General advice needed, new structural column next to existing load bearing wall.

[liam1369]

Hi All,

I am looking for some general advice in relation to the new foundations for a column which forms part of a new goal post frame allowing for an opening.

I believe we will need to underpin the existing wall as the new column pad will go below. Please see image below.

Does anyone have any advice for this detail and whether other elements or factors need to be considered?

 

[1503-44]

Can't you just design an eccentrically loaded footing that does not require placement under the existing wall? What you have there will probably eventually pick up some wall load and be eccentrically loaded due to that condition anyway.

 

[liam1369]

Hi, yes I have designed it as so, the existing brick corbel and foundation will be broken out regardless, therefore, I think the above is most suitable?

It is small home improvements so I am not going to be introducing any transfer structure etc, will be easier to underpin I believe...

 

[Liam Nesson]

Im with 1503-44, don't touch the load bearing wall unless you have too, can you join the 2 column bases with a wide grade beam instead? How big are the loads and soil bearing pressure? I assume you will need to needle the wall to install that foundation. Also i am not used to seeing unprotected steel below grade.

https://www.eng-tips.com/viewthread.cfm?qid=4081

 

[1503-44]

I thought it was for the Green Bay Packers and not embedded in dirt.

Don't undermine your wall. That won't turn out well.

 

[kipfoot]

I think the way you have it is the simplest solution. First, consider taking the step of creating an opening in the bearing wall where the footing will be. Then, you'll have the space to pour the footing and save the complication of having shoring in place while you're trying to pour the footing.

Assuming your footing accounts for the line load from the wall, you then fill the space under the lintel with masonry.

 

[liam1369]

I think the beam does not resolve the issue and creates the same problem anyway.

It is typical to excavate in 1m lengths under the existing and stagger the concrete underpinning.

The steel beam connecting the two will not solve anything as the column / beam will sit flush against the wall.

Yes the footing will account for the line load from the wall.

 

[liam1369]

Hi Kipfoot,

I believe during the works there will be needles at higher level with acrow props supporting the wall for these works to be carried out. They will have this all on site for other works so this will be taken care of.

 

[Enable]

Can you perhaps explain the problem that you are seeing? I am with the others and not understanding why a simple footing extension or beam at the same elevation of the original footing would not suffice. I'm literally building a concrete wall in front of an existing deteriorated wall on a commercial project right now, where the wall takes substantial loads and we are doing the eccentric footing extension protocol.

I am not necessarily adverse to underpinning of a sort in 1m strips. I just don't understand why you need to do it in this case. Also, underpinning usually involves leaving 2" or so of space from the underside of the existing footing to dry-pack to avoid loss of contact between the new concrete/old footing (dry-pack doesn't shrink away but concrete will). In your circumstance you won't have that opportunity to dry-pack so the concrete will shrink away from the underside of the footing leaving a nominally sized gap (probably not terrible given the amount of shrinkage you could expect given pour size but I go back to the question of why introduce problems).

 

[liam1369]

Hi Enable,

Ok I will look into the eccentric design, that does seem a bit simpler. I was thinking with the concrete surround below ground level etc it will be better just to cast a footing and then underpin the existing wall in the same process.

Enable, can you please show how your new concrete wall interacts with the existing wall and footing? Perhaps a section? As you say something that clarifies the ‘existing footing extension protocol’

Yes the dry packing is a very good point.

Liam

 

[1503-44]

I just can't see why a "home" installation of a goalpost should be worth such trouble. If I can't do it at home simply and easily, preferably cheaply and without risk, I usually won't do it. Cant you just U-bolt it too the wall? Apparently our priorities are completely reversed.

 

[liam1369]

I am getting a lot of queries from building control to confirm I am not underminning the existing wall in any way. I believe I am not providing enough information specifically to their concerns.

The client want to fully remove the existing return walls, thus the steel goalpost solution has been proposed.

I will be bolting it to the wall, this is more for transfer of lateral forces and effectively providing a buttress to the larger wall panel formed.

Keeping on point, I have sketched my two proposals, first one being my initial idea of underpinning the existing wall and designing the new pad to take the loading from the new column and existing load bearing wall.

Option two will involve extending the existing footing effectively, however, due to the column being eccentric putting a lot of pressure to the back end, this will then combine with the pressure from the existing wall and potentially undermine the existing wall. I believe this is what building control are concerned with and I see their point.

The column is taking a bit of load ~ 50kN (5 tonnes), therefore, I do see his concern.

 

[Celt83]

In my opinion your initial option of shoring and underpinning the wall would be the best option here.

Be careful bolting the column to the wall unless they are slotted connections, the bolts will be trying to vertically support the new column.

Option 2 - isn't really possible there is no good way to provide continuity between a new concrete foundation and corbelled brick so they will not act as a single foundation unit.

Make sure you provide enough clearance from the wall for the column base plate.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Enable]

I absolutely missed that you are dealing with a brick corbel for a footing. In that case, Celt83 is correct that making them act as one would be a difficult challenge. That being the case I would not attempt option 2 as is.

There is nothing wrong with option 1 that I can see (other than you need to control shrinkage as mentioned before). However, for me a more economical solution would be to auger new caissons on the interior side and not touch the wall at all (except for some minor cutting of interior corbel). That way everything is independent and we don't have to worry about shrinkage (I mean...we do but that's a different topic all together ). It's quite easy: bring in a hand auger and rip some 16" or so caissons side by side to support the footing on. Have bar in the footing span from caisson to caisson.

I say 16" because if you have 200 kn/m2 bearing capacity and your load is 50kn: 50/200 -> 0.25m2 = 3.14r2 -> r=0.88m / 2 -> 0.44m ~ 16" which is a standard auger size.

But if you think it's cheaper to underpin the existing wall then I say go for it. I'd probably pour the pad 1" shy of the actual underside of the footing then make a port to pour in liquid non-shrink grout after to achieve full bearing.

For what it's worth here's a section from the job I am on right now. Very simple extension using rebar to dowel into existing. But it is all concrete.

 

[Celt83]

not a huge fan of the extension details, as they usually aren't fully baked.

Governing code in my Region would be ACI, in that detail posted by Enable the bar would need to satisfy both flexure and shear friction. For the shear friction provision the bar needs to be fully developed on both sides of the shear plane which doesn't appear to be the case or possible in that detail. That said there looks like a continuous wall over the joint so you could probably make a rational argument that it's not a slip plane.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[liam1369]

Hi Enable and Celt83,

Thanks for the responses. Yeah if it was all in concrete I would have doweled and extended the existing footing. I was wondering why everyone kept saying that when I mentioned the brick corbel and existing wall .

I think underpinning will likely be cheaper as it will be a small contractor with minimal equipment. Yes, I will advise on the non-shrink grout / packing from bottom of corbel to new concrete footing.

Also Celt, yes, I will put provisional slotted holes to ensure it only provides lateral restraint and doesn't start trying to transfer load into that wall.

Thanks great responses both.