I found this design for a bridge and thought that it could be adapted to the form work of an earthen roof.
Good idea/ bad idea?
I’m thinking of constructing it on a small scale.
From what I understand about structures, it looks like this whole thing would lock together.
I’m thinking of constructing it out of treated gum pole. It would sit in an earth dug platform, so I’m guessing that the load from the roof would disperse into the earth on the sides.
I’m not sure of the size I’m going for yet, probably around 5 meters.
Final head height somewhere around 2.4 meter. Back of structure recessed into the soil 1.6-1.8 m with clerestory window for lighting, front will likely be mostly glass/ windows.
Has anyone tried anything similar, thinks that it won’t hold up structurally, or know if any other ways that this bridge design has been adapted?
Leigh Martin wrote:... I found this design for a bridge and thought that it could be adapted to the form work of an earthen roof.
Good idea/ bad idea?
I like where you're going with this, and can only respond with a partially qualified "I don't know". Meaning I am not an engineer but a few years back I found some software [doesn't seem to be available anymore and is copyrighted so I can't share it] and designed a gambrel-style roof with it which the building inspectors improved no-bid deal. Ordered the trusses and off we went, saving 30% of materials and increased the roof's loading factor by a similar margin.
There's also something called a "mansard" style roof that does the same thing on all four sides, plus geodesic domes, so I don't see the build difficulty being terrible. Anyway, dinking around with that software but not looking at the lock-in gravity bits, the numbers look good, provided I've guessed right on pole diameters, etc. you might choose to use. Wondering about the side anchoring, however. If this is to be an in-ground fixture, you might think about "footing the roof" separately from the below-ground walls, i.e., maybe have concrete or similar pillars that they are tied to but compatible with the overall design. That would take a whole lot less than full concrete footings, methinks.
Another thread, but at some point after I finish acquiring a new home but older home property (next month), I'm also going to be looking for a "permies" property to build a mini-park/community garden kind of thing, and can visualize where I'd use something like this both to teach a bit of STEM and just to show some cool design in what would basically be the exposed part of a "walk-in veggie cellar". This will be a fun experiment to add to my list of projects.
This is a fine line of thinking; but bear in mind that once the structure is up as a self-supporting structure, you'd still want to pin those logs together with some long screws, particularly if it's going to be a residence. The modern building codes require it due to earthquake induced collapse risks, even in many areas that don't have earthquakes. Of course, that's just in the United States. Your locality may vary. As a bridge, it would be an acceptable risk because the odds that anyone would be on the bridge during an earthquake are low. As a residence, I would imagine that wouldn't be an acceptable risk.
John C Daley wrote:
The important fact is that the abutments must not move.
John, could you elaborate on why you say the abutments must not move?
I can. If the abutments can shift under load, even a little, the self-locking nature of the logs can become un-jammed. If this happens, maybe the structure just finds a new stable state, or perhaps it becomes "fluid". If it becomes "fluid" anything can happen, including collapse while under load.
I once watched a television program about this type of bridge. In the program, it was stated that this was a Chinese invention from a dynasty of the distant past. Not surprising as similarly great engineering ideas arise spontaneously in different regions at different times in history. I like how this type of structure becomes more rigid as more load is brought to bear on it.
The stability of this design depends on the cross-pinning logs not being able to slip out of place, so if the parts are notched or otherwise locked together, it would not depend on rigid abutments. It is not an arch, even though it is arch-shaped.
Using this design as support for an earthen vault would likely provide such massive weight at the foot of the span that the ends of the structure would be well-supported and not easily moved, as long as there are good thrust blocks to spread the load of the post ends and keep them from punching into the subsoil. This does presume that the base of the arch shape ends at or below the exterior grade; if above grade, there would be great force trying to push the walls outward as the structure flexes a bit taking up the overburden load.
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