Thread by @25kV

THREAD: recently @EurostarJustinp and @EurostarGeorge mentioned windspeeds in relation to #OLE design. In response I promised them a thread on #wind.

No sniggering. Yes, you at the back. I saw you. STOP IT

This is that thread.

1/

OLE is exposed to a number of enviro factors, and wind is one that we spend most time worrying about. It's common for industry types to jokingly refer to OLE as "the wind-blown wobbly wire" - and there is some truth in that.

But wind affects a lot more than just the wire...

2/

…but lets deal with the wire first. OLE must be kept within specific horizontal limits relative to the pan - and therefore to the track. Broadly, in the UK, the wire must ALWAYS be within 400mm of the track centreline.

Doesn't sound too hard does it? THINK AGAIN DEAR READER
3/

The first thing that eats into the ±400mm allowance is basic geometry. Those track %$&*rds insist on using CURVES. But our wire only goes in STRAIGHT LINES. Those straight lines form chords on the curves.
4/

Where we can, we set the stagger up so the centre of the chord is over the track centre - but thats not always possible due to other constraints. So that eats into our position allowance.

But what's this all got to do with wind? I'M GETTING TO THAT
5/

Unfortunately as well as insisting on going round curves, our paying customers also expect trains to work when it's windy. The wire is pushed away from its normal position - and ironically, forms a curve. Specifically, a parabola.

And that eats further into our allowance
6/

Finally, we have to account for a geometry feature called stagger effect. I won't go into what that is here - life's too short - but suffice to say it eats a bit more of our allowance up. 7/

This diagram shows how the still air position - "midspan offset", the wind deflection - "blowoff", and the stagger effect accumulate in a single span of OLE. At all times, in every span, we must respect the ±400mm limit.
8/

As a designer the only variables we can use to reduce the deviation of the wire from the track centre are: wire tension (as it goes up, blowoff reduces), stagger (tweak it to bring midspan offset down) or span between structures (reducing it reduces blowoff).
9/

BUT THE WIND GARRY, THE WIND. GET TO THE POINT FGS
OK lets talk about wind! How do we decide what windspeed to use? Lots of people assume that we go out with an anemometer on a windy day and measure it.
10/

Luckily we don't have to - smart people have looked at wind measurements across the UK and charted it in a handy map. Unfortunately the map is in the dreaded Eurocodes which means we have to go and read them.
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I hated doing this so much that a while ago I plotted them on a g**gle map instead:

drive.google.com/open?id=1PiVy4…
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So what do these represent? They are the 1 in 50 year 10 minute mean windspeeds. Lets break that down:

They represent the predicted maximum wind gusts, averaged over a 10 min period, that will be seen during the most extreme storm you will see over a 50 year period.
13/

So that's a seriously rare event. Too rare, in fact; we do use the 50 year value (we'll get to how later), but making sure the wire stays on the pan is a service reliability issue, not a structural failure issue.
14/

So we can reduce the period that we use to 3 years, and that reduces the predicted speed (a bit). Hurrah!

Well, no actually. Lets have a look at those windspeeds; and let's pretend that we are going to electrify the route nearest my house, in not-very-windy Wiltshire
15/

The base windspeed where I live is about 21.6 metres per second. Not a unit you'll be used to dealing with. Let's turn it into mph.

That's 48mph - a fairly fierce storm. Things get worse the further north you go - by the time you reach Glasgow its 55mph.
16/

A recent standards change means we now also allow for directionality - the fact that in the UK, south-westerly gales are stronger than northerlies. So the windspeed also depends on the compass bearing of the railway
17/

You might be saying "so what - I've been out in winds like that". But we're not done yet - these base windspeeds are normalised to remove the effects of exposure on embankments, funnelling in valleys and other local effects.
18/

So we modify the base windspeed to account for local features - on a viaduct windspeeds of 100mph are possible.

We turn this speed into a wind pressure, then calculate how much it moves the wire. Too much movement means we must tweak staggers or reduce structure spans
19/

So far we have only addressed wind pressure on the wire - if we get this wrong, we have a dewirement.

There are worse things that can happen.
20/

As any building designer will tell you, wind pressure can do much more serious things to structures. So we assess wind load on those as well. Because the structure will be there for at least 50 yrs, we use the 50 year return period, which gives a higher windspeed.
21/

The resulting wind load on OLE structures can often be the dominant load when sizing masts and foundations.
22/

Unfortunately, even when we get all of this right, we're not out of the woods. PUN ALERT. When trees fall onto the line, the best outcome is a mere block to trains. The worst is a dewirement - there's no way to cater for these shock loads.

So best cut those trees back!
23/

High winds also bring foreign objects onto the railway; sheds, trampolines, gazebos - it isn't possible to design OLE that is impervious to these.

SECURE YOURS TRAMPS PEOPLE!
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(photo credit: Alan Baxter)

So that's wind - something OLE engineers take very seriously.

I. SAID. NO. SNIGGERING!

As always, you can find out more about wind and OLE by downloading the book from ocs4rail.com/downloads. Sections 10.10 and 10.14.2 refer....

ENDS/