The article makes it seem like it's a common practice in the industry. Pretty scary! I don't really understand how it saves time, though... I'd think the bottleneck is the heavy-lift crane, isn't it?
It's not hard to imagine that the practice is common. I know I'm cutting corners and don't know anything about construction, but if you make a little mental game out of it. If you have 20 men on site, dismantling crane probably shuts down most of the work on site, so you have to send most of the 20 men home during dismantling. You hire an outside contractor to dismantle the crane, so you probably want it to only take a day to cut costs on outsourcing and worker leave, you bring the dismantling crew in the morning to remove the pins and the lift the pieces off on the afternoon. There are thousands of cranes all over the world and there are probably dozens erected and dismantled daily. If we only have a disaster every couple of years, out of the hundreds of operations, that's when the practice get's exposed, it's like lottery but you win every time if you don't happen to hold the winning ticket. Truth of it is, many of us roll the dice with those odds.
In the AvE comments there's a lot of people claiming that this is NOT a common practice--because its incredibly dangerous. It just takes a gust of wind when those pins are out to cause massive damage.
Except now somebody's boss is going to site this article to show it's common practice (I'm only partially kidding) or only remember the removing pins saving time part.
I'm reminded of the LA TV news shorts reminding people not to modify their lighters to continuously spew flame and throw them out the window on the freeway. The alignment of stupidity and (anti)social incentives to risk other people's lives is a dangeous thing. Antivax rallies come to mind.
Disclaimer: I know almost nothing about this business. I do think the following could be a good explanation:
If removing a bolt takes 2 minutes and moving from corner to corner (at height, carrying a power tool capable of unbolting the bolts) another 2, you can gain 2x8+3x2=22 minutes per section removed by removing the bolts before the dismantling crane is attached to the part. Multiply by 6 or so on a day, and you can go home two hours earlier (or pay all personnel involved for two fewer hours), and (possibly the most important factor) rent that heavy-lifting crane for two fewer hours.
Also, you probably are right that the heavy-lift crane takes more time to do its work than removing the bolts. If so, the workers who loosen the bolts are mostly waiting, but they are still being paid.
If removing a fully tightened bolt takes more than one person (say because the tool used to do so is big and heavy; these bolts probably are inches in diameter), while removing a loosened one can be done by one, it also pays to have two persons loosen all bolts in the morning, send one home (or to another job) and only pay one in the afternoon to remove the loosened bolts.
AvE did two videos on the collapse, both very good. The comments are excellent, and worth skimming even if you’re generally allergic to YouTube comments. There are explanations from several ironworkers who actually erect and dismantle cranes, as well as from managers and foremen.
Removing the bolts doesn't really have an impact on strength. A lack of a bolt can cause wear on a pin as the structure flexes, but typically unless you're leaving it for years that wouldn't be an issue. You can inspect the wear the next time you use the pin.
Removing pins weakens the structure substantially. However when the crane isn't in use, it doesn't need anywhere near all of it's strength, so removing one of two pins is fine.
I would guess in this case, they removed 2 of 2 pins, leaving gravity the only thing holding everything in place, which is plain stupid.
Simply loosening bolts to “hand tightened” has led to crane collapses. (The article under the Worthton accident on page one summarizes expert witness testimony to this effect)
In general, removing even one element from a truss structure substantially lowers safety margins. Also, crane collapses often happen during assembly and disassembly, since that is when unexpected things often happen. Unnecessarily removing redundancy from the structure increases the chances of such collapses.
Can you explain the difference some more? I think of a bolt as a pin with threads, that gets secured with a nut on the other end. A pin has no thread and gets secured by putting another piece through a hole in the end of it.
So they have different failure modes (a bolt can get its thread stripped, or vibrate the nut off) but otherwise can be used interchangeably. What's the structural difference you are referring to?
Right. Bolts are usually weak in shear but strong in tension.Their job is to create high friction between two pieces; that friction joins the two pieces as one.
I believe the metal sleeves that the bolts go into are the pins.
Edit: I probably shouldn't have said "sleeves". That has more than one meaning in this context. I'm talking about hollow pins, secured by bolts when I say "sleeves".
I think it's right. He seemed to be describing hollow pins with threaded holes. The pins hold the sections together, the bolts screw into, and steady the pins.
Watch this video for a good breakdown of what people think happened: https://www.youtube.com/watch?v=cexN2-T6dxY&feature=youtu.be he has a followup video with additional pictures sent in from the scene that seems to corroborate his thoughts
This might be a stupid question, but how is the assisting crane finally disassembled? Is it a different type that doesn't need disassembling in the same way?
Yes, the assisting crane is a mobile crane where the boom pivots up and down about the base. Some have booms that need to be disassembled piece-by-piece, so you just pivot it down until the top is resting on a flatbed trailer, then unbolt it.
These mobile cranes can reach just as high as tower cranes, but can't carry as much, can't reach out over the tops of buildings, and take up more room on the ground. Hence the need for tower cranes.
Another interesting advatange of tower cranes is that they can grow/shrink themselves by using a special jig; that's how they're able to negotiate 200 story skyscrapers. That jigs tends to be too convulted to use for smaller projects, in which case they use a mobile crane as seen here.
sometimes they’re loosened all the way
to “finger tight,” sometimes a few pins
are removed ahead of time
[...]
“It saves time, which equals money.”
I'd have thought the time it takes the supporting crane to lower the top section to the ground would have been greater than the time it takes to remove eight bolts. After all, presumably the guys doing this for a living have the best tools available.
Does anyone know why removing the bolts is so slow? Or is the supporting crane actually really fast?
Thinking about it, they need to remove eight bolts per section, then the section itself, then eight bolts for the next section, and so on. I could have expected them to save time by removing the bolts of the second top section in parallel to the supporting crane removing the top section, but removing the bolts of all sections only makes sense when the supporting crane is actually faster than the bolt-removing guy/team.
Something else might be going on -- maybe they did not want the workers who were removing the bolts to wait for the supporting crane and sent them home to save on wages or gave them other work to do.
My first guess was that you have to climb back down to the ground between each section being removed, for safety reasons (don't get hit by a swinging section of crane). But in that case you'd still have to climb to remove the last two bolts, so it doesn't really save anything.
It might be along the same lines as your mom telling you to do the washing up now rather than later when you need the dishes.
It's the same effort now and later, but putting it off makes you look lazy.
Someone was probably keen to impress the boss and say "Look, we've removed all the pins so that cranes ready to come down. Nothing else to do now! 2 days ahead of schedule!".
I would guess it saves crane time for the supporting crane. If the bolts are loosened before the supporting crane begins operating, the total start to finish crane time can be less.
Of course, the tower's resistance to overturning moment is reduced during that time - to what appears to be fatal results in this case. Not worth it.
And a follow-up after new pictures became available further confirms this: https://www.youtube.com/watch?v=7Wo7J_buZUE
The article makes it seem like it's a common practice in the industry. Pretty scary! I don't really understand how it saves time, though... I'd think the bottleneck is the heavy-lift crane, isn't it?