[Strawbale] Green roofs on strawbale buildings
derek at unm...
Fri Sep 17 20:37:51 CEST 2010
While Rob Tom's proposed experiment is entertaining, evocative, and a
good illustration of why we shouldn't build a wall with uncompressed
broadloom carpet, I think there is some merit in looking at the
original question in light of both laboratory testing and case studies
of existing strawbale buildings. My observation is that the
loadbearing element of a plastered strawbale wall can vary somewhat.
I am a believer that the stiffest element of a composite will take the
loads when the force applied exceeds the capacity of the more resilient
elements. Bruce King's book "Design of Straw Bale Buildings" has
references to multiple structural tests that confirm this view. "The
Last Straw Journal" has published a number of these tests over the
I am also a believer that unplastered strawbales are subject to creep,
that is, that they will continue to compress under a static load over a
long period of time. Thus, one would expect that even if the plaster
skins are not strongly loaded initially, over time they would come to
take on more and more of the load, even without the dynamics of snow,
ice, and soggy green roofs.
On the other hand, if you visit a number of loadbearing strawbale
buildings, I suspect you will observe, as I have from time to time,
walls where there is a continuous gap between the plaster and the roof
bearing assembly (RBA), both inside and out. This gap exists, because
plaster shrinks as it dries/cures. Without special attention and
detailing, as RT recommends, wet-applied plaster is likely to recede
slightly from every plaster stop and edge, including the bottom plate
and RBA. It is fairly common, in my experience, that a continuous gap
is present a couple of years after construction and building occupation
in owner-built homes, and in some cases, the gap has been filled with a
non-bearing flexible caulk.
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. The post is suspended
from the beam, hanging with a small amount of clearance above the stone
that is supposed to be supporting it. My guess is that the beam shrunk
enough as it dried, that it lifted the post off of its support. Or it
may be that the pier settled, or both. One of these buildings shows
the continuous gap between the plaster and RBA described above, while
the other has a variable, non-continuous gap. So both of these
buildings in fact have load-bearing strawbale walls, even though roof
loads were intended to be carried by the post and beam frame. In the
case with the continuous gap, it appears that the bales themselves are
taking the load, while in the case with the variable gap, I suspect
that the plaster and the bales share the load.
This doesn't contradict Rob Tom's thesis that the stiffest elements
tend to take the load, but it does point out the difference between
strawbales and uncompressed broadloom carpet. Strawbales are pretty
dense and stiff, and are able to support some percentage of the roof
loads by themselves. Potentially 100%. There are historical records
of strawbale buildings remaining unplastered for over a decade of use.
We don't know how much creep the walls experienced, but not enough to
be commented on in the historical descriptions.
In plastered strawbale walls, the plaster may be taking all of the roof
loads, or it may be that the bales are taking all of the roof loads, or
each element may be taking some of the loads. As the loads increase,
and as time goes by, I would expect the plaster to take on an
ever-increasing proportion of the roof loads. I would also argue as RT
recommends, for careful plaster application and detailing, so that the
plaster takes the load as soon and as completely as possible. But I
think evidence is good that in a fair number of real-world cases, the
bales themselves may be taking a significant part of the roof loads
during some periods of time.
OK, RT, tell me where I've gone wrong.
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm...
--On Friday, September 17, 2010 11:38 AM -0400 RT <ArchiLogic at yahoo...>
>> Would you mind providing a reference for that? I have no trouble
>> believing that the stiff plaster takes dynamic loads, but I don't see
>> why it would take the static loads, given precompressed bales and all
>> static loads applied before the plaster is applied. So experimental
>> evidence would be very valuable.
> Just curious as to how you think a SB wall assembly differentiates
> between the dead and live components of the gravity loads to which
> it must respond and then proceeds to direct the straw portion of
> the wall to deal with only the dead load component of the applied
> loads ?.
> (And I don't think it's reasonable to assume that all of the dead
> loads will be in place at the time of plastering. ie think of
> multiple storey structures as a "for instance")
> To anyone who has any doubts about the harder/stiffer elements taking
> the loads I would suggest a simple demonstration they can do on
> their own to confirm:
> Find a piece of deep pile broadloom or carpet and place a few small
> stones into the carpet in a manner that the carpet strands stand
> proud of the small stones.
> Then place a chair or ladder next to the piece of carpet, take off
> your shoes and socks and climb up and then jump onto the
> stone-studded carpet.
> When you recover, comment as to which element (hard stone (analagous
> to the plaster in a SB wall assembly) or the compressible carpet
> (analagous to the straw) took the load .
> As to official in-lab test data, any of the compression resistance
> tests done on plastered wall panels (in North America, Europe, AUS
> or NZ) will provide the same "evidence".
> Those same tests will also show the importance of proper detailing of
> the plaster to deal with the expected failure modes (ie Euler
> buckling, localised crushing, delamination etc.)
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