Donald:
I agree with a lot of your points, but I disagree with a few of your conclusions.
The strength of any building material is always important. While snow loads can be significant. Most every climate and every geographic location has its own issues to deal with. Every building should be designed to withstand whatever forces it encounters. Hopefully any building is adequately strong that nobody has to "worry" about it, but that's a pointless semantic argument, hehe. Regardless of the wording. I think we always should be aware of what various materials are good at.
A beam on top of a wall serves several purposes.
1. It distributes all loads above the beam evenly throughout the wall. This is important because almost every house design has at least some uneven loads above a wall. There are the obvious, like an air conditioner mounted in the attic, or a partial 2nd floor. There are the more obscure, like wind loads being stronger on the upwind areas and weaker in the downwind areas. There are many possibilites. In any case, it is best to spread those loads out. This is especially true with papercrete. Papercrete can often flex a little. A large point load on top of a wall may tend to sink into a papercrete beam and create an uneven area everywhere above. This could easily cause trusses to break, and roofs to leak. In fact, if one part of a papercrete wall carries more load than another part of a papercrete wall, it could easily lead to cracking stucco and plaster (if stucco and plaster are the surface treatments. By spreading the loads out over an entire wall, everything reacts in unison. Keep in mind that roof trusses and roof rafters can be considered a bunch of individual point loads. Some roofing materials can actually be heavier than many snow loads. Concrete roofing tile may be the heaviest. Those beautiful ceramic roofing tiles that are popular in the Southwest and in Mexico aren't exactly lightweight either. Don't get me started about the constant fluctuatons that come from wind loads.
2. A beam on the top of a wall also acts as a wall stiffener. This acts to prevent a wall from bowing inward or outward. Again, this is another common cause of plaster and stucco cracking. It's typically not vertical loads that cause wall bowing, it's horizontal ones. You would be surprised how many tons of lateral pressure a very unassuming wall may need to resist.
3. The beam also acts as a strong attachment point for whatever structural members are above it. Roof trusses being the most obvious.
4. When built properly, the beam on top of the wall is directly tied down to the foundation. We shouldn't be counting on the tensile strength of the papercrete wall to hold things down. This concept has been discussed previously on Papercreters. I even got out my crayons to draw a rather ugly looking picture describing one method of attaching a wall beam to a foundation. (There certainly are other methods that what I described.) Here is one thread where we discussed it:
http://groups.yahoo.com/group/papercreters/message/2581
If you scroll down to the bottom of that page, you will see the responses people posted to that message.
When considering everything that a beam is asked to do. My personal opinion is that a papercrete beam simply isn't up to the challenge in practially every situation I can think of (at least in any structure of any size). There are better materials to perform as a beam. They need not be particularly expensive either.
--- In papercreters@yahoogroups.com, "donald1miller" <donald1miller@...> wrote:
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> I'm reading this discussion on tensile-compression strength with interest. It would seem that we don't have to worry too much about the compressability of a papercrete wall unless one lives in an area that gets a tremendous snow load and such an area might not be a good choice for pc except as infill and well protected from external elements as most areas that get a lot of snow also get a lot of rain in the spring and fall. The other force to deal with is the lift force. A conventionl framed 2X4 wall with a double top plate is not inherently strong but it combats the lift forces because it is anchored to the concrete footing. If a pc wall has rebar anchored in concrete say every 4 feet or so and with a bolt welded to the top of the rebar to attatch to a plate thus compressing the wall somewhat, it will not require a humongous plate to attatch the roof to as it will be anchored securely to the concrete.
> As for making a beam out of pc and when thorougly dry placing it on two blocks and standing in the center, that is not how a beam would be stressed on top of a wall. There would be a wall under the entire beam so it wouldn't break in the center as it would in the experiment. As for lift on the roof, I live in La Paz County Arizona [The Arizona Outback] and I am not too worried about snow loads and excessive rain. I deal with very severe winds and short, extreme rains in the summer. Most of the storms don't have much rain, only a lot of dust. I am going to do a tradional southwest adobe type structure with no roof overhang and canales to run off what water will fall on the roof.
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