Curved walls are not very affordable for wood construction, but they are a somewhat natural choice if you are working with shotcrete, particularly in an earth sheltered application. But even if the curved walls are easy, many other aspects, from surveying to carpeting, are much trickier. Tricky translates into expensive. I will keep track of curve related issues as I build and put them here.
Damns are curved to better hold back the enormous pressure of the water behind them
If you try to balance a playing card on its edge, it will fall over, but curve that card and it will stand. When building a concrete wall, a curved wall is self bracing. Damns are curved because it allows them to hold back much more water pressure. Earth sheltered homes with flat roofs need at least 12 inches of concrete, but curved roofs can be as thin as 4 inches.
Curves also look and feel very appealing. Humanity has only been living in wooden boxes for a few hundred years.
“The idea of putting a square home under the earth made no sense. Caves are not boxes, and the box is not a shape that lends itself to the immense load of earth above.” ~William Lishman
On the other hand, curves bring lots of issues. While laying out our curvy house, we had lots of issues making things work. It is just not as easy to lay out a wedge shaped room as it is to lay out a square one. It also added lots of difficulty for the architect. In fact, if my house were rectilinear I might not even have needed an architect.
Looking ahead to construction, I am sure that siting the house will be much more difficult due to the curved walls. I can’t just string a rope to make a straight line. Digging a curved trench will be more difficult than digging a straight one. Framing will require curved track which costs more per ft than straight track costs. Placing floor joists is more difficult because each joist is a different length and angle from the wall. Some constructions systems (like pre-stressed concrete planks) are out of the question, and others (like curved ICFs) just get a lot more expensive.
Rectilinear homes rely on the principles of the post and beam, but when the beam between two posts is curved horizontally, it results in torsion. the weight above the beam doesn’t just push the beam down, it applies a rotational force to twist it off its posts.
In a curved wall, window lintels are not just a simple beam. They need to be specially fastened to the posts to resist this torsion. I only have windows in the curved front wall of the house. Here the curve is really just to be continuous with the other earth sheltered walls. To reduce this torsion effect, I have kept the wall above the windows as light as possible. It is the only section of wall made of steel studs without shotcrete. I also put beams between each 40 inch wide window so that the max deviation at the mid point between supports is only about an inch.
The Formworks earth sheltered homes all use curved back walls to support the earth. But most have very simple layouts and simple flat front walls. I am treading into dangerous territory with my curves, but at least I am aware of it.
Lets review March…
As I mentioned in my earlier post, I had met with the architects and engineer at the start of the month and reviewed a bunch of stuff. Most importantly, we came to agreement on a few things, including some changes they wanted and some that I wanted (and had already asked them for months before). I came home with a new set of prints that included the 12 sheets of house plans, plus 6 sheets from the engineer. I could see that the architect had already made a good effort to target the issues I had reported in my big “catch up” review… Of course, I found some new issues, especially as I got to the later pages, but it wasn’t too bad. I carefully went thru sheet by sheet and itemized all the issues over several emails.
Mean while, I was trying to get a meeting with a shotcrete company in michigan (who has since asked to have his name removed because of negative comments on youtube about his work). I figured I would run the plans by him to make sure it made sense to him while I still had the architect and engineer under contract. In had been a few weeks since I had received anything from the architect, so I asked for an update of the plans to show the shotcrete contractor. Not many of the corrections I had asked for were implemented yet, but they had added two new and interesting sheets.
One of the sheets included images of the sunshade… The sunshade had been hard to see in the elevations, but now that they had done a detailed view, it was clear that they had not understood what I meant when I said the front corners would sit on pilasters between the windows on the side walls. Hopefully we will get all these details sorted out soon.
This month, in prep for my meeting with the shotcrete contractor, I did a lot of planning the order and process of construction. It suddenly occurred to me that the large concrete ribs were sized as if they would be set on the finished slab floor, but in order to keep it from being damaged, the floor would not be added until long after the ribs were in place. The foundation drawings showed footings for the ribs, but nothing was indicated between the footings and the ribs. This was particularly bad on the half of the house with a basement. Similarly, the walls (and the steel ribs within them) would need to start from the footings, which most of them already did. The problem was that the exact depth of the footings is not known until they are poured, so if we based the arch heights on the drawings, they would almost certainly be wrong and require us to build a stem wall which would weaken the structure. I contacted the architect with solutions to both problems and he agreed. We would add pilasters to support the ribs, and extend the bottom of the ribs down 6 inches below floor level so we could later pour around them. For the steel arches in the walls, we could order them a bit long and cut them down to shape. I will also try to delay ordering those as long as possible and will perhaps have the actually installed footing depths at that time.
20 gauge steel studs with open web, $0.47/ft
I also noticed that some of the drawings still specified steel studs in the vertical walls. This idea had come up in November, but one of the concrete guys (Ken Veera) had been worried that the steel studs would be flimsy and deform under the impact from the shotcrete. They would also cause weak planes in the concrete (like cold joints). The idea of using metal lath should stiffen the wall structure up a bit. But before asking the architects to change this again, I figured I should come up with a better idea. I decided to try and find more suitable metal studs. Previously my metal stud research was based on reducing heat loss thru the front wall. Now I shifted my focus and my search terms to look for stronger studs that would resist deflection. I ended up discovering studs from Marino Ware that were 20 gauge structural steel studs, and had the cutouts that would reduce heat loss if used on the front wall. Also, the frequent holes allow the shotcrete to pass thru reducing deflection and preventing the cold joint effect.
Anyway, back to the shotcrete guy. He is a busy guy and was somewhat hard to get a hold of. I guess that could be a good sign (I don’t want the contractor who is just sitting by the phone). That week, he was actually putting in the Beaver Habitat at the Detroit Zoo. Eventually he squeezed me in between a day at the zoo and an evening at a pool show. I just had 50 minutes to chat and show him the plans.
He seemed quite comfortable with the plan I presented, which included 20 gauge studs to frame the walls. He was also fine with shooting the basement from the outside, so I could have the nice planar specfinish over metal lath interior walls; as long as the slope of the excavation outside was greater than 45 degrees for at least 8 ft. We talked about the shooting tower, the bedrooms, the garage, the “light well” and the eyebrows. We talked about order of operations including how his guys would handle working without the floor in the main living area. We talked about storage for his equipment, as well as what equipment I would need to rent (articulating man lift, crane, etc.). I asked what other aspects of the job he would like to bid on and he said he was interested in all the steel frame work and the floors.
I asked him several times if any of this scared him, but he said it did not. Like me, he is an optimist with a “can do” attitude, so I am not really sure if he should be scared, but just doesn’t know it yet.
We also talked a bit about when we might start. I told him that I hoped for a June start, but that means the architect would need to be finished in the next 3 to 5 weeks (end of April). I emailed the architects an update and asked if they thought they would be ready, but they have not responded for over a week now. I will call them next week.
Since I finally had some final drawings for the ribs, I took another shot at getting quotes on getting them precast by professionals. The last time I had looked for quotes, one of the replies told me that they didn’t feel they had the right PCI Plant Certified or APA Plant certification. I used each of those terms as a Google search and found a lot more precast concrete companies that did carry that certification. While browsing thru these sites, I learned about the different finishes (I am interested in a sand blasted or acid etched finish), terminology, etc. With this new info, I sent off requests for quotes along with some details and drawings to half a dozen companies. One replied the next day and quoted me $4K per rib. I have 10 of them, so that adds up to more than I want to pay. The majority of the companies never bothered responding. Another company in IL called back to tell me that they wouldn’t ship that far. But a couple days later, I got a call from a company in Wisconsin that I had not contacted because I thought they were out of my area. Apparently, the one in IL had passed my info on to them. This Wisconsin company is very organized. They have called me a couple times to discuss finishes and immediately sent me a 7 lb box of concrete color samples (Sherri and I agreed on “sandstone”). However, it has been a couple weeks and I have not received a quote yet. I suspect that the shipping will be relatively high (~400 miles). Since doing that search (which didn’t get me a lot of call backs), I noticed that some of the companies were in the Architectural Precast Association, which gave me a bunch more companies to try eventually.
If I can get each rib for a reasonable price, I would rather pay someone else, but if not, I will make the forms and precast the ribs myself. I figure each rib requires less than $500 of concrete and rebar. Each form may use a few hundred dollars worth of supplies, but those can be reused. Check out this page for more info on that.
This house was framed with steel. Click on it for a closer look at the open web studs and joists.
This month I also spent some time at drywall suppliers… I was not looking for drywall, but just those steel studs. These out of the way suppliers have much better options and prices than Home Depo. Presumably, they save money by hiring unhelpful people and I faced the same sort of product ignorance and lack of interest I found in my other sourcing adventures. The guy behind the desk would tell me they didn’t sell something or such and such a product didn’t exist, so I would give him the part number to look up in his computer. One guy actually argued with me for a while before actually entering it. He told me it didn’t exist, but I noticed a poster of it on the wall. He didn’t even believe the poster. He just kept saying I could drive around back and look for myself. Eventually he entered it and I got my prices. Other guys just asked me to write it down and they would get back to me. Guess if they did?
While researching steel studs for the walls, I also got interested in steel studs for the floor joists over the basement area. The drawings specify 2×12 lumber, but I found I could also switch to wood I-beams (straighter and lighter or steel joists from Marino Ware. The 2×12 or wood I-beams cost about $1.50 per ft. The steel ones cost about $3.50/ft, but would never burn or rot. I only need 65 beams totaling 605 linear ft, so the price difference isn’t that much in the big scheme of things. The advantage of steel is that you tell them all the lengths up front, so they do the cutting for you and there is very little waste compared to buying a bunch of 16′ or 24′ wood joists. I am still thinking about it and will research more.
I also spent some time looking at spiral stairs and steel grates and a few other things…
Everyone’s favorite part of the site…
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Meiso No Mori in Japan… Designed by Toyo ito.
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Meiso No Mori in Japan… Designed by Toyo ito.
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Tama Library, also in Japan and by the same architect, Toyo Ito.
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I like these euro copper chimney pots and would be thrilled if my chimney ended up looking this good, but they sure are pricey…
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I just thought these were fun.
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I thought this “earth sheltered table” took it to another level with the water feature.
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I would love to get something cast like this… I am looking on craigs list.
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If we go with a metal lath grating across the hole in the storm room floor, there are many types of expanded metal lath available
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It occurred to me that a 6ft expanded metal lath patio table would fill the steel grate position nicely… And probably for a fraction of the price… I am not too serious yet, but give me a few months.
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A steel spiral staircase…
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Here is a copper trellis. I plan to use larger copper pipe, specifically 3/4 inch Copper pipe, TYPE L, Hard
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I liked this sun room…
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The architect drawing of the rib. Note the construction circles… The rib was designed to be drawn easily on the shop floor
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The engineers drawing of the rib showing the required reinforcing
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The process starts with a steel form. Steel is great when you want to pour a lot of arches, but I will make mine of less sturdy stuff.
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The steel form is placed over the rebar spine
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Assembly is fairly rapid and the bridge quickly takes shape.
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This house was framed with Steel studs
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The architects version of how the the ribs meet the central tower.
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The architects version of how the ribs meet the central tower.
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A rough visual test to show that they arches combined with the ribs to form a vault that expanded radially, but with a constant peak height.
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Crane for 600$/day
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An articulating boom lift will be very important to our construction site. Rental is $1850 per 4 weeks.
The architect drawing of the rib. Note the construction circles… The rib was designed to be drawn easily on the shop floor
This is a rough rendering of the inside (living room)…
My unusual umbrella design uses some “Euclidean egg” shaped ribs erected around a central tower. vaults will span between the ribs forming an umbrella like structure that will distribute the earth loads down to the ribs. These interesting architectural features allow me to have an open plan layout without needing to span the space in one large vault.
The idea is to precast these flat on the ground and erect them into place. If I can find a precast concrete company to do the work for me at a reasonable price, I will gladly pay. But I suspect it will be expensive and I will need to do this work myself.
The engineers drawing of the rib showing the required reinforcing
If I can order from an architectural precast company, I can relax knowing that they have lots of experience working with precast concrete. They know about the mixes and the finishes and they have all the equipment to cast the pieces and move them around, as well as the space to store them until I am ready to position them. We we are ready, they will deliver them at what ever rate we can handle assembling them (all on one day I would guess).
If I precast the ribs myself, the first concern is having the space to setup the molds. The ribs are heavy and cranes are expensive, so it would have been nice if I had the space to setup the mold, pour the rib and then just leave the >5000 lb rib right where it is sitting. I would move the mold to another spot on the floor, pour again, etc. Then I would only need to call the crane out for one day to erect and position all the ribs at once. I have a lot of ribs to make, so if I only used one mold to make them one at a time, it may take me 6 months while the rebar is placed and the concrete cures in sequence. I am thinking that I should probably make 3 or 4 molds to speed up (parallelize) the process. I could fill several molds at once from a single cement mixer delivery or I could be setting up the rebar in one while the previous one is curing. Laying on their sides, The ribs are only one ft deep, but cover an area about 13′ x 16′. I could fit only 4 at a time on my garage slab. I don’t plan to pour the floor for the rest of my house until much later, so I will probably need to move one set out of the way before I can pour the next. This may mean several additional crane rental days at probably close to $1000 per day.
I also must consider that if I do end up using my garage slab to pour the ribs, I won’t be able to erect the Quonset hut and close it in for storage purposes until all the ribs are done.
Unlike the bridge example below, my ribs will be architectural elements of my home. This keeps them in a a more gentle environment in terms of temperature swings and corrosive elements like salt or acid rain. However, the arches will be under much closer visual scrutiny by my guests (some of whom may be concerned about the tons of concrete and earth over their heads). I am sure any flaws will be even more visible to my critical eye. So the final finish and other small details will be important.
Visually, I am most concerned with the difference in the finish between the steel troweled side and the the other three sides that will be poured against the mold. Most precast architectural components are designed so that the steel troweled side is the “back” of the piece and never seen. Speaking to various experts, I am told that the difference will be minimized if I use a lighter and finer cement. I actually got some samples (shipped in a 7 lb box) from one of the architectural precast companies and Sherri and I compromised on “sand stone”. I also plan to sand blast the all sides of the final piece to remove that poured concrete look and simulate something that looks more like the texture of sand stone.
Structurally, I have the engineered drawings for the rebar spine that should be more than strong enough once the rib is in position and loaded as expected, but I am a bit concerned about the process of putting it into place. I will need some sort of rebar hooks in the spine for the crane to lift it by. I asked the engineer to position these, but he is under a fixed price contract and declined. he said that was something the precast shop should work out. I plan to cast #5 rebar loops right into the spine of the ribs which will complicate the mold a little so it can be demolded after the hooks are set in place. I have some ideas. ;^) The hooks will end up helping to tie the ribs into the vaults and will not be visible in the final home.
Crane for 600$/day
Placing the precast rebar will take a crane. I have already shopped around for crane rentals. I can rent one to use myself for about $600/day or $4000 for 4 weeks. I will also look for one that comes with a skilled operator. The cranes will lift 25 tones 45 feet into the air, so it shouldn’t have much problem positioning the ribs. I had hoped to use the Articulating Manlift that I will already be renting to help setup the rest of the arches and put the shotcrete crew in the right positions, but it will only lift 1/10th of what the ribs weigh.
I will illustrate how it was done on the Hang Tua Bridge in China. Sorry for the quality of the images, but the process was the best documented of any example I could find.
Like many other bridges, this one is made from precast open spandrel concrete arches.
The process starts with a steel form. Steel is great when you want to pour a lot of arches, but I will make mine of less sturdy stuff, most likely poly-carbonate and plywood. This form is very similar to mine, including the open spandrel.
Next, the rebar spine of the arch is wired together. This looks just like what the engineer specified for my ribs.
The steel form is placed over the rebar spine and bolted to the steel base of the mold. In my case, I plan to bring the form in several manageable sections and then cross brace to hold it all together.
Concrete is poured into the mold to encase the rebar spine. The rebar spine is lifted off the bottom with hooks so the concrete can wrap around all sides of it. Vibration is also used to ensure that the voids are removed from the concrete.
The concrete is steel troweled off and allowed to cure. My form would have a similar process, which means that the finish of the steel troweled side would look a bit different than the other 3 sides poured against the form. Unlike the bridge components, my arches will be right in my living room. I will use a light color and sandblast the final concrete to try and hide this difference.
After the forms are demolded. The rebar sticking out of the spine will help tie it into the deck above. It looks like these may have been added after the form was removed (drill and grout). Since my forms will come away horizontally instead of vertically, I can wire some directly to the rebar skeleton before I pour.
In this sequential process with a single steel form, cost is reduce but time to manufacture all the ribs is increased. The concrete arches are stacked to wait while the next one is being poured and cured. Concrete continues to gain strength even after it is removed from the mold, so this time is not wasted.
Finally, erection day comes around and they are delivered to the job site and a crane is used to carefully position the ribs.
Assembly is fairly rapid and the bridge quickly takes shape.
Rectilinear homes rely on the principles of the post and beam, but when the beam between two posts is curved horizontally, it results in torsion. the weight above the beam doesn’t just push the beam down, it applies a rotational force to twist it off its posts.
