More than 100 people died on Friday when a crane toppled over during a storm and crashed on to the Grand Mosque in Mecca. 

An engineer on the construction site around the mosque described this as an act of God. "It was not a technical issue at all," he told Agence France Presse. "I can only say that what happened was beyond the power of humans. It was an act of God and, to my knowledge, there was no human fault in it at all."

But let's look at the known facts. There were certainly strong winds at the time. On Thursday – a day before the accident – Saudi meteorologists had issued a warning of bad weather: "Cloudy skies and thundershowers, accompanied by winds around 50 km per hour raising dust, [are] expected over the next few days."

In fact, the winds turned out to be stronger than that. According to Arab News, speeds of 60 km (37 miles) per hour were recorded in Mecca on Friday. Even so, that is not particularly severe or unusual weather. It's Force 7 on the Beaufort Scale, ranking as a "moderate gale" or "near gale".

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"Backward stability for cranes has always been a serious consideration and often misunderstood or overlooked"

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Cranes cannot operate safely in strong winds but there is no suggestion in any reports that the crane in question was operating at the time of the accident. For non-operating cranes, though, winds of the kind experienced in Mecca on Friday should not be a serious problem, so long as the cranes have been set up correctly. 

There's a huge amount of construction work taking place around the Grand Mosque and photos of the area show numerous tall cranes – maybe as many as 20. These all survived Friday's storm ... with one fatal exception.

Most of the surviving cranes are tower cranes, fixed in position, but the crane that toppled was different and somewhat special. Known as a "crawler crane", it was capable of moving around the site using tracks similar to those of a military tank.

A video from 2013 shows the crane in its operational state on the Mecca site and describes it as the world's second-largest crane (a claim that I haven't been able to confirm).

The crane in question is probably the LR-11350 made by Liebherr, a leading Swiss-based manufacturer. The name "Liebherr" can be seen on the crane in one of the photographs and if it's not actually the LR-11350 it's a very similar model. Technical details, as well as some diagrams, can be found in a large PDF file on Liebherr's website.

As can be seen from the diagram above, the rear of the crane has to be fitted with weights to prevent it from tipping forward when lifting heavy objects. Tipping forward is an obvious risk that manufacturers and crane operators are well aware of and take steps to prevent.

But a careful look at photographs of the collapsed crane in Mecca shows something rather surprising: it didn't topple forward but backward. This might sound improbable but in fact it can happen quite easily, as an engineering blog explains:

"Backward stability for cranes has always been a serious consideration and often misunderstood or overlooked. The tendency is to carefully study the capacity charts to assure frontward stability when planning a lift. 

"It is somewhat counter intuitive to worry about backward stability as all the lifting is being done over the front. In fact, there are almost as many crane accidents that are a result of 'loss of backward stability' as there are for all the rest combined.

"All mobile cranes are subject to a loss of backward stability ..."

The point here is that because of the way weight was distributed it would take a relatively small force to overbalance the crane and tip it backwards compared with the force needed to tip it forwards or even sideways.

Whether the wind on its own was strong enough to topple the crane is a question for the investigators, and at present we don't know what steps contractors took to secure the crane in the light of the weather forecast.

However, a report in Arab News suggests there was a contributing factor. The crane's heavy hook appears to have been hanging loose in the wind and, according to an engineer quoted by the paper, it "began swaying and moved the whole crane with it, toppling into the mosque".

The hook can be seen in the video below which shows the crane toppling, and a clang is heard at 0.37 when the hook hits the crane. Unfortunately the video doesn't show the position of the hook before the crane began to fall.

   
Posted by Brian Whitaker
Sunday, 13 September 2015