Steel column pad base questions

[Tim Edwards]

Hello,

I am in the process of building an extension to the rear of my house. Part of the project involves opening up the rear of the house by installing a 8 metre steel beam which will be supported by a column either side to form a portal/goal post system. This has been designed by a structural engineer and building control have approved it. The main foundations for the walls have recently been poured and brickwork up to DPC complete.

My next task is to install the columns before the main floor slab can be cast. The engineer has specified a concrete bad base of 1200x1200mm wide/long and 700mm deep, there will be one of these under each steel column. I am struggling to understand how exactly these will be located as the engineer has not been very specific. I have limited experience with something like this so was hoping someone on here could help point me in the right direction.

I have spoken to building control who seems to think the pads will need to as deep as original house foundation, this is fine expect the location of the pads will interfere with that original foundation, below is a sketch showing how I think the portal will look. Note nothing is to scale.etc:

Here is a more detailed look at the foundation:

If the above sketches are accurate, this would mean that I must cut and dig out a portion of the original house foundation in order to install the pad base in that location. Obviously the pad base will be much wider than the foundation (which is around 600mm). can anyone confirm if this is correct and if so is there anything I need to be aware of when doing this as currently the original wall is still in place and not supported?

Also on the second sketch I have marked the column away from the remaining wall as the pad base will need to be butted against this foundation, is it possible to dig under the remaining walls foundation in order to move the steel closer to the wall and maximise the opening?

As above I have limited knowledge of this so may be asking obvious or stupid questions but I am just trying to get a better understanding of how these pads need to be built.

For reference, here is an extract from the structural engineers drawings which shows the pad base detail:

Appreciate any suggestions.
Thanks

 

[stevie888]

Your SE is clearly showing a 1000 dig with 700 depth of concrete (and it would generally be inferred if not clearly stated that this is subject to on site confirmation by BC that the ground at this level is satisfactory).

Depending on how old the house is you may well be deeper than its foundations though there is an argument in this situation for founding the pad to the same depth to lessen the risk of differential movement.

The area of the pad is determined by the applied load and the allowable bearing capacity of the soil. The depth is determined by soil type and presence of trees while the thickness is largely a matter of convenience. If the thickness of the concrete exceeds the distance form the column base to the pad edge then there is no need for any reinforcement as the shear strength of the concrete is enough. This is usually the case in simple domestic pads and saves the hassle of trying to compact backfill in small areas

In your case you are crudely putting 8m of wall onto your 2 pads and your exg founds are 600 so your SE has said 8x0.6 = 2x1.2x1.2. However this really is lazy approach as it's quite normal to have 450 wide founds under a hoiuse which would equate to 900x1200 pads or even less if the actual loads were considered.

The column would ideally be central on the pad and the pad would usually be square for convenience but different options can be accomodated in the design process and a good SE will aim to make the build as simple as possible.

I'd ask the guy if he really needs such an area. BC aren't going to tell you this even if they think it. You then need to create the required area of additional foundation: I'd probably look at a strip each side of the existing, thoroughly clean the side, add plenty of dowels and leave the existing wall and found alone as much as possible. Finally a short section of steel to span across the old and onto the new on each side to avoid any localised shearing effects.

All this is up to the builder but I read you're DIYing: I can't decide if you're brave or daft, probably a bit of both, as many seasoned bulders would be a bit concerned about an 8m opening.

 

[Tim Edwards]

Your SE is clearly showing a 1000 dig with 700 depth of concrete (and it would generally be inferred if not clearly stated that this is subject to on site confirmation by BC that the ground at this level is satisfactory).

Depending on how old the house is you may well be deeper than its foundations though there is an argument in this situation for founding the pad to the same depth to lessen the risk of differential movement.

The area of the pad is determined by the applied load and the allowable bearing capacity of the soil. The depth is determined by soil type and presence of trees while the thickness is largely a matter of convenience. If the thickness of the concrete exceeds the distance form the column base to the pad edge then there is no need for any reinforcement as the shear strength of the concrete is enough. This is usually the case in simple domestic pads and saves the hassle of trying to compact backfill in small areas

In your case you are crudely putting 8m of wall onto your 2 pads and your exg founds are 600 so your SE has said 8x0.6 = 2x1.2x1.2. However this really is lazy approach as it's quite normal to have 450 wide founds under a hoiuse which would equate to 900x1200 pads or even less if the actual loads were considered.

The column would ideally be central on the pad and the pad would usually be square for convenience but different options can be accomodated in the design process and a good SE will aim to make the build as simple as possible.

I'd ask the guy if he really needs such an area. BC aren't going to tell you this even if they think it. You then need to create the required area of additional foundation: I'd probably look at a strip each side of the existing, thoroughly clean the side, add plenty of dowels and leave the existing wall and found alone as much as possible. Finally a short section of steel to span across the old and onto the new on each side to avoid and localised shearing effects.

All this is up to the builder but I read you're DIYing: I can't decide if your brave or daft, probably a bit of both, as many seasoned bulders would be a bit concerned about an 8m opening.

Firstly thanks for the detailed repose, much appreciated and very useful.

Since writing the original post I have spent a fair amount of time researching this and have some ideas of how to approach it. I do like your idea of utilising the existing foundation as this would both make the job a lot easier and also not disturb the current wall (something I have been worried about). I will speak to the SE about this and will also mentioned off centering the column as that could also work.

I agree the beam is very large and maybe I am an idiot for attempting this as a DIY but I've done my research and hopefully everything will work out. The house is an awkward design so it really needs this opening to improve the space.

Thanks again,

 

[Dereekoo]

In your case you are crudely putting 8m of wall onto your 2 pads and your exg founds are 600 so your SE has said 8x0.6 = 2x1.2x1.2. However this really is lazy approach as it's quite normal to have 450 wide founds under a hoiuse which would equate to 900x1200 pads or even less if the actual loads were considered.

Stevie out of interest 1.2 x 1.2 = 1.44m2 so 2nr = 2.9m2 which is less than 8 x 0.6.

Similarly 0.9 x 1.2 x 2 = 2.2 which is less than 8 x 0.45

Since both calcs give a discrepancy factor of 1.6 am I missing something?

 

[Dereekoo]

I agree the beam is very large and maybe I am an idiot for attempting this as a DIY but I've done my research and hopefully everything will work out. The house is an awkward design so it really needs this opening to improve the space.

2m 4m or 8m its all the same really just the logistics of placing the beams a bit more involved , as long as its structurally sound and all the i's are dotted and tee's crossed its going to be a great project