Cob: Re: Cob & Bool Re: stones in walls:

[otherfish]

To all,

I sent this yesterday intending it to go to the whole list, but apparently
hit the wrong send button.  It's in reply to an earlier response from
Lootvik.

.....................
Hi Lootvik,

Here is some data re seismic performance of adobe walls.  I think it is
reasonable, in the absence of similar data specific to cob, to use this
adobe data towards a theory of how cob walls may perform in an earthquake.

.............
In 1995 The Getty Conservation Institute issued the report: ³Seismic
stabilization of historic adobe buildings².  In a nutshell, the report gives
us the following knowledge about adobe walls:
adobe walls will crack in a severe earthquake;
the wall height-to-thickness ratio effects wall performance;
a ratio of<4 will remain standing even if cracked by an earthquake;
a ratio of 4-6 (typical of historic adobe buildings) will usually remain
standing if cracked;
a ratio of 6-9 lessens in stability as the ratio increases and is likely to
collapse in strong earthquakes;
a ratio of 9-12 is likely to be unstable and collapse;
a ratio of>12 is unstable;
walls supporting a load from above are more stable than those not supporting
a load.
The stability methods tested in the Getty report and their earthquake
severity performance results are (in a range of 1  to 7 as  moderate to
severe): 
a stiff bond beam is effective if it has a positive connection to the wall
top (1-4);
a flexible bond beam performs better than a stiff bond beam (2-5);
horizontal straps along the walls act effectively to restrain the wall once
it cracks (3-6);
a flexible bond beam with vertical ties performs best (1-7).
Of the model structures which were shake table tested, the best performing
stability methods were a roof diphragm combined with either a flexible bond
beam with center wall ties, or a flexible bond beam with vertical and
horizontal straps on the outside of the walls.  These methods show a
survival potential of almost 3 times that required by the UBC.  This is
promising indeed.
................

The comments / supposition I made in my previous post about Bool cob walls
are based on this Getty Report info.  The 3 - 5 : 1 ratio range was from my
memory of this data & it seems as tho I did'nt remember exactly what the
report said.  If I recall correctly, adobe buildings with a 2 - 3 : 1 wall
height to thickness were the ones that historically have survived
earthquakes without significant damage.

As to your question of how abode wall quality, & via extrapolation, cob wall
quality will effect this I can only surmise.  I'll make a logical leap & say
YES, quality of construction will effect the seismic resistance, but to what
extent, your guess is as good as mine.

The point I was wanting to make is that if one choses to use the Bool
technique, & place rocks in a cob wall, prudence says to be sure to space
the rocks out so as to not have less than the 4 - 5 : 1 height to thickness
ratio of continuous cob between any two stones.  By this technique, the
spacing distiance between stones should be determined by the wall height.
To have less cob between the stones will reduce the ratio & thus possibly
compromise the seismic strength of the wall.

Hope this answeres your question.

Regards,
john fordice
TCCP 


on 1/24/03 11:21 AM, ~Lootvik~ at lootvik at usermail.com wrote:

> If you're drawing a parallel with concrete walls, you're probably right
> John.  I know that engineers freak out when you put big rocks in a
> structural concrete wall.  What is the tensile strength of cob?  Most
> likely it varies a lot.  I think the embedded rocks are a great idea.  Like
> if you go without a plinth, they would shield from backsplash along the
> base.  Or above the plinth or just for looks.  I'd worry a lot less with
> rough-surfaced rock, than say river rock, which wouldn't bond as well.
> 
> That 3-5 ratio is a mighty big range.  Is the 3 for building with "bad"
> adobe cob re etc.?
> 
> otherfish wrote:
>> Phil & Charmaine,
>> Altho I don't know zip about Bool wall construction, I feel that a bit of
>> logic will help in deciding how to do it.  It's pretty clear that the
>> strength of cob ( and other forms of earthen construction ) derives from
>> being built as massive walls.  Thaditional cob walls were THICK.  This is
>> true of adobe also. Tested data of adobe walls shows that a minimum
>> thickness to height ratio of 3 to 5 is fairly stable in seismic conditions
>> if there are no other stability measures included in the wall construction.
>> This means that you want the wall to have a proportional thickness of 1 foot
>> for each 5 feet of height.  Given this as the proportion of thickness (mass)
>> of wall cob that is stable, it stands to reason that if you place rocks
>> within a cob wall, it will be in keeping with proven stability proportions
>> if no cob part of the wall is less than this same thickness.  So if your
>> wall is 5 feet high, don't have less than 1 foot of continuous cob BETWEEN
>> any stones you place in the wall.  As your wall increases in height keep
>> this same proportion of non disrupted cob between stones.
>> 
>> This is supposition & not proven, but to me stands to reason.
>> 
>> john fordice
>> TCCP
> 
> 

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<TITLE>Re: Cob & Bool Re: stones in walls:</TITLE>
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<TT>To all,<BR>
<BR>
I sent this yesterday intending it to go to the whole list, but apparently hit the wrong send button.  It's in reply to an earlier response from Lootvik.<BR>
<BR>
.....................<BR>
Hi Lootvik,<BR>
<BR>
Here is some data re seismic performance of adobe walls.  I think it is reasonable, in the absence of similar data specific to cob, to use this adobe data towards a theory of how cob walls may perform in an earthquake.<BR>
<BR>
.............<BR>
In 1995 The Getty Conservation Institute issued the report: ³Seismic stabilization of historic adobe buildings².  In a nutshell, the report gives us the following knowledge about adobe walls: <BR>
adobe walls will crack in a severe earthquake;  <BR>
the wall height-to-thickness ratio effects wall performance;<BR>
a ratio of<4 will remain standing even if cracked by an earthquake; <BR>
 a ratio of 4-6 (typical of historic adobe buildings) will usually remain standing if cracked; <BR>
a ratio of 6-9 lessens in stability as the ratio increases and is likely to collapse in strong earthquakes;<BR>
a ratio of 9-12 is likely to be unstable and collapse;<BR>
a ratio of>12 is unstable;<BR>
walls supporting a load from above are more stable than those not supporting a load.<BR>
The stability methods tested in the Getty report and their earthquake severity performance results are (in a range of 1  to 7 as  moderate to severe): <BR>
a stiff bond beam is effective if it has a positive connection to the wall top (1-4);<BR>
a flexible bond beam performs better than a stiff bond beam (2-5);<BR>
horizontal straps along the walls act effectively to restrain the wall once it cracks (3-6);<BR>
a flexible bond beam with vertical ties performs best (1-7).<BR>
Of the model structures which were shake table tested, the best performing stability methods were a roof diphragm combined with either a flexible bond beam with center wall ties, or a flexible bond beam with vertical and horizontal straps on the outside of the walls.  These methods show a survival potential of almost 3 times that required by the UBC.  This is promising indeed.<BR>
................<BR>
<BR>
The comments / supposition I made in my previous post about Bool cob walls are based on this Getty Report info.  The 3 - 5 : 1 ratio range was from my memory of this data & it seems as tho I did'nt remember exactly what the report said.  If I recall correctly, adobe buildings with a 2 - 3 : 1 wall height to thickness were the ones that historically have survived earthquakes without significant damage.<BR>
<BR>
As to your question of how abode wall quality, & via extrapolation, cob wall quality will effect this I can only surmise.  I'll make a logical leap & say YES, quality of construction will effect the seismic resistance, but to what extent, your guess is as good as mine.<BR>
<BR>
The point I was wanting to make is that if one choses to use the Bool technique, & place rocks in a cob wall, prudence says to be sure to space the rocks out so as to not have less than the 4 - 5 : 1 height to thickness ratio of continuous cob between any two stones.  By this technique, the spacing distiance between stones should be determined by the wall height.  To have less cob between the stones will reduce the ratio & thus possibly compromise the seismic strength of the wall.<BR>
<BR>
Hope this answeres your question.<BR>
<BR>
Regards,<BR>
john fordice<BR>
TCCP <BR>
<BR>
<BR>
on 1/24/03 11:21 AM, ~Lootvik~ at <FONT COLOR="#0000FF"><U>lootvik at usermail.com</U></FONT> wrote:<BR>
<BR>
> If you're drawing a parallel with concrete walls, you're probably right <BR>
> John.  I know that engineers freak out when you put big rocks in a <BR>
> structural concrete wall.  What is the tensile strength of cob?  Most <BR>
> likely it varies a lot.  I think the embedded rocks are a great idea.  Like <BR>
> if you go without a plinth, they would shield from backsplash along the <BR>
> base.  Or above the plinth or just for looks.  I'd worry a lot less with <BR>
> rough-surfaced rock, than say river rock, which wouldn't bond as well.<BR>
> <BR>
> That 3-5 ratio is a mighty big range.  Is the 3 for building with "bad" <BR>
> adobe cob re etc.?<BR>
> <BR>
> otherfish wrote:<BR>
>> Phil & Charmaine,<BR>
>> Altho I don't know zip about Bool wall construction, I feel that a bit of<BR>
>> logic will help in deciding how to do it.  It's pretty clear that the<BR>
>> strength of cob ( and other forms of earthen construction ) derives from<BR>
>> being built as massive walls.  Thaditional cob walls were THICK.  This is<BR>
>> true of adobe also. Tested data of adobe walls shows that a minimum<BR>
>> thickness to height ratio of 3 to 5 is fairly stable in seismic conditions<BR>
>> if there are no other stability measures included in the wall construction.<BR>
>> This means that you want the wall to have a proportional thickness of 1 foot<BR>
>> for each 5 feet of height.  Given this as the proportion of thickness (mass)<BR>
>> of wall cob that is stable, it stands to reason that if you place rocks<BR>
>> within a cob wall, it will be in keeping with proven stability proportions<BR>
>> if no cob part of the wall is less than this same thickness.  So if your<BR>
>> wall is 5 feet high, don't have less than 1 foot of continuous cob BETWEEN<BR>
>> any stones you place in the wall.  As your wall increases in height keep<BR>
>> this same proportion of non disrupted cob between stones.<BR>
>> <BR>
>> This is supposition & not proven, but to me stands to reason.<BR>
>> <BR>
>> john fordice<BR>
>> TCCP<BR>
> <BR>
> <BR>
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