Historically, electrocution has been the primary cause of crane-related death by a wide margin. There is little room for debate where electricity is the cause of a fatality or injury. This fact notwithstanding, it must also be acknowledged that a crane can overturn or fail structurally.
The reasons why cranes overturn can be grouped into two general categories. First, the crane can simply be blown over by high winds. Such occurrences are infrequent and generally relate far more closely to weather conditions than to lifting related factors. The majority of tip-overs occur because the load being lifted simply exceeds the crane’s stability limits, or the strength limits of the soil beneath the crane. Unlike weather-related overturnings, both of the latter two occurrences depend on the size of loads being lifted by the crane.
As a crane approaches the limits of its rated capacity chart lifts by lifting heavier and heavier loads, the focus of the crane and load weight, known as the centre of gravity, moves ever closer to the crane’s tipping point. Once the centre of gravity of the mobile crane and load weight moves beyond the rated capacity and then outside the tipping point, the crane will overturn. Just prior to tipping, the entire weight of the crane and hook load are concentrated on two outriggers, the toes of the crawler tracks, or one crawler side frame. While tipping due to simple overloading remains an ever-present danger, the failure of the underlying soil to support the increasing concentration of pressure exerted by the outriggers or crawler tracks is an equally critical factor in overturning.
Where the foundation under a crane is concerned, the two key questions that must be answered are 1) what loads will the crane impose on the soil? And 2) is the soil adequate to withstand these loads? Right or wrong, more often than not the experience of the crane operator has been the sole source relied upon for the answers to these questions. As valuable as this experience is, once the operator arrives at the site and begins to set up the crane, it is generally too late to undertake anything but the most superficial soil preparation. Preplanning is crucial.
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“Even lifelong crane users may not have the engineering background – or necessary weight data – to mathematically determine foundation loadings” |
Even with time to spare, foundation loadings are not easy to calculate. Even life-long crane users may not have the engineering background—or, for that matter, the necessary weight data—to mathematically determine the foundation loadings for each lifting condition. In the past, crane manufacturers were the primary sources of guidance in the soil support area. Today, they have taken a major step forward in making foundation loading information more universally available to users. They now deliver the mathematics for calculating outrigger and crawler track loadings either within the electronic load indicating systems on their cranes or on separate computer disks. This allows even non-engineering personnel to determine the foundation loadings anticipated for any specific crane application. It gives users easy access to the answer to the first key question: what loads will the crane impose on the soil? When this information is provided in a timely fashion to a project manager, it allows the involvement of the necessary geotechnical personnel to answer the second key question: is the soil adequate to withstand these loads? This is preplanning. It raises the safety bar for everyone involved in lifting operations.
For decades crane users have understood intuitively that the weight of the counterweight makes a crane tail-heavy when the boom is at a high angle and no load is being lifted. This condition has been used to quickly check the adequacy of the soil on which the crane is deployed. By swinging the counterweight over each outrigger or crawler track in turn, the weight of that counterweight increases the loadings on the underlying support surface to reveal any signs of settlement (and hopefully allow correction). This practical, albeit crude, method has now been complemented by the easy availability of more technical support information from manufacturers to allow verification of the foundation adequacy under a crane.
Will cranes still tip over? Of course. As long as mankind succumbs to the temptation to push its machines to their limits, overturnings will happen. However, new levels of product support from crane manufacturers can now be employed by crane users to preplan in some of the key areas that historically were left more to chance. This technical support makes it easier to prevent surprise overturnings and permits users to focus on preventing tip-overs initiated by deliberate misuse.
Who can compute loadings
Even lifelong crane users may not have the engineering background – or necessary weight data – to mathematically determine foundation loadings
Shifting sands Shifting sands
