Kind of a heavy subject, but I thought I'd give it a shot anyway.
I am trying to hire someone to do a few foundation concrete mass underpinnings, and to feel confident I'll get a good job. I want to understand a couple of things about the job itself.
One concern about this process, is creating and maintaining a positive connection between the old slab and new underpinning. How is this typically done here in NZ?
I'm reading that dry-packing the top 50mm is still considered the most reliable way to create positive contact across the whole surface. It requires an extra day, though, because the pour needs to dry a bit first, before ramming the dry-pack in. I know noone I've talked to intends to use this method, because they have said the job would take 1 day.
The engineer has specified 2 steel rods to connect the slab to the underpinning. Are we relying on this to create and maintain the contact between the two concrete surfaces?
My second concern is that soil compression underneath the new underpinnings will cause some minor sinking, and will pull the connected slab downward and crack it. Are there techniques used during the process, to avoid loose disturbed soil at the bottom of the hole?
If there any people here that are knowledgable about this, I would appreciate advice.
trad,
Jan 25, 10:22am
I think you could get some helpful replies to this if you gave more info on the job. Eg: is this a single story timber house on a concrete slab on the ground? Is the underpinning being done because of settlement? Area settling or number and depth of underpinnings? We currently have no idea of the scale of this job.
bcohen,
Jan 26, 6:06am
Oh, sure! The job is quite small. I have a 2 story house, where the second story is supported on poles, rather than in the normal way which would be to have the second story supported on the first story. The bottom story has a poured slab with no footings, which acts as floor for the rooms below.
The upper story has a couple of location where it's sagging, because of excessive spans between the pole supports. These locations are along an exterior wall of the house, not the interior.
The simplest way to correct this problem is to simply bring down some mid-span supports to the floor of the first story, within the wall. The issue is that there are no footings in the first story slab to take the weight.
So, therefore, underpinnings. But simple things are never simple as they appear. Done badly, the secondary effects are worse than the problem you started with. Anyone else noticed that?
Anyhow, in my first message, I asked my questions, about creating positive contact to the underside of the slab, and about minor soil compression under the new underpinnings causing the slab to crack. I don't know what the proper techniques are, to avoid problems with these 2 things, and therefore I can't determine that the person I hire knows them.
If anyone can help me with these questions, please do!
trad,
Jan 26, 8:42am
Is the second story floor a concrete slab? And it is this slab that is sagging at the outer edge? Sounds as it the outer beam is non-existent or not beefy enough. Have you considered correcting this by putting a (larger) beam between the existing poles? (maybe fixed to the side of them). How do you propose to take out the sag or are you going to live with that or jack it back up?
If the existing structure is ok for earthquake resistance you maybe could put a prop between the upper floor beam and the lower floor slab - having beefed up the this slab locally where the prop is to go to spread the additional load.
If existing earthquake strength ok, you are not expecting the prop to take lateral or uplift earthquake loads, only dead loads, so only required to 'hang together' in an earthquake, therefore nominal upper and lower connection ok.
You also have to consider that the upper floor slab may need to be beefed up locally to take the additional point load from underneath. You only seem to have considered the lower slab strength.
bcohen,
Jan 26, 10:23am
The upper story is not a slab, it's just bearers spanning the poles, with joists on top of that. All wood. It's very lightweight construction, open beams and sarking, chalet style, but up in the air.
The prop you are talking about is EXACTLY what I am trying to achieve, and the beefing up of the lower slab is exactly what I am asking for clarification on. Can you help with my two questions about that?
BTW, the engineer has already verified that this is an acceptable solution, and will serve to support the upper story. She has also designed proper connections for the new columns (props). . I'm going at this one step at a time, the first step being these underpinnings. She does not have advice on who to hire for the job. I think there is more skill to this than just having the proper license. You might say "Hire an underpinnings expert!" Well, I've tried. The job is too small for the big shops I've tried, they don't want it.
The expense and the risk of accomplishing getting a larger beam into these locations, is huge. I could explain why, if you really want to know. The prop is exceedingly simple. But the underpinnings need to be done right. If the slab is damaged in the process, I would better have done nothing!
trad,
Jan 26, 1:59pm
Normally slabs are made thicker locally to take point loads. What that is aimed to do is spread the point load over more ground and also make the slab locally stronger so that it can take the load and spread it out to where the slab becomes thinner again.
You could perhaps cut some slab out and do this. Or perhaps a spreader of some sort could be utilized (concrete, timber or steel) connected to the slab thus making it locally stronger and spreading the load.
I talked about upper and lower connections in previous post. I called them nominal. What your engineer is proposing for top connection sounds adequate as it is only a prop (ie will not tale lateral or uplift equake loading)
[quote]She does not have advice on who to hire for the job. I think there is more skill to this than just having the proper license. You might say "Hire an underpinnings expert!" Well, I've tried. The job is too small for the big shops I've tried, they don't want it.[/quote]
What about a small (perhaps one man) house repiler? How many props to do?
[quote]The expense and the risk of accomplishing getting a larger beam into these locations, is huge. I could explain why, if you really want to know. The prop is exceedingly simple. But the underpinnings need to be done right. If the slab is damaged in the process, I would better have done nothing![/quote]
Accepted.
As I asked, are you looking at living with sag or jacking it up? I imagine that the only problem with the sagging is the unsightliness of it. Normally beams are made bigger solely to prevent or lessen this; a smaller beam being adequate strength-wise.
So my question now is - why are you looking at doing this job? If cos of unsightliness, have you considered a new slightly lower ceiling or something similar?
bcohen,
Jan 26, 3:55pm
There aren't any repilers local to me, at least I can't find them. Of the 3 smaller specialists I successfully contacted from down in the city, one was too unpleasant to communicate with. The second one I really liked, but he became unresponsive after the first visit. But even then, he said he wouldn't use the dry-pack method. I didn't push it. But I believe there isn't enough height to get a good pressure head for the overpour method. (You could possibly accomplish it by building a chute, then cut the excess off). The third one said "Can't you get anyone closer?"
I really wanted to cut a piece of slab out and just put in another pole. My logic was, if the upper story is independent, lets keep it independent. But the engineer said no. I talked to her boss and he also said no. I pushed pretty hard, and they said no, it's not the right way to do it. And then I didn't realise how hard it would be to get a specialist.
I've looked at the existing slab btw, and it hasn't so much as a hairline crack in it, it looks great.
What is a spreader, that you mentioned above? I'd like to look that up.
I am looking at 4 props total. Which means 4 underpinnings total. The engineer told me I could easily just get 2, but they'd land smack in the middle of my 2 windows, so I opted for 4, one on either side of each window.
I will probably slowly jack the sagging end wall up by about 10mm, when I do that part of the job. And I do mean slowly. I intend to allow the house time to reconform. Once everything is all properly supported, I intend to insulate the roof. It's been hot as the fiery gates here this last week.
And, this is more than a cosmetic issue, otherwise I'd probably find some way to hide it. The house isn't broken yet, but I am convinced that eventually it will be if I don't fix it. And I certainly can't do anything that would add weight.
So I'm still stuck with these 2 questions. I don't see success in using an overpour method, but none of these people are talking about dry-packing the top 50mm. How does a kiwi do this job, to ensure that full reliable contact is made with the underside of the slab, across the whole underpinning?
And, what is the proper way to ensure that the newly disturbed soil is sufficiently compacted to not result in settling which will pull the slab down and crack it?
skin1235,
Jan 26, 7:44pm
whats in the lower floor walls, apart from the windows of course ( how has the upper floor managed to sag if the lower walls are from the slab to the underfloor structure)
almost sounds like those lower walls are not weight/load bearing, and you are intending to insert at least 4 columns into them, then reclad them so those columns are not visible - if so then why not put your spreader on top of the slab, spread the weight across the entire slab, the sq inch pressure would then negate the need for underpinning?
skin1235,
Jan 26, 7:49pm
your spreader could be a simple 10X4 , but what length? to go from one side to the other, a 10X4 will spread a load across a large area ( probably bigger than the bearers or joists above, yes?)
skin1235,
Jan 26, 8:04pm
have you considered that concrete shrinks as it sets - sure you can dig a footing to whatever depth you desire along under that slab, and you can scrape the bottom of that so there is no loose soil, all the way down to bedrock if thats your desire, fill it with concrete, and 10 days later you'll be able to slide the blade of your square the full length under the existing slab along the top of your footing Personally I'd shy away from digging anything out from under the slab, its already compacted as well as it can be, be interesting to know what size perimeter footings that slab has and if there is any steel in them, any damp course included etc, underpinning will require any damp course to be broken, how will that be remedied Was this built as a dwelling room, or is it a now converted under floor garage and originally was exposed poles with no provision for weight bearing in the slab
bcohen,
Jan 27, 8:06am
That's exactly one of my questions, although it wasn't shrinkage I was worried about as the cause. If you believe the literature, when mixed right shrinkage is pretty negligible. But shrinkage or settlement, I'd have worse than a gap, because there will be rebar connecting the old slab into the new underpinning, the gap would instead turn into a cracked slab, because it would be dragged down. That's one thing I'm worried about.
My 2 questions were, how to avoid the possibility of dragging the slab down and cracking it, and how to make sure you have positive contact with the slab, across the whole underpinning. And I'm going to guess that your answer is, you can't!
You have the picture exactly, the slab was poured after the raised pole house, and filled in to be used as rooms. The slab has no footing, as it is not intended to be weight bearing. However, having removed the lining downstairs it can be seen that it is taking some of the weight. Which is where this all started.
I had a slab scan done, and there is steel mesh in there, and a few rods, additionally. There is damp course underneath, I found it when checking for footings.
I think i get what a spreader is now, from what you said. It's a big huge piece of timber laid down, that spreads the load evenly across the length of the edge of the slab. I still wonder if that would be sufficient to carry the additional weight I'll be adding, when I build the roof up for insulating.
How would you go about adding support to the upper story, instead of underpinnings? I'm open to alternatives.
pauldw,
Jan 27, 9:39am
If it is comparitively light weight, you are only removing a 10mm sag in a beam and you have an engineered solution maybe you are over thinking the issues.
What is the fear of the bottom slab cracking? Chances are that it is already cracked along control cuts.
trad,
Jan 27, 9:50am
You could accept that the load is moving onto the lower floor walls and construct a good footing under the outside edge of the lower walls, the aim being to take some load off the slowly settling poles.
You talk of sagging. I took this to mean the sagging of the upper floor beams. Ie the beam ends staying still but the middle of them moving down. But now I am thinking you may be talking of the settlement of the original poles. Is this correct?
I assume the house is on quite a slope and the slope falls away from the side of the house where the problem is.
bcohen,
Jan 27, 9:56am
The issue is, to bring down some point loads onto the slab below. I've examined the slab, and it has no cracks. There aren't any control cuts in it. I've never actually seen control cuts in a slab. Only on driveways and such.
bcohen,
Jan 27, 10:08am
Yes creating that footing reliably is what this question is about. "good" is the keyword. The engineer has engineered footing sections to take the point load from the upper story, to be installed underneath the edge of the slab. Theoretically this works grand. On paper, the numbers work. In the real world it requires 1. a full contact be maintained across the surface of the new underpinning, and 2. that the new underpinning not sink. So my questions were, how to do these two things, so that I can be sure the contractor I hire has the skills to give me a good job.
But it's not a problem of the poles settling. It's simply that the span between them is too large, so the timber beams spanning between can't take the load. They're technically beyond their maximum load now.
trad,
Jan 27, 11:02am
It sounds as if you are looking for a solution whereby the new props will go from underside of beam to the new foundation ground level with provision for jacking to 'take up the slack'. This could possibly be provided by flat jacks which I know little about. They are probably expensive.
If you put such full length props in and no provision for jacking then there will be settlement for a while before the new props take their full load. However these props will be independent of the ground floor slab that has been cut away for the props to be positioned.
If the designer of the solution cannot see how to overcome a basic part of the design, I think it is a bit much to expect the contractor to 'know what to do'.
When you say that the beams are "technically beyond their maximum load now," I assume that that is there deemed safe working load which has big safety factor incorporated. If they were approaching ultimate load capacity the engineer would at least not allow anyone in those lower rooms.
pauldw,
Jan 27, 11:24am
The beams are spanning a distance that is wide enough for 2 extra supports yet the sag is described as 10mm so far. Who determined that they are beyond the limit?
Control cuts in concrete are often placed under wall plates so not seeing them doesn't mean there aren't any.
bcohen,
Jan 27, 1:22pm
She didn't say run for your lives, or anything. If I hadn't wanted to put up insulation, I definitely wouldn't be going to so much trouble. I need to build up the roof to make a space for the insulation. Too much weight.
bcohen,
Jan 27, 1:24pm
The engineer told me the numbers show that some of the support is over the limit, certainly by modern standards. it's a pretty old place.
That's interesting about control cuts. I didn't know that.
trad,
Jan 27, 3:23pm
If the engineer wanted no settlement she should make this clear in the contract documents as well as how this is to be achieved. This could include alternatives. By leaving the method up to the contractor she is effectively saying method is not important. And if there is difficulty getting a contractor experienced in this type of work this is doubly important.
In any event treatment of the upper surface of the new foundation would not eliminate settlement of the ground under the new foundation under load. Perhaps she considers this not significant.
budgel,
Jan 27, 4:31pm
I wonder if you could cut holes in your pad for your new piles, drill into the ground, pour your compression pads in the bottom of the holes, let them go off enough to gain strength, then do a second pour once the new piles are in place with the beam jacked up slightly to accommodate the beam sag. This second pour could be up to say, 75-100mm below your slab surface, which then could be topped off as essentially a cosmetic finish. This is essentially putting two pole hose piles in after the fact. The slab is of secondary concern as it is not structural. I am not sure what sort of connection to the slab your engineer envisaged with his two reinforcing rods going into the pile footing. There are some solutions using epoxy .
skin1235,
Jan 27, 7:19pm
you've certainly got a bit of a problem there haven't you, the slab was not built to be load bearing - any steel in it will only be to maintain integrity of the slab, - and it has no footings around the sides so making it become weightbearing ( on instruction from this engineer) is ludicrous, especially when her instructions are just add a bit piecemeal, but can't tell you what is required to do that, suggests go engage some ' expert' I'd say she's full of shit, and unless you can bypass the slab entirely you're going to end up with the cracks you envisage The slab is not built to be weightbearing, trying to add an underpinning beneath it is not the answer, Assume this is one wall, how far between existing poles and how long is the entire wall that will need treatment
to do it properly you will have to bypass the slab - cut a section out, as suggested above by another poster - drill down with quite a large footing/pile and bring that to the slab surface - and for feks sake don't tie it into the slab You then put an adjustable 'jack' on that when it is fully cured, ( 30 days min) and wind it up over a period of a few weeks or months
be very careful if you try to de-stress the upper beams by way of temp jacks alongside the position for the new jacks - that will put a load on the slab that it is not accustomed to, and it could very easily crack and tip the edge out - exactly what you're trying to avoid
The use of a spreader on top of the slab would suffice IF the slab had footings around the edge with some steel designed for weightbearing included - it doesn't, and without the proper steel structure within you will tip and crack the slab
the 'jack' will stay there permanently and should be able to be clad past eventually, so it won't be visible from inside or out - you're going to have to strip sections of the wall, inside or out to do it, probably inside, cos its easier to modify
cannot understand why an Inspector would give such poor advice, nor can I understand the next up the chain saying that isolating the slab from weight supporting structures is 'not the way to do it', seems they have no idea what they're talking about - and don't give a fek either
skin1235,
Jan 27, 7:36pm
whats the bit about the roof, Chalet style, assume exposed beam inside, ply sheet under iron outer, ie no air space for insulation, again, assume, you want to create an air space you can fill or partially fill with insulation Thats not an easy task, the existing beams are a very vital bit of the infrastructure of the building, it wont' be as easy as lift the iron, lay some 6x2 purlons, and screw the iron back to the new purlons.
not even sure if any reputable crew would want to do that job but going by the poor advice you've already had from the authorities they'd probably pass it without thought
Consider putting polly foam slab between existing beams internal, rebate them for a small batten, after slabs are in fit small batten and lay in a cover of ornate finish or ready to paint finish, you will have reduced the amount of beam visible without reducing the strength and integrity of them and the iron above - if it is more desired to have the full beam exposed then add a fake fillet below your new now insulated visible ceiling to bring it back to the same sized exposure
if the weight of a few sticks of wood and insulation is going to compromise the building then get ta fek out of there now, yep the inspector didn't say run for the hills, but if the structure is already weak enough to concern you why are you even considering staying there, put a digger in there and tear it down before it kills someone
skin1235,
Jan 27, 7:42pm
one integral part of Chalet builds is the floor, extra weight on the roof does not come down on only the walls - it tries to spread the walls apart, the connection between the wall and the floor is utterly critical when considering adding more weight to the roof - if the walls spread and the floor doen't hold to the walls the whole thing goes eggshaped very quickly, a normal build has top and bottom plates, and internal walls to support the support material for the roof, Chalet builds have to be tied together very well, and stay tied together - one of the reasons most Chalet styles have a large wooden cord between the beams
skin1235,
Jan 27, 7:53pm
by Chalet I mean full Chalet, not just a steep pitched roof on top of standard walls, on full Chalet builds the roof comes right down to floor level There was a craze a few years back for half Chalet style, expose beams in lounge and dining area, 6mt plus to apex of ceiling, ( normal walls with a steep pitched roof ) and a half story in one end, bedrooms over the top of the service areas, maybe one bedroom down the bottom, stairs to upstairs all nice and ornate, bloody steep too
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