Tagged with Structure

Steve’s Ingenious Dome Home

My parents sent me this link a few weeks ago, knowing I would appreciate it.  At first glance, it was interesting.  A guy built a pretty cool dome home (in Thailand) from cement blocks.  He only spent ~$6000 for the basic structure and got it done in about 6 weeks…  The home looks nice and organic, but it is not earth sheltered and it didn’t have to deal with our cold weather.  Not a lot of lessons that I could apply in North America…

But then I saw this…  And I realized that someone on the team, maybe Steve or one of the guys he was working with, was quite brilliant.

This pivot arm is key to the whole thing...

This pivot arm is key to the whole thing…

Instead of working with complicated geodesic shapes, he is just using regular concrete blocks. Before starting the construction, he placed 3 of these posts, each with a pivot arm.  Then he could just swing the arms to mark the footings.

StevesDomeHome_02

Then he placed each row of blocks using the end of the pivot arm to set them exactly to form a sphere.

StevesDomeHome_04

 

You can see more pics on his site, but eventually, he had several intersecting concrete domes.

StevesDomeHome_25

 

Then he just coated them, inside and out, with stucco for  a nice smooth finish.

Brilliantly simple.  I probably still can’t apply it directly to my build, but I sure do find it interesting.  Maybe I will experiment with building something else this way…  When I have time ;^)

And here is a video tour of the completed project… (and some practice embedding youtube video)

Basement?

Contents
  1. Basement under the home in the Earth?
  2. Count the cost
  3. Reality update
  4. Structural considerations

Basement under the home in the Earth?

Most earth sheltered homes do not have a basement…  This is mostly due to concerns about natural lighting, depth to the water table, etc.   Structurally speaking, two buried floors would experience a lot more lateral forces from the earth.  If you don’t want to rely on a sump pump, you also need to dig deeper drains, etc.

However, in my case, with a nice big hill of sandy loam soil that would have relatively low lateral forces and great drainage.  I also liked the idea of using QuadDeck ICF (Concrete) flooring that would act like a “shear floor” against lateral loading.  After not having the benefits of a basement in my current home, adding one seemed like a good idea to me.  I just had to work out the egress exit/natural lighting and figure out where to give up space for the stairs and I was ready to go.

For the stairs, I tried a number of locations before I figured that out.  I explained that process in a post a while back.

For the basement egress, since the basement was only on the North side, I had to put a window well on the north side.  That was the side I originally planned to bury, but since I had such nice views there, I had already relaxed on that and put in a few windows.  Now I would need to put the basement egress directly under one of those windows.  Rather than a small “vertical shaft” window well, I thought it may be more interesting to put in a larger conversation pit.  I could use the pit to get closer to the side of the hill and perhaps actually end up with an egress with a view, as well as a cool sheltered place to hang out.

Count the cost

While it is true that a basement is a relatively inexpensive way to gain square footage, mostly because it doesn’t need an additional roof, it does still need its own walls and floor and electrical and plumbing and that all adds up.   also, the suspended floor over the basement costs considerably more than the slab on grade floor that would be needed without the basement.

Eliminating the basement would also simplify the construction process starting with a much simpler excavation,  shallow drainage pipes, etc.

My specific design only called for a partial basement.  I hoped to limit the complexity, but because I have a sandy site, the engineer specified a slope of 1/2.  Meaning that my 10 ft deep basement will affect the construction for 20 ft around.  The design with the basement required more expensive step footings, taller stem walls, two levels of french drains, etc.

On the north side, the egress window was a challenge for earth sheltering the house because I needed to be a lot more careful about retaining the earth around it.   The plan looked good in 2D, but my 3D model revealed some concerns about the scale and cost of the retaining walls  that will be required to keep earth from spilling into the basement.

In my original gantt chart (building schedule), I planed to spend 1/5th of my costs and a month of my schedule on the basement…  Knowing that I wouldn’t have the option to come back and decide to add a basement later, and generally adding space to an earth shelteted home is difficult and because it would make a lot of my passive HVAC stuff work better, I decided to go for it.   

Reality update

The basement is in now and I can say that the costs were well estimated.  Shotcrete went considerably over estimates, but I saved money in other areas and ended up with a fairly affordable basement in the thirty-something-dollars per square ft range for a rough basement.  If I decide to plaster the walls or finish the floor, that will raise costs, but I don’t need to spend that money until I need the space.  At that point, it will seem like a bargain compared to adding space from scratch.

However, I didn’t factor in what a disruption it would be on the building site, and therefore to the building schedule.  The excavators and footings contractors didn’t like the step footings.  I checked with the building inspector and he didn’t like them either.  This meant that I was not able to do the footings all at once.  Keep in mind that the basement is only under a portion of the house.  I would need to backfill the basement before I could do the footings for anything else.  Before I could backfill, I needed the basement shotcrete, waterproofing, plumbing, septic field (and the trench to get there), etc.  Each of these things had delays, especially the septic field which slowed us down by a month due to a gravel shortage and trouble getting the septic tanks ordered.  By the time we got the footings in for the rest of the house, it was pretty much the end of the summer construction season.

The silver lining is that the basement was a bit of a trial run.  We got to see shotcrete applied in a less critical area of the home.  The resulting mess has lead to some adjustments in the plan for the main level.  If we had started with the main level, where the walls are higher and the shotcrete also needs to be applied overhead, it could have been much worse.

Structural considerations

With lateral loading on either side of a shear wall or shear floor, the connection across that support dramatically effects the deflection in the walls.    It is important that the basement wall acts like a single element from footing to the roof.  If there is a joint between the basement floor and the main floor, the shear floor between them will not be nearly as effective.

(I will come back another time with some illustrations)

Arch Forms

Rib_01As you may recall, my design features pre-cast concrete arches to support the heavy earth loads and let me have open spaces without requiring large spans.  I had some old posts about how these will be built, and even my own experiment to build quarter scale models.

I did get quotes on having these ribs done by professional concrete pre-casting companies. One even sent me nice faux stone concrete samples, but when the cost estimates came back, they ranged from $40K+shipping to $80K (with shipping) for the 10 ribs…  I thought that was ridiculous considering that each rib only used about 130$ worth of concrete and less than 200$ worth of rebar.  I asked the companies how many forms they would make and what they would make the forms out of… All three said that 10 was a small order, so they would just make 1 form out of wood…

Obviously, they were charging way too much and I was going to have to take this into my own hands…  I had designed the ribs to be cast easily in a 1ft deep form.  I could do this.

But first, I wanted to make a computer model to figure it all out.

My model revealed that the cost of reusable parts, assuming I went with a rather expensive Melamine base and 2 layers of 1/4 inch smooth plywood for the side walls, would be about $575.  I would probably make 2 in order to cut down on crane visits (the crane will have to come at least 5 times with 2 forms).   Then each rib would require about $350 worth of rebar, concrete, etc.  I also decided that I would need to buy a concrete polisher (wet) ($200) and a sawsall concrete vibrator attachment ($50).

Rib_00_PartsList

All told, that would mean about $1150 for the 2 forms, plus $3600 for the rib materials, plus $250 for tools, which gives less than $5000.  If I add 20% to cover misc, it comes to $6000.   I plan to work out a deal with the concrete company to rent their crane for a reasonable cost.  They currently use it to place pre-cast septic tanks which are about the same weight (35 cuft at 130 lbs each is 4550 lbs, plus the weight of the rebar).

Along the way, I thought about things like layout, materials, form removal, etc.  For instance, I plan to build these on the front half of the garage slab.  I will build the back half of the quonset for use as a shop, and then cover the front half with a large tarp to keep the rain out.  I can then remove the tarp so the crane can pick up the ribs more easily.  I plan to use some #4 rebar to create hooks on the top of the casting.  In order to remove the form later, I will need a slot in the form that I will plug with pieces of scrap insulation during the pour.  For a base, I plan to use melamine sheets that will provide a non-stick surface.  I will then need to polish both sides of the form to get a similar finish on the trowel finished side of the concrete.  I plan to build two forms, one left handed and one right handed…  There are a few different configurations to lay these out next to each other to minimize the space needed, I think I will go with a 24×24 layout that will require 5 sheets in the top layer, 6 in the middle layer and 1 in the bottom.

Here are some pics with a few more details…

Curved Walls

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

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.

TorsionRectilinear 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.

Dangerous-Curve-Ahead

March Update

Contents
  1. The plans…
  2. Sourcing
  3. Eye Candy

Lets review March…

The plans…

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.

Sunshade2

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.

I plan to "frame" the vertical portions of the walls with these metal studs with metal lath backing and rebar reinforcement, then I will get the shotcrete guys to encase it all in concrete...

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.

 

Sourcing

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 studs

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…

Eye Candy

Everyone’s favorite part of the site…