Steel column on existing block cavity wall

11 Feb 2010
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United Kingdom
Seeking clarity on above before engaging engineer for calcs.

I have built two houses so building is not new to me, I have built a steel boat so steelwork is also familiar but I seem to have a mental block when trying to marry the two.

The job entails widening two picture windows in a 10" conc block cavity wall such that they meet at the corner of the building. The existing corner/column of block work ( approx 1m x 1m wide) will be replaced by a 4/5" steel box column supporting two inverted T section steels ( 8" flange) which will sit under the existing conc lintels, the web section inside the cavity.

My thought was to remove the block work down to dpc in way of the corner, pour concrete L shaped seperate pads (thus keeping the cavity) and bolt the sole plate to these new pads, block work then made good around them. New cill height would be 450mm off dpc and new span is 2.2m approx, the column sitting mid way on the wall. I am not seeking calcs for the steel on this but more a better idea of the methodology of introducing the steel column to the existing building.

Does this appear reasonable to you steelmen out there or am I missing the obvious once again. Any thoughts or advice would be most welcome.
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You need to put this to Building Control for approved.

You are in effect replacing a two meter run of existing foundation with a single point load. Without knowing the nature of the existing foundations this is crazy.

Just get it done properly - The principle is fine providing you design this from the ground up. I suspect you will have to take out the wall in the corner all the way down to the foundations, possibly modifying the foundations before coming up again. Unfortunately it may mean the floor in the corner will also have to come up and I suspect you are trying to avoid this! - Don't risk it. A collapse will not be covered on your insurance.
You need to caclulate it but off the top of my head it doesn't sound sufficient to me. I've just done a similar one yesterday with 3.3m of steel beam (that's half of a 6.6m steel) onto a corner column and it came out as a 600x600x600 pad. You've also got to consider the suitablility of the existing foundation?
Thanks for response.

Building control will be involved, structural wall and wider opening and a structural engineer will provide the calcs and ticket for BC. As it will be me that will feel at first hand the particular law of physics which relates to roof falling on head a, 'proper job' will be done. You are making assumptions when you do not have all the info to hand. Not your fault, I was trying to keep the original post as short as possible.

The foundation is known, 6" strip conc footing with double 4" conc block cavity filled with conc to just below dpc, a standard build as I know it. If I understand point loads correctly, the dead load will dissipate at an angle of 45' from the end of the column outwards into the surrounding block work and down to the foundation which equates with what is in place at the moment, though it is a much wider "column "it is also much heavier.

The SE will I am sure produce figures and design detail to suit the project, but I am old enough to know that just because you have 'best in show' attached to your ear, you may not prove to be so when laid alongside a few roast potatoes.

I seek to improve my own knowledge about a, for me, hazy aspect of building technique before I talk seriously with the 'professionals', just so that they know that I will know if they are not what they appear to be.
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I was trying to keep the original post as short as possible.

Great knews, I admit after reading the original post I really got the impression you only thought you knew what you were doing.

The actual deadload you have is not changing and if the Building Control officer agrees with the method of transfer to the ground you would get approval. Personally I would prefer to see the loads being transferred to regular shaped pad! (As Jeds). It is an interesting deviation from the "norm". If I was the building control officer, I regret I would not be comfortable with it and pass it upline for the chief to approve!

It is not just the Theory they need to consider but how in future what is done may be interpreted and further changed.

I've just done a similar one yesterday with 3.3m of steel beam (that's half of a 6.6m steel) onto a corner column and it came out as a 600x600x600 pad.

Do you mean by that an H section split to form the T or just that 3.3 is half of 6.6, I'm missing the sense of that comment.

600 x 600 x 600

A cube of 2 foot for the column, is this a new build with no existing foundation or a multi story?. If you have the time I really would like to see some more detail on the build and your calculation, in any event I do appreciate your input. It just hit me, you mean 600 legs each 600 wide
What about the cavity............?


Great knews, I admit after reading the original post I really got the imp...........

An element of truth in that comment which is why I may try and flog this way past it's death until I absolutely know what I am doing.

.......the loads being transferred to regular shaped pad

By that do you mean rectangular or square as in Jeds.

I think I should have posted full spec initially.

The strip footing from the drawings was 550mm but the JCB ony had a 2ft bucket so the width is 600mm+
The depth from the drawings was 150mm but in reality it is 6"+. I know this to be true because I did it some 15 years ago.

The house is a modern variant of a but 'n ben, 1 1/2 story, one window is in the gable wall with no roof load, just the block work. The other is in the side wall with just 8" of lintel, brick and wall plate plus the roof load, reduced by a 13" x 7" pitch pine beam mid span under the joists. Assuming that the roof load was the same as the gable load common sense tells me that the column will be taking half the weight of each steel + the dead load above, or the full weight of the gable steel plus the block triangle above.

Am I right in supposing that you are an active BC officer, if so I do thank you for your comments and would welcome any further input you may care to offer. I have searched quite widely but have been unable to find any drawings/illustrations of a similar project.
By that do you mean rectangular or square as in Jeds.

Am I right in supposing that you are an active BC officer


I meant a base that had the load centred on the foundation so Rectangular or Square would be fine. I visualised your's as being L shaped and becuse it was originally designed to serve a different layout.

No I'm not an active Building Control officer. - I have worked with them studied with them even had the local senior BC officer turn up on my doorstep because he spotted my name on my extension and came along to say Hi! and do the final. - Old students back together!

Think about a square pad foundation deep enough to bear the weight of what is being put on it whereever placed. Great when the load is central. Now put the same load on the edge of the foundation. What could happen now is that the foudation will rotate because you have introduced an uneven moment into the load. - Hope you get my drift!
Sorry to confuse. What I meant was a 6.6m long beam. Half the load goes one way, the other half the other. (well, not strictly half because this included point loads which means the reactions are different - but they are approximately the same) So it was the load from half the beam. Which was about 54kN.

Taking the load capacity of average firm cohesive clay (from BS 8004) as being 150kN/m² I get 0.15N/mm². Divide the load (54kN) by the capacity (0.15) and you get 54000/0.15 = 360000mm. The square root of which is 600 or 600x600 square.

It is customary to make the depth of the pad equal to the width.

Now, at this stage this is only a rough guide - something to aim at. The final decision will be made when exact ground conditions are known.

Hope you get my drift!

I do indeed, easiest way to tip a dinghy in a calm!


The mist begins to clear, your meaning is crystal regarding lendth.

I was not aware of the width/depth connection and your figures/working turned on the lights. My loads are considerably less than yours and should comfortably fit within the existing conc strip with some additional work. I was guilty of npt seeing the column as a seperate entity and calculating accordingly.
Thanks to you both once again.

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