Knock through/Padstone in floor needed

12 Feb 2009
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United Kingdom

I am in the process of ordering steels as I am about to knock through an existing wall. At present this external wall leads onto a flat roof extension via a door, the plan is to open this are up fully. I have had the calcs done for the steels which are fine. Now, my problem arrises with the spec of the padstone to be inserted into the floor so that a vertical riser can bolt to it. The size of this padstone is 750 x 750 x 300 thick.

The building inspector has stated I dig down approximateley 700/800mm until i see the start of the external foundations and then call him out to check and once, happy I can fill to 300mm. The problem is once I pour a GEN 3 Cement, in effect make a 300mm thick padstone, what happens with the other 400mm? the vertical column is to fix via a base plate and anchor bolts to the padstone?

1. Do i cut out a 750 x 750 x 300 hole in the floor and lay DPM then pour a GEN 3 Mix (Not sure If I can mix this myself and if so what is the ratio) onto the DPM, fix the vertical riser with anchor bolts and then refill with wither ground and then screed of fill the remailing 400mm with the same mix?

also looking at the plan does this need to be cast on 2 stages as there will already be a foundation running down the middle, or would the ned padstone rest ontop of the existing foundation?

I have attached the image of the area in question.

cheers guys your feedback would be much appreciated!

View media item 50611 [/img]
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If you have had your steels calculated - presumably by an SE - then why hasn't he given you these further details?
The SE has not done a proper job if he has given you the beam sizes, but not told you how to support them.
The horizontal steels are to be supported by a 150 x 200 x 600 pads stone fixed into the wall on one side and welded to a vertical column on the other. However the vertical column is to be bolted to a pad stone in the floor measuring 750 x 750 x 300. The building inspector has informed me to dig down to the foundations aprox 700/800mm, but my query is once I dug down 700/800mm either side of the existing wall what do I do about the existing foundations of the external wall? do I remove x amount of bricks sitting ontop of the existing foundation and a slab/cast a slab through these removed bricks ontop of the existing foundation? If so what do I do about the rest of the hole? In theory there would be a 750 x 750 x 400 empty cavity?
I think a Gen 3 mix is just a standard foundation mix, so: 1 part cement to 6 parts ballast.

If you only have 400mm to backfill you may as well fill with concrete.

If the pad is wider than the existing foundation I would pour it on top and make it upto 750mm by letting the concrete run down each side of the existing.

If the existing is 600mm you would only have 75mm down each side so you could make the pad 900mm say to give it a bit "more meat" :)

Just seen your 2nd post; yes knock enough bricks off to suit.
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Thanks for the info. I have just dug down on one section of the wall, ended up digging 600mm until I hit the foundation. I would say the foundation overhangs the engineering bricks (sticks out) approx 150mm. So in theory I have a wall 120mm, cavity 70mm and wall 120mm totalling 340, with an overhang of foundation 150mm each side so my foundation slab equals 640 wide. I have been asked to insert or cast a 750mm wide padstone. Would you advise on casting straight on top? At present I have dug down flush so there is the existing foundation then earth straight after. Would this be fine? Also ;) do I wait for the slab to set once cast (through 750 wide 300 high engineering bricks also) drill and anchor bolt my vertical riser, them fill the remaining cavity void with cement ballast mix, in theory burying the bolts in a 400mm thick concrete, which is bolted into a 300mm concrete block! Phew hope hat makes sense!
Does the pad need to be below frost if it's within the envelope of the building?

Plus what Tony said.
It doesn't seem worth casting another, 750mm, foundation if the existing is 640mm. I would be inclined to bolt the steel to your existing one.
Why not expose it all & check if the BI is happy.
If casting a new one you'd have to go down the sides of the old.
Wait for it to set before drilling your bolts in, then fill with more concrete :D
If you have had your steels calculated - presumably by an SE - then why hasn't he given you these further details?
The SE has not done a proper job if he has given you the beam sizes, but not told you how to support them.
My thoughts too.

Has the S.E. not provided a section drawing?

My thoughts are you dig down and possibly under the existing found's and fill the hole in with concrete to say within 450mm or so of dpc. Let it all set then drill the bolt holes once you have your column and base plate.

the building inspector is not in a position to overrule the S.E. unless it is a simple common sense decision. It is the S.E. that is underwriting the project so deviating from his specification is risky.

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