The Folly

Aah, the Folly.

The site is split into 2, with a bus lane running through the middle. The resulting building is one side of the bus lane, but all the space for their Portakabins, deliveries and so on is the other side.

And they don’t want to have to walk round. That might take 30 seconds.

So instead, they are all going to get super fit by climbing up and down stairs all day to go over a bridge between the 2 sites. You remember it’s going over a bus lane, yes? Most of the buses are double-deckers, so the bridge has to be twice the height you would imagine. That’s a lot of stairs.

Going over bridge, apart from being far more effort physically, is almost certainly going to take significantly longer than walking round. Particularly when you do it for the 20th time that day with legs that have had enough of climbing stairs.

To give you an idea of the scale of it, here it is looking from the Salisbury Road (North) end:

IMG_1089

And from the Interchange (South) end:

IMG_1086

Not small. It’s taken weeks to build.

You may notice the footpath entrance under the white bit of wood in the 2nd photo above. This is a covered tunnel the whole length of the site, right up to Salisbury Road.

When they have a crane lifting, say, several pallets of bricks from one side to the other, if the crane drops it all, you won’t get killed by being hit by tons of bricks, and buried under them.

Instead you will get killed by a collapsing tunnel that can’t take the impact of a few tons of pallets+contents being dropped from 20m and hitting it at considerable speed. For the physicists, there’s quite a bit of potential energy in them thar bricks at 20m, is there not?

( Editor’s note: can a chemist please advise on the amount of dynamite containing the same energy as 2 or 3 fork-lift pallets of bricks dropped from 20 or 30m? A physicist can help you with the potential energy bit, your job is the chemical energy. Then we can imagine putting a suitable bomb on the roof of the tunnel instead. )

I, for one, am not convinced that a tunnel made entirely out of scaffolding planks and metal pipes which are only clamped to each other is strong enough to withstand that impact. So you will still be dead, but buried under a load of steel and wood as well as pallets of bricks.

And don’t even think about what happens when the crane “drops its load” onto a bus. They are basically made out of tin plate. The roof of a bus is about as strong as the roof of a tent. But you know that already, everyone has seen a news photo of a bus that has driven into a low bridge; the top of the bus just crushes totally where it hits the brige.

We should do what many countries do in such situations: build a little shrine (complete with candles) at each end, so you can at least say a prayer before entering. The prayer will be at least as effective as the tunnel.

3 thoughts on “The Folly

  1. Harvey’s calculations…

    tl;dr approx 1 stick of dynamite or 1 M67 hand grenade.

    Browsing suggests a pallet of bricks weights 1-2000kg. Given the density of bricks and the size of the stack on a pallet I reckon the top end of that is nearer; lets say 2000kg.
    E=m*G*h and G=10m/sec^2 as near as damn it, so the potential energy E=400kJ.

    Now actually that’s not much in chemical energy terms. Think – petrol/diesel driven crane, how many times could I lift this on a tankful of fuel? Answer, many, many times; so that potential energy is equivalent to not much petrol/diesel.

    How much explosive? TNT is the easy one, 4.2GJ/tonne (i.e. 1000kg) or 4.2MJ/kg, so the potential energy is the equivalent of about 95g of TNT, about a 4cm cube, a out *half* the charge in a hand grenade. Not that big an explosive charge.

    Actually your problem is not mainly the *energy* – a decent bowl of breakfast cereal, creamy milk and some sugar is probably as much *energy*. You problem is *momentum*. From say 20m again it will hit the ground at about 20m/s with a momentum of 40,000 kg.m/s and your squidgy body isn’t about to stop it without a lot of grief. It weighs a lot more than you do, and accelerates the top of you cowards the bottom of you, with unfortunate results.

    In relatively low velocity collisions like this momentum transfer is the big issue. In very high speed collisions the kinetic energy becomes the big issue (which is why meteor craters are round even in an oblique hit.)

    Physics lesson ends.

    Like

  2. I think it’s more likely that the bridge is built for health and safety. One of the biggest causes of accidents on a building site is people being hit by vehicles. When they get the site up and going properly they will segregate pedestrian areas from vehicles. This has been extended to people being hit by buses and therefore a bridge. The time it takes is always completely irrelevant when you are talking about health and safety. They may have already had a near miss or comment from a driver which would make this happen.

    As to the strength of scaffolding, the forces it can withstand are very large and are dictated by the density of the poles used and the amount of cross bracing. A fall may damage the poles but is unlikely to deform enough to hit people underneath. The support has been made double with cross bracing on it so it seems this has already been thought of.

    Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s