The skeletal steel frame of my earth sheltered house is critical to its success, and getting from the plans to hard steel on site took a lot of effort and is therefore worth a post…
I’ll probably end up putting notes about the more nitty gritty details and lessons learned into the technotes and design tips section of this site… When I am done learning.
In the meantime,
here is the video.
Steel frame basics
So, the structural design of my house is essentially Shotcrete sprayed over a steel frame skeleton. You may not see this often, but you can find many examples on line.
Check out some examples on the Formworks site (their facebook page is excellent), or the site of their brother company, PBS. I would love to know the falling out story behind that split, but it is clear that the brothers did their early work together because a number of homes are shown on both sites. After the split, the Formworks designs get pretty interesting, while the PBS designs stay pretty simple in terms of geometry.
Formworks uses 3×4 I-beams to span vaults of up to 50ft across. They weld Z-brackets on the side of the IBeams to make it easier to add the rebar later. Special brackets are bolted to the slab foundation and then the I-Beams are slipped into place and bolted to the brackets. The rebar is dropped into the z-brackets and they are hammered down to lock tight. They also have a variety of specialty hardware for bolting the IBeams together, attaching floors, etc. It is a pretty good system, aimed at helping the “do-it yourself” earth sheltered home builder be successful, and refined over many years of actually building these sorts of homes…
So, why would I try to improve on that?
Well, for starters, I didn’t really like the large flat front parapet wall design that many of these homes end up with… I guess it looks pretty good if the home is mission style, but I was wanting something that looked a little more integrated with the earth. I wanted the hard edges a bit more broken up. It is just personal taste, but I wanted something more like a single story “Hobbit home” than a 30 ft tall “Lonely Mountain” edifice. This meant that I was going to be keeping my heights and spans low. I also couldn’t resist the idea of mixing and matching in a whole bunch of different arch forms. The resulting jumble of small arches required a new approach.
My wife and I have talked a few times about how stringing together 4 Quonset huts would have saved a lot of time and money. I would be done and writing my earth sheltered book by now if we had gone that route. I actually had some pretty good ideas for how the exterior could be softened (we were going to go “modern wave”), but we couldn’t agree on a good way to finish the interior and we had already put so much thought into the other plan… But maybe a simpler plan would be a better idea for you?
My overly complicated design includes 3 groin vaults, 3 apses, 10 radial vaults (of 3 basically different sizes), a portion of a “toroidal” vault and one simple vault over the mud room. In some cases, those steel arches are sitting on precast concrete ribs or spanning shotcrete walls. It was a lot to plan out.
Square or Round tube?
Originally, I had planned a mixture of both square and round tube (based on tangency to the ceiling below), but I ended up switching to all square tube because it is stronger in its primary load direction (because there is more material where the peak stress occurs) and costs less… But I later discovered that it is harder to roll-form (without deforming or collapsing). Paradoxically, some places insisted on the steel being thicker (0.1875 instead of 0.12, or 50% heavier) so it would be easier to roll. The company I ended up with just took their time and the deformation is barely visible.
The radial vaults spanning the curved ribs were the most tricky to plan because I needed to adjust the radii of the steel arches for the location and curvature of the precast concrete ribs they were sitting on. I ended up deciding to have a level interior and used a simple formula (Rarch=2πrθ/360) for the arch radii at each location, but used the 3D model to calculate the length of the spacers between the arch section and the curved concrete ribs.
Again, in order to avoid the “flat south façade” look that too many earth sheltered homes end up with, I wanted the dirt to spill down around the bedroom windows. I decided to “miter” the corners. It seemed like I should use properly shaped steel to get that miter shape right and that required ordering “ellipse” shaped frames. This turned out to be quite a bit more difficult than the regular arches.
They basically made them by creating pieces with simple radii and then welding them together. Then they tweaked them a bit (more of an art than a science) to get them to match a full sized template that they had asked me to make. The end result cost about 3 times what I would have paid for a simple arch, so I hope the shape they give the bedrooms is worth it in the end.
To make the template, I used the two foci and a string method (shown in the video). The coolest thing I did there was make a little car out of Lego that had a place to hold the Sharpie and two pulley wheels to hold the string as it went around. This made drawing a smooth curve much easier.
When it came to quoting me for the arches, it became clear that most companies were quoting me per “roll”, regardless of the length of the roll or even the size of the steel. Since I had a number of half arches for forming my apses and the corners of the bedrooms, it was clear that I could save some money by ordering those as full arches and then cutting them in half later. I made all the drawings this way… For instance, that ellipse piece will be cut in half as a corner piece before it is installed. It is also easier to weld the legs on straight if I do it before the pieces are cut in half.
Some of the companies were strictly rolling and wanted me to do everything else myself. The one that I ended up with including the cutting in the base quote, but would charge me 50% more to weld the pieces together. Since I would still have other welding to do anyway, and because I kind of like doing it, I decided to do all of it myself (or with friends and family).
For the next few weeks, I will be focused on getting these arches welded together, and the bedroom ones in place. I am also still working on the precast rib forms and trying to get the quad deck guys to come out…
Noun: the occurrence and development of events by chance in a happy or beneficial way.
“a fortunate stroke of serendipity”
(happy) chance, (happy) accident, fluke; luck, good luck, good fortune,fortuity, providence; happy coincidence”the consequence of serendipity is sometimes a brilliant discovery”
I first discovered the concept of earth sheltering serendipitously while looking for something else on the web. Something tangential. I ended up stumbling across Peter Vetsch. Maybe others will stumble across my site? Why do I care? I would eventually like to put a book together and figure that more traffic to this site can’t hurt…
Anyway, speaking of tangents… My generator has been causing slowdowns lately by shutting down on my often. Anyway, last weekend, I got tired of walking back and forth to restart it and decided the problem had to do with the RPM setting being too low. My generator doesn’t have a nice little screw to adjust the throttle, instead, I needed to loosen and reposition the whole throttle arm.
For one second, I ran the RPM up too high. I quickly shut it off to avoid damaging the engine, made the adjustments and got the RPM where I wanted it. The original problem was solved… But then I noticed that I wasn’t getting any power out of the plugs…
I didn’t have my multi-tester out at the property with me, so I ended up bringing it home… I worked it out and then decided to make a video to show how. More people search the web for info on fixing generators than looking for earth sheltered homes, maybe this will introduce some serendipity.
Brushless alternators use a capacitor to introduce a charge into the windings. This gets them excited so the alternator will produce electricity. When my engine RPM surged, it surged the alternator and the capacitor blew to protect all the electronics downstream…
If the alternator is working, it produces a ~5 volt difference (plus or minus a volt or 2) across the capacitor. So, I just had to check the AC Voltage with my multimeter. It turned out the voltage was OK. By process of elimination, I decided to order a new capacitor.
My original was covered with a layer of rust, so I couldn’t read the specifics. I checked the internet to find the right one for my generator, a 40µF, 370VAC generator capacitor. The genuine PorterCable part costs 60$. Now that I knew the specific properties, I was able to search based on that and found a Genteq knockoff that only cost 16$ with delivery. It claimed to be just as long lasting (60,000 hours), and like the much more expensive OEM product, it is also “self-extinguishing” so I won’t die in a terrifying fireball if it fails.
Its physical dimensions are a little different, but it fit. And more importantly, my generator works again.
Now my generator can do its job and give me electricity on my jobsite.
At the moment, the construction schedule has 3 main paths.
1) On the east side, we are getting the steel arches rolled. We can start to erect those after the excavator comes in and sorts out the grading issue, but they are waiting on the frost laws to allow them to carry their heavy equipment on the roads.
2) On the west side, I am getting the Quonset hut ready as a work shop to build the wooden forms for the precast concrete ribs.
I probably have a months worth of work on each of these sides before the paths converge on the middle.
3) In the middle of the house, I need to get in these steel posts that will support the ring beam that will eventually support the precast concrete ribs that will eventually hold up the steel arches and shotcrete that will form the radial vaults.
This weeks video is about setting up the posts for the center section.
I’ll put the story in here to provide a little more detail and to make it text searchable for Google ;^).
In this region of the earth sheltered house, the load from earth above (not shown) is directed, by the radial vaults (made of heavy shotcrete over steel arches), down onto the precast concrete ribs.
On the outside edge of the house, these radiating concrete ribs (which weigh almost 5000 lbs each) are sitting right on 4’x4′ concrete pads made of strong reinforced concrete 1 ft thick. However, at the middle of the house, the high end of these ribs are set into a central tower made of shotcrete (which will carry most of the final loads) over a steel skeleton (which will carry all the loads during construction before the shotcrete is in place). This image also shows the QuadDeck ICF floor forms. When concrete is poured over these, it will help lock the steel posts into position.
The load of those 10 heavy ribs runs thru the ring beam and down the structural steel columns into the shotcrete basement wall below.
The 3D CAD model shows a nice flat surface where these pipes attach to the top of the basement wall.
The reality is that the shotcrete guys did not do a great job of squaring off the edge of this wall, probably because it was 9 ft tall and hard to reach. I needed to fix that. I setup some cardboard forms to the level the wall should be at and backfilled with hydrolic mortar. It was thin enough that it was pretty much self leveling.
The next step was to prepare the steel bases. I bought a box of scaffold bases for $4 each, but the (nail) holes were too small to fit the anchors thru. I drilled them out using the lowest setting on my drill press and some lubrication.
Drilling steel requires low speed and lubrication. I lubricated with thread cutting oil. The smallest container I could find was much more than I would need and only cost ~4$, the drill bit was 7$, so I used the lubricant generously in order to preserve the life of the bit. I found applying it directly to the bit was the best way. If the bit started to smoke, I would stop the drill and add more oil. If the speed/pressure and lubrication are correct, long spirals of metal will come off. This indicates that the steel is being cut and not just wearing away the bit.
Then it was time to head back out and mount the bases to the wall. I was using sleeve anchors that require you to drill a hole first, and then drop the anchor in the hole. It is important that the sleeve be completely below the surface. When you tighten the nut, the sleeve is expanded and presses against the sides while the nut pulls what ever you are attaching down toward the sleeve… I was in a rush and didn’t put one of the first anchors in properly. There was probably dust in the hole. But I couldn’t pull it out again either. With the sleeve too high, the nut tightened against it before pressing the metal to the concrete, so it would never be tight. Eventually, it will be under another ft of concrete, so I was not too worried about it, but I put the rest in more carefully.
I was also careful to make sure each scaffold base was level so it would be easier to plumb the columns later.
My steel posts were ready a couple days earlier than promised (a first for me on this build), so I picked them up in my trusty trailer. They weighed about 100 lbs each and put the trailer near its official limit, but it felt like it pulled easily.
The following weekend, my parents visited with my sister.
The mission was to move the steel posts into position on the base plates, hold them steady and level and then tack them into place with my welder. The first one worked pretty well. But the welder jammed on the second one. I couldn’t untangle it, so we had to cut out all that welding wire and re-setup the machine. From then on, I always checked the wire spool every time I moved the welder to make sure that it was not tangled.
This third post was the one on the less stable base, so we added extra bracing to keep it plumb until the concrete is poured around it. We had a lot of experience pluming steel last summer, so everyone knew exactly what to do and it only took a minute or so.
My sister and I both enjoyed the welding, so we took turns welding vs holding… Some times on the same post if it was tricky to access a certain point. The most time consuming part of the process was simply adjusting the scaffolding, etc. so we could reach what ever we needed to reach.
It took us about an hour to get the first 5 posts in, but they were on the side of the circle that was easy to reach. We were expecting the back side, were we had to balance on the wall, to be more difficult, but it was pretty easy too.
After all the posts were in place, I was worried that someone may give these a good push and cause them to shift… Yes, they are welded to steel plates bolted into strong concrete with 3 inch steel anchors, but an 8ft long 100lb lever would probably be able to pry those out.
I wanted to weld the ring beam on right away, but each half was 320 lbs and I couldn’t think of a safe way to get them into position.
So we decided to weld some rebar between the posts as a temporary solution. This was nice and easy and I really liked the look of it, so it will be sad to have to cut most of it off again when we need to put the door bucks in.
I was not sure how far my tank of shielding gas (Ar and CO2 for the MIG welder) would go, so we only did tack welds. That turned out to be a good decision because it ran out just before we were done. Without the gas, the final few welds looked pretty ugly.
After that, we put in some work prepping the shop for building the forms, and I will talk about that another time.
The next step in this center section will be getting in the QuadDeck ICF (insulated concrete form) floor over the basement. I am having a painful time getting that scheduled.
The rest of the steel arches are still being rolled and I will soon start on the forms for the ribs.