Fitting RSJ - Support

[chrisctk]

I am building a single story front extension on my house (mainly DIY), and now thinking about how I am going to install the RSJ. I have had a structural engineer do the cals and I will be installing 203x133 UB25 beams (two of them bolted together because its a cavity wall) - beams will be about 3800mm, with the opening being 3500mm (100mm bearing one end, 200mm bearing the other).

At one end the beams will be sitting on the internal cavity wall (running 90 degrees to the RSJs) so that the extension wall is flush without a protruding pier. The SE told me to fit a 800mm long padstone, 100 wide and 215mm deep. Hopefully simple enough to do - half of this padstone will be fitted in the existing wall, and the other half in the new extension wall.

At the other end, I apparently need to knock down the existing cavity wall and build a 300x300 solid pier. Apparently, due to other openings in the existing wall, its not solid enough to be retained as a pier.

Would really appreciate some advice regarding (a) supports required and (b) the order to do things in.

I was thinking of using acrow props with timber needles to support the wall when doing the work - probably 5 of them on each side with the needles running through. The space above where the RSJ is going is tight (due to upstairs floor), so I was hoping to use 100mm x 75mm timber for needles (so only have to knock out single brick) - does this sound strong enough?

In terms of order, I wasnt sure whether it would be best to fit the RSJ first, then knock out the wall and build the supporting pier (with the RSJ supported on acrow props while I build it), or to initially install just one needle and knock out and rebuild the 300mm section of wall for the pier and then deal with the RSJ.

Hope all makes sense!

Cheers

 

[noseall]

As long as you can wriggle the steel beam into place and that you have some solid ground for propping, then you are as good as sorted.
Look at it like the beam is now doing what the wall once did. As long as you can safely support that beam, upon ground that is not going to give, you should be fine. For example, we often install beams then retrospectively install the pads beneath, whilst propping said beam.

 

[chrisctk]

Cheers. Will 75mm x 100mm needles do the job, or do they need to be bigger? Seen some people refer to 100mm x 100mm needles, but I don't have the space to squeeze these in. Would only be about 65cm between the needles, so hopefully weight well spread until the beam takes over.

 

[tony1851]

Why not hire Strongboys instead of using pairs of acrows with needles?

They make things a lot easier as you are only supporting from one side.

Is your inner skin made of sun-dried mud brick? 800 long for a padstone is overkill, even for low-strength celcon/thermalite.

 

[chrisctk]

Yeah I have been considering strongboys, but my understanding is that they old hold 1 skin, so if I had them outside I would still need to find a way of supporting the inner skin. I have read that I can potentially support the inner skin by just proping the ceiling (the ceiling joists run into the wall at 90 degrees), although I also read this can be problematic if you don't know how well the joists were fitted (ie if they are not packed into the wall and therefore could allow the wall to slip a few cm).

Given all this, I thought props and needles would be easier, but let me know if I am missing a trick.

Yeah I thought 800 was excessive, but no real hassle to fit so may as well stick with the SE designs (will also be spanning a small cavity where new wall joins existing) so better safe than sorry.

 

[tony1851]

Strongboys can be used to support both skins - they're long enough.

(in this instance, the inner skin is supported via props under the floor joists).

 

[chrisctk]

Cheers Tony.

In terms of the RSJ beam, the SE have spec'd that the two beams need to be secured together using PFC diaphragms welded to one web
and secured to other web via 10mm thick MS endplate welded all round with 2No. M16(8.8) bolts. Is this overkill - I have seen others on here just refer to drilling and joining the two beams with M16 bolts, using cut down scaffold tube as spaces. Would be a lot easier and cheaper to just order pre-drilled beams and use spacers rather than sort out welded diaphragms, but not sure if its wise going against the SE design. Any thoughts?

When I spoke to the SE, they said their design meant the two beams worked with each other rather than acting as two separate beams, but given both the beams are sitting on the same padstone (and are each far stronger than the weight they are supporting) I cant really see how this makes much difference.

 

[tony1851]

Beams are usually fixed together to prevent sideways buckling, but in practice this is only a problem for relatively slender beams over long spans. In a domestic setting
over a span such as yours, and using the 203 x 133s, that is overkill IMO. It will be expensive in terms of fabrication costs; also, how will you get the bolts in? (unless I've misread your post). Unfortunately, if you use the scaffold-tube method - which would be perfectly fine - the SE will disclaim any responsibility, so you may be stuck with his detail.

 

[chrisctk]

Thanks Tony - you echo my thoughts exactly . I have been trying to get more clarification from the SE (eg how the bolts will go in), but they only seem interested in helping if I pay their hourly rate (which seems a bit off to me given I am just trying to interpret their technical jargon).

I am inclined to just go with bolts and spacers and lose the SE insurance, assuming building control will sign off. Can't help that feel the SE is more interested in their insurance policy renewal than practicality of design.

 

[tony1851]

. Can't help that feel the SE is more interested in their insurance policy renewal than practicality of design.

You have hit the nail on the head.
Some SEs will overspec the design to make the risk of failure 1 in 50 million instead of 1 in a million, and the client pays unfortunately.
Yes, his design will be OK structurally, but so will many simpler and cheaper ways.

 

[chrisctk]

Yeah, just been quoted £798 for the SE design to be fabricated, vs £204 for simple beams pre-drilled for M16 bolts (and I am sure I can find some scaffold pole and cut it to size myself for spacers).

Any advice on the spacers - 600mm centres OK? Single M16 bolt or two (one above other)? All in a line along the centre of the web or at different hights along the web?

 

[chrisctk]

In case its of use for anyone else - having spoken to the structural engineers again and challenged the complexity of the diaphragm design, they have changed their minds and are now happy with spacer tubes at 400mm centres staggered 50mm above and below the centre line. Was really worth challenging them, as the fabrication needed for their original design was going to cost £800, whereas the simper design with steel spacer tubes is nearer £200!

 

[tony1851]

Nice one! Well done.

 

[noseall]

Isn't it refreshing when expensive artistic license is usurped by economical common sense.

 

[chrisctk]

Almost ready to get started on this now, just want to double check the temporary support...

I am thinking of using strongboys on the outer wall skin (one every 70cm or so), then support the inner cavity skin by using scaffold planks on acrow props holding up the floor joists (as they run 90 degrees into the wall) - as far as I can make out basically the same setup as Tony's photo above. Am I missing anything - anything I need to check (eg do I need to rip the plasterboard off the ceiling and check that the floor joists are fitted into the wall OK etc)? In case it's relevant, its a 1985 house with 75mm cavity and celcon lightweight block for the inner skin. Cheers!