[GSBN] Steel mesh in clay plaster + earthquake safe fire shelters

[oasis at oasisdesign.net]

Hi, RT...

Thanks for raising these points. 

I'll respond in the same sequence:

1) high embodied energy/ greenness

There is no doubt steel is higher embodied energy *per pound.*  However, it only takes a few hundred pounds of wire to reinforce a 100,000 lb cob building. This reinforcement has a high likelhood of a) enabling the thing to get a permit and be built in the first place, as compared to some other, far higher embodied energy building, and b) keep the building from falling down and needing to be rebuilt after an earthquake, which takes quite a bit of resources. When the building does come down, the cob could be washed off and the steel recycled. Steel is one of the most recycled and recyclable materials of all industrial materials. 

Many "natural" buildings have an unnatural fraction, sometimes quite a large one. A steel reinforced concrete footing of the type often found below straw bale or sometimes cob buildings has a lot more weight of steel, not to mention the concrete. Relativistic arguments, I'll admit, but a helicopter to the store for milk it's not.

2) Cheese cutter effect

A myth with respect to dried cob with long straw and properly sized reinforcement, I believe (despite how wonderfully it works on wet clay in the ceramics shop).  What is properly sized reinforcement?  Still being worked out, but 1/8" wire mesh on 6" centers is a better fit to cob than say, 3/8" rebar on 18" centers. For plaster, with chopped straw, we've got this 1" square mesh with REALLY fine stainless wire that's OK..more ideal would be 1/2" chicken mesh with the same thin wire. It seems that the inclination of engineers is to go fat and far with reinforcement...recently heard about 5/8" bars on 4' centers. These are relatively likely to blow the wall apart.  

Check this crazy photo of a test brick not only being held together by 1/2" square hardware cloth wrapped around the top, bottom and ends, but still holding 500 lbs of load[1].  I don't know what  to call the opposite of the cheese cutter effect is, but the wire here is clearly holding all the pieces together rather than severing them apart. (here's a straw only reinforced brick in typical fracture[2] for comparison)

3) Bamboo/ organic reinforcement is better

The practice of ecological design essentially boils down to doing what makes sense in the context. In some contexts bamboo et al are better, in others some form of steel will yield the overall optimum resolution of all relevant considerations. I am not aiming to debunk straw reinforcement--on the contrary the ultimate goal we're aiming for is a menu of options along a continuum from most to least organic, from which the builder can choose the most organic option that can meet the structural requirements. 

Peace, 

Art

Quoting RT <archilogic at yahoo.ca>:

> I've no experience in building with earthen plasters but I must 
> confess that I did find the notion of high embodied-energy steel 
> reinforcement for earthen plasters, incongruous --not only because of 
> the obvious Greenie issues but because using 200 - 400 MPa steel 
> reinforcing with an earthen mix whose compression resistance might be 
> in the range of 20 - 40 kg/ cm^2 strikes me as being akin to to using 
> a Sikorsky Sea King helicopter to go to the corner store to get a bag 
> of milk.
>
> I would have thought that a reinforcement strategy would include 
> bamboo if only because it provides a larger cross-sectional area 
> against which the low-compression-resistance earthen plaster is 
> bearing. ie I would think that the relatively small diameter wire of 
> steel or plastic mesh would create a scenario not unlike a wire 
> cheese cutter going through a block of cheese, if push comes to shove 
> (as in a seismic event).
>
> I had a vague memory of reading about the U.S. Army doing some 
> studies with bamboo as an alternative to steel in reinforced concrete 
> in the '60s of the previous millennium and a quick Google turned up 
> the following document as one of the first hits:
>
> http://www.romanconcrete.com/docs/bamboo1966/BambooReinforcedConcreteFeb1966.htm
>
> In looking through the document, the following sentence in the intro 
> caught my eye:
>
> ============ copied material ==================
> " Bamboo was given recent consideration for use as reinforcement in 
> soil-cement pavement slabs in which the slabs behave inelastically 
> even under light loads"
> ============= end of copied material ==================
>
> The rest of document appears to contain plenty of useful, practical, 
> factual info. like:
>
> ======= more copied material ===============
>
> " When using whole culms, the top and bottom of the stems should be 
> alternated in every row and the nodes or collars, should be 
> staggered. This will insure a fairly uniform cross section of the 
> bamboo throughout the length of the member, and the wedging effect 
> obtained at the nodes will materially increase the bond between 
> concrete and bamboo."
>
> =========== end of more copied material ==========
>
>
>
> -- 
> === * ===
> Rob Tom                                        AOD257
> Kanata, Ontario, Canada
>
> < A r c h i L o g i c  at  Y a h o o  dot  c a  >
> (manually winnow the chaff from my edress if you hit "reply")
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Links:
------
[1] http://oasisdesign.net/shelter/cob/slideshow/pages/8088.htm
[2] http://oasisdesign.net/shelter/cob/slideshow/pages/8081.htm
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