KHouse Modern – Foundation Progress

If you are an architect, and you like to get things built, getting the foundation going on a project is one of the most exciting times to be found. It’ been a while since I gave everyone an update on the KHouse Modern project, but it hasn’t been due to lack of anything going on – quite the opposite. A whole lot of something has been going on, but this house has a fairly significant and complicated structural system for a residential project and the work is deliberate and methodical.

So let’s take a look at what’s been going on since construction started – the foundation.

The short description is that the site had to be cleared (architectural speak for scraping the dirt and landscaping away as needed), the piers have been drilled (holes dug into the ground down to rock which will be filled with concrete and rebar) and formwork is getting set. Formwork is a structure, usually temporary, used to contain poured concrete and to mold it to the required dimensions and support that concrete until it is able to support itself. I should probably explain some of the other parts that make up a pier and beam foundation since it seems that so many people don’t really know what they are or how they work but this post will already be bordering on head-explodingly long so I’ll save that for another time.

Let’s get to the pictures shall we? This is the easiest way I know to illustrate and explain this process.

Drilling piers

At this point, the site has been cleared and the drilling rig is on site drilling holes. This project has mostly 16″ diameter piers and due to that size, a big drill bit is needed.

tying rebar cages for the pier

While the piers are being dug, the rebar cages are being built off to the side. This massive ring of … rings, is individually tied to 4 long pieces of steel rebar at a spacing of every 12″. This part is obvious for two reasons – the first is obvious (we’ll get to the second reason in a minute): steel is an important part of the piers that are being dug. The steel cages are built and placed into the holes and concrete is poured in to complete the structural assembly (you know, because concrete is strong in compression and steel is strong in tension but hey – nobody cares about that but structural engineers. *note* structural engineers will be dying to elaborate on this … don’t let them).

drilling 16 inch piers

The process of drilling this piers is pretty quick – in our case 60+ piers we completed in two days and that’s the second reason it’s important to have the rebar cages ready to go as you are drilling: the hole gets dug, the steel goes in, followed by the concrete ON THE SAME DAY! You don’t dig these holes and then leave them open so the contractor only drills piers that he is prepared to complete on the same day.

pier holes with rebar cages

Here’s a look at the rebar cages sitting in the holes. Fascinating?

foundation piers in place

Two days later, 60+ piers are in place and we are ready for the next big step in the foundation process: formwork.

starting the formwork

When the formwork goes up, an interesting thing starts to happen: the building takes shape … kinda. You start to get a feel for the house as you can physically move around the site and imagine the spaces (i.e. “I am standing in the bathroom”). In fact, it is quite common for me to walk through the site at this point with the owners and play the “this is where the [fill in the blank] would be” and it is a very exciting moment for everyone.

Site Meeting to discuss structural progress

Of course, getting the foundation as close to perfect as possible is the goal and having a job site meeting with the structural engineers, the contractors, and myself is a good idea … it’s not much fun because structural engineers are super boring but they do add value to this part of the process.

structural drawings on site

This is the foundation plan for this project – beautifully rendered by the contractor. The contractor has a lot of notes on this page and I learned that this is the reason why the contractor doesn’t want new sheets issued when revisions are made – they don’t want to transfer all this data onto the new sheet.

KHouse Modern BIM terrace view Jan 2014

I thought I should include one of the rendered images to remind you what this corner of the house looks like – it’s a little easier to visualize than this next picture …

grade beam formwork

This is the same corner of the house as the rendered image above – do you recognize it? It might take some visualization skills …

grade beam formwork

This house has a crawl space underneath it that’s over 48″ deep – this is where our mechanical equipment will be placed and distributed. Part of the programming requirements for this house was that the owner wanted to house to be elevated from street level – this means that the finished floor will actually be about 24″ higher than the top of the formwork you see in these images.

pier detail - drilling into solid rock

This site is basically sitting on bedrock – the deepest pier we have is 10′ deep and 36″ of that length is embedded into the bedrock. That is pretty salty my friends.

drilling 12" diameter piers

I mentioned that we had 16″ diameter piers on this job – which isn’t all that common actually, at least not for residential work in Dallas. Part of the reason we had that site meeting with the structural engineers is that the contractor wanted to change some of the 16″ piers to 12″ piers so that he could use a smaller drilling rig (the bobcat pictured above). He made this request so that he could avoid damaging the tree canopies in a few areas and he needed to get the structural engineer to sign off on the additional smaller 12″ piers (which he did – whew!).

pier top with sonotube cap

This is looking down at the top of one of the 12″ piers. You’ll notice that there is a brown ring around the pier – it’s only about 12″ deep and it’s in place to make sure that the concrete at the top of the pier doesn’t spill out (what we call “mushrooming”). We want a nice clean and nicely formed cap on our piers.

pier rebar cages detail

Here is a close-up look at the rebar cage for one of the 12″ piers …

grade beam formwork towards basement

This picture is looking back over the site – I am standing in a precarious spot taking this picture … 6″ to my left and I am falling into the basement.

side view of soil

If I had fallen into the basement, this is what I’d be looking at – the cross-section through the soil and rock of the project site. We hit rock after drilling for 4′ … like I said, this will be a salty foundation.

KHouse Modern BIM Alley view Jan 2014

Time for another finished project image reminder – this is looking up at the rear of the house from the alleyway. To the right you can see the basement level garage. As the site falls away from the street, the garage is halfway carved into the site which is easy to see …

digging out the basement

… in this picture. I think this is a pretty interesting photo, it also shows that our drawings were correct in terms of the topography (go Revit! … <sigh> I can’t believe I just said that)

basement desk

In this picture, I’m looking down in the basement level (plus, I always take pictures of job site desks for my growing collection). If Dallas had a rainy season, we would be in it. Since this basement is sitting completely within the bedrock, it basically doesn’t drain water. Right in the middle of the photo you can see a red with blue hose pump to remove the water. The concrete foreman in this photo is also doing something that I don’t see too often on our projects – he is building 2×4 wood stands to tie off his string level lines.

building stands to tie off level string

It might be hard to see the string lines in this image but they’re there. At the rear of the driveway, as it connects with the alley, you can see some of the 2×4 structures that have to be built in order to create the spot where the level string lines will get tied off. These string lines basically act as visual guides for the guys building the formwork as they move around the site. They use laser levels and survey equipment to locate everything and then run these strings as a reference.

hot pink formwork level string

It’s not rocket science but I will tell you that I am glad that color has been introduced into the strings – makes them a lot easier to see. There are literally dozens of lines crisscrossing all over the site and it’s easy to walk into one (or 10) as you are moving around the site. If you’re worried that they’ll break free when you walk into them, think again. They are tied around nails that are driven into 2×4′s and formboards that are extremely secured to the ground. It’s far more likely that you will garrote yourself than break one. It’s kind of embarrassing if you walk into them as well because those strings are hard to see – to someone looking at you from across the site, you simply looked like you are having a fit as you drop or stop awkwardly in mid-stride. Stupid architect …

concrete formwork loose ties

The things you see sticking out from the plywood above are called “form ties” and the ones we are using on this project are for light-duty (2,00 to 3,500 pounds) and is known as a “snap tie”.

concrete formwork tie rod detail with notes

Looking down on the snap tie, it easy to see how it gets its name – there is a weakened point in the rod so that when the concrete has been poured and is ready to have the formwork removed, the tie is simply “snapped” off at a weakened point. The washers you see in these photos press up against the inside face of the plywood formboard – this is what maintains a consistent distance between the two pieces of plywood. Since we will have a 12″ wide concrete beam, the snap ties are ordered with the washers spaced 12″ apart – just behind the washer on the image above and to the right, you can see a small ring on the tie rod that keeps the washer from sliding down and becoming closer than 12″ away from the opposite side.

setting concrete formwork brackets

This is the other side of the formwork boards from the snap-tie photo above. These are the guys that are putting up the brackets and 2x framing that will hold the plywood together under the weight of the concrete when it is first poured and while it is curing.

concrete formwork bracket detail

Here’s a close up look at one of the formwork brackets – you can see that the bracket swivels around the head of the snap-tie. This allows the snap-ties to be pulled tight while allowing some construction tolerance and flexibility on site. When I was on site for my field report (and I was taking these photos), I asked the concrete contractor a question:

Bob: Hey – is there was a special name for these “things” (holding up one of the brackets.)
Concrete Foreman: Yes.
Bob: Uh … great. Could you tell me what it is?
Concrete Foreman: That’s called a “bracket”.
Bob: A “bracket” … really? That’s it huh? There’s no special name for them?
Concrete Foreman: Well, we (gesturing to his crew) call them “cerditos” – that’s Spanish for piggy.
Bob: Okay … those are brackets.

So there you have it – as comprehensive an update as I am willing to assemble. You are now caught up to date on our progress and my condolences for making this such a long post. I did debate on whether or not I should cut it up into to posts – but then you’d have two long and possibly boring posts rather than one long one. Next time I promise I won’t go so long between updates and we’ll focus in on something a bit more bite-sized.

Cheers,

Bob AIA signature