I'm getting close to putting a bond beam on a cob structure I'm working on. In Earthbag Building (Pg 112-115) they talk about using strapping/banding on the last few courses in lieu of a concrete bond beam. Outside of the book I haven't found much on using strapping for bond beam. Does anyone have experience in this area?
Concrete or portland cement bond beams have several issues, leaching, efflorescence, rust jacking, fly ash or other pozzolans helps. Heavy weight at the top of low compression walls is not good. A lighter wood bond beam would be best especially if you are in a seismic zone above SDC "C", or high winds above 90 MPH gust.
You could strap the rafters to the walls, and to courses of blocks, to aide in reducing up lift forces. Someone is confused if they think straps replace bond beams. They serve different functions. BB distributes load to a wider area uniformly and reduces local rafter/joist pressures on the wall. Straps/Bands resist uplifting and/or lateral loads.
In Austin with high heat and humidity I would not use steel in contact with earth. Driving some FRPs (see below) into a wood BB would take out lateral loads, not rust jack, and have much better properties than steel rebar. For uplifting, if that is what you need, Simpson makes a hurricane ties that is galvanized steel that will eventually wear off the zinc coat. Zodiac in Houston can make some straps out of ECR fiberglass and has rebar of higher tensile. If you have a weak clay as binder use some type s lime at the wood BB to COB, the MGO in it will bond well to cellulose.
Thanks for the response Terry. I realize the mistake in my previous post. Earthbag Building mentions using poly strapping to affix a wooden bond beam to the top few courses rather than using a concrete bond beam.
I don't have any cob experience, but of the many historic adobe homes that I have worked on, not a single one was strapped at the bond beam. The wooden bond beams are usually 2-3" thick and mortared to the top of the wall with the same mortar as was used on the bricks. The rafters are then toe-nailed to the beam.
But, I've never worked in tornado country so you may need something like what Terry describes in windy Texas, especially if you use lightweight prefab trusses and OSB for your framing.
Done it with (and is Common in) strawbale construction.
Good ones are pricy. Harbor freight he a cheap one. You may be able to rent one for a day from a tool rental place. It's a great way to attach wooden bond beams/top plates. You may want to drill out and Chase some conduit through your walls for the strapping to slide through--depends on your cob mix but the straps might saw through your wall when you crank them tight. Poly seems as strong as the metal but easier to use.
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