Why Cranes Collapse

Cranes are dangerous. Any time something  goes up, there’s a chance it might fall down.   Keep that in mind next time you climb a ladder.  But lifting stuff up and getting it back down   safely is pretty much a crane’s only job. So  why do so many of them fall down? Let’s walk   through some of the biggest crane disasters in  modern history to try and understand. I’m Grady,   and this is Practical Engineering. In today’s  episode, we’re talking about crane failures. This video is sponsored by HelloFresh,  America’s number 1 meal kit. More on that later. Cranes are the backbone of just about every  construction project. All construction can be   boiled down to material handling: taking delivery,  storing, moving, and placing all the pieces and   parts of a project. Of course, you can do a lot  of that with your own sweat and muscles, and there   are even tools to provide a mechanical advantage,  allowing one person to lift a lot. But, anyone   working in the trades will tell you that there are  plenty of jobs only a crane can accomplish. Heavy   equipment amplifies the amount of work that can be  done. That’s so true with cranes that the question   at most construction sites is not if a crane  will be mobilized, but when and what type. Because they are so pervasive and they do  such a dangerous job of lifting massive   objects high into the air, occasionally cranes  fail. In this video, I want to walk through   some of the reasons these failures occur,  using historical accidents as case studies.   I’ve got some demos set up in my garage with  toy cranes, but these accidents are no joke.   Crane collapses lead to property damage,  injuries, and fatalities every year. But,   they’re almost always preventable, so I’ll do  my best to explain what could have been done   differently for the cases described. By the  way, this is part one of a series on cranes.   A future video will cover rigging and the things  that can go wrong between the hook and the load. The first reason cranes fail is improper assembly  or disassembly. Most cranes on construction sites   are temporary. They’re not staying when the job  is done. That means they either arrived under   their own power (called mobile cranes), or they  were shipped on trucks and assembled on-site,   a process that can take days or weeks depending on  the size of the machine. A crane can be extremely   vulnerable during assembly or disassembly since  all the components aren’t fully bolted together.   A recent collapse in Seattle happened when a team  disassembling a tower crane prematurely removed   the pins holding sections of the tower together.  Presumably, this was done to speed up disassembly.   However, when winds picked up that day,  major components of the crane were completely   disconnected, being held together just by gravity  and loose sliding connections between members.   It didn’t take long for the crane to collapse,  killing four people and injuring many more. In most cases, these cranes are expertly  engineered for worst-case conditions. But   the manufacturers are rarely the  ones installing them on site. So,   they provide detailed manuals for the crews  assembling and disassembling them. Unfortunately,   those manuals aren’t always followed to the  letter. One of the worst crane disasters in   modern history happened in 2008 in New York  City. Crews were working to attach a tower   crane to a building when a major component of  the attachment hardware, a heavy steel collar,   suddenly fell. As it fell, it crashed into the  attachment points below, breaking them from the   building and allowing the whole crane to overturn.  The cause of the accident was simple: the crew   assembling the crane didn’t follow the manual.  Specifically, they didn’t attach that collar to   the crane according to the manufacturer’s  guidelines while it was bolted together   and attached to the building. Seven people died as  a result. Another collapse in Battersea, England,   in 2006 also happened because the operator was  using the wrong manual to assemble a crane.   Instead of an eight-ton counterweight, they  used 12 tons, putting the crane way out of   balance. Eventually, the bolts holding the slew  ring failed, and the entire boom broke free,   killing the operator and another person who  was just fixing his car on the street nearby. Once a crane is set up, the challenges  in keeping it that way aren’t over.   Many crane failures happen during everyday  operations, and one of the biggest causes   is overloading. Every crane has limitations in how  much weight it can handle, but it’s not as simple   as something like a bridge that has a single  load limit. Not only can most cranes have a   wide variety of different configurations - like  different counterweights, jib lengths, and boom   sizes - they also, by the very nature of their  job, move. They slew, luff, telescope, traverse,   boom up or down, etc. And more importantly,  their load limitations depend on these movements. Every crane has a tipping line - that’s the line  at which the machine will tip if overloaded. Any   increase in weight outside the tipping line  destabilizes the crane if not balanced on the   other side. The further away the load is from the  tipping line, the greater the moment or torque   on the crane. This is easy to demonstrate with  my model. Using a spring scale, I can estimate   the load required to tip the crane at different  distances from the tipping line. Plotting distance   and force, we get roughly a straight line. That’s  because torque is the product of length and force.   As the distance from the crane goes up, the force  required to tip it over goes down proportionally. Cranes have a few tools to keep the load in  balance. One is counterweights. These weights   oppose the torque by balancing it on the opposite  side of the crane. Another tool is outriggers.   When there’s enough room to use them, these arms  extend the tipping line, bringing it closer to the   load and thus reducing the length of the lever.  The crane can hold several hundred grams with   the outriggers extended, but it can barely  even hold up the spring scale without them. All those factors add up to a lot more than  any operator can be expected to keep track of,   which is why cranes have load charts.  Reading these charts is pretty simple,   as long as you’re looking at the one that  matches the configuration of your crane.   Look at the furthest radius your hook will be  from the centerline of the crane during the lift,   and you’ll see the maximum allowable load the  crane can handle. Of course, most modern cranes   have sensors and electronics that can help an  operator keep track of this on the fly. Load   moment indicators tell the crane operator  when they are getting close to the maximum.   Many cranes will even lock out specific  movements to prevent the crane from tipping   or sustaining damage from overloading. That  doesn’t mean it doesn’t happen, though. In 2016 in Manhattan, a crawler crane fell over  as it was being laid down due to high winds.   The boom and jib of this particular crane could  be set down on the ground if things got too windy.   But, even with nothing on the hook, the  load chart doesn’t allow the boom to be   lowered below 75 degrees. Unfortunately, the  operator dropped the boom below this level,   and the crane fell, killing one  person and injuring several others. Even if the crane can handle the load and do  it stably, that weight doesn’t just stop at   the base. It has to be transferred to the ground,  and surprisingly, sometimes the ground can fail.   The geotechnical engineers call this a vertical  deformation, but you can just say the ground moved   when it wasn’t supposed to. And this can happen  in a couple of ways. The first is settlement.   That’s when soil particles compress together when  subjected to a load. It’s usually a slow process,   but it can create issues at a construction  site over time. Settlement can be solved in   one of two ways. Sometimes compacting the subgrade  before using it as a foundation is enough to make   sure that it won’t compress further over time.  For clay soils that are difficult to compact,   it’s usually best to replace the top layer  with something more stable like crushed rock. The other type of vertical deformation  is called a bearing capacity failure.   In this case, the soil particles actually  slide against each other in a shearing motion.   These dowels represent soil particles, and you  can see what happens if the earth can’t withstand   a vertical load. The particles below the base  get forced downward while the adjacent particles   bulge up on the sides. Here’s an example using a  tower crane. With the hook at the end of the boom,   the crane can hold 150 grams without  tipping on a stable surface. But,   when set on top of loose sand, things aren’t  quite so static. The soil isn’t able to support   the two feet in compression. Instead, it  gives way, allowing the crane to topple. In 2012, this crane fell over while lifting a  part of a ship in Vietnam. The cause is obvious:   the ground wasn’t strong enough to withstand  the load. This accident killed five people.   Geotechnical engineers can estimate the  bearing capacity using simple tests,   so there’s no good reason this should  ever happen. If the ground can’t handle   what’s required for the crane, the solution is  simple: distribute the load over a larger area.   This is often done using steel plates or  wooden constructions called crane mats. Water also affects bearing capacity. The  strength of a soil is primarily a function   of friction between soil particles. If water  gets into the space between those particles,   it pushes them away from each other, reducing  this friction and weakening the soil. If you’ve   ever stepped in the mud, you have some intuition  about this. In 2013, a massive crane helping with   the construction on the Brazil World Cup Stadium  collapsed while lifting a roof section into place.   The cause of the collapse was a bearing capacity  failure of the soil beneath the crane exacerbated   by several days of heavy rainfall. So, keeping  the site well-drained from water is essential,   and again replacing crummy soil with a stable,  free-draining material is usually best practice. The final cause of crane collapses that I want  to discuss is wind, or rather, neglecting the   power of the wind to overload. In 2019, a severe  thunderstorm led to the collapse of a tower crane   in Dallas, killing one person and injuring several  others. The fault for the accident is still   being litigated, but it brings up one thing many  don’t realize. Most tower cranes are designed to   withstand very high winds, but only under certain  conditions. When winds are high, operators have to   disengage the clutch so the crane can freely point  into the wind. This is called “weathervaning.” If   the boom is locked, it can end up broadside to the  wind, significantly increasing the forces on the   crane. In the video of the 2019 collapse, you can  see two cranes pointing in different directions,   and only one of them failed. That suggests that  an operator may have forgotten to secure the   crane properly or the crane malfunctioned  and couldn’t weathervane into the wind. In September 2017, three cranes at  separate construction sites in Florida   all collapsed on the same day due to winds  from Hurricane Irma. All three cranes were   the same make and model, and they were all  secured to weathervane as required. Luckily,   no one was injured at any of the sites since  they were shut down due to weather. The winds   during the storm were more than 125 miles per  hour or 200 kilometers per hour in some places.   That’s far more than the maximum design wind speed  for the cranes, which was 95 miles per hour or   about 150 kilometers per hour. As the saying goes,  there’s always a bigger storm, and in this case,   Hurricane Irma just delivered winds that were  well above the requirements and codes that cranes   are required to meet. However, since only this  single type of crane failed during the storm,   investigators recommended some changes to the  design that may make them safer in the future. Situations like what happened in Florida are the  exception, not the rule, though. As I mentioned,   nearly every crane accident is easily preventable.  In 1999, one of the largest cranes in the world   collapsed during the construction of Miller Park  in Milwaukee, now called American Family Field.   The crane was lifting a 510-ton roof section  of the stadium at 97% of its rated capacity,   but it wasn’t the just weight of the  structural members bearing on the hook. Steel assemblies can appear pretty slender, but  that doesn’t mean they can’t catch the wind.   Watch what happens when I put my leaf blower on  this steel truss. It’s a little hard to see the   spring scale, but a little bit of wind goes a long  way. By my estimation, the wind is adding about   15% to the load on the hook. It’s more than the  margin of error when you’re operating at maximum   capacity, and that doesn’t even consider  the huge horizontal loads affecting the   crane. These machines are rarely capable  of withstanding much force in any direction   other than straight down, so their load charts  generally disallow operation when winds are high. On that fateful day in 1999, the crane operator  neglected to include the additional force from   wind loading when assessing the crane’s capacity,  and so it ended up overloaded. A safety inspector   happened to catch the entire collapse on  camera. Three construction workers were   killed during this incident, and a sculpture of  them still stands at the stadium in their memory. As you may have noticed, there are  technical reasons that cranes fall over,   but there are also underlying human factors as  well. That’s why it’s best practice to create a   lift plan any time a crane is used. That involves  taking the opportunity before the load is in the   air to consider all the aspects of the lift  and what could go wrong: weight, dimensions,   the center of gravity, and lifting points of the  load, the path it will travel during the lift,   the capabilities of the crane, outside factors  like wind, and communications during the whole   process. For complicated lifts, these plans  can take days or weeks to prepare with detailed   engineering reviews to ensure that nothing  goes wrong once the load is on the hook. It’s time for everyone’s favorite segment of   me trying to cook while my wife  tries to capture that on video. Yeah just wad that up. Yeah that looks good.” With a helper who (let’s be honest)  isn’t really that helpful - not to   mention our currently broken microwave  - dinner time in our house is a little   chaotic. That’s why we’re thankful for  HelloFresh, the sponsor of this video,   for converting cooking from a chore into  our favorite thing to do on date night. “Easy there cowboy” We are indecisive eaters, meaning neither of us  likes to be the one to decide what’s for dinner.   It’s nice to have HelloFresh curating delicious  and healthy recipes so we don’t have to. “Do you want sound effect or no sound effect?” The pre-portioned ingredients mean  there’s less prep and less food waste,   and the packaging is mostly recyclable  or already recycled content. HelloFresh   also helps us get dinner ready quickly  when we haven’t had time for planning,   prep, and shopping - not that we  are easily distracted or anything. Go try it yourself at HelloFresh.com and use code   PRACTICAL14 to get 14 free  meals, including free shipping. “You’re practicing on yours first, right?” Supporting our sponsors helps support this  channel. That’s HelloFresh.com and use code   PRACTICAL14. Thanks, HelloFresh, and thank  YOU for watching. Let me know what you think.

Channel: Practical Engineering

Views: 1,151,980

Rating: 4.9519353 out of 5

Keywords: crane, construction, Crane collapse, assembly, disassembly, overturn, counterweight, overloading, jib length, slew, luff, tipping line, outrigger, crawler crane, boom, vertical deformation, crane mat, Brazil World Cup Stadium, weathervaning, Hurricane Irma, Miller Park

Id: LxdjSG5IFds

Length: 15min 39sec (939 seconds)

Published: Tue Aug 03 2021

Reddit Comments

[u/Troutrageously]

This guy makes great stuff. Love his channel.

[u/mormotomyia]

Kranplätze müssen verdichtet sein!

[u/paddyZ_99]

Always amazing, informative and enjoyable videos. The first video to watch when I have some time left.

[u/turbodeeznuts]

This was profoundly depressing to watch.