Casting concrete padstone in situ

[Peter500]

I need to put a 6m steel (203 UC) on a corner that is at a strange angle (approx. 20 degree angle IIRC). As I'm using 7N blocks, the structural engineer said I won't need a padstone. However, as the steel will sit right in the middle of the corner, I'm keen to cast a padstone in situ to avoid putting it directly on the weak point. I've attached a picture of what I'm planning to do (it's not great but I hope you can understand what I'm doing with the steel). I have a few questions:
1. Given that 7N blocks are adequate to support the load, would a standard mix be fine (e.g. C20), or should I get some bags of C40 concrete?
2. Does the padstone need to be symmetric to spread evenly on both walls or is it fine to match the blockwork bond (see picture)?

 

[noseall]

I need to put a 6m steel (203 UC) on a corner that is at a strange angle (approx. 20 degree angle IIRC). As I'm using 7N blocks, the structural engineer said I won't need a padstone. However, as the steel will sit right in the middle of the corner, I'm keen to cast a padstone in situ to avoid putting it directly on the weak point. I've attached a picture of what I'm planning to do (it's not great but I hope you can understand what I'm doing with the steel). I have a few questions:
1. Given that 7N blocks are adequate to support the load, would a standard mix be fine (e.g. C20), or should I get some bags of C40 concrete?
2. Does the padstone need to be symmetric to spread evenly on both walls or is it fine to match the blockwork bond (see picture)?

View attachment 301329

Why don't you construct a shutter box at the base of that wall, right on the angle? It'll give you the correct shape. Might be a bit heavy though.....

 

[robinbanks]

C20 is 20N; what's your concern with it?

Why does the drawing show beds/perps around the pad stone (maybe the bed/perp representation is a part of the sketchup component? what is the odd sliver at the bottom?), yet the description says cast in situ?

How are you tying the other courses together across the continuous perp?

 

[George_engineer]

I would get a steel spreader plate and bed it onto a high strength grout. This will mitigate the weakness in the perp.

 

[bsr]

I would get a steel spreader plate and bed it onto a high strength grout. This will mitigate the weakness in the perp.

Sounds sensible. What would you use, 6mm?

 

[jeds]

Agreed, steel plate is best option. I'd go 8mm.

 

[Dereekoo]

Why a steel spreader plate, the underside of the beam acts as a spreader plate to the padstone which then spreads out the load to the underlying blockwork. If the reasoning is that this spreader plate helps in protecting the corner perpend in someway (if one side wants to subside more than the other then you have a major structural problem ) then by that reasoning you could argue that spreader plates should be fitted across all bed joints down the perpend. Similarly why high strength grout the 7N brickwork is supported on normal strength mortar so what will high strength grout achieve over and above normal mortar.

 

[jeds]

The spreader plate spreads the load wider than the beam width. It would be, say, 400 wide instead of 200 wide, so bridges (and ties together) the joint and puts the load further onto the blocks. Bedding the plate means the load is distributed evenly under the plate. The plate needs to be thick to stop it deflecting, which would defeat the object. The engineer has already said it's not strictly necessary so this is peace of mind for the OP rather than a structural solution.

 

[Dereekoo]

The spreader plate spreads the load wider than the beam width. It would be, say, 400 wide instead of 200 wide, so bridges (and ties together) the joint and puts the load further onto the blocks.

That is the function of the the padstone. In fact assuming the padstone is 225mm deep the OP could make it 200 + 225 +225 wide and the load would be spread over 650mm of the supporting blockwork and there will not be any concerns with deflection (through shear or bending) of some nominally selected piece of steel plate.

 

[Peter500]

That's a really good collection of replies from all of you. Thank you
@noseall that's a very interesting idea but my wall is almost up to steel height and I don't want to rebuild it . The difficulties I can see with it is that the shuttering would probably need to hold over 1T on concrete. I would then need to make sure it doesn't blow down, given that it'll be around 3m tall. I'm sure it's possible, but maybe a little OTT for this
@robinbanks My only concern with C20 concrete is that the paperwork from my SE says that padstones should be C35 concrete. However, that's probably only when they're specified. I've butchered the drawing quite a bit now, so the mortar isn't quite right. The silver below the concrete are small pieces of blocks (and would sit on a bed of mortar). These small pieces would leave me with 225 for the concrete and also allow me to easily put the wall ties on that course below the concrete. My walls are tied together with a wall tie on each course (in addition to the ties between each skin).
I like the steel plate idea. I'll have a think about it. I have a load of spare ballast sitting in a bag, so the concrete wouldn't cost much at all.

 

[robinbanks]

small pieces would leave me with 225 for the concrete

This is the only bit I don't really get. The concrete doesn't care if it's 225 high or not; I'll just as happily set if it's 250 high, so rather than faffing about with slivers of blocks under the concrete, just shutter the entire area off and pour concrete into the hole until it's 25mm proud of the top.. ?

Concrete also doesn't care where a wall tie is: it could be sticking out 150mm above the base of the hole and the concrete will still set around it..

 

[Peter500]

This is the only bit I don't really get. The concrete doesn't care if it's 225 high or not; I'll just as happily set if it's 250 high, so rather than faffing about with slivers of blocks under the concrete, just shutter the entire area off and pour concrete into the hole until it's 25mm proud of the top.. ?

Concrete also doesn't care where a wall tie is: it could be sticking out 150mm above the base of the hole and the concrete will still set around it..

I know concrete can be any height. I would be a bit worried about putting a hole in the shuttering for the wall tie. I don't want the cavity filling up with concrete .
Also, although the extra concrete wouldn't be much, it would add to the weight that the shuttering would need to support. IIRC it's about 35mm but I need to re-measure. I haven't done concrete like this before (I've done foundations, shed base, and oversite), so I'm a bit cautious

 

[George_engineer]

Why a steel spreader plate, the underside of the beam acts as a spreader plate to the padstone which then spreads out the load to the underlying blockwork. If the reasoning is that this spreader plate helps in protecting the corner perpend in someway (if one side wants to subside more than the other then you have a major structural problem ) then by that reasoning you could argue that spreader plates should be fitted across all bed joints down the perpend. Similarly why high strength grout the 7N brickwork is supported on normal strength mortar so what will high strength grout achieve over and above normal mortar.

Beams shouldn't be sat over perps because any out of level and one block will be taking all the load. And the perp mortar shouldn't be treated as load bearing in this situation (too weak), reducing the bearing area of the beam. A 100mm wide beam on a 100mm block has a 10,000mm^2 bearing area - a perp could be 1,000mm^2 - 10% of the area.

Half the reason I spec a padstone is that I know the beam will be sat fully onto a robust block - in some cases you could get away with a 10N block rather than a 30N padstone, but a padstone ensures it is correctly lines and leveled and the blockwork is built around the padstone.

A steel spreader is an alternative to a concrete padstone - that oculd be eliminated completely. A steel spreader is wider than the beam and gets the load into the adjacent blocks just as a concrete padstone does. Including not loading the perp joint.

 

[jeds]

That is the function of the the padstone.

No shlt? The OP can decide which is the easiest option; cast an angled pad stone or use a steel plate. I know which I'd do.

 

[Dereekoo]

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