When engineers are designing buildings, bridges, or other large construction projects, a lot of thought is given to the environment. Some of these considerations might seem obvious, like designing a skyscraper in San Francisco to tolerate earthquakes, building a stadium in New York City to hold up not only its own weight but the weight of several feet of snow on the roof, or constructing bridges in any coastal area to be able to tolerate salt spray. Not everything is this straightforward, though. Not only do the structures themselves have to tolerate the environmental conditions they are in, but the equipment that is used to build them must tolerate these conditions as well, specifically the large cranes that are often semi-permanently attached to their construction sites.
Perhaps the most extreme example of this in recent memory was during Typhoon Manghut as it hit Hong Kong. There were several large construction cranes that didn’t fare too well with the high winds. At least one toppled as a result and catching the free-spinning of another on video is more than enough to make you gasp. Other videos of construction cranes surfaced from this typhoon showing some concerning, but surprisingly well-designed, emergency operation of the same type of crane.
THE CRANES THAT BUILD TALL BUILDINGS
Before digging into the considerations for extreme weather, it’s worth discussing some of the different types of cranes that might be found on a construction site. The large cranes used to construct skyscrapers (and the ones seen in various states of distress in the linked videos) are known as tower cranes. These are fixed to a concrete base specifically built to hold the crane during the course of construction. Once the base is complete, the parts of the crane are lifted by a lighter-duty crane and assembled one section at at time. Parts of the mast may also be fixed to the lower parts of the building for added strength. These cranes commonly have a fixed jib (the horizontal part at the top that holds the counterweight and the payload) with a rolling trolley. Other cranes at the site may have luffing jibs that can move up and down, and some cranes are mobile and can be disassembled onto a truck for quick and easy movement from site to site.
At sites with many tower cranes operating simultaneously, they are all built at different heights in order to keep the jibs from colliding with one another. Ideally, they should all be far from other structures as well, but in a place like Hong Kong with such high population and building density, it’s often impractical or impossible to build reasonable margins of safety into these cranes, especially during storms.
WEATHERING THE STORM
The different heights also help out during times of high winds, like typhoons or hurricanes. These structures are designed to deal with the same environmental hazards that the buildings they’re in will tolerate, so if a storm is on the way the cranes can safely remain aloft. In this situation though, the slew ring is unlocked which allows the jib to freely rotate like a weathervane would. This allows it to naturally have its lowest cross-sectional area facing into the wind and take as little stress as possible, and it allows it to change direction and be in the optimal position as the storm moves past. Since the towers are all at different heights, they won’t collide with each other even during a bad storm. You read that right, the free-spinning crane linked to above is performing as designed!
In theory, it’s possible to build a tower structure that can withstand a hurricane on its own. Any tower crane built in a cyclone-prone area is built strong enough to withstand high winds. Most tropical storms are well-predicted, often giving construction crews more than a week of advance notice prior to landfall. Therefore, since some tower cranes can be disassembled in just a few days it is possible to have the cranes dismounted and out of harm’s way in time. Of course, it’s expensive to disassemble a crane like this and then put it back up, especially in the event that a storm changes path and doesn’t hit the construction site at all, but it’s likely less expensive than dealing with an uncontrolled tower collapse.
That being said, sometimes things don’t go according to plan. If the concrete in the crane’s foundation wasn’t exactly the right mix, or the rebar wasn’t installed properly, or the anchor points weren’t as secure as they could be, there can be issues. While minor issues like these will likely go unnoticed (if they exist at all) during ideal, clear-sky conditions, under the stress of something like a hurricane any small flaw in engineering or construction will pop up. Even extreme wind gusts can tear down a properly constructed crane, and hurricanes have a pesky habit of spawning tornadoes when they make landfall.
Don’t fall into the trap of assuming that lax building codes or construction standards are the culprit. Developed countries that have robust building codes, permitting, and rigorous engineering standards aren’t immune to sudden failures of cranes due to bad weather. Last year during Hurricane Irma, two tower cranes collapsed during the high winds. One crane operator suggested that a failure was due to a tornado that spawned within the hurricane, which could easily have exceeded the maximum sustained wind rating that the hurricane would have produced on its own. But it’s also important to note that the center of Irma didn’t pass through Miami, and if it had we likely would have seen many more tower crane collapses.
Additionally, it was reported that these cranes would have taken two weeks to dismantle, which might be a design flaw (or a poor choice) in an area where the building code specifies that structures must be designed to withstand 150 mph wind, and where construction site operators might only get a week’s notice that a large hurricane is heading their way. Learning from these disasters makes it easier to choose cranes that are more easily disassembled, built stronger in anticipation of future hurricanes, or even limited to operation outside of hurricane season. Although avoiding hurricane season altogether is likely unfeasible for large construction projects.
WHEN EVERYTHING GOES RIGHT
Some of these principles played out last month during Hurricane Florence as well. Many tower cranes are deployed in Charlotte, which despite being far inland compared to Miami still uses cranes that are rated to withstand 100 mph winds, according to an article in the Charlotte Observer. As the city prepared for probable tropical storm force winds, the cranes were set to pinwheel freely and all signs and banners were removed from them to cut down on wind resistance. No cranes were lost, although the storm did not impact Charlotte as heavily as Irma impacted Miami.
Using proper design standards and understanding one’s environment is critical for building large structures, and not just for the structures’ sake but for the equipment used to build them. It’s possible to design and implement common-sense building practices to limit the number of crane failures, but some outside influences (like tornadoes) are hard to design around and it’s unlikely we’ve seen our last crane failure as a result of a natural disaster. The best we can do is build to high standards, and take into consideration the environment we’re in. And maybe take out a good crane insurance policy just in case.
- by: Bryan Cockfield