when any one of several conditions is present. If a crane is not set level within a manufacturer's specifications, the crane could tip, especially towards the low side. Another common condition is inadequate crane support, especially on soil. If the supporting base sinks, the crane will quickly get out of level, which could lead to a tipping accident. The base failure could be soft soil, inadequate blocking, failure of an underlying structure such as a bridge, trestle or pier or several other conditions.
Another common cause of mobile crane tipping is exceeding the crane's gross capacity. Each manufacturer provides a load chart that specifies maximum capacities for each boom length, radius, base, boom type and counterweight. Understanding these capacities and remaining well below the limit is essential to safe crane operations. Another relatively common event that could cause a tip-over is a sudden loss of a load, sometimes due to rigging failure. The recoil from the lightened boom could cause the crane to tip over backwards.
Tipping could also occur from the following less common conditions:
- Improper crane assembly
- Missing counterweights
- Wind
- Base setups less than those specified in the load charts
Structural Failure
Structural failure of mobile cranes has represented an increasing percentage of crane accidents. Each crane manufacturer specifies crane assembly methods, requirements and limitations of components, and gross capacities for each configuration. Most structural failures are a result of failing to follow manufacturers' instructions. Structural failures can include actual failure or breakage of the boom, failure of hoist lines or, in many cases, hydraulic failure. Regardless of the actual failure type, these accidents can be catastrophic due to their sudden onset and the operators' inability to compensate.
According to North American Crane Bureau (NACB), two of the most common causes of structural failure are the result of exceeding the gross capacities on the load chart or improper crane assembly. Load chart capacities can be exceeded when the load weight is not known or when the crane capacity is not properly calculated. Other conditions include excessive dynamic loading, which can occur with excessive operational speeds, braking or duty cycle work such as during concrete bucket operations.
Load charts are predicated on matching specified conditions and equipment. Some lattice boom cranes allow more than one boom type, each with its own chart and capacities. Multiple types of running rope may be allowed, again, each with its own capacity. Adding parts of line can increase capacity, but not without increasing friction loads, which also must be accommodated. Boom extensions, also known as auxiliary boom heads, jibs, flys, dead sticks or other devices, can greatly affect crane capacity, whether in use or erected but not in use. In many cases, the operational notes included in load charts may significantly reduce crane capacities when compared with the simple tables with which many of us are familiar.
Other causes of structural failure for mobile cranes are wind loading, improper assembly and use of improper components.
Improper crane assembly can take many forms. Mixing different components, from either different cranes from the same manufacturer, or from other sources, can produce unpredictable results. Failure to include all of the bolts, substitution of materials, or using damaged materials could lead to a structural loss.
Tower Cranes
Tower cranes are a category of their own. There have been several well-publicized tower crane wrecks this year, each with fatalities. Without discussing specific events, tower crane accidents have many similarities, as well as some differences, with mobile crane losses.
Tower cranes usually enjoy an advantage over mobile cranes. The masts for tower cranes are set on level, fixed bases as specified by the manufacturer. Tower cranes also operate with a series of computerized capacity limiters that prevent overloading. To the extent possible, engineering controls are installed in tower cranes to prevent accidents.
Human ingenuity, or at least "best intentions," occasionally finds its way into many systems, including tower cranes. Based on our review of many accident investigations and loss reports, we found that some accidents have occurred when variances from manufacturers' specifications have been made in crane bases, assembly methods and operations, or when operators have overridden safety devices.
Other errors that have led to tower crane incidents include improper jumping, also known as increasing the mast height, improper assembly or disassembly, improper rigging of components, and substitution of improper materials or parts. Each tower crane manufacturer supplies an operations manual with their equipment. These documents are often 300 or more pages, which may dissuade some from reading the entire contents. However, most tower crane accidents are the result of a deviation from the manufacturers' instructions.
Remedial action
While a variety of rules, procedures and regulations govern crane operations, none is more important than manufacturers' instructions. The first steps in reviewing best practices should be to compare current field activities with the manufacturer's minimum requirements. These reviews should include crane assembly, operation of the equipment and inspections. In most cases, proper preplanning, oversight and following regulatory and manufacturers' requirements will prevent crane-related accidents. These valuable tools are engineered with safety in mind, but when shortcuts are taken or proper steps are not followed by trained personnel, crane wrecks can happen.
The information in this publication was compiled from sources believed to be reliable for informational purposes only. All sample policies and procedures should serve as a guideline which you can use to create your own policies and procedures. You should consult with your own attorneys when developing programs, policies and procedures. We do not guarantee the accuracy of this information or any results and further assume no liability in connection with this publication and sample policies and procedures, including any information, methods or safety suggestions contained herein. The subject matter of this publication is not tied to any specific insurance product nor will adopting these policies and procedures ensure coverage under any insurance policy.
Construction Business Owner, February 2009
