In Mecca, the Grand Mosque at the city’s center was already filled with visitors. An hour before sunset, a blustery storm was whipping through the city’s buildings, half-built high rises and thicket of tower cranes.
The storm toppled a large red and white crawler crane. It fell backward and landed on the mosque’s roof. At 5:20 pm on September 11, the crane hit hard and the collision shook loose tons of concrete and debris onto the pilgrims and visitors inside killing hundreds.
The crawler crane’s geometry is blamed for its structural failure. The superstructure, where the operator sits and where the crane pivots, rests on two tank-like treads. Attached to the front of that superstructure the long arm that carries the crane’s load, which the cables connect to a mast—extending from the back of the superstructure. The cables move the boom up and down, but the mast is what keeps the weight balanced. Balancing depends on counter weighting. Sometimes the mast is enough but superstructures often get loaded down with huge concrete and steel weights to balance. The outriggers or extendable feet give it a wider base, for increased stabilization.
Despite their low center of gravity, crawler cranes are far less stable than teetering tower cranes. A crawler crane has to be on firm ground and level within one percent and soft ground will throw a crane out of balance.
The Physics of Tragedy
The wind is a crane’s greatest foe, and even a perfectly set-up structure is susceptible. This is because the boom acts like a giant lever that the wind can push upon. When the boom is higher up, the wind pushes the crane over with less force.
On the evening of the collapse, Mecca experienced sustained winds around 25 mph. But, tall, clustered buildings that surround the Grand Mosque caused channeling effects forcing the wind to go faster in order to go between them.
Too many cranes in too little space force the operators to leave the boom high up without tying them. Finally, the crane being incredibly heavy, the mushy ground could become the cause of an accident. Even outriggers wouldn’t be much help if they aren’t seated on the solid ground. Even the best engineers can’t control the weather, but they can try to accommodate it through safety procedures. The shifty gust that caught this crawler off guard was devastating, but the damage could have been averted.