Placing a RSJ on a 3.6N AERATED CONCRETE BLOCK (Thermalite)

23 Jun 2018
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United Kingdom

I am taking a corner out of the kitchen on the ground floor which will be supported by a 'T' RSJ section. The largest RSJ (203 x 203 x52) has a span of 5m. One end will be on the original 9inch brick pier and the other will be on a new 300mm wall (inner leaf - 100mm).

The builder has built the inner leaf with 3.6N Aerated thermal blocks where the RSJ is to be placed perpendicular. I was expecting this section to be built with concrete 7N blocks. I am concerned as according to the calculations, there will be a force of 51.4 kN, however the builder is confident this is ok. There will be a padstone on the wall of a size to be determined.

Is it ok (and normal) to place RSJ's perpendicular onto a thermalite wall and will it be able to withstand the load?

Could I extending the RSJ to the concrete outer leaf so that the the RSJ sits on both leafs ?

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Yes it’s normal to support a beam perpendicular on the inner leaf of a wall, but the structural engineer who is doing the beam calcs should specify the masonry strength and padstone size and not the builder.

3.6N thermalite block sounds a bit light for supporting a big point load, but really depends on length of the wall, padstone size etc… Has your SE not not done all this?
Build as per the plans. Only deviate with the prior approval of whoever did the design and the building inspector.

Non of this "Oh it will be OK I've done this before" nonsense.
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I have calculations and it mentions bearings, but I dont know how to read them.
Ask your architect to explain via email.

There should be a specific description somewhere in the calc's describing the pad-stones in relation to end bearings. It's not unusual for beams to sit on 3.5n walls but rarely directly onto the block. Refer to the calc's to see if there was any special dispensation regards the wall construction/make-up. They often describe the beam landing zone as pier 'A' or pier 'B' or some such.

Oh and sack the builder if he can't read the calc's. (y)
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Not a problem - it just needs a suitably-sized spreader to distribute the load to the blockwork. Extending the beam to sit on the outer skin as well would cause more problems than it solves.

Assuming the 51.4kN figure is a 'factored' load (ie increased by the required safety factors) a 440 long x 100 wide by 140 high padstone would be fine.

(The SE also needs to do a check on the strength of the blockwork skin approximately half-way down below the padstone; and also be satisfied that removing the wall doesn't compromise overall stability).
The 51.4kN figure is 'factored' load. The inner wall where the RSJ will sit on is currently 7 blocks high and the bricklayer said he will use concrete blocks for the remaining 3 courses just below the padstone (as I was worried about it).
Thanks for 440 long x 100 wide by 140 high padstone, that has helped.

I would have thought, extending the rsj to outer 7N leaf by 75mm would be good. I wouldn't expose the rsj to the weather as it would be 25mm from the end of the outer leaf. I know this is frowned upon because of thermal conductivity issues but would this not help disperse the load?
Definitely don't extend it, steel is super conductive so you really want it to be inside the insulation, otherwise it will cause a thermal bridge. Our builder had to do a load of rework because they cast pad stones across both leaves and were laying a beam almost all the way to the render.
The architect said sometimes you get beams raining on you if they're conducting the heat out of the insulated area quick enough.
the bricklayer said he will use concrete blocks for the remaining 3 courses just below the padstone (as I was worried about it).
And then your next worry is the different thermal properties in the wall and stained decorations.
the bricklayer said he will use concrete blocks for the remaining 3 courses just below the padstone (as I was worried about it).
Also, contrary to popular belief, an aerated block will be just as strong as an aggregate concrete one if they are rated to the same compressive strength.
No benefit at all in swapping out 3.6N AAC blocks for 3.6N concrete ones.
There might be a benefit in using 7.3N blocks (AAC or aggregate concrete) but not if you use a pad stone as Tony described.
The only question that remains is whether the wall is strong enough with the load spread at 0.4h (as Tony mentioned). Three courses of concrete blocks won’t help there either.
Hi, I was under the impression from Tony, that putting the RSJ on the aerated block was ok providing its placed on a 440 long x 100 wide by 140 high padstone and the wall will be able to take the load. Have I misunderstood ?
I've attached plan and calc's for B3.Under Bearing Details heading, it seems calcs have been worked out at 20N/mm square (I assume brick). But in reality I've used aerated 3.6kN blocks. Is this an issue?

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I don’t think you’ve misunderstood anything, what do you think you’ve misunderstood?

Tony and I both said that the wall panel should be checked about halfway down the panel (0.4h), as well as directly below the bearing. It’s probably fine, but Superbeam (the software the SE has used) doesn’t do this check. Tedds does though.

The calc shows a brick bearing - not sure why the SE assumed the inner leaf would be brick, as inner leaves are almost always built using blocks (and the drawing shows blockwork).
Using 3.6N blocks instead of 20N bricks is only an iissue in the sense that you will need a bigger pad stone than the one shown (as already discussed).
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Why is the padstone shown yellow marked as 150 wide? Is the inner skin blockwork 100 thick? If so, part of the padstone will be overhanging one side or the other, and the effective contact area would be 300 x 100, which would be OK for 7N block, but the extra width seems pointless.

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