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GSBN: Digest for 7/9/07



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-> Re: GSBN:Cemento (combined responses)
     by Martin Hammer mfhammer@...
-> Re: GSBN:Cemento (combined responses)
     by Derek Roff derek@...
-> Re: GSBN:Cemento (combined responses)
     by Graeme North ecodesign@...


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Date: 9 Jul 2007 12:04:09 -0500
From: Martin Hammer mfhammer@...
Subject: Re: GSBN:Cemento (combined responses)

Rikki, did you ever imagine you would get such mileage out of your Cemento
subject? . . . .

Derk Roff wrote:
>> A cement grade beam on a rubble trench saves a lot of concrete, with
>> potentially superior performance.

Mark Piepkorn wrote:
>          To be fair, I guess we also have to say that a full-depth
> concrete foundation offers potentially superior performance to a
> rubble trench with a grade beam, too. It just depends.


I see pros/cons for rubble trench foundations compared to concrete
foundations as follows:

A. Advantages
1. Less cement used
2. Self draining (especially with an integral perforated pipe)
3. No additional excavation and materials needed for drainage
4. Potential use of salvaged material such as broken concrete, or of local
or site available rock
5. Less expensive (materials and labor) (depending on where rubble comes
from?)
6. Allows some flexible movement in an earthquake (?)

B. Disadvantages
1. Less bearing capacity (The loads are delivered to the ground through the
contact area between the rubble and the earth.  For a given foundation
width, there is less such area compared to a solid concrete foundation,
because of the voids in a rubble trench foundation.)
2. Allows too much movement in an earthquake (?)
3. Can't be used for piers in a pier and grade beam foundation (?)


Re: less bearing capacity in a rubble trench foundation, I estimate it has
70-85% the capacity of a solid concrete foundation. (With the same
foundation widths and soil bearing capacities.)  This is an educated guess
without rigorous analysis or calculation.  I've yet to see a published
number for this.  Has anyone else?  In most cases for SB buildings (and
other modestly sized buildings) I don't think the diminished capacity is a
problem, or you can always make the footing a little wider if needed.

Re: earthquakes, I list potential movement as an advantage and disadvantage.
Controlled or limited movement in an earthquake is good thing, to absorb
some of the energy with little or no damage.  But too much movement can, of
course, be a bad thing.  I wrestle with this important question.  I think it
depends on loads and the details of the wall, stem wall, and rubble trench.


Martin Hammer
California








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Date: 9 Jul 2007 14:17:50 -0500
From: Derek Roff derek@...
Subject: Re: GSBN:Cemento (combined responses)

Hi, Martin,

Thanks for your summation, with which I agree.  I would like to hear
more on one point.  You say:

> Re: less bearing capacity in a rubble trench foundation, I estimate
> it has 70-85% the capacity of a solid concrete foundation. (With the
> same foundation widths and soil bearing capacities.)

I am comfortable with your estimate, but I request that you say more
about the "same foundation widths" part.  In my limited experience,
foundation widths don't tend to be equal.  Soil bearing capacity is
almost always sufficient for a foundation width much narrower than the
bales.  So the architect and builder are faced with a dilemma.  A
concrete foundation as wide as the bales increases materials costs.  A
narrow foundation requires some additional detail to support the full
width of the bales.  I think the latter is the more common choice.

Widening a rubble trench foundation to the width of the bale will
usually incur a lower cost penalty.  Most of the rubble trench
foundations that I have seen for SB houses were made at the bale width
or wider.

My conclusion is that most rubble trench foundations for SB are wider
than concrete foundations for SB.  I wonder if your experience is
similar.  Using your 70-85% estimate, and the greater width of the
rubble trench, I'm thinking that the bearing capacities would be
similar.  Again, I welcome your comments.

Derek


- --On Monday, July 9, 2007 10:01 AM -0700 Martin Hammer
mfhammer@... wrote:

> Rikki, did you ever imagine you would get such mileage out of your
> Cemento subject? . . . .
>
> Derk Roff wrote:
>>> A cement grade beam on a rubble trench saves a lot of concrete, with
>>> potentially superior performance.
>
> Mark Piepkorn wrote:
>>          To be fair, I guess we also have to say that a full-depth
>> concrete foundation offers potentially superior performance to a
>> rubble trench with a grade beam, too. It just depends.
>
>
> I see pros/cons for rubble trench foundations compared to concrete
> foundations as follows:
>
> A. Advantages
> 1. Less cement used
> 2. Self draining (especially with an integral perforated pipe)
> 3. No additional excavation and materials needed for drainage
> 4. Potential use of salvaged material such as broken concrete, or of
> local or site available rock
> 5. Less expensive (materials and labor) (depending on where rubble
> comes from?)
> 6. Allows some flexible movement in an earthquake (?)
>
> B. Disadvantages
> 1. Less bearing capacity (The loads are delivered to the ground
> through the contact area between the rubble and the earth.  For a
> given foundation width, there is less such area compared to a solid
> concrete foundation, because of the voids in a rubble trench
> foundation.)
> 2. Allows too much movement in an earthquake (?)
> 3. Can't be used for piers in a pier and grade beam foundation (?)
>
>
> Re: less bearing capacity in a rubble trench foundation, I estimate
> it has 70-85% the capacity of a solid concrete foundation. (With the
> same foundation widths and soil bearing capacities.)  This is an
> educated guess without rigorous analysis or calculation.  I've yet to
> see a published number for this.  Has anyone else?  In most cases for
> SB buildings (and other modestly sized buildings) I don't think the
> diminished capacity is a problem, or you can always make the footing
> a little wider if needed.
>
> Re: earthquakes, I list potential movement as an advantage and
> disadvantage. Controlled or limited movement in an earthquake is good
> thing, to absorb some of the energy with little or no damage.  But
> too much movement can, of course, be a bad thing.  I wrestle with
> this important question.  I think it depends on loads and the details
> of the wall, stem wall, and rubble trench.
>
>
> Martin Hammer
> California
>
>
>
>
>
>
> ----
> For instructions on joining, leaving, or otherwise using the GSBN
> list, send email to GSBN@...HELP in the
> SUBJECT line.   ----
>



Derek Roff
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@...



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Date: 9 Jul 2007 18:08:37 -0500
From: Graeme North ecodesign@...
Subject: Re: GSBN:Cemento (combined responses)

I fully agree with the notion of regarding cement as a precious material to
be used with care and wisely -
Hence I regard it as being really dumb to use it in earth buildings as a
stabiliser to make what is effectively poor (I often use a different word)
concrete

I hate large foundation trenches filled with more of the material that
necessary where it is mostly used as for bulk stuffing, esp under wide straw
walls.
 
So back to rubble foundation trenches - has anyone any idea of how these may
be designed to work in a seismic area where the soils are often wet, and
where we need to keep the bottom of strawbale walls well above the ground
(say 300mm) to keep them dry?

As it happens I have been seriously asked to look into this,  this very
week, but most engineers here are stumped -

Here is a more thoughtful response than some - maybe there is some
information to be gathered from out there on this list?

I have seen rubble trenches a lot in books on eco building from the US.
I have no experience with them. I do not know of any recent buildings
that have rubble trench footings in this area. They have significant
advantages in areas subject to heavy frost which is not such an issue
here. With a rubble trench footing you generally need some kind of
concrete footing. I generally always recommend reinforced concrete
footings. A reinforced concrete foundation beam is required for the
earth walls to act as shear walls.  I am a great fan of the concrete
raft type footing and have  utilised them now in
several earth walled buildings.

Cheers,


Graeme,
Graeme North Architects,
49 Matthew Road,
RD1, Warkworth,
New Zealand 0981
Ph/fax +64 (0)9  4259305

www.ecodesign.co.nz



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