footing for blockwork supporting a beam

[b00gieman]

I bought an old sandstone cottage built around 1750 that has rear and side extensions, probably built in the 1940s/50s. Its been one fun find after another, from rotten timber lintels to no lintels to having to replace the whole first floor joists after finding they are undersized (3" x 3") with various notches and lap joints. The house will be our longterm family home so spending some time making it right before moving in.

The latest discovery is a beam in the side extension that is supporting some blockwork above and the purlins for the side extension roof. It was covered in pine panelling. I was expecting it to be steel behind but found a 5" x 2.5" wooden beam. I thought it looked undersized so consulted with a structural engineer who agreed and suggested putting 100 x 8mm steel plate on either side of the beam. I also want to add a blockwork (pillar, column, pier or whatever you want to call it) on one side, tied into the wall to slightly reduce the span and make sure the steel plates can extend over the support. There are some minor cracks in the plaster on that wall, starting where the beam is pocketed in so I think adding strength here would be sensible. I think that wall is an origional sandstone wall that was used for stables. I’ll make the column 215mm depth starting with the first block on its face and use screw in ties throughout. On the other side I managed to get the steel slightly into the pocket and the wall there looks fine (newer and made of brick) so happy to leave it as is.

The room has a concrete floor which I don't think is particularly thick, maybe 70mm. So I don't think it would be solid enough to start the blockwork on. I have little experiance with foundations so wondering what you all would recommend for a footing for the blockwork column?

 

[Notch7]

Well the concrete floor is a slab rather than a foundation so not designed for a point load.

I wonder what foundation the house has if any as such!

If a modern house you would simply transfer the load to the outer wall down to the house foundation - but in your case a new internal wall / pier probably wants a new foundation / pad to sit on.

It also depends on soil type and any nearby trees.

 

[b00gieman]

Well the concrete floor is a slab rather than a foundation so not designed for a point load.

I wonder what foundation the house has if any as such!

If a modern house you would simply transfer the load to the outer wall down to the house foundation - but in your case a new internal wall / pier probably wants a new foundation / pad to sit on.

It also depends on soil type and any nearby trees.

I don't think there is much of a foundation under this wall and the main origional house but its been standing for 250+ years and hope it will for another 250

According to an agricultural map the ground is "slowly permeable seasonally wet slightly acid but base-rich loamy and clayey soils". There aren't any large trees closeby, nearest would be 20m+ away.

Is it time to consult the structural engineer again? He (and my bank balance) are going to be sick of hearing from me

 

[Notch7]

I don't think there is much of a foundation under this wall and the main origional house but its been standing for 250+ years and hope it will for another 250

According to an agricultural map the ground is "slowly permeable seasonally wet slightly acid but base-rich loamy and clayey soils". There aren't any large trees closeby, nearest would be 20m+ away.

Is it time to consult the structural engineer again? He (and my bank balance) are going to be sick of hearing from me

Unfortunately there isn’t really anyway to know what the load bearing capacity or stability of the concrete slab….and to be honest a floor slab isn’t designed to support a significant load.

 

[b00gieman]

Unfortunately there isn’t really anyway to know what the load bearing capacity or stability of the concrete slab….and to be honest a floor slab isn’t designed to support a significant load.

Thanks. I'm intending to take up the concrete there and put in a new footing but of what? Should I dig out an area a few sizes larger than the block to a depth of 0.5m and fill with concrete and start on that or would it need to be deeper or larger? Sorry, I'm comfortable making concrete but I haven't had to make a footing before and not sure where to start. Happy to over engineer a little.

 

[cdbe]

I'm not an engineer but can't understand what you're doing, If the beam is overspanned then the flitch plates will address this - it doesn't need to extend into the wall or onto a bearing near the wall unless you're concerned about shear. Minor cracks on the wall could be historic from the initial loading, especially if at the top rather than the bottom and if necessary could be resolved by inserting a length of lintel or similar under the beam to spread the load across the wall (obviously if the wall has cracked in half below the beam or is sinking etc you have a bigger problem).

 

[Notch7]

Thanks. I'm intending to take up the concrete there and put in a new footing but of what? Should I dig out an area a few sizes larger than the block to a depth of 0.5m and fill with concrete and start on that or would it need to be deeper or larger? Sorry, I'm comfortable making concrete but I haven't had to make a footing before and not sure where to start. Happy to over engineer a little.

Depth should be down to load bearing ground

size would be a guess without an engineer to specify. I know a pad to take the load for a steel post might be a 1 metre x 1 metre pad. Depth might be 1 metre - that’s what is often specified for basic underpinning.

A strip foundation for a single wall is typically 450mm wide, but thats typically for a longer trench you would want to go bigger as it’s just a pier size - I would be tempted to go say 750mm x 750mm is ground is good and dig down 500mm and see what the ground is like.

 

[b00gieman]

I'm not an engineer but can't understand what you're doing, If the beam is overspanned then the flitch plates will address this - it doesn't need to extend into the wall or onto a bearing near the wall unless you're concerned about shear. Minor cracks on the wall could be historic from the initial loading, especially if at the top rather than the bottom and if necessary could be resolved by inserting a length of lintel or similar under the beam to spread the load across the wall (obviously if the wall has cracked in half below the beam or is sinking etc you have a bigger problem).

The structural engineer said the steel should extend over the supports for shear. Thats why I pushed them into the pocket on one side and will build a column on the other. I've attached a picture of the beam with the steel attached. I've also attached a picture of the wall with the hairline crack where I intend to build the column. I've put the acrow in place until I build the column.

 

[b00gieman]

Depth should be down to load bearing ground

size would be a guess without an engineer to specify. I know a pad to take the load for a steel post might be a 1 metre x 1 metre pad. Depth might be 1 metre - that’s what is often specified for basic underpinning.

A strip foundation for a single wall is typically 450mm wide, but thats typically for a longer trench you would want to go bigger as it’s just a pier size - I would be tempted to go say 750mm x 750mm is ground is good and dig down 500mm and see what the ground is like.

Thanks, thats really helpful. When you say load bearing ground, what am I looking for?

 

[b00gieman]

Just to follow up on this, I dug a hole over double the size of a block on its face and went down about a meter, surprisingly to the same depth as the wall foundations. Filled this with concrete, covered with dpc and built the pier on top up to the beam, tieing into the wall along the way. I knocked off the plaster to find the hairline crack was nothing, only a crack in the plaster. Stone behind was fine. All very solid now and building control are happy.

 

[Notch7]

Just to follow up on this, I dug a hole over double the size of a block on its face and went down about a meter, surprisingly to the same depth as the wall foundations. Filled this with concrete, covered with dpc and built the pier on top up to the beam, tieing into the wall along the way. I knocked off the plaster to find the hairline crack was nothing, only a crack in the plaster. Stone behind was fine. All very solid now and building control are happy.

Nice, solid job

 

[George_engineer]

Just to follow up on this, I dug a hole over double the size of a block on its face and went down about a meter, surprisingly to the same depth as the wall foundations. Filled this with concrete, covered with dpc and built the pier on top up to the beam, tieing into the wall along the way. I knocked off the plaster to find the hairline crack was nothing, only a crack in the plaster. Stone behind was fine. All very solid now and building control are happy.

What's the beam? Looks like a reverse flitch beam.