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Rat Trap Bond walls in Adobe

 
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Hola,

I'm interested in using this bond for an adobe block home.

https://theconstructor.org/building/rat-trap-bond-construction-advantages-disadvantages/37334/#:~:text=Rat%20trap%20bond%20is%20a,as%20a%20Chinese%20brick%20bond.

Architect Laurie Baker invented the rat trap bond for the Indian subcontinent. There it's made with fired brick or laterite, and is claimed by Baker to have the same structural qualities as regular solid bonds. The advantage is it uses 25% fewer bricks and also provides trapped air for insulation.

My thought is that this system can fix the poor insulating qualities of adobe without any additional material. My plan currently is to build the roof with a Nubian Vault, so it'll be adobe bricks. But that requires quite massive sidewalls to bear the arch load.

Would an adobe rat trap wall have the load bearing capacity to bear the arch weight? How could I test this? Build a doghouse or some sort of scale model?

Also what size adobe brick? In Iran they make hollow walls with 20x20cm bricks, but only for internal non load bearing walls. I'm thinking the standard brick size 8x4x2 would work better than the large standard American bricks.

Also, I'll be building in Southern Arizona, climate zone 2, no freezing.

Thanks!
 
master pollinator
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Location: 5,000' 35.24N zone 7b Albuquerque, NM
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I enjoyed learning about "Rat Trap Bond" and seeing the animation about the method. In my world, the marvel of adobe is that the compressive strength of the walls sandwiched between heavy foundation and upper bond beam makes this building method accessible to regular folk using clay/sand soil with lots of wiggle room. Take away some of that density and lots of uncertainties emerge. The "Rat Trap Bond" would not work with silt, sand, clay, organic fiber adobe blocks commonly made in New Mexico. But something about the air space seems really interesting. I look forward to hearing from engineering experts who could explain the issues and possibilities.
 
pollinator
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Location: Bendigo , Australia
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Its a myth that adobe has poor insulating values.
The test for R values is designed to work with traditional insulating materials and fails with adobe.
Adobe works differently by holding and releasing heat to moderate room temperatures.
If you dont accept that point, then fit an external cover wall with battens, traditional insulation over the adobe, almost like a weatherboard structure.

I have built many homes in Australia with adobe, we call it mudbrick.
Depending how you make them the sizes are different
- wet mix 15 x 5 x 10 inches because a pair of feet in gumboots with fit in the mould for compaction.
- 8 x 4x 12 for a drier compressed earth brick sometimes referred to as CEB.
- Middle eastern ones are often smaller almost 4 x 4  x 6
Overall weight for handling purposes is something to consider.

Also think about building a roof initially held up by steel poles. It gives you a surface to work under in shade, you have a water catchment system and you can store things under it.
Finally you could live in it it hard pressed by wrapping hession or canvas around a few poles.
\Good luck with it all.
 
gardener
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I think the suitability of this bond will depend on the size of adobes and the way it is used. For vault support, I think it would be safer to modify the bond such that half of the tie bricks are partials: two short ones to make the surface pattern and one the length of the cavity and placed in line with the tie bricks above and below. In effect, this would be a solid wall with straight vertical cavities. You would get about 1/3 of the wall really solid top to bottom. It would be more work than the regular rat trap bond, but not more difficult, and would have the same amount of insulating cavity.
This would depend on the strength of the adobes compared to the bond between adobes and mortar. If the adobes are distinctly stronger than the joints, this might not work so well.

If you need greater strength to resist the horizontal thrust of a vault, you could double the rat trap pattern, with a central wythe that has tie bricks going alternately inward and outward, and the inner and outer wythes modified as described above. This would give a very thick wall, which would only make sense in a large structure where the wall bulk would be small in comparison to the interior volume. Apparently the maximum span for a standard Nubian vault is about 11 feet (3.3m), and the walls are supposed to be 60 cm thick (2 feet). https://www.lavoutenubienne.org/-technical-corpus-78- -> Basic Technical Rules. Using a single instance of the rat trap bond on walls this size would require adobes two feet long.

Another point, which I was considering mentioning even before seeing the pdf above, is that vaults like this do not just depend on geometry for stability, but mass in the walls. Medieval flying buttresses typically did not just arch up to the base of the main vaults, they had an extension on their top which weighted down the buttress arch and helped it resist sideways thrust. The Nubian vault requires the same, with heavy walls extended above the springing of the vault. Lowered mass in the walls might be counterproductive structurally.
 
Todd Fletcher
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Wow, what a great response, this place really does rock!

It sounds like the potential benefits in insulation are not needed in my climate, the thick solid walls doing the job and being structurally sound. I wouldn't consider it for the lower brick count alone, not worth the risk.

I've heard of double walled barns in Colorado, perhaps rat trap would have some use for those folks.

Yes, I've considered a metal roof shade! In my area metal roof 2 car carports are within my budget.

Basic plan is: take a year off, move to the land in camper while I build it all up. I have a couple of sons who can be badgered into helping. But it's at least a year or two off; I have not bought the land yet.

Thanks again for the super helpful response!

Todd
 
John C Daley
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I missed the note about the vault structure.
There is a recent comment elsewhere about it and I included some notes.
The walls do not need to be massive, the vault is shaped so only vertical loads are on the walls.
Earth structures, vaults design

basic-technical-rules
The load-bearing walls which support the vault are built of successive courses of large bricks and have a minimum thickness of 60 cm.
The courses are made up, in their width, of bricks laid alternatively lengthwise and width-wise, bound with earth mortar.
Joints between bricks must alternate from one course to the next to avoid any possibility of joints lying one above the other.

Various openings are made in load-bearing walls for doors and windows, and also for alcoves and cupboards to give extra storage space and to reduce the quantity of bricks and mortar
needed.
The gable walls are built with large bricks (35 - 40 cm long) which define the thickness of the wall. They must be built with a very slight inwards incline (of about 1 cm per metre of height). These openings are completed with lintels in the form of arches, formed over a barrel or an assembly of loose bricks.                              
The arches are made with small mud bricks and closed (in the case of alcoves or cupboards) by a 20 cm thick partition. The arches must be built before the start of the vault construction, and must follow the predicted slope of the vault.

The vaults are constructed without formwork or shuttering! They are built up with small (24cm x 12cm x 4cm ) mud bricks made from good quality earth, such as that used for granaries.  
These bricks are placed course after course against the gable walls to form a vault which is basically semi-circular, except for the final segment at the stop of the vault, which is slightly ogival, forming a strong catenary shape.
Once the vault is completed, the masons build 'buttresses' by raising the load-bearing walls by 8- 10 courses of large bricks, and filling the void created on the flanks of the vault with earth
mortar and broken bricks. This 'loading' of the vault must be executed with care, especially if a flat roof terrace or another storey is to be added on top.

The height of the 'buttresses' can vary according to the client's wishes, but must reach at least 2/3 of the height of the vault. The best solution is to raise them to the
height of the vault, thus forming a more or less flat roof terrace, because the more the loading on the vault, the stronger it is, and the flatter the roof, the lesser will be the effects of erosion during the annual rains.
 
Glenn Herbert
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It would be interesting to read the whole article, but it is behind a paywall. I have doubts about the idea that walls supporting a vault could have no sideways thrust; by definition, the lower edges of a vault must contain the load from the center, which can only travel through the line of the vaulting. The only exceptions would be if the vault were rigid, in which case it would be a bent beam and not a vault, or if there were a tie across the base of the vault which took the sideways thrust. Neither of those are practical in earthen masonry.
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