How much load can 100mm2 of 3.6N block take?

29 Jul 2014
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United Kingdom
I appreciate that this is something that I should speak to a SE about. I've had SE calcs done and this included padstone design for the builder. Looking back at photos and the SE calcs I don't think the builder has done what was requested.

We have a 3m knock through on the gable end of the house (so no roof load, but a lot of block) and floor joists notched (badly) into the steel.

Looking at the calc the SE requested a 200mm long pad stone on each side of the steel (2 steels, one for each leaf) onto the block below. As one side of the steel sits on a wall that runs parallel with it, the builder has put a single padstone over the cavity rather than running along the length of the block and built up to the padstone with thermalite type 3.6Nmm2 block. So the padstone rather than spreading the load over 100x200mm x 2 padstones, reduces it to two 100x100mm 3.6N termalite blocks.

Not sure if this is standard practice, other builders appear to suggest that they rebuild up to the padstone with stronger block work. Any idea how much weight will each 100x100mm carry?

Looking into it, I think 1kg = 10N. So does 3.6Nmm2 over 100x100mm equal:

100 x 100 x 3.6 = 36,000N capacity over a single face. Divide this by 10 to give 3,600kg potential load per 100x100 block?

Looking at the weight of blocks, looks like they're about 150-200kg per m2 for each wall. So say 3m x 6m to top of the house = 3 x 6 x 200 x 2= 7200kg spread over 4 x 100x100mm blocks, each of which can take up to 3,600kg in ideal conditions.
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I was away when we did maths, but here goes;

Firstly, the 3.6 strength quoted for the block can be increased by a factor of 1.25, but then also has to be reduced by a factor of 3.5, so the design strength will be around 1.25, a lot less than 3.6.

Second, your calculated loads have to be increased by various factors, typically 1.4 for the dead loads and 1.6 for the live loads. How that would work out in your case I wouldn't know, but a bearing of 100x100 on aac block does seem dubious to say the least.
Thanks for the response, wasn't expecting one :)

That explains the multipliers involved when trying to interpret the existing calcs.

I've got another SE booked in for a look at this as the original SE calcs have 8kN of load at the end of each steel (I think).

My guess is, even if you knock out 3m of existing wall and put a steel in. The walls above never fully sits on the new steel and the load passes to the columns. The steel is there to catch the block as it eventually starts to sag a little to sit in the new place.

If you were building up from scratch off the steel, the steel would sag a little with the weight of the block and possibly have the full 8kN of load on either side. Putting a new steel in, it never had that initial compression so the wall needs to move to put it under that load. Either that or builders pre-load the steel before packing out the gap. I've read about some hammering in wooden wedges.
Depends how you fit the beam. I always get builders to fully load the beam with folding wedges and remove supports before packing.
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Yeah makes sense, think that's something I'll do if I can bring myself to do a "next time" on a build.

I had two beams butted up against each other. As it was a two block thick wall with no cavity.

The builder had only pointed up one side of the beams and told me he was happy with it as he'd made contact points with the wall above. After about a week he still hadn't removed the skirting board to point up the other side. I decided to pull the skirting board off the other side as the technique he said he'd used for pointing, I couldn't see how he'd force cement in 20cm deep. Found that he'd literally just pointed the gap to the first beam, both of them were pretty much supporting fresh air over a 3m gap. Max amount of mortar I found was 2cm deep at a couple of points. The contact point he referred to was about 1cm2 about 1m from one corner. Hence why I'm not confident about these 3.6Nmm2 blocks he's put the padstones on.

I knocked in a bit of roof slate and then pushed in a 3/1 mortar mix. As I've tapped the slate between the steel and low points with a hammer, I doubt I've put enough load on it by comparison to wedges. Seemed better than just 20-30mm of fresh air though.

Think I'll quiz the SE about this and see what his thoughts are.

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