Welding advice

[chris_d]

Dear members,

I need to have two 15mm thick steel plates butt welded together in situ (structural steel). The butt weld joint will be approx 215mm long. The joint will eventually lay directly on a concrete padstone that will be cast in situ.

I have yet to cast the padstone but one of the plates (an RSA) is fixed in its final position and cannot be moved.

Question: Can 15mm steel plate be butt welded from just one side/face of the joint?

I ask this because it will be significantly easier to cast the padstone before the second plate is positioned but with the compromise that the welder's torch only has access to one face of the butt joint. Conversely, welding the joint before casting the padstone permits access to both faces of the butt joint but makes the padstone much more difficult to subsequently cast.

Another consideration is whether it is dangerous to weld steel where the joint location is in direct contact with concrete?

I tried to call my local welding shop earlier today to discuss the feasibility of such a weld but they had difficulty understanding the context over the phone.

Any guidance gratefully received,
Chris

 

[jasonB]

Surely this is something for your structural engineer to detail and the BCO to approve.

Can you not weld or bolt a angle plate to the underside of your long plate and then bolt that to the underside of the joist flange using large CSK bolts if you want to keep a flush top surface, I've recently done some structurat steel and teh engineer spec'd M16 Csk bolts so we could have the minimum protrusion below the steel.



Jason

 

[chris_d]

Hi Jason,

The Structural Engineer specified the welded joint! This is because the surface area over which bolts would otherwise be used is too small to resist the forces at play.

Unfortunately, a complicated two stage build forced me to this irritating compromise situation - there was no way to place them already welded together in a single action without various other schedule and cost impacts. The structural setting is actually far more complex than the diagram shows with rafter ends currently fixed to the RSA and the far end of the RSA has a welded return that is bolted to another padstone that has been built into a new gable end wall!

Any other welders out there?

Cheers,
C

 

[Boatfixer]

I would say a qualified yes it should be possible to do a full penetration weld from one side if the faces are properly prepared but there may be some problems with the concrete standing the heat. I know my workshop floor has blown bits of concrete where I have been welding on it.

Graham

 

[No skills]

You really need to speak to a good mobile welder to find out for sure, IMO I think its possible with enough amps on an arc welder but I would imagine the joining faces would probably need to be chamfered first - if the poored concrete is going to interfear with the grinding of the chamfer then obviously that will need doing before the mixer gets going.

Again, please speak to whoever will be doing the welding first!!

 

[chris_d]

Thanks Graham and 'No skills'.

I shall take a printout of the above picture to my local welding shop tomorrow and ask about the feasibility of a 'full penetration weld' assuming the faces to be joined have been pre-chamfered.

Thanks again,
Chris

 

[jasonB]

Did you get the other steel from a fabricators, most of them will come out and do site welding.

As said you will want to grind a good chamfer on the mating edges, should not be a problem welding close to teh existing concrete.

Hitachimad on here is a steel fabricator, not seen him about much lately but you could pm him or try his welding forum

http://www.mig-welding.co.uk/

Jason

 

[chunkolini]

I dubt anybody will be wiling to do the weld in contact with concrete.
this would take some time and out a lot of heat into the concrete. I have heard of concrete shattering in this kind of situation.
I have foud doing small quick welds on the floor out of laziness you can hear and feel the concrete cracking and trhowing splinters about.

Sounds like a bad move to me.

Just remembered this post from the mig welding forum http://www.mig-welding.co.uk/forum/show ... hp?t=20435 a great source for any metal bothering queries.

Chunko'.

 

[barkwindjammer]

No welding required
this butt joint unless it involves a levered over shot doesn't need to be welded at all, if there is a levered overshoot then 1 inch stitch welds would suffice-but before the padstone is cast in situ !, the small aggregate in short slump concrete like this would be the equivelent of heating 9mm shell ordnance :shock: -tell your architect he is a t*t, most of them object strernly to this kind of 'constructive criticism' :wink:

 

[chris_d]

Hmmm. I was quite happy until I read the comments posted earlier today. I was happy because this morning I spoke to a mobile welder who said there should be no problem in achieving the weld and recommended that I embed a strip of fireproof board (eg Vermiculite) within the padstone beneath the joint location. The welder has a diesel powered generator on a trailer that can produce 400Amps for arc welding which is capable of joining up to 18mm thick steel at up to 25 feet from the trailer.

If the fireproof board isn't up to the job then the concrete splintering/blow-out risk highlighted by today's posters is particularly pertinent as the concrete will be pretty green/damp given my intended schedule.

I shall keep you posted.

 

[No skills]

Mmmm 400 amps

The fireproof board under the joint sounds like a good (experienced) suggestion - has the welder done similar on other jobs?

 

[jasonB]

Another option may be to create a void below the weld area, say cast in a bit of 25mm polystyrene insulating. Dig this out once the concrete has set, do the weld and then pour in a grout slurry to fill the void and give support to the underside of the joint.

 

[Eric The Viking]

It's still not clear what forces the weld will be subjected to - straight shear, twisting, bending, or what? It doesn't look like it's under a lot of force, but as you say you haven't shown everything. Are you going to bolt the joist to the concrete pad eventually?

Could you go one better than a heatproof pad by leaving an air gap under the steel at the corner?

Your welder will most probably grind the mating surfaces to a "V" then tack and fill with weld. this ensures 'full penetration" from just the one side and should be pretty strong. You'll get a better weld with MIG than just arc (less slag), but if it's just to hold the two elements together in position, that may not matter.

I'd worry most about not being able to get enough heat into the weld with the concrete there, but flying debris wouldn't be fun either - if it's cast it will be wet for a while too.

 

[Davon]

I think I'd do it this way

Davon