[Cob] finding stats

[Shannon Dealy]

On Wed, 23 May 2007, claysandstraw wrote:

[snip]
> <snip: for structural calculations cob = adobe>
>>
>> While this may be fine for getting you past the code, it will in many if
>> not most cases cause you to do things which are structurally
> unnecessary.<snip>
>
> ...well the thing is that by the time your walls have enough mass you have
> far far exceeded the structural requirements for either cob or adobe.  So if
> the adobe standards are strong enough then why go any further for code
> purposes?

You missed the point, the adobe codes require that structural 
reinforcement be used which while it may be necessary for adobe to meet 
the structural requirements is not necessarily needed for cob to meet the 
structural code.

>> [snip]
>>> 1. structurally, bond beams are worth it
>>
>> I would say yes, if needed, in many cases where adobe needs it, cob would
>> unquestionably not need it.
>
> ... I think "unquestionably" is a bit stong.  I can tell you based on my own
> "shake the building test" that my cob columns were stonger after the bond
> beam went on than before.  I'm not saying that the building would have
> collapsed without a bond beam, I'm just saying it was clearly stonger once
> the bond beam was on. Now it has been well argued that cob can form a bond
> beam, from what I have seen this is true.  But 6" of concrete and steel will
> always be stronger than 12" of cob... in extreme circumstances of roof load,
> earthquakes etc. this *might* be an advisable compromise in the use of
> concrete.

Again, you missed the point, properly built cob IS unquestionably stronger 
than adobe, and therefore, there will be cases depending on structural 
needs (which are regional), where a cob wall would be sufficient without 
the bond beam where adobe would not.  I am in no way implying that a bond 
beam will not make it stronger, nor am I implying that a bond beam 
should not be used with cob (and I said so), but forcing everyone who 
builds with earth to use a bond beam just because one would be needed if 
the building was adobe, is a silly and unreasonable constraint.  The use 
of bond beams and other structural reinforcements should be based on the 
actual needs of the site and material used rather than blindly applying 
cement and steel to anything that looks like adobe.

The point here is that cob is not adobe.  Will applying the adobe code to 
cob work?  Yes, however, you MAY incur unnecessary structural and 
environmental costs due to overbuilding for blindly applying a code 
written for a different material.

>>> 2. the distance that headers bear on walls beyond the window opening is
>>> quite crucial
>>
>> Agreed, though that distance is in large part a function of the
>> compressive strength of the material, and again, cob's compressive
>> strength (as well as shear and tensile strength) is greater than adobe.
>
> ... I would consider the flex resistence of your header material (probably
> wood) as the limiting factor in any window opening scenario.  I have yet to
> see any headers crushing the wall below them, but I have seen several
> headers deflected from the load above.  When a header extendeds into the
> wall on either side: in order for the middle to bend downward the ends want
> to lift up... the further the ends extended into the wall on either side the
> more cob they have to lift before the middle can go down.

An interesting point, and certainly a consideration for lighter weight 
headers with heavy loads.  My headers generally don't see that much of a 
load (the wall doesn't go that much higher) and the headers are pretty 
heavy, so flex really isn't an issue.  Different designs may have more of 
an issue.

>>> 3. stem walls should rise above interior floors
>> [snip]
>>
>> This needs to be kept in context, the only reason I know of for
>> stem walls
>> to rise above the interior floors is to keep water away from the base of
>> the walls in the event of flooding due to a pipe breaking or some other
>> plumbing problem.  In a building that will never have plumbing, I know of
>> no reason to apply this rule.  There may be a good reason to do so, but I
>> have yet to hear it.
>
> ... well, first of all, how many cob buildings get a roof before their first
> rainstorm?  ... and (secondly) what happends in 50 years when the roof gets
> a leak and no one is around to fix it and water rains in?  or (thirdly) when
> the roof burns and the fire deparment comes and hoses the hole thing down
> and leaves 3" of standing water inside the house?
[snip]

Frankly, most people I know get something rigged over the building long 
before rain is a significant issue, I'm often surprised at how many 
people build a roof first.  I'll concede the fire department and roof 
leak, however, it's only an issue if it is left standing there for a 
prolonged period, which is highly unlikely with the fire department 
senario and not even particularly likely with the roof leak as both cases 
require large quantities of water and no one around to deal with the 
problem.  Of course it could happen.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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