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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