[Strawbale] vaulted construction

[RT]

On Fri, 21 Aug 2009 15:29:19 -0400, <espacone at unich...> wrote:

> Mark Asheim in California did some seismic tests years ago. He had to
> build a vault for a private house. He is a professor somewhere in
> California.

>>   donald mchardy wrote:

>>>   I'm looking for some info on vaulted SB contruction.  I have some
>>>   experience in "conventional" SB (small, load-bearing) but would like
>>>   to learn something of vaulted construction (no "roof"?).

Perhaps the most useful bit of information about vaulted SBC that one  
should have under one's belt is that the two California houses built in  
this configuration back in the early days ... both required extended time  
frames and over-sized budgets to complete.

One of the major "challenges" that must be addressed is the fact that  
straw bales are not really a material that is suited to creating a pure  
compression structure.

While it is true that bales do have sufficient compression resistance for  
the axial loads to which a residential-scale wall is subjected in most  
situations, the difference between a vault and a wall is that there really  
isn't a point where bales will stop compressing in a vault configuration  
and since bales are not uniform from one bale to the next, the vault will  
not simply tighten-up as in a vault made using masonry materials. Sudden  
collapse of the vault will occur when weakest bale is distorted to the  
point where it folds in on itself, subsequently opening up on the bottom.

The early California vaults addressed this issue by using steel rebar to  
create what was essentially an exoskeleton.  One of them experienced a SB  
fire as a result of trying to weld the rebar exoskelton. (Later, YakWoman  
(aka Keller Lerner) substituted bamboo for the steel).

Another major challenge that needs to be addressed is that vaults are "all  
roof".

Simply plastering the bale vault and applying some coating to the plaster  
is not going to provide the weather resistance necessary to keep the straw  
dry and rot-free.

Pre-formed metal roofing (about the only real viable option (while  
acknowledging the existence of things like Kevlar membranes) will in most  
cases (ie unless custom formed) need to be laboriously clad with smaller  
and quite likely, site-formed, custom-made pieces if the curvature of the  
vault is anything beyond "gentle".

I would venture that the most prudent approach to SB vault construction  
would be to construct a fully-clad roof structure first and then build the  
bale vault underneath, utilising the structure of the over-roof to provide  
support (where necessary) for the bales.

The engineer (David Mar ?) for one of the California SB vaults (and who  
was given an eng assoc award by his peers for his design/analysis work  
that went into the SB vault) did a write-up for The Last Straw back then  
and it should be available from TLS. It may even be available on the  
contractor/builder's (Skillful Means) website.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  ChaffY a h o o  dot  C a >
(manually winnow the chaff from my edress in your reply)
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