Tag Archives: Structure

Clearing out the skylights


Posted on January 27, 2017 by



In order for the formwork to be as strong as possible while the shotcrete was going up, I had chosen not to make a hole in the rebar cage for the skylights.  Instead, we just built the skylight forms above the steel structure.  Now that it is too cold to work outside, it is the perfect time to cut the steel out of the openings and remove the skylight forms.

The Video

The Gallery

Another round of shotcrete forms


Posted on October 15, 2016 by

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…


Rebar and Shotcrete over the Quonset Hut


Posted on July 15, 2016 by

Part of my earth sheltered home design included burying a Quonset hut.  Actually, this was the easiest and cheapest part of our build and if we had done the whole house this way, we might have been done the first year.

This post is for phase 3 of building our earth sheltered Quonset hut.  In phase one, we put up the steel Quonset (with a little help from our friends).  Phase 2 was getting up the Fox Blocks ICF endwalls.  That part was interesting, but took more time (and several posts).  We chose ICFs for the endwalls because they made the transition from wall to parapet for retaining the earth very straight forward.  Wood end walls would probably have been cheaper and faster, but not nearly as durable..

I have seen Quonset huts buried directly (with just a pond liner for waterproofing), but I wanted to put in a layer of reinforced concrete first. Phase 3 is to cover the quonset in reinforced concrete before we can bury it in phase 4.  (actually, I guess there was a waterproofing step before we can bury it also).  In this way, it is really the reinforced concrete that supports the earth load.  The Quonset hub becomes fancy form-work and an interior finish.

Here is the video about phase 3.

The Video:


Hot:  The day we put the insulation and plastic on the roof was officially the hottest day on record for our area…  The reflective Quonset hut was making it feel twice as hot and the heated metal was melting the duct tape that we used to tape down the Styrofoam. I edited the water breaks out of the timelapse, but they were about every 15 minutes.  Fortunately, we got clouds and cooler weather for the second half.

Crew:  Just  a quick note that I appreciated all the help I got on this portion of the build, both amateur and professional.

Rating:  My Quonset is rated for 75 lbs per square ft.  I am sure the engineers were thinking snow load and not concrete, and there is probably a significant safety factor built into that.  Either way, I am sure I significantly exceeded the official rating by about 50 lbs/sqft. Fortunately, my shotcrete was also sitting on the footings, shotcrete above that was really sitting on the shotcrete below, etc. The stiffening shotcrete really formed an arch from footing to footing and is probably not loading the Quonset hut significantly. Maybe if all the concrete were wet (no internal stiffness) at the same time, we might have had a problem, but instead, the shotcrete was curing and carrying load as it was applied.  Spreading the shoot over the 3 days probably helped.

Crenelations:  The Quonset crenelations (groves) were about 7.5 inches deep.  Across the top sections, we decided to fill with polystyrene strips 4 inches thick, 10 inches wide and 24 ft long.  Some quick math, and I can tell you that we placed more than 7 cubic ft of Styrofoam in each groove, which means we reduced the load on the top of the roof by about 1000 lbs per groove.  There were 20 grooves, and I filled 16 with Styrofoam, so that is a 16000 lb reduction.   I left 4 of the crenelations without Styrofoam (and added extra rebar) so they would have stronger hoop beams across.  We filled the concrete to a depth at least 4 inches above the crenelations, so the concrete in the crenelations was nearly 12 inches thick.

Plastic: The plastic liner was primarily to keep concrete from directly contacting the Quonset, and secondarily to provide an extra waterproofing layer.  I wasn’t fanatical about it.  The plastic is tough, but not impervious to puncture.  I have heard of people using pond liners with a heavy felt underlayment, or even grinding down all the bolt ends that could puncture the plastic.  Pond liner and underlayment can easily cost more than 50 cents per square ft (and the Quonset surface is well over 2000 sqft).  Our 6 mil plastic sheet cost less than 1/10th of that, so when it got a few little holes and tears I didn’t worry about it too much.  I had thought that the plastic would make it more difficult to walk around on top of the Quonset (because it could slip), but actually, it improved the walking conditions quite a lot.  I sagged the plastic to allow it to properly fall into each crenelation when the rebar and concrete were added.

Openings: The Quonset hut is a nice strong shape to work with (mine was an “S-Type”, the “Q” type would have been even better, but less practical without the straight stem walls).  Loads are transferred around it like pressure on an egg.  As you know, any crack in an egg weakens the overall structure tremendously.  I planned to have 3 openings in my Quonset shell, two skylights and a side door (to the mudroom). The most stressful time for the Quonset would be when it was covered with 46 yards of wet concrete (186,300 lbs) and two tons of rebar.  You do not want to have holes in the Quonset at that time.  Instead, I just created bucks to keep the concrete out of these three areas.  Later, I can come back and cut the Quonset steel with a grinder to make the openings.

Rebar: Each crenelation got 4 pieces of #4 rebar.  One vertical piece was “inside” each crenelation, centered and about 4 inches from the inner surface.  The other verticals were at a layer about an inch past the outer surface of the Quonset hut, spaced 8 inches apart.  The horizontal rebar was mostly tied to the outside of the vertical rebar and spaced no more than 12 inches apart.  Some horizontal pieces were placed first, against the surface.  These first horizontal pieces made it easier to place the outer layer of vertical rebar without things falling inside the crenelations.  They were kept off the Quonset hut by placing a few 1.25 inch rebar chairs (according to code).  We wanted most of the horizontal rebar further out where it could help hold more shotcrete up.

The other important rebar code section to worry about is overlapping the pieces.  We had a 40 ft long Quonset hut with 20ft long pieces of rebar.  I just placed them end to end and then came back later and tied 4ft long segments along that seam.  That was more than enough to overlap both sides by 40 diameters (code).

Electrical and Plumbing: I didn’t include it in the video, but we also ran plumbing and electrical before adding the shotcrete.  These were then inspected.  Part part of this was running white vent tubes up the side of the Quonset so they would come up the skylight curbs.

Balance: The Quonset hut shape holds even pressure very well.  It will actually get stronger when I put the balanced earth load on top…  But while applying dynamic load (such as the shotcrete) it is important to apply it evenly to both sides so the Quonset hut isn’t pushed flat.  Our shotcrete was poured over 3 days.  The first day got most of the short vertical walls done (our Quonset hut is an “S type”).  This stabilized the base and got us ready for the second day where they added most of the shotcrete, including filling some crenulations all the way across.  In the video, you can see them working one side and then the other, back and forth.  This required moving the heavy hose, which is tiring.  They made their lives a little bit easier by toeing the hose with the lift whenever they could. Then the crew left for the weekend (not ideal) and came back on Monday to finalize the top, add the shotcrete for the skylight curbs and do a final coat for smoothing.  All this time was partially because they also shot the bedroom at the same time (upcoming post), but planning to shoot things in a balanced and paced way is a good idea even if you could shoot faster.

The Compressor:  The shotcrete is moved to the wall in two ways.  The mix truck dumps the concrete into a hopper where it gets pumped (by a very expensive concrete pump) thru the hose to the nozzle.  In the nozzle, the “nozzle man” injects compressed air to blast the concrete at the wall.  Part of the magic of shotcrete (the strength, lack of cold joints, etc.) comes from the way it impacts/compacts the wall particle-by-particle.  Unfortunately, the compressor broke down soon after starting on the Quonset hut and somewhat spoiled the day (but somehow didn’t reduce the cost).  Ironically, this same compressor had broken down when they did the basement shotcrete, which was the last time the crew had come out.  They already had a concrete truck on site with 8 yards of concrete in it, so we decided to pump it out.  I knew that this wasn’t quite ideal in terms of speed of application or strength, but it did work and I didn’t have to worry about cold joints, since it was the first bit to be applied.  The next day, they rented a nice new compressor for only 60$ / day and it ran flawlessly.

Hard work:  Pretty much everything about shooting shotcrete is hard work.  Moving the hose, aiming it all day, keeping everything flowing, finishing the surface, repairing any issues with the equipment, cleaning up, loading up, moving scaffolding, etc.  It is all hard work and I appreciate the effort of the crew. Certainly I would suggest anyone think twice before deciding to take on this part of the build themselves.



These are pictures taken from my cellphone or time-lapse screenshots…  Enjoy.