Replacing Bearers and Piers

[silentC]

I'm after some suggestions on what to do with the situation under my house.

When my grandparents bought the place about 35 years ago, the previous owner had excavated and built an office under the house. The house is on a slope, so there was plenty of headroom on the downhill side - in fact this is where the laundry was. He excavated back up under the house between the piers, replacing one with a 4x4 post, poured a slab and built his office.

Last year, we were invaded by termites so we pulled the office out. This photo shows what we found. As you can see, the footings for this row of piers are about 3 to 4 feet above the floor level.

[silentC]

Here's a view from further back. You can see the row of piers in the middle distance and in the foreground the first of the second row. The post that he used to replace one of the second row of piers is in the foreground. What I'd like to do is to replace that first row of piers but at the same time, get rid of the second row altogether. This will give me a lot more room under the house.

[silentC]

Looking back the other way you can see the second row again with the post. Behind that, is the original external wall. The blue tiles you can see are where the original laundry was. I will relocate the hotwater service and demolish the slab it's sitting on to give me one level throughout.

[silentC]

I have a couple of ideas on this:

1. Replace the piers one by one - prop, demolish the pier, excavate, pour new footing, replace pier. Cheapest and least disruption. I can do it myself. But - I want to get rid of the second row if I can.

2. Replace first row as above then replace bearers from there through to the back wall with RSJ.

3. Leave all as it is and put in steel alongside the timber. Then remove old piers and bearers.

The other problem is that I'd like a wall along the room where the 3 to 4 foot bank is currently. If I could prop all four bearers along the first row of piers, maybe I could knock out all the piers and build a block wall underneath for it all to sit on, then put in RSJs from there to the back wall. The other alternative is to replace the piers then fill in between with blocks.

I was also considering steel posts instead of brick piers.

Any ideas?

[journeyman Mick]

Darren,
you'll have to get an engineer to check it out for dimensions of posts etc. but I'd be inclined to replace the first row one by one with steel - 100x 100 (x5mm wall?). As you say you can prop them, demolish the brick pier, bolt the steel post in place and pour the footing. Once you've replaced all of them I'd be bolting flat bar or channel to the side of the bearers. I've done this before on old Q'landers. Got a fabricator to supply the steel, cut to length in pairs and pre-punched with holes. Just a matter of getting the steel in place, sandwiched either side of the bearer, clamp in place, run a drill through the holes in the steel through the timber and bolt it all up.
Building a wall instead of the posts is a little trickier but still quite possible. You'll have to do it first. Prop bearers both sides of existing piers. You will need to leave some form of access to behind the wall to remove the props and to keep an eye on future termite activity. After you've propped it you can demolish the piers, dig your footing, pour concrete, lay blocks (have a few beers). Don't know what's required for tie down your way, but your engineer will advise, also how much steel the blocks will need and how many cores need to be filled. Don't forget to place DPC (alcor) on top of the wall (and maybe a membrane against the dirt). The DPC will act as a termite barrier if fitted well, forcing the termites to build a track across it, making them easier to spot. Doesn't sound like much fun at all!

Mick

[silentC]

Mick,


Doesn't sound like much fun at all

That's why I've put it off for over a year but something has to be done!

Thanks for the input. I'd better get the local engineer (yes, there's only one) to come out and have a look. Good idea to bolt steel to the bearers. That'll be a lot easier than trying to replace them.

Funny you mentioned Queenslanders. I was looking at one in Wynnum late last year that has been raised and it gave me a couple of ideas. They used RSJ throughout though and it must have been a mother of a job. Solid as a rock.

Cheers

[journeyman Mick]

Darren,
they'll often replace thebearers completely when they raise them (or shift them and raise them), makes it fairly easy then.

Mick

ps. "them" = the house. just reread and it was a bit ambiguous.

[bob w]

Darren
Steel piers are available in two sectional sizes depending on the length required. One brand I know of is "Unipier" from BHP (I think) They have an inbuilt ant cap and are a two piece set up that makes them adjustable and reasonably easy to use. Especially with the headroom you have. Most builders supplies would have specs on these. If you have any trouble getting details let me know and I will get it for you
Regards
Bob:)

[silentC]

Bob,

Thanks. I'll check out the local supplier and see what I can find.

[bitingmidge]

I was always taught not to replace the timber bearers completely, rather to bolt a channel on the side as per Mick's original post, although most house removalists these days do simply replace the beams with RSJ's or whatever, without any apparent problem.

The reason seems a bit academic, in that the original floor will have "settled" into the shape of the old bearer, which in Qld at least would have been installed green and would have shrunk 12mm or more. Shrinking is not consistent along the length of the bearer, resulting in differences in height of a few mm for each joist, (even if the joists were "sized" during installation).

Given the amount of movement that takes place while the installation is happening, and the fact that the new span will have different deflection characteristics all of this is probably a bit academic, but old habits die hard!

Steel columns are readily fabricated out of nominally 70 or 90 SHS, "fabricated" is a loose term which means cut to length, with a rectangular plate welded on the top to form a flange, and a square sole plate on the bottom to provided a key into the concrete footing. They can then be galvanised. No flange needed if they are to be welded to the new beam.

Installation is dead easy. After all the neccessary digging propping etc, bolt or weld them in position hanging in their footing hole, and back fill with concrete. If you plumb them first there will be no need to provide temporary braces even.

I assume that you wanted a block wall to act as a retaining wall at the back of the space. Another method of holding all this up while you do so, would be to use 70mm posts as above, and build them into the block wall as very large bits of reinforcing :D. You will need to use "H" blocks to surround the posts. At the risk of stating the obvious, don't forget to drain and seal the bit of wall in contact with the ground.

Oh, and re-read Mick's advice re access and termite inspection/proofing.

Cheers,

P

[silentC]

What I had planned to do was to excavate the bank back at a 45 deg. slope from the base of the new piers so that it's well away and allows easy access for the termite guy. The wall is really just something to hold up a few powerpoints and tool cabinets. Blocks are my preference because I don't fancy putting timber down there after what happened to the last guy's efforts.

One problem I forgot to mention is that the bearers are all single span and lap on top of each pier, so there are actually three staggered lengths in each run. This might present a problem when it comes to bolting on steel. I wont be able to get a full run on both sides with full contact on all three bearers, but I could run it flush and put in packers on alternate lengths. I'll get the engineer to spec that I think. He might as well earn his consultation fee.

For drainage, because there's a good slope, I'm assuming I can just dig a trench below the floor level behind the wall and run some ag pipe. Or maybe a half-pipe gutter. We don't get much moisture under there unless it rains for days.

[Frenchie]

I believe that the engineer will prefer a continuous beam to be used if you want to remove the existing piers. This will mean that from what I can tell from the photo's the steel would run below the bottom of the existing bearers. You may be able to run the steelwork beside the existing bearers, but you will need to notch out the others and have additional support cleats fabricated for support to the engineer's requirements.

Are the short beams actually bearers or were they placed as infill in the previous exercise (office construction). It seems unusual to have bearers forming a cross like what is shown unless they are required for structural support of roof/wall loads from above. Anyway, if you have an engineer look at it you will be able to determine quickly if they are structural or otherwise.

I have restumped our Qld'er recently allowing new rooms to be built underneath. Used 75*75*5 SHS for the columns with 450 dia x 900 deep concrete footings. Bolted channel sections (200 & 250PFC) to the existing bearers so as I could shift locations of columns to suit room layout.

[silentC]

I think that the reason for the cross members was his attempt to reduce some of the bounce in the floor. They are not supported other than skew-nailing to the bearers. I would be taking them out and if necessary putting an extra row of bearers in between. It probably needs it anyway, because the floor is quite bouncy.

There is one load-bearing wall above the piers and it is in line with the first row (the row I want to replace). This wall supports a second storey wall and 1/3rd of a roof load as well.

[IanA]

All that lovely room under your house.

You're making me drool!!!

[silentC]

OK, the engineer came round this morning and had a look. What he has recommended is pretty much in line with Mick's suggestion. We're going to bolt a channel to the bearers, blocking it out where it's staggered. He doesn't think it will need one on each side.

To keep the section size down, we're going to have a row of steel piers, probably in the middle. To compensate for that, he reckons I should get in a dingo and excavate the whole bank behind the first row of piers, which would give me another 10 sq. metres of space. That makes a total of about 60 sq. metres.

We can do all that without removing any existing piers, then when it's finished, I can dig out the bank and remove the old piers at my leisure (funny definition of leisure). It will need a retaining wall all the way around because the sub floor will be above floor level in a lot of places. He can design one so that I wont need to underpin the footings, which will be excellent. You just have to stagger the retaining wall to give the bank a bit of mass on the inside of the sub floor.

$700 for the onsite inspection plus pier, beam and retaining wall designs. Money well spent I reckon. Downside - requires a construction certificate.