Tides and engineering

[grant hutchison]

Not sure whether this is a Q&A, Astronomy or General Science. Mods should feel free to move it.

A friend just sent me a clipping from p12 of something called Professional Engineering, issue of 7 Mar 2007.

The Moon has posed BAE Systems an unusual problem during the final assembly of the Typhoon jet fighter. On the Lancashire coast, at Warton, where the aircraft is assembled, the movement of the tides and the moon's gravitational pull have resulted in the tight tolerances being thrown out of alignment.
Martin Topping, final assembly operations manager, explained: "Every time the moon pulls the tide in and out, the ground moves by between one and two millimetres."
So BAE has spent £2.5 million putting in alignment facilities which use laser trackers and computer automated jacks to cope with the distortion. ...

It seems quite improbable to me that tidal flexing could induce a significant strain in a 15m airframe resting on a concrete floor.

Anyone know anything more about this?

Grant Hutchison

[Ronald Brak]

It seems quite improbable to me that tidal flexing could induce a significant strain in a 15m airframe resting on a concrete floor.

Anyone know anything more about this?

I get the impression that it's not tidal changes in the airframe that's causeing the problem, it's tidal changes in the earth's crust.

[grant hutchison]

I get the impression that it's not tidal changes in the airframe that's causeing the problem, it's tidal changes in the earth's crust.

That's what I think, too.
But it seems, off the top of my head, that any tidal flexing of the ground and concrete floor along the length of the aircraft would be too small to produce measurable engineering difficulties because of relative movement of parts of the airframe.

Grant Hutchison

[Ronald Brak]

Hmm, I don't know what they are doing. If it is the moon's gravity over the length of the airframe then just standing next to it should cause similar problems.

Maybe they got a laser to measure tolerances that they set up 100 meters away for some bizarre reason?

Or maybe every four weeks someone parks a big truck by the side of the building?

[grant hutchison]

I'm guessing, assuming that this is a true story, that the significant factor must be the movement of a large mass of water nearby, causing the ground to dip by some measurable amount.
But is the entire factory really rocking by a millimetre or two along the length of the airframe? (Yikes.)
Or is the effect much smaller than that, but the Typhoon being built to micrometre tolerances? (Wow.)

Grant Hutchison

[Ronald Brak]

But is the entire factory really rocking by a millimetre or two along the length of the airframe? (Yikes.)

That wouldn't surprise me actually. Not that I know exactly what stresses and deformation a building might go under, but I do know concrete buildings can flex due to wind etc. and it's possible for what we regard as solid stone can have a tiny amount of give in it over enough length.

[publius]

This is a very different different type of flexing, but something that I just don't like the feel of is locally they've been building bridges, and a couple of large shopping mall complexes that are allowed to flex and move with changing stresses.

Sitting at a redlight on some of these new bridges, you can feel it swaying, and it is a most disconcerting feeling. You think the bridge is about to fall. And same thing with one of these shopping malls. Standing on the second floor, every few minutes a little "wave" will propagate through and you'll feel it.

They say these flex designs are much better than very rigid designs. But it doesn't feel that way.

-Richard

[grant hutchison]

... but I do know concrete buildings can flex due to wind etc. and it's possible for what we regard as solid stone can have a tiny amount of give in it over enough length.

Maybe it's just my physical intuition letting me down, but it seems like tidal water movement on a neighbouring shore should be several orders of magnitude below wind forces on the building itself: wind applies big forces, if you integrate it out across the surface area of a building.
It just feels (and it's no more than a feeling) that a factory floor that shifts significantly because of the local tides should be shifting because of the movement of machinery within the building, too.

Grant Hutchison

[grant hutchison]

They say these flex designs are much better than very rigid designs. But it doesn't feel that way.

This famously caused the closure of the Millennium Footbridge across the Thames. The side-to-side forces generated by a lot of people walking made the whole thing sway to an extent that felt hideously unstable, and they had to shut it down while they installed dampers.

Grant Hutchison

[Doodler]

Concrete slabs flex as they age. One of the key elements of slab design is a control joint. Nothing more than a sawed crack about two inches deep across a pattern that carves the surface of a slab into smaller segments. The idea is the acknowledgement that no matter what you do, concrete slabs will crack. There are no ifs, ands, buts or countermeasures, you just do your damnedest to be ready for the inevitable and try to control the path of the crack.

Tidal flexing is something I hadn't heard of before, but if they can demonstrate it, I won't argue with them. So much more flexes a slab, though. Vehicles moving on it, temperature and humidity variations... Temperature variations are the worst. Get one area constantly warm, and another constantly cold, and you're asking for headaches. The amount of motion is pretty miniscule to the scales of human sense detection, but the forces at work are very real.

[Trebuchet]

Boeing is said to have had similar problems at its plant in Seattle, next to the Duwamish river. This is almost at the river mouth so the river is tidal. The plant is in a valley bottom which would have been tidal marsh originally. So the concrete slab floors move a little with the tides.

[Irishman]

Since water is more flexible than rock, we see the tidal effect strongly in the oceans of the Earth, but barely at all in the ground. However, the rock does bend, by as much as 30 centimeters (about a foot) up and down twice a day!

http://www.badastronomy.com/bad/misc/tides.html

So the Earth surface rises and falls about 30 cm twice a day. The differential shift in the shop floor is measured at 1 to 2 mm, which is clearly affecting tolerance to the micrometer.

[torque of the town]

This famously caused the closure of the Millennium Footbridge across the Thames. The side-to-side forces generated by a lot of people walking made the whole thing sway to an extent that felt hideously unstable, and they had to shut it down while they installed dampers.

Grant Hutchison

I found that bridge problem fascinating, It's main cause was "synchronized walking"


David.

[Larry Jacks]

And don't forget, aerospace tolerances on high performance vehicles are very tight. When you're working with micrometer tolerances and keep detecting variation, you start to wonder why. Seems they found the answer. It was probably something they never had to deal with before.

[Peter Wilson]

Wonder if the moon creates any "unusual problems" for LIGO ?

[Ronald Brak]

I found that bridge problem fascinating, It's main cause was "synchronized walking"

I understand armies have their soliders break step when crossing bridges. (Or at least some of them do.)

[HenrikOlsen]

Different problem, in this case it was side-to-side swaying which hadn't been seen in bridges before.
If the frequency is near to normal walking steps, ans even very small swaying exists, people will automatically sync their steps to the oscillation to help keep their balance, and it happens in a way that adds energy to the oscillation.