There was just enough decent weather left in the season to prep and shotcrete the walls around the perimeter of the central circle.  We had to start with the formwork, and since these walls had simple curvature (rather than compound curvature), we decided to use OSB board screwed to vertical steel studs.  Of course, rebar was added in and tied.  I’ll get into details and lessons learned later in this post… And there is always a gallery of pictures at the end, but first, the timelapse video.

The Video

Extra bits

Steel Studs

These MarinoWare steel studs I have been experimenting with are a bit of a mixed success.  They are great for holding the formwork, rebar, electrical, etc. And I like to think that they provide some reinforcement for the concrete.  On the negative side, the shotcrete crew were not always able to properly encase them and possible voids in the walls along these studs probably reduced the wall strength and provided a path for water to channel.  I wouldn’t call them a total failure, but I have decided not to use them in this same way for the central tower.  Instead, I will brace that formwork from the outside.  I will continue to use these studs for the South wall because the design there has these outside the concrete, supporting the rigid insulation formwork.

Screwing into Steel Studs

Pre-drilling would just take too long and it would probably drive you crazy trying to keep the boards aligned to get the screws thru the pre-drilled holes.  Self-tapping drill point screws are what you need.  They should be long enough to get thru the form materials (3/8th inch OSB in my case) and still bite in nicely to the steel.  In places where you put an extra layer of OSB (such as to thicken the joints), you will need an extra 3/8ths of length.  Also, to save money, you want to use the shortest and smallest screws that will work.  Finally, the key thing is the head…  I recommend the Hex-Washer-Head because it will be held securely by your driver without any slippage or cam outs.  Do not buy Phillips head screws, you will seriously regret it.

Personally, I ended up mostly using TEKs #10 x 1 in. Zinc Plated Hex-Washer-Head Self Tapping Drill Point Screws that cost about 4 cents each.  If you can get them in bulk, you can probably reduce the price to half that.

Plumb and Braced

For some reason, the camera kept moving on to the next area before we would complete the important job of plumbing and bracing each section of wall.  However, that step was important and worth a small section here. The walls had some natural stability because of the curvature, but we still needed to brace them against the force and vibration of the shotcrete and they were not always naturally plumb.  On dirt, we could just brace with a 2×2 or 2×4, screwed to a block on the wall and then screwed to a stake.  However, for most of this job, we had to brace on the concrete deck and didn’t want to attach the bracing to the deck and risk damaging our radiant tubing…  The solution was to place a board on the deck, screw the diagonal brace to that, and also screw a horizontal brace and tie it back into the wall.  This worked pretty well.

In one location, the wall was out of plumb and took some serious pulling to try and force it plumb.  Some combination of the boards, studs, and rebar was fighting against us.  I ended up just using a strap and a come-along to pull it.  Part of that strap is still embedded in the concrete wall, but at least it is plumb.

Welding Rebar

I get a lot of negative comments from people saying that it is “against code” to weld rebar.  In reality, the building code has about 85 pages devoted to welding rebar.  The code talks about what types of Rebar you can weld (mine has a little “w” on it to indicate that it is weldable), diameters (most welding rules don’t kick in until #7), what types of welds you can do (butt joints are not acceptable), pre-heating, exceptions based on engineering approval, etc.  I was going to get into it a bit here, but it is probably better to just suggest you read up on it yourself.

While I agree that the heat treatment that comes from welding does influence the ductility and other material properties of the steel, in certain situations (some of my situations), it does make things much better.  I know what I am doing, everything is inspected, no need to worry about it.

The Gallery

Here is where we put some of the pics we took during this period…

from time to time, I’ll just take a photo of my screen so I can have the measurements handy in my phone… Some times it is faster than trying to transfer a proper screen shot…

The Hex Head screws are much much better

For us, this build is pretty focused work. But while we are out there, the kids will often find time to play or read a book in some odd place.

Working on the side wall in the guest room. We had this one shot from the kitchen across the basement stairs.

Me setting up the camera in the kitchen…

I think I may have kept too many pics of David setting up the camera…

Sherri gets goofy some times…

Joe working on the dining room wall.

David Setting up the camera. He likes this job.

Here my father is working on the wood forms and my mother is tying rebar… Probably not the retirement they expected.

Sliding the rebar into the short wall section.

For this little side wall, we put in all the rebar pieces and then cut them down to size.

David setting up the camera for a shot

David was climbing around on the tower steel and took this interesting pic of Hunter walking thru the site.

Another shot of the playroom apse before it got started. You can see all the steel arches leaning against the rigid insulation in the background.

I separate out these jobs, but they often overlap. On this day, I was working (For ANSYS inc) from the site because the stucco guys were working on the garage. In the evening, I probably switched to welding the playroom apse or something like that.

I originally set everything up around this middle post, then I welded the outer edges in place.

To form the apse, I had to weld the thicker rolled steel arches to the thinner galvanized steel studs… It doesn’t have to hold forever, just long enough to be encased in concrete. It took a little practice to be able to weld the different materials reliably. I did little patches like this ever few inches along each edge where they met.

This looks like it was from my timelapse, but the rest of the video was lost… It happens sometimes.

This is one of those pics that happens when your 10 year old is just walking around with a camera.

Welding the rebar

apse steel ready

Apse steel from above

Outside of the guest-room wall. We shot the walls from the outside to try and avoid messing up the deck.

The inside of the guest room forms.

We try to avoid it, but occationally we are all working within a very narrow distance of eachother…

For some reason, Hunter likes to pose for the timelapse… Just while he is walking by in the middle of the shoot. The easter egg photos are mine now.

Friends playing while their parents helped out…

The north side, ready to shotcrete.

This gap between the back of the kitchen wall and the Quonset hut will be filled with a lot of earth.

This pic is trying to show that I cut away the fox blocks and drilled the horizontal rebar into that wall to tie everything together structurally.

The connection between the guest room wall and the mudroom…