Re: [Strawbale] Green roofs on strawbale buildings

[RT <ArchiLogic@yahoo...>]

On Fri, 17 Sep 2010 14:37:51 -0400, Derek Roff <derek@unm...> wrote:
[snip]
> I know of two post and beam strawbale houses, where one or more of the  
> posts does not touch it's pier at the bottom.
[snip]
> OK, RT, tell me where I've gone wrong.


Sheesh, I come in to relax a spell with a nice cuppa tea and I find this  
message from that pesky Derelict *demanding* an answer.

I'm going to guess that the P&B SB home in the Derelict's example is

(1) relatively small (or if not, has relatively short-span joist or rafter  
bays and/or
(2) subject to relatively small live and dead loads

I think that even the most sceptical amongst us knows that reasonably  
dense straw bales (ie compressed to  120 kg/m^3 or more) provide  
sufficient compression resistance to demonstrate minimal to negligible  
deflection under the stresses imparted by the dead loads that are  
associated with modest-sized residential structures and in situations  
where the live loads are relatively small (say 2.4 kPa as a number pulled  
out of my hat, unsubstantiated by nothing more than my gut) in conjunction  
with those relatively short spans ... may very well exhibit minimal  
deflection as well -- initially.

But beyond certain stress levels (as can easily happen with the larger  
spans, larger building sizes and/or higher gravity loads) the bearing  
capacity of the straw will be exceeded no matter how well they were  
precompressed.

Any chart from compression tests of unplastered bales that show deflection  
values under incremental loads will show where that "tipping" point falls.

True, simply using wider bales thereby reducing stress values may work in  
some situations but in most scenarios space consumed by structure is not  
unlimited so excessively wide walls or grossly over-sized round bale  
"columns" are not a feasible option.

Now, I'm hoping that someone else here will comment upon Brian Waite's  
instinctual reservations against structural bale sandwiches (although I do  
agree with his preference for utilising door and window jambs as slender  
columns).

I don't have a problem with load-bearing SB walls. The problems arise when  
people can't get the roof onto those loadbearing bale walls quickly enough  
to avoid rain-wetting of the bales. Short of a temporary over-roof over  
the building site, few "rain management" schemes are actually as effective  
as hoped.

(Tea time is over.)

--
=== * ===
Rob Tom
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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