Structural Steel Grades

[i_am_fubar]

I take it your a structural engineer then?

I've got a good grounding in engineering in general, just self-training in structural for this one. And figure there's no harm in going a bit over kill to ensure what I have IS strong enough.

It'll be a marked improvement on what I have at the moment which is a rotting heap of compost (literally is that in places, pics to follow).

The over 10kN for tensile is based on the steel failure rate, not the brick. the studding will be the last thing to go out of anything else, penny washered and nylocked on.

Reason for steel behind the ledger is that the roof is currently build into a wall over the toe of a boot lintel with no additional support. So there are 5 joists resting on the cantilevered lintel that should have been upgraded during the original build. But budget doesn't extend to fixing that now.

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• i_am_fubar
• 18 Jul 2013
• 2

Boot Lintel

Assuming a worst case failure of that lintel (so the ledger is unsupported for a 2m length in the middle. The steel is the difference between a 3.8mm defelection and a 0.7mm. Again, probably over kill, but it does mitigate the lack of proper lintel.

Reason for resin over any sort of expanding anchour system is because the wall was excesivly damaged when the old joists were installed. There is also a door frame just above the roof, so only about 3-4 rows of bricks above fixings. Compressive strength will be fine as the ledger will be attached below the position of the old joists, but I'm a bit dubious about the tensile strength and don't want to risk splitting the brickwork.

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• i_am_fubar
• 10 Jul 2013
• 1

Note door above flat roof.

And I wouldn't do that Tony... Well, any time soon. I was looking into a distance learning course

Fubar.

 

[RonnyRaygun]

Interesting what you are saying about the boot lintel.

I wonder if it would be possible to completely avoid loading the boot lintel with the new roof structure. They obviously aren't designed to receive any additional load, and although it might not have moved yet because of the load from the existing roof, now might be the time to unload it.

Maybe you could install enough fixings into the ledger either side of the lintel and just nominal screw fixings above the lintel itself. Obviously the stresses would have to be checked.

Or maybe a beam spanning side to side picking up the joists. More expense, but you could do the calcs yourself

 

[i_am_fubar]

Just a total lack of understanding on behalf of the original builder. There is a RSJ covering a similar span half way down the living room that was part of the original build.

This was the thinking behind the steel. With the inclusion of the steel backer, I would hope to not just massivly ease the loading on the lintel, but actually re-inforce the wall above it that was comprimised when the old joists were built in.

Even if there were no fixings into the wall above the lintel, center deflection was coming out at 0.7mm I think (and that's assuming point load rather than distributed)

For the beam, do you mean span from side wall to side wall of the extension, and NOT anchour to the house wall (for any vertical load bearing)? If so, that's not going to work as the extension goes past the corner of the house, so the last 3 joists will need to be wall plate to wall plate. NOT wallplate to ledger.

I'm currently doing a model of the plan that will clarrify some points, I'm also a little torn on how to do the soffit ladders. Will ask once the model is done.

Just as an aside, is it acceptable to have any of the roof loading on the outer skin of the house?

Regards,
Fubar.

 

[i_am_fubar]

So, my preference is still to use 100x100x10 steel. However, I have found a cheap source of 120x120x5 steel.

The MOI is higher for the 120x120, but it does create the issue of being wider than the brickwork:

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• i_am_fubar
• 22 Aug 2013
• 1

The asymetric forces deforms the steel unevenly (no direct load on left vertical element). However, the amount is negligible in this situation (less than 0,01mm at 15000N/m^2)

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• i_am_fubar
• 22 Aug 2013
• 1

Would this be acceptable from a structural stand point, or would I be better off biting the bullet on the 100x100?

Similarly, I can get 200x6 steel for the ledger easier than 150x6. The strength increase (125000mm^4 above 150mm) will be beneficial, but there will then be 50mm of steel coming down the wall inside. The plaster will cover it, but again, would this be acceptable?

Cheers,
Fubar.

 

[tony1851]

On that scale, the asymmetric load on the larger section would not be significant.

 

[RonnyRaygun]

Wow, FE Analysis on a box section lintel in a domestic situation!

What do you think of that Tony?

As Tony says, it's not going to be an issue. For strength you would need to check what additional compressive stresses are being created in the outstand element, and check it doesn't exceed the limit, but it's going to be fine.

As for the 200x6mm plate, would it be possible to have the addtional 50mm above the timber? It will get covered by the lead flashing won't it?

What software is that Fubar?

 

[i_am_fubar]

Dang, I just realised I've been making the PCB board mistake by calling it FEA analysis. Gods damn it >.<.

It came with the latest version of works CAD package (Autodesk Inventor 2014. Very nice package and each business license comes with a home license for 'working' from home), seemed daft not to use it

Got no problem with the strength of the beam. Compression of mortar (as the steel will be bedded on) will be well in. Concern is with the encroaching 20mm into the cavity. It won't be a cold bridge in the truest form, but will impact the cavity thickness (currently only 70mm). If someone would be kind enough to give the thumbs up or down for this, I can get on and pester my company directors to sell me the steel, would be nice to start this weekend

I could go above the ledger, but it's a warm deck, so it'll be covered by insulation, not flashing. That IS a cold bridge there, and I would rather not meddle with condensation risks. Figure I can glue some plaster board to the steel and skim over that. If not, the coving will cover it anyway.

As another (4) aside(s) to the steels,

1) I've read mixed opinions on the need for wall plates for joists. The plan is to put perhaps 1 or 1.5 courses of engineering brickes (cut as nececery) over the steel, then wall plate tied and bolted down, then joists onto that. Sound good?

2) I've currently got this joist arrangement planned (noggins excluded):

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• i_am_fubar
• 22 Aug 2013
• 1

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• i_am_fubar
• 22 Aug 2013
• 1

I'm not 100% sure if the fascia ladders are right. On the far (left) side, it's not too bad as the wall plate is about half way from joist to fascia. On the near (right) side, the joist is almost right next to wall plate, so any load applied to the edge will be amplified on the joist & fixings.
(this is why I was asking about the outer skin taking load).
Facias are goign to be 18mm ply with uPVC over the top due to depth (300mm+).

3) Where do regs stand on closing the cavity off on single story extensions these days? Rather not put the extra cost in if I don't need to as I see no fire spread risk.

4) I plan on using 6mm ply / 110mm PIR / 18mm OSB3 for the deck. For additional strength, I'd like to bond these together as they're laid for additional mechanical strength. Would contact adhesive be best, or another type?

Cheers folks,
Fubar.

 

[tony1851]

Wow, FE Analysis on a box section lintel in a domestic situation!

What do you think of that Tony?

And I'm still using a slide rule

 

[i_am_fubar]

Despite owning two of them, I've never worked out where the 'equals' button is :/

 

[i_am_fubar]

Looking through this document:

http://pdfsdb.com/pdf/ground-floor-extension-internal-alterations-amp-loft-conversion-25250527.html

Don't know how official this guide is. But a concern:

Section 5.10 states:
Lintels to openings in all walls shall be obtained from Catnic Lintels Limited, of types indicated on Architect&#8223;s drawings, correctly sized for the span of the opening, with 150 mm minimum bearing at each end and d.p.c. over all in accordance with the manufacturer's recommendations. All Lintels to have a three course blue brick pad-stone under bearing.

Seems a bit off as IG do them too :/

(Still need the go no-go on steel encroaching on cavity)

Fubar.

 

[RonnyRaygun]

I'll let someone else answer the other questions, but there won't be a problem with 20mm encroachment. Single beams often carry cavity walls so your 20mm isn't an issue. You could shove a load of insulation inside the box section.

That link is just a specification written by a particular professional, and can be revised for a particular project, but it has no bearing on what you can do.

 

[i_am_fubar]

Cheers Ronny, Legend.

Steel being cut as we speak and hopefully fitted by end of bank holiday

I've got some spare glass fiber I can pack in the box as well as using it to close in ends of joist voids.

Thanks again,
Fubar.

 

[i_am_fubar]

Well, roof has been raised. Old lintels removed, steels fitted. All look good.

Next up is to fit the ledger when the steel arrives and the wall plates around the rest of the walls (I know not technically needed for a flat roof, but for ease of fitting the joists to the wall over the steel, worth it)

While I'm 99% sure that a ledger is the way forward on this job, I know it's not the strongest way to mount joists, preference always being to bed them into the bricks. 2x M12 studs per joist, as a general rule, for a flat roof terrace is fine isn't it? Building control won't start flipping out

Just having a look at the Rawl website, they seem to specify maximum fixture thicknesses for their resins (36mm in the case of below)
http://www.rawlplug.co.uk/index.php?option=com_content&task=view&id=23&Itemid=33

As I need 53mm (47mm timber + 6mm steel), would the M16 or M20 be more suitable than M12?

Cheers,
Fubar.

 

[tony1851]

would the M16 or M20 be more suitable than M12?

Cheers,
Fubar.

Overkill. IMO.

 

[i_am_fubar]

Right, so M16 is overkill.... But, it's only another £10 to go up to that size with the suppliers I have, so unless there's a reason NOT to go bigger, I think I'll go with M16.

Only technical issue is that I'll need to borrow a blacksmith bit from work to drill the steel, but no biggy.

Cheers Tony,

Fubar.