Why do cranes topple

The manufacturer's specifications will dictate when a crane needs to be brought down into safety mode to protect it from toppling over. But there are no blanket regulations about this, and that number may be higher or lower than what a city mandates, creating confusion. Pritchett suggests having the city set a level, but deferring to the manufacturer's specifications if they recommend lowering the crane even in lower wind speeds.

According to Barth, there's more to consider. Sure, high winds and sudden storms have been known to tip crane booms, but high winds can also create side loading of the crane boom, another reason for collapse. Aviad Shapira, professor of construction engineering and management at the Technion-Israel Institute of Technology in Haifa, tells Popular Mechanics that securing a crane under high-wind conditions is more complicated than it appears, especially if wet snow gathers on the crane, adding weight.

So many factors come to play with wind, such as the fact that wind speed is generally greater the higher you go and that wind funneling between buildings can speed up gusts. Even the very act of taking down a crane changes these dynamics. Cranes are designed to handle their highest wind loads in their built and secured state. The process of pulling them down takes them out of that state, opening up room for even more uncertainty regarding the wind, sail effect, and weight loads.

Wind isn't the only worry, of course. Another key crane experts point to is the need to fully understand the work site conditions. However, it is the crane operator's responsibility to ask the site superintendent about any hazards such as utilities, fresh uncompacted fill and more.

They may not be recognizable to the eye but could pose a threat of ground failure. Whether you're working in downtown New York or on a windmill in Arizona, Pritchett says, every site carries its own hazards, which is another reason generic regulations simply can't handle the diversity of conditions.

For example, some parts of a city may have soft ground. Everything about crane setup, from wind considerations to loads charts, must account for that. Statistics from the United States Dept. But crane collapses and their consequences—everything from property damage and lost time on a project all the way up to injury and death—aren't new to the modern-day crane, of course, with data showing a fairly consistent pattern.

What's changed is the size of the machines. Shapira says accidents involving today's bigger cranes may indeed be more dangerous. But it isn't because larger cranes are falling more, it is because when larger cranes do fall, the outcome of the fall can prove more daunting.

This is particularly true in dense urban settings. One of the largest failures in crane history came from the collapse of "Big Blue" in The foot-plus crane was installing part of the roof of Miller Park, the Milwaukee Brewers baseball stadium, when gusts up to 35 mph sent it toppling, killing three workers.

If you think the cost is high, simply compare it to the cost of a crane tipping over. In November a ton crane collapsed during construction of the new football stadium in Sao Paulo, Brazil. Two workers were killed. The crane manufacturer concluded the cause was ground support failure due to heavy rains in days preceding the lift. See aerial photo on the right.

Matting should be engineered, considering sheer stress and bending. It is not reasonable to assume the matting is stiff enough to provide even distribution across the total area under the mats. An extreme example of this is when the mat bends to resemble a Chinese wok. The raised edges are not taking any load. It is somewhat counterintuitive but when using multiple layers of mats, placing each layer parallel to other layers provides better distribution than placing them in perpendicular layers.

A common, reliable method for determining the capacity of the soil is to have a reputable geotechnical company perform borings and determine the capacity of the soil. Then you can compare the allowable soil capacity to the crane ground reaction and design a load distribution or matting system to suit. In some cases matting is not enough and improvements will need to be made to the soil. Soil improvements could consist of a modest layer of compacted crushed stone fill material or could be as complex as installing piles tied in to a supporting concrete slab.

In all cases, I recommend a minimum design safety factor of 1. Radar Sounding of the set up area should be considered when working in older industrial facilities and if there is a possibility of underground voids, tanks, abandoned structures or natural cavities. The pictures above show a classic example of ground bearing failure that resulted in the loss of life for five workers in Vietnam in There is no evidence of any matting or significant soil improvement.

Some contractors, such as Bovis, specify the use of anti-collision systems on their multi-crane sites. Erection and dismantling of tower cranes are periods of high risk. They require particularly close attention to the manufacturers' instructions, Mr Phillips says. Bolts must be loosened in the right order, for example. Tower crane collapses tend to be highly visible, spectacular affairs, often having serious consequences for workers and the project. When mobile cranes overturn, in contrast, they regularly go unreported if no one is injured.

The damage may remain hidden behind site walls. But a visit to the US website craneaccidents. According to leading manufacturer Liebherr, the most common cause of mobile cranes falling over is lack of support to outrigger pads - for instance if they are positioned on soft ground, drain covers or unknown underground cavities such as sewers or pipe cavities.

Calvin Morris, Ainscough Training's senior management instructor for cranes and lifting, says that while blaming ground conditions or the operator is 'the easy answer' there are a host of interrelated reasons, many to do with management and supervision, that explain why mobile crane accidents persist.

The consensus of the experts consulted is that the most important aspects of crane accident prevention are equipment maintenance, personnel training, job planning and site supervision. Tower cranes can have lives of 20 or 30 years, and all the erection, dismantling and transportation over those years take their toll. Metal parts corrode and electrical parts wear out. They also naturally want to fall down. The dynamic forces that develop in the joint areas increase far beyond those originally designed by the crane manufacturer.

A daily comprehensive check of all the tower crane parts is required. Paul Phillips confirms that clips and pins can work themselves loose and need to be checked regularly. He says that as a result of recent accidents in Dublin and Riga, Latvia, HTC now puts cable ties around the R clips on its cranes to prevent them popping out and allowing pins to work themselves loose.

Rigging equipment, such as slings and shackles, also needs to be checked regularly and discarded if showing critical signs of wear. It is not just operators but also banksmen and those responsible for erecting and dismantling cranes who need to be trained to recognised industry standards. Calvin Morris, Ainscough Training's senior management instructor for cranes and lifting, points out that even a trained operator may not be familiar with the particular details of a type of crane he may be operating for the first time.

On the flip side, he may have been operating it for so many years that complacency develops, and lazy shortcuts develop, as with most motorists. The appointed person responsible for lifting operations also needs to be trained, although training alone may not be enough, says Mammoet's Bryan Cronie. Courses vary in length and a Construction Plant-hire Association initiative to standardise appointed person training will help, Mr Cronie hopes.

But I've noticed in the past year or so that clients will wait until the last minute before awarding the contract, so you don't get the planning time you need. With mobile cranes, clients might want to save money and call in a smaller machine than is really needed. With tower cranes this is less likely to happen since they stay on site for much longer, but proper planning is still required. Paul Phillips says tower cranes have sometimes fallen over because the users have not adequately compensated for factors of safety when designing the base.

This was because they did not properly understand the manufacturer's instructions. Planning will also ensure the crane is used correctly.