To the free folk of Middle Earth: Thank God the winter is almost over!
March is finally here and I can see the light at the end of the tunnel. I keep thinking of the time in Moria when Gandalf didn’t know which way to go. He restarted his journey with the Fellowship following his nose. The air seemed different in a certain direction. Sometimes during the winter months I get a little lost, if you will, but once March comes around I can see the light or smell the change of the new season coming. This particular winter wasn’t such a bad one for me though. But I digress.
I turned to a new page on my Hobbit calendar. It is the classic one inside of Bag End looking out of the front door, I believe by John Howe. This picture was one they tried to replicate in the movie and is remarkable for its architectural beauty.
Hobbit houses are beautiful but the fascinating thing about them is what you don’t see. There is much more to a Hobbit house than meets the eye . The engineering that goes into it is significant and was much more of a challenge than I had anticipated. The passive house aspects of the design created a different set of challenges to the structure that I usually mulled over on my commute to my job in the city every day. I came up with some great ideas while heading to work. Lets just stick with the structure today.
The structure. The Hobbit house I will be making is essentially an arched highway overpass. I wanted to make the span 32 feet across and about 46 feet deep. Now instead of cars going over this bridge we have dirt and snow. These loads are substantial. Soil weighs in at about 109 pounds per square foot (psf),concrete is 150 psf and the snow load I used was 70psf. I wanted to use a 9 inch concrete slab. When you add the numbers up you are talking about a structure that has to support almost 300 psf. That equates to a vertical load of almost 440,000 pounds! I don’t want to get too technical here but on a regular house the roof system is basically a triangle on top of two outside vertical walls.The vertical weight of the roof and the snow load come down on the roof and try to “flatten” it out,pushing the two outer walls away from each other. This force is resisted by the bottom chord of the triangle which are usually in the form of ceiling joists or collar ties. I did not want any collar ties in my Hobbit house. What I did was create a series of concrete buttress along the walls to resist the lateral force pushing the walls out. (You see buttresses on the outside walls of churches and cathedrals a lot) I also made the footings four feet wide to spread the vertical load on the soil. I wanted to keep the soil load under 3000 psf.
Here in New York you need a stamped set of drawings either by a professional engineer or architect in order to get a building permit. So I took my set of design drawings that I had completed and brought them to an engineer. There aren’t a lot of engineers out there and there aren’t a lot of Hobbit houses out there either. Their proposal was twofold. One , analyze the structure and see if it is safe. If that all checks out: redraw my design drawings and make any necessary changes to meet code.
Sort of as a side note. In my head I was always thinking that this type of house would be something that would last forever. Did you notice the house I demoed in the last post. Six years after the owner’s passing the house was ready to fall apart. I wanted to over design my hobbit house structurally so it would last 1000 years. That is one of the reasons why I chose heavier than needed snow and soil loads.
So I met with the design team and went over what I was doing and what my thought process was and how I came up with the design. One of the things that I didn’t really figure on was that this was going to be “their” design. In other words if the structure fails they will be liable. Which kind of gets back to what I said before about over designing the structure.
A few weeks after this meeting I got a call from the engineer working on it saying I should come in and that he didn’t think the structure was workable in this form. My heart sank. According to his calculations the horizontal force pushing the walls outward could not be overcome by the buttressess I had penciled in. They also said that because of the site conditions (the house being next to a stream) they were worried about the soil condtions where the footings would go. They wanted to have soil borings done to make sure that the soil could withstand the snow and soil loads that would be placed on the footings.
If I remember correctly I think he said that horizontal force was on the order of 10,000 pounds per linear foot of wall. He wanted to put collar ties in or supports in the center of the building to mitigate the loading. But I didn’t like this at all. We also tried larger buttresses but they were massive and complicated the building of the walls even further.
So the wheels started turning.I had to find another solution. I talked to my brother and he had a couple of good ideas. We are both civil engineers but wound up getting into NYC high rise concrete construction. Then an engineer who we know gave me the number of another engineer who he said is a concrete genius. I talked to the guys I had hired and they said they would have no problem working with someone else to come up with a solution.
This guy was unbelieveable. He came up with a solution within a week and that is what we did. Thanks Nat T! What he came up with is a tapered slab. The slab would start at 9 inches and thicken to 16 inches over the walls.The walls would also thicken to 16 inches. Originally I had 10 inch concrete walls. The intersection of the walls and arched roof would be considered a rigid frame. The only other thing I had to add was horizontal rebar through a 6 inch concrete floor slab which would be tied to the walls. This is essentially the collar ties but you won’t see them. This design eliminated the buttresses and will make building the footings and walls ten times easier. So the structure was really a collaborative effort and very satisfying.
By the time all this was said and done another almost 3 months had passed and it was nearly November! So I took a pass on getting my building permit and decided to wait until spring to start. That’s where I am now.
I did have soil borings done. Here’s a picture of the rig they used to do them.
The borings actually came back OK. The only problem was on one side of the house I had rock and the other was soil. The engineers were worried about a difference in settlement between the two. In other words, where the footings sat on rock there would be almost zero settling over time and on the side where the footings sat on soil there might be an inch or two of settlement. This could ultimately cause cracking in the roof which we obviously don’t want. The solution, a six foot wide footing on the soil side. It’s a little extra concrete but that will let us sleep better at night.
I’ve had a number of people ask about building a Hobbit house. It’s not to be taken lightly. There are some serious forces at work here that are considerable and could be dangerous if not considered by someone who is not trained in this kind of work. Those forces can change significantly depending upon the span and support system you or your engineer choose. The one thing that I like about my design is that it is a completely open floor plan. When the structure is finished you will basically be left with one gigantic room with no intermediate supports.This enables you to arrange the rooms however you want.
There were a number of other items that were problematic during the course of design but most of them would be hard to describe without sitting down with paper and pencil. So for now lets just call it a day. L ike I said there’s more to Hobbit houses than meets the eye.
Enjoy your weekend and week!