Pad foundation

[joe88]

Hi, can some one please run me through the build-ability of the padstone foundations(footing F01- green hatch). Obviously the existing rear wall of the house is their at the moment and we are wanting to build and make the extension watertight before knocking the rear walls down and installing the steel.

 

Its the right hand side pad that I'm concerned about as i believe this will have to be done when the extension foundations are poured. can they do this from the outside(underpin) or can it be poured in two stages?, our kitchen is the room that this backs onto and we don't want to rip it out just yet.

 

The SE has stated " 1000x1000x300dp rc35 pad foundation with 2 No. A393 in bottom", the opening is 6.5m so quite large, im not sure if a box frame might be better design for us.

 

 

Thanks in advance.

 

[saveasteading]

Ask your Engineer if the pad can be asymmetrical, ie more of it outside the existing house. 

That will let you dig more easily, and perhaps all from outside in one operation.

Alternatively it might become more rectangular.

That may require it being a little deeper, or reinforced.

 

Yes you can normally pour it in parts as all the load is presumably downwards. In that case you would probably put some ties bars in to link fully (push them into the earth face  before pouring, or drill in afterwards),

 

This should  be simple to redesign  if you explain why you want to do it.

[joe88]

14 minutes ago, saveasteading said:

Ask your Engineer if the pad can be asymmetrical, ie more of it outside the existing house. 

That will let you dig more easily, and perhaps all from outside in one operation.

Alternatively it might become more rectangular.

That may require it being a little deeper, or reinforced.

 

Yes you can normally pour it in parts as all the load is presumably downwards. In that case you would probably put some ties bars in to link fully (push them into the earth face  before pouring, or drill in afterwards),

 

This should  be simple to redesign  if you explain why you want to do it.

 

Saveasteading, Thanks for your suggestions, ill go back to him with them and hopefully we can go with one of them options. it would be great if we can do it all from the outside as it would save messing with concrete at a later stage and just use one readymix load

[NandM]

@joe88 - what did you end up doing?

 

My SE has also specced pad founds and am looking at ways I can minimise disruption of the existing living room floor. The one alternative he has come back with is to use wall piers and possibly with underpinning - but these are would stick out too much. 

 

Also - if there is no reasonable alternative, then how much of the existing floor would need to be dug up?  And would cold bridging be an issue? 

Edited February 9 by NandM

[George]

A box frame as mentioned above - keeps the foundation much narrower, but has a higher steel cost. You'd only be able to do it for B1 but I'm guessing that's the problem one. 

[Gus Potter]

12 hours ago, NandM said:

My SE has also specced pad founds and am looking at ways I can minimise disruption of the existing living room floor.

From what you posted that looks like an expensive mess by the SE. These pads and the disruption look like they will cost you a fortune. It also looks unbuildable at a sensible cost. It's not just the structure it's moving electrics / plumbing,  cutting floor joists and making good.. the absolute mess it will make during the works that needs to be reinstated at a cost.

 

From time to time I see stuff by other SE's where I think.. fair enough.. but this is ringing alarm bells. The other big one is the sideways stability of the building and the knock on costs.

 

Can you post a full set of plans. By that I mean the whole lot.. all floors and sections etc.

 

If I'm jumping to conclusions and wrong on this I'll say so and take the flack.

Edited February 9 by Gus Potter
2 dots!

[NandM]

On 09/02/2024 at 13:29, George said:

A box frame as mentioned above - keeps the foundation much narrower, but has a higher steel cost. You'd only be able to do it for B1 but I'm guessing that's the problem one. 

 

Sorry for the late reply - I need to enable notifications.

 

By box frame, do you mean something like the attached option the SE suggested? 

[NandM]

@Gus Potter - I've attached the plans and the SE drawings for all the floors.

The whole disruption is the biggest concern. The living room is fully finished and the disruption will probably affect at least a third of it.  We have a solid floor, so fortunately no electrics to disturb and no pipework underneath.

 

 

 

Edited February 13 by NandM
corrected order of pics

[Gus Potter]

On 09/02/2024 at 22:01, Gus Potter said:

If I'm jumping to conclusions and wrong on this I'll say so and take the flack.

I maybe need to take a hit on this one as had no idea that there was so much stuff going on above and the loft is getting converted. I'll eat some humble pie as promised.

 

I'm minded to make a box frame at ground floor level.. after that.. the drawings just show some beams. The box frame at ground floor will help what is going on above.

 

In terms of buildability and cost I can't say anymore other than..  You MUST get your SE to site, let them take their measurements and open some stuff up, your SE needs to put forward a buildable, sequenced and cost effective solution to this. It's not the Architect's job nor the contractor's job. A job like this might need 20 drawings with details on each drawing. You then may want steel fabrications drawings.. a full set of these add another 25 -30  drawings that go to the fabricator.

 

A fabrication drawing package has drawings that show all the beams with the bits welded to them. These are often called Assembly drawings. To make up an assembly you often have a beam ( say called a shaft) so it has its own drawing showing holes ect and then you have plate drawings ( the end plates /  stiffeners etc) Thus each assembly has sub drawings.

 

Now all of that may cost you say 6.0k - 10k plus structural calculations. But if you skimp on the design information (that forms the contract) and you lose a week on site for three men (2 skilled + 1 labourer) that could cost you 2.5k.. now you can maybe see the value of the professional service?

 

On the other hand you can produce too much design information. Contractors are people too and if they think they have some nutter of an SE / Architect overseeing the job they will just add on more. It's a people business also.

 

You'll maybe need to pay the SE more and if they don't engage and it becomes apparent that they have no idea on the cost /buildability of things then need you then sack them pronto.

 

Yesterday I went to a job and met a highly competant Contractor for the first time.. say @saveasteading they vetted me (as an SE) just as much as I vetted them! Get a good builder / contractor to meet with the SE and let them fight it out while you watch, listen and learn.

 

Edited February 21 by Gus Potter

[saveasteading]

8 hours ago, Gus Potter said:

met a highly competant Contractor for the first time

There is more than one. Oh I see what you mean.

[George]

On 13/02/2024 at 10:24, NandM said:

 

Sorry for the late reply - I need to enable notifications.

 

By box frame, do you mean something like the attached option the SE suggested? 

 

No they've suggested leaving a masonry pier at each end. It'd be a much cheaper way of doing things and architecturally would separate the living room off, which is a good thing. 

[NandM]

Hmmmm....not sure what you mean by separating the living room off. The new living room extension won't be open plan in to the kitchen, it'll just be en extension for the living room.

 

But I've gone back to the SE to get an actual number for how big these piers would be. 

 

*Edit: the pier sizing is in the diagram and would just be too big*

Edited February 21 by NandM

[George]

They can't really go less than 665mm without a steel goalpost or box.

 

If masonry piers aren't acceptable then a steel box frame is likely to be a better option to reduce foundation size.

[NandM]

Thanks @George - I'll get back to the SE and see what he comes back with.

[NandM]

The SE has come back with the following, which is at odds with what I've read so far. I thought the beam needs to be encased, but here the SE is saying we need to create a new foundation.

"A box frame might be feasible, however the bottom member will require a mass fill trench foundation approximately 700-800mm wide dependant on the loads, which will be casted central to the bottom transfer beam resulting in excavation on the existing property."

[George]

The encasement and foundation are essentially the same thing, just depends how far down you need to go to get to competent ground.

 

700-800mm is thinner than the pad and should be able to be mostly not under the existing walls, which I would have thought was the main issue.

 

Last one I did was 600mm wide but I think you've more load coming down onto that part of the building. Ultimately it will need to be what it needs to be.

[Gus Potter]

 

On 21/02/2024 at 09:40, saveasteading said:

There is more than one. Oh I see what you mean.

Worded that the wrong way.. there are plenty competant Contractors.. this was a new one.

 

@joe88 A few things that would help a lot would be to get some gridlines on these drawings so we can see how things line up.

I would say this to all budding Architect's.. put some gridlines on your drawings when sending to the SE, the builder, QS etc. If you don't it just wastes all of our time. Your job (Architects) is to communicate essential information in a way that is easy for others to follow, it's a skill that you need to develop, oh and it's good for self checking drawings. On the other hand SE's, QS etc need to reciprocate.  A lack of gridlines demonstrates etc that either you don't care or don't know what you are doing. 

 

@joe88.. it still looks a mess. Are there some existing walls under the line of B1 and B2? Your image of the existing ground floor plan seems obscured by the side bar? A lot of folk say that a box frame is more expensive.. in terms of steel cost.. it can be but on a job like this it may not be as it adds stiffness that saves beam weights, reduces connection costs, makes it more buildable etc further up. Your SE says you need an extra bit of found.. trench... but.. why the beam B1 & B2 in the first place? To be honest I've not spent the time trying to line stuff up on your drawings and trying to second guess.

If you have some existing founds the soil under they will tend to be more consolidated (more bearing capacity).. say each side of a window. The bottom of the box frame in the middle of it's span tends to act to stiffen the structure above in terms of.. in layman's terms the bottom member of a box beam doesn't put that much load on the ground near the middle of the span.. hence we may put crawl holes here for solum space access and ventilation. I mention ventilation as it is a classic bit for getting tripped up by BC.

Just glancing again at what you propose.. Column C2 is to be bolted to the wall with resin anchors.. does anyone know how that work will be sequenced and how the steels will be measured by the Contractor so that it works and can be built?

 

Beam B9 spliced at third points.. would be interested in seeing the welding on this connection and associated cost. My gut feeling is 20-25% in from the end of the span so it becomes more of a shear type connection. and thus cheaper. If you see anything like "full strength weld or "full pen weld"" annotated on the drawing then that needs tested. It could add £800- £1500 onto the cost of the steel if a small fabricator has to get the inspector in.

The third splicing rule is always a flag.. could be a lazy Engineer and is worth examination.. or could be due to getting say beams into a confined space.

 

A lot of folk say that a box frame is more expensive.. in terms of steel weight cost.. it can be but on a job like this it may not be as it helps stiffen every thing above and reduces the associated stress and deflections.

I could go on but you have a complex project that is fraught with difficulty. From experience you could end up paying for things that won't get built on site. It all could end up in a massive argument and spoil the enjoyment for you. Before I was an SE I was a Contractor for 20 years so can see this from both sides.

If I was you I would start talking to Contractors and finding the ones that know their stuff. You can then feed back that info to your design team. Even if you offer to pay a Contractor to visit you it will be well worth your while.

I'm sorry if I come over as a bit harsh but it's worth grasping the nettle now. On a job like this you will need a lot of details.. fixings./ guidance notes that tie in with the Architectural side... so you get what you are paying for.

 

 

 

[Gus Potter]

On 22/02/2024 at 15:18, George said:

Ultimately it will need to be what it needs to be.

@George.. What is your gut feeling on this one?  Do you fancy stepping up to the plate?

 

Unless the SE et al has been paid enough to the design and detail job properly it is not going to work out well.. you know it.. I know it! The Architect has to sit back and let the SE sort it out.

 

For all..there is no point in trying to hide behind fact. SE's like us have been cowed in recent years... the result is that out Clients are not getting a good service as we are not sticking our guns.

 

@joe88The SE needs to get their arse to the site and make sure it can be built at a reasonable cost if at all? And the Client @joe88 needs to understand the financial risk of not doing so.

 

@joe88 Please take my advice.. you won't lose anything from testing my views but you could lose a lot if you don't. It may be  that you need to pay more. I'll direct you to Ruskin here and hope. Quote below.

 

The below was written a long time ago but still applicable in the modern age.. I see this day to day where folk are get ripped off.. it's getting worse not better. The professionals that are supposed to be helping you are more concerned in covering their arses. I stick to "old sckoool" stuff and have a niche market but my fees are higher.. but in the round I always aim to save you more than you pay me and will ensure your house is safe to live in. Ruskin mentions "lawful prey" take note!

 

There is hardly anything in the world that someone cannot make a little worse and sell a little cheaper, and the people who consider price alone are that person’s lawful prey. It’s unwise to pay too much, but it’s worse to pay too little. When you pay too much, you lose a little money — that is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot — it can’t be done. If you deal with the lowest bidder, it is well to add something for the risk you run, and if you do that you will have enough to pay for something better.”

 

@joe88 this is part of my day job so I'm not shitting you!

Edited February 24 by Gus Potter

[Tetrarch]

I had exactly this problem where my SE specified the foundations below. After a good discussion we've opted for a picture frame instead of goalposts for the beam where the blue squares are and we had 600mm trench dug to 1200mm below DPC that satisfied the SE for the remainder. This way we didn't have to undermine the foundations of the existing structure that have been prefectly happy for 53 years.

[George]

18 hours ago, Gus Potter said:

@George.. What is your gut feeling on this one?  Do you fancy stepping up to the plate?

 

Not for free!

 

But a box frame feels the right solution here. There's not much lateral stiffness in the structure otherwise and they could get the box installed first before taking out all the rest of the structure, and the other steelwork is tied back to it.

 

With suitable trial puts I think I'd consider reusing the existing foundation.

Edited February 24 by George

[NandM]

On 09/02/2024 at 22:01, Gus Potter said:

If I'm jumping to conclusions and wrong on this I'll say so and take the flack.

 

@Gus Potter - can I get your final thoughts on this - or do you agree with George that it appears to be OK.

[saveasteading]

23 minutes ago, NandM said:

appears to be OK.

That's really all you should expect on here: an overview.  Beyond that there are site specifics,  complexities and liabilities, and it is time to pay a local expert.

[Gus Potter]

On 16/03/2024 at 12:48, NandM said:

can I get your final thoughts on this

 

On 24/02/2024 at 20:38, George said:

But a box frame feels the right solution here. There's not much lateral stiffness in the structure otherwise and they could get the box installed first before taking out all the rest of the structure, and the other steelwork is tied back to it.

Agree with George above.. I would look at the benefits of a box frame before ruling out.