Ordering the Steel Arches
Posted on April 27, 2015 by
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.
Radial Vaults:
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.
Elliptical Arches
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…
