Rubber Isolators

[Welderpaul]

Rubber isolators are required between mild steel and stainless steel parts to stop bimetallic corrosion between the two metals.

A drawing asks me to fit rubber isolators between bare mild steel and galvanised parts (on a structural job). Necessary?

Also, where a stainless tube meets a galvanised/powdercoated part - is an isolator required there?

 

[madkayaker]

i guess if there on the drawing then you need to fit them?

 

[Welderpaul]

i guess if there on the drawing then you need to fit them?

Not always - It's design and build contract and i have been requested to value engineer as far as possible...just another way of saying do the job, as cheap as possible, but don't change anything. Theres a lot of these - and they are a stupid price for what they are, plus they have to be stamped out.

Basically put together a design proposal based on the original design intent for approval. Everyone looses track of what we originally priced for (except us!) so we all make a few quid...

 

[Hitch]

Between bare steel and galv, i suppose it ought to be isolated, the zinc of the galv will react and rust prematurely.

Why is some galv, yet what its bolted to isnt?


Stainless meeting powder over galv.... thats a difficult one. If theres a chance the powder will be damaged (a tight fitting spigot/bracket) i guess it would be beneficial, otherwise probably not that important.

The zinc will always be the sacrificial part, unless your working with magnesium. Thats the only thing higher on the galvanic table iirc.

 

[Paul.]

Handy galvanic table here, these sort of things really should be decided by the engineer, whatever you do there will be some contact of dissimilar materials, bolts washers etc,

 

[madkayaker]

i see

 

[Welderpaul]

Why is some galv, yet what its bolted to isnt?

The galv is pre formed heavy scetion deck sheets, and the plain mild is the frame of a building. The building frame is unpainted as the design calls for a heavy section for strength/stability such that the steel would not corrode beyond acceptable limits during the life of the building. (Already in the spec) Its gonna be crap putting up a frame thats already rusty, but theres no coatings in the bill of quants so its not going on! The deck is thinner, realtive to the steel, and so is galv.

 

[Hitch]

So the deck sheets sit on the mild? and they want isolators on the bolts?
How about the joint between frame and deck, non isolated?

Id be inclined to suggest just bog standard bzp bolts rather than galv, with no washer.
Seems pointless to go the extra mile when your bolting to a rusty frame.

That is some serious penny pinching, not even blast and prime?

 

[Welderpaul]

The isolation pads go under the ribs of the deck which contact the steel. The deck cannot just be shot-fire fixed to the steel as normal - it contributes to the temporary stability until the concrete is poured so is bolted - no mention of isolators on the fixings.

We want to use blasted and primed steel, but we have been told it is to be bare steel only. To the lay man we are doing a crap job! Plus its fairly impressive so was going to make a nice picture for the re-branding excersice.......
I understand they are going to go massivly over budget with the frame cos they didnt allow for the roof steel when they priced the job! so penny pinching all the way. They have been hanging back from giving us the nod to fab until steel prices started coming down, then asking us to do all the design connections, model the frame, purchase the steel, fab and erect with two weeks notice. Sorry to disappoint but it doesnt work like that!

 

[Hitch]

If its that tight on budget, its cheaper to go with what they have spec'd, than your time to go over it.

 

[Welderpaul]

If its that tight on budget, its cheaper to go with what they have spec'd, than your time to go over it.

There is money to be saved - for one thing the original design shows the same section size used for the columns all the way up the building, and as we know, the loads get less on the columns the further up the building you go

 

[Hitch]

But sometimes, the cost of splicing smaller sections to larger ones outweighs the cost of continuing the heavy UC right up to the top....

I once made a large building, all end plates were 50mm thick. Apparently, it was cheaper to massivly over spec the plate, than work out individual calculations

Crazy world we are in

 

[Weldingartisan]

Perhaps the steelwork is left bare for spray on fire proofing to adhere to? W/A

 

[snowcat]

The spec seems strange, I used to do a bit of steel bashing in my time,(porta frames) things like perlins are always galved and bolted straight on the purling plates, M/S, never know any premature failure there......

it used to be standard to use massive over calcs on most of the frame, as hitch said its cheaper than buggering about, some of the reason was everything was done to a massive tolerance, for Assembly, when your running 80' eves at 30' in the air, all sorts of bits and pieces get eased with the gas to make em fit, so this is all accounted for, very common for stations to end up being cut and shut where the F**** ground workers cant use a leval.......bolts holes to be gassed etc etc........ ahhhh happy days.....

 

[Welderpaul]

Yeah, I hear that the Structural Engineer in question is not very experienced.
Great.

 

[Hitch]

On the over engineering... i did a fairly big shed (3 storeys) for Twinlakes park a few years ago. All the end plates were 50mm thick, apparently it was easier to do that than work out what they needed to be
Easy for them to work out, not easy to make- thats a lot of drilling

 

[Welderpaul]

On the over engineering... i did a fairly big shed (3 storeys) for Twinlakes park a few years ago. All the end plates were 50mm thick, apparently it was easier to do that than work out what they needed to be
Easy for them to work out, not easy to make- thats a lot of drilling

Thats gotta add a lot of cost onto the job - Simple end plates - 10/15mm plate - cropped and punched v. profiled 50mm plates - drilled to suit.

Nahhhh. The man in the office needs to get busy with the pencil.