Construction sites can be dangerous places. That is why under most circumstances, access to the sites is strictly limited to workers who presumably know what they're doing, and even then, worksite injuries and deaths can occur as temporary structures or machinery such as cranes can get out of control.
But what if the site you're working on is regarded as sacred by your religion, and in a few weeks hundreds of thousands of pilgrims are going to visit it? Putting up "closed for construction" signs isn't an option.
This is the dilemma that those in charge of the Grand Mosque (Masjid al-Haram, in Arabic) in Mecca faced as this year's hajj (obligatory pilgrimage) approached. When upwards of a million people are expected to crowd into a few dozen acres of ground, the potential for disaster is always present. And in years past, stampedes of pilgrims have on occasion led to the deaths of hundreds of people caught in panic-stricken rushes. Improvements to the structures used can help with crowd control, and so areas near the Grand Mosque have seen a lot of construction activity in recent years. That is one reason why the Grand Mosque was surrounded by numbers of tall construction cranes last Friday, Sept. 11, shortly before the time of evening prayer at 6:30. At least one of these was a "crawler crane" mounted on a mobile platform that could move on tank-like treads.
Around 5 PM, a thunderstorm approached the city and brought heavy rain, lightning, and high winds. Although the central part of the Grand Mosque surrounding the Kaabah (the black cube at the center) is open, much of it is covered by in a ring-shaped multistory structure that affords protection from the weather.
While details are not yet clear and await investigation, apparently about 5:30, winds became strong enough to overbalance one of the crawler cranes stationed just outside one of the Grand Mosque's walls. Videos shot at the time show the crane as it toppled onto the roof of part of the mosque, crashing through the ceiling and landing with its top inside the mosque's inner open area. Unfortunately, hundreds of people were in the path of the collapse and were killed or injured when the crane knocked down masonry as it fell. As of Sept. 13, the death toll had risen to 107, with over 200 injured. King Salman of Saudi Arabia has stated that once an investigation of the tragedy is complete, the findings will be made public.
First, our prayers and sympathy are with the injured and the relatives and friends of those who died. Accidental deaths are always tragic, but especially so when victims were engaged in a religious pilgrimage made obligatory by one's faith. There is some comfort at least in the knowledge that the pilgrims who died were engaged in what they considered to be a holy act.
From an engineering point of view, this incident has several lessons that can be learned.
First, crawler-type cranes can be less stable than other types with bases that are anchored to the ground. The crawler crane is obviously more flexible and easier to position, but this convenience comes with a price: less stability, unless great precautions are taken to ensure that the crane's rated load and maneuvering envelope are strictly observed. And even if this is done, unpredictable wind loads such as are present in a thunderstorm can tip the balance of forces away from stability.
Prudence might have suggested that with all the cranes around, someone should have kept an eye on the local winds and issued an evacuation order if the wind exceeded a certain speed. But that might not have helped, for a number of reasons. First, winds in a thunderstorm can change minute by minute, and it's possible that a sudden gust was responsible for the crane's collapse. But evacuating a complex as large as the Grand Mosque would have presented its own problems, including the possibility of inducing exactly the kind of panic that has led to deaths in stampedes in the past. So although evacuating the area might have prevented some loss of life, it might have contributed to it as well.
The other alternative would have been to use only cranes that could withstand higher winds. This might mean either using only stationary ground-mounted units, or shorter crawler cranes that are sturdier in high winds. While either of these options would cost something in terms of workplace efficiency and schedules, in retrospect it would have been a price worth paying.
Like airports in expanding metropolitan areas, the Grand Mosque complex in Mecca is likely to be under construction in some sense for an indefinite time. Given that it is, the authorities in charge of it are under an obligation to see that nothing like this tragedy can ever happen again. Unlike the whims of mobs, engineering involves calculation, prediction, and the ability to plan ahead. While engineers cannot foretell every eventuality that could lead to disaster, the investigation of the Mecca crane collapse may show how it could have been prevented. If it does, the engineering staff in charge have their work cut out for them to make sure that pilgrims can worship safely in the holiest city of Islam.
Sources: I referred to news articles on the collapse carried by several outlets: CNN at http://www.cnn.com/2015/09/12/middleeast/saudi-arabia-mecca-crane-collapse/, the BBC at http://www.bbc.com/news/world-middle-east-34236662, Al Jazeera at http://www.aljazeera.com/news/2015/09/saudis-probe-deadly-mecca-crane-collapse-150912125336576.html, and The Guardian at http://www.theguardian.com/world/video/2015/sep/11/aerials-of-mecca-crane-collapse-reveal-damage-video, which has a cellphone video showing the crane falling amid heavy rain. I also referred to the Wikipedia articles on the Grand Mosque and the climate of Mecca.
