Forth road Bridge nuts

Found this earlier today

All bridge nuts must be replaced
Forth Road Bridge (Pic: Charlie Simpson)
Nine out of a total of 1,888 nuts are cracked

An interim report into the failure of nine heavy-duty nuts on the Forth Road Bridge has found all 1,888 similar nuts on the crossing should be replaced.

Bridge authority Feta ordered the investigation after inspectors found nine cracked nuts during a routine inspection last year.

Engineers FaberMaunsell carried out lab tests and a study of similar issues on suspension bridges across the world.

The nuts are used to secure 192 "cable bands" to the bridge's main cables.

Cable bands are metal castings over which the steel rope hangers holding up the roadway are looped.

Each cable band is held in place by a number of 35mm diameter-high steel bolts, pre-tensioned to a load of about 80 tonnes.

http://news.bbc.co.uk/1/hi/scotland/edinburgh_and_east/7999853.stm

My question is how do you tighten up a nut to 80 tons without it slipping off?

Presumably they don't have five burly guys swinging on the end of a 10 foot shifting spanner

Edited By Ady1 on 31/03/2012 10:53:06

I think this is the BBC and its Journos getting the nomenclature confused! The cables that support the roadway are all pre-tensioned by the makers and cut to precise lengths using special measuring devices under the tension load to less than a mm tolerance.

The cables are then terminated in 'Glands' with an eye for fitting to the supports and the cable glands are shaped internally to follow the twist of the cable.

I think there is a conical wedge put into the end of the cable to lock it in place.

This can all be done flat on the ground and the cable then hoisted into position.

There are about 8 nuts and bolts on each gland and the recomended torque for the 39mm bolts would be about 370ftlb, easily done with a big torque wrench. Biggest problem is the nut driver splitting, which was quite common when I was doing 1" bolts to 180 ftlb.

The hub retaining bolts on my Passat front wheels are tightened to a very high figure in the 100's which I do with a piece of ali tube over the wrench.

Overtightening just stretches the bolts and they can increase length by 10%. As they said the new bolts and nuts had smaller profile and were harder materiel and the the loads would increase because of this.

This is really a non story as ongoing maintenance would take care of this and the public would be unaware of it.

Clive

They tighten the nuts using a compressor.

A couple of years ago we walked across the bridge, and there was some engineers working on one of the towers, they had a compressor at the bottom with pipes going up inside, I got talking to the guy who was operating the compressor and asked him what they were doing he answered tightening the nuts, he explained that they do this every year and after five years they replace all of them, you could hear the compressor kicking in as each nut was giving the treatment.

No idea what they were using to tighten them I sould imagine some sort of air gun like they use on car wheels.

Stew

I know a thing called a torque mulltiplier was used to tighten wheel nuts on trucks and buses years ago,do'nt know if they still use them. I think it was a planetary gear device to reduce the amount of effort needed to tighten big stuff.

Dek

Gentlemen,

I volunteer at a Museum called Internal Fire http://www.internalfire.com/ at Tan-y-Groes about 25 miles south of Aberystwyth where we restore and show big diesel engines built by Allens of Bedford and we use a Torque Multiplier to tighten the big ends of the engines.

Martin P

Not sure where the mystery is for tightening nuts/ bolts.I run the engineering section in a factory processing plastics.I insist all my workmen and contractors use a torque wrench at all times when tightening up critical fasteners.We have wrenches that go up to 1500 Newton meters plus and also torque multipliers which increase the applied torque by a factor of over five times. There really is no need to swing on spanners with metal tubes or smack them with hammers etc.(although I have seen this done on many occasions). If you were to look on the bolt manufacturers lists they will give you the maximum permited torque for their products. Over tightening is not required as it can seriously compromise the fastener. I have produced lists showing the settings required for the different sizes and types of bolts we use.Some of our bigger dies have 40mm high tensile bolts and these are taken up to quite high tensions (still below the manufacturers limits) all we use are the correct tools for the job! Simples.

Hi Martin,

we were over your way a year or two ago, and I saw the signs to 'Internal Fire' when heading south. We were going to call in on the way north but I couln't spot you!

Neil

I live in Bedfordshire and go for several days at a time, our opening for the new year is next Easter Sunday and the museum is open until the end of October, during this period there is a sign both north and south direction just north of Tan-Y-Groeson the A487. It is well worth a visit if you are into large industrial engines including a working Proteus Gas Turbine Generating set removed from Festiniog.

Martin P

I checked the date just incase....

Hi Stub did that sing not start with an E and not In

Have a nice day everyone

Bob

Perhaps it was 'Infernal' Bob!

Neil

my old man recons that in the local pit they had nuts of about 6 inch across flats and these where turned with a special spanner about three feet long with a large blob of metal on the end of the handle designed to be hit with a big hammer or mundy hammer as some seem to call it

The large bolts that we used to connect the flanges of the propellor shaft sections of large tankers were not tighted by using thie nuts. The bolts were stretched along their length by known hydraulic pressure which also reduced their OD and allowed them to slide into a their 'fitted' hole and then the nuts nipped up before the pressure was released and the bolt clamped the flange.

Someone I helped out this evening found a way to loosen tight wheelnuts. Get punctures in both left wheels then drive 10-20 miles until there is no tread left on the tyres and a huge holes appears in them causing enough noise for your to notice

Toi bhe fair they have only been driving for a month, but i'd love to know how they got two punctures and didn't notice for that long.

Neil