Sharing information and promoting earthbag building
|Making Bond Beams for Earthbag Buildings
Questions answered by Kelly Hart
Q: What is a circular bond beam? What is that made of and how does it integrate into the bags?
A: Bond beams can be created in a variety of ways, but probably to best with a circular structure would be to form a trough at least 6" deep and as wide as the bags on top of them, suspend two rings of 1/2" rebar running down the middle of it, and then pour this full of concrete. Periodic rebar pins pounded down into the bags, with their tops bent over in such a way that they will also become imbedded in the concrete, will connect this bond beam with the circular wall.
Q: How deep would you pour the concretet in the bond beam for the top of a garden wall? I'd like to keep it to a minimum as we're on a tight budget.
A: Bond beams are usually between 4 and 6 inches deep, but for a wall of this sort I would suggest the lower figure since it will not be bearing any weight.
Q: Do you use any kind of bond beam if you are only building walls with the earthbags? How would a roof be attached so that it is not lifted in high winds? (We do get tornados and hurricanes here fairly often)
A: I didn't use any bond beams in my house, but then I built domes. There are several different methods of attaching a standard framed roof to an earthbag wall. This can be accomplished with a reinforced concrete bond beam and tie bolts, or simply using a wooden top plate that is strapped to the earthbags several courses down. I think this is an excellent way to rigidify and stabilize the wall and provide a good, level base for the roof structure.
Q: My intention was to pour a bond beam at the 8'6" height and then proceed to pour the second floor. Is the bond beam really necessary or will the pour be my bond beam?
A: You should be able to create a single monolithic poured second floor with the function of a bond beam, as long as it is adequately pinned into the bag wall and reinforced with rebar running along the wall.
Q: I've decided to build a small earth bermed dome (14-16ft diameter) as an experiment prior to starting on our full size home here in Texas. Other than experimentation with the technique itself, the dome will provide shelter while we're working on the other house (we might even decide it's adequate as is and not build any bigger.) I've been looking at the pictures of the interior of your pantry, thinking of using a similar manner of construction. Did you use any form of bond beam or cable to secure the bottom ends of the beams before stacking the bags on the "roof"?
A: No, actually I did not; those beams were just "toenailed" into the bags with some long rebar spikes. For a larger building, or for more weight put on top than I did, a bond beam and/or cable might well be a good idea.
Q: I'm about to build a superadobe roundhouse for my mother in a rural/slum area in the Philippines. This is my first project, and I have most things planned and all set for building next week, except for the bond beam. Any suggestions or simpler alternatives to a reinforced concrete bond beam for the roundhouse?
A: For a roundhouse it is hard to imagine what could be used to create an adequate bond beam at the top of the wall other than reinforced concrete. Most other options generally involve sections of straight wood or steel which obviously won't work with a curved wall. I suppose than one could create sort of polygon with short sections of these materials, but that could be a lot of work and would not ultimately be as strong as reinforced concrete.
Q: How do I connect the roof supporting structure to the earthbag walls? can I drive the rebar staves directly through the wooden beams and into the earthbags? (and would they not damage the earthbags or cause ruptures in them?)
A: If you pre-drill the wooden beams to receive the rebar it is usually fairly easy to pound the rebar through the bags without real damage to the wall.
Q: If we use a poured concrete bond beam, how is the roof assembly anchored into it?
A: The most secure attachment method would be to embed J-bolts into the concrete, and then use these to anchor the roof assembly.
Q: How deep should nails be to secure additional steel tubes next to the steel bond beam for a lintel? Is there a minimum depth into the earthbag nails should penetrate?
A: I think that 2 to 3 inches should be adequate for this.
Q: I'm building an earthbag structure and am concerned about the size and stability of one of the walls. The wall is 30' long. The building is 14' high on one side and 8' on the other. This wall is straight and I'm trying to avoid using buttresses on this wall. Is it necessary to use buttresses every 10', rebar every 4-6' AND a concrete bond beam?
A: On a wall like you describe I would suggest using all of the precautions that you outline, especially since it is a rather high wall. I suggest placing the bond beam at the 8' height all around, and then stack more bags above that for the rest of the wall.
Would you then suggest that I attach the roof to the bags via cleats?
I suggest making another bond beam at the top of the tall wall to connect your rafters to.
How do I connect the upper stakes to the bond beam at 8'? Specifically, the bond beam needs to dry before stacking on top of it, so how would I pin it to the layers above?
A: You can leave protruding pins embedded in the bond beam that will pierce the bags laid over them.
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