There is a combination of things that stop cranes from tipping over. Base, structure, and physics are a few reasons. But that’s very simplified. Let’s delve much deeper.

Before we explain how cranes stay upright, we need to categorise the two styles of cranes that we’ll cover.

These are:

• Static cranes
• Mobile cranes

Static cranes generally arrive on site in pieces and get erected there, where they’ll stay in one position until finished with. Tower cranes, for example.

Mobile cranes are able to travel around on wheels or tracks, either with loads on (pick and carry) or between lifting. Some examples are crawler cranes, all-terrain cranes (more commonly known as mobile cranes), and rough-terrain cranes.

We’ll now break down each separate category, plus combined, to explain the topic.

Static Cranes

Bases

It only seems right to start at the bottom. Tower cranes have highly engineered bases that help prevent the cranes from falling over.

The most common are large cubed concrete foundations that the tower anchors to. But there are various other types of bases. You can learn more about them here.

Ties

Sometimes, when tower cranes reach a certain height, they get tied into the building that they’re constructing, to give extra stability. Otherwise, free-standing cranes that are too high can sway and bend a lot, which puts stress on the mast.

A frame locks in around the mast, and tie beams that are attached to this frame get bolted onto fixed anchor points on or in the building. One or several of these ties can be placed on the mast, depending on how high the crane is.

Weathervaning

One obstacle tower cranes constantly have to deal with is wind. When experiencing extremely high winds, as well as when operators finish their shift, they must weathervane the cranes. There are various other terms for this, such as putting the crane out of service, or putting it into free slew. This allows the crane to be blown around by the wind. (The slew is the 360° motion it makes).

When cranes are in operation, a slew brake is engaged until the operator makes the crane slew, then it releases to allow the movement, then engages again once stopped, to hold the crane in position.

If this brake stays engaged during high winds, there is resistance against the wind, which can snap the jib, bend the jib over the back of crane, or cause too much strain on the mast, causing the crane to collapse.

Anti-collision/zoning systems

Multicrane projects should have zonings systems programmed into the tower cranes to:

1. Prevent loads from sailing over the site’s boundaries into other property or public places.
2. Prevent crane jibs from colliding.

If cranes collide, the jibs could snap and fall, which in turn could collapse the cranes.

Anti-collision systems can either warn the operator when the crane gets close to another (passive), or automatically stop the cranes from getting too close (active).

Mobile Cranes

These types of cranes that travel on the ground tend to tip over more than tower cranes, because they’re not fixed down, plus the ground conditions and travelling pose more risk.

As with tower cranes, we’ll start with the ground to cover this section. Every type of crane that travels on the ground must have a suitable surface to operate on. Heavy equipment that transfers that weight into small points of pressure like tracks, wheels, or outriggers, etc, can cause vertical deformation of the ground. In other words, the ground can move, sink, or give way.

Crane mats

If the ground is unsuitable to bear the weight of heavy machinery, crane mats get put in place. These could be timber, hardcore, concrete, pads, or whatever spreads the weight of the vehicle over wider areas and gives a solid surface to drive on.

Outriggers

Wheeled cranes gain more stability when using outriggers to hold the pressure of the crane and the load it’s holding. These outriggers are legs that either drop down or stretch out from the crane, and lift the crane level so that the wheels are slightly off the ground.

Low centre of gravity

The laws of physics show that objects that are short and wide have less chance of falling over. Let’s use crawler cranes as an example. They have wide bodies, with tracks that protrude out even wider than the body. Even each track is wide.

Laying the jib down

Slewing mobile cranes aren’t able to be left in free slew like tower cranes. The slew gets locked in place. Because these cranes are on the ground, the jib/boom could collide with a structure or building if the wind were to blow the crane around.

So, when mobile cranes finish a shift, the operator will retract the boom to prevent wind catching it.

Also, if wind is predicted to be very excessive, crawler crane operators will lay the crane’s jib completely on the ground if there’s space.

Static and Mobile Cranes Combined

Structure

Crane jibs/booms are made of high-grade, high-density steel that is structured to withstand a lot of force, but with enough flex to allow for movement. They can bend slightly to help prevent snapping. Tower crane masts can also twist slightly to allow for wind pressure on the side of the jib, as well as the torque that the crane uses when slewing.

Counterweights

This is a simple case of physics. Counterweights, otherwise known as ballast weights, counteract the weight that gets lift by the crane. The weights are positioned at the back end of the crane to even out the weight distribution when stress is put on the front end of the crane when loads are lifted.

The weight of the counterweights needs to be more than the crane’s maximum lifting capacity.

With tower cranes, when no load is on the crane hook, the crane’s natural resting position is usually leaning backwards because of the rear ballast weights. When loads are picked up, the top of the crane moves to a more central position, making the tower plumb.

Safety precautions

There is software built into cranes that prevent them working beyond their capabilities, which helps to stop them from tipping over. For example, it can be catastrophic if cranes lift excessive weight in relation to the ballast weights.

So, a Load Moment Indicator (LMI) or Rated Capacity Indicator (RCI) will prevent the crane from lifting the load if it’s too heavy.

Also, some tower cranes get derated when working close to rail lines. This procedure is usually requested by Network Rail. The crane’s computer gets programmed to reduce the crane’s usual maximum lifting capacity. This is to prevent the tower crane from falling onto the rail line if it lifts beyond its capabilities.

Crane operator skills and judgement

There could be all the safety precautions in the world, but if a crane operator is too inexperienced, ill-skilled, negligent, or makes bad judgements, the risk of them tipping their cranes over is high.

Here are some examples:

•  If you were to travel down a slope in a crawler crane with a heavy load on the hook, would you travel forwards or backwards? The sensible option would be to spin around to point the load towards the top of the slope.
• An experienced operator will inspect the ground before they travel on it. If the ground is unsuitable, they’ll refuse to drive on it, then report it to a supervisor or management.
• There are lots of judgement calls crane operators have to make when operating cranes in windy conditions. They might need to take the crane out of service if the wind speeds exceed what the crane manufacturer recommends operating in.

Or they might choose to only do selective lifts on loads that won’t be too affected by the wind.

Also, if operating a luffing jib, they might choose to not bring the jib to minimum radius, which would take the jib up almost vertically. Having the jib in this position would make the crane top heavy and susceptible to tipping over if wind pressure is too high.

These are only some examples of a crane operator’s potential decision making.

Load charts

Because it’s important not to exceed the crane’s lifting capacity, crane operators must be able to have an idea of what can be lifted at specific radii. This is where load charts are useful.

The charts give information about the crane’s configuration, as well as what maximum weight can be lifted at each radius (and heights for telescopic cranes). So an operator can check this information prior to doing a lift to make sure the lift is possible.

Examples of Cranes Tipping Over

Big Blue – Miller Park

In 1999, a Lampson LTL 1500 crawler crane, more commonly known as Big Blue, was attempting to install a near 500ton roof section on what was then the Miller Park, which is now the Milwaukee Brewers Baseball Stadium.

At the radius the crane had taken the roof section to, it was working at 97% of its lifting capacity. Not only that, the average wind speeds measured at 26mph, with gusts going into the mid-30s. The jib was only rated to 20mph.

The crosswinds caused a large side-load on the jib, which put too much strain on the crane, causing it to tip sideways, killing three workers and injuring five others.

Battersea

In 2006, a tower crane in Battersea collapsed because bolts connecting the slew ring to the mast failed. The bolts had already previously failed but not enough of an investigation as to why that happened was carried out. A thorough investigation could have prevented the next catastrophic incident.

Reports state that the crane firm had installed counterweights for a bigger crane after reading the wrong manual. They used 12ton counterweights instead of 8ton. This weight put too much strain on the bolts, causing the crane to collapse, which killed the operator and a man washing his car.

Liverpool

A 2007 tower crane incident in Liverpool killed a site worker and trapped the operator in his cab for half an hour when the crane collapsed.

The likely sequence of events of this incident is reported to be:

• The crane was lifting a light load with the jib nearly vertical, a high gust of wind raised the jib, which released tension on the luffing rope.
• This luffing rope then came off the sheaves and jammed in the reeving system. The crane driver attempted to jib down, but because the rope was jammed, slack formed as the luffing drum released the rope at the other end. This formed a loop at the rear of the back jib.
• The jammed rope then freed up, which sent the jib into free fall because there was no tension on the jib.
• This caused a huge shock-load, which bent the jib. The bolts holding the main crane assembly failed and the slew ring fractured. The crane assembly then fell, landing upside down on the building below it.
• The concrete ballast weights fell out, killing a worker in the building.

This highlights the decision-making aspect spoken about earlier.

Conclusion

As you can see, it takes a lot to keep a crane from tipping over. Some things are out of our hands. Machines fail and weather is unpredictable. But most incidents are as a result of inadequate planning, unclear responsibilities, or unsafe use.

Method statements and lift plans get put in place to ensure safe methods of work, but these guidelines are not always adhered to.

If companies and individuals followed the correct protocols outlined in these guidelines, far fewer incidents would occur.

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