[GSBN] Load-bearing straw bale idea

Derek Stearns Roff derek at unm.edu
Fri Jun 23 12:17:18 CDT 2017


I was wrong in my last message, saying the weight on a gable roof ridge beam is 1/2 the total weight of the roof.  I should have said the roof beam takes 2/3 of the weight.  (This is in a roof using simple rafters, as Rikki indicated, rather than trusses.)

Derek

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>




On Jun 23, 2017, at 11:09 AM, Derek UNM <derek at unm.edu<mailto:derek at unm.edu>> wrote:

Hi, Rikki,

It sounds like your architect plans what we call a ‘gable roof’ in the US.  I don’t know if anyone has tried the design shown in your architect’s drawing, but I would advise against it.  It asks bales to do what they are least good at- resisting point loads/forces.  A ridge beam in this kind of roof framing will bear about half the weight of the roof.  That isn’t too bad for a lightweight metal roof on a sunny day in summer.  When the roof is covered by two feet of snow, or clay tiles, or a couple of layers of asphalt shingles, a large amount of weight is concentrated on one spot in the top bale, and on the gap between two bales on the second row.

The thin wooden panels shown will help a little bit, but they, too, will flex under load.  When roofing loads are distributed evenly, I don’t think that ‘creep’, the slow deformation of bales under prolonged load, is a severe problem in most strawbale houses.  In this design, creep would probably be a factor, and the ridge beam could get a little lower each year.

If the pyramid of bales shown in the drawing was faced with plywood on the inner and outer faces, it would increase the resistance to deflection by about 1000 times, I’m guessing.  This would be a much more reliable method of transferring the roof loads to the wall, and then to the foundation.  It might be necessary to shave those bales down by the thickness of the plywood, to get the dimensions that you want from the bale and plywood composite.

Other architects and engineers on this list may have better ideas.  This is the best approach that I have thought of, which remains very similar to the architect’s drawing.

Best wishes,
Derek

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>




On Jun 23, 2017, at 9:06 AM, Rikki Nitzkin <rikkinitzkin at gmail.com<mailto:rikkinitzkin at gmail.com>> wrote:

Hi all,

I forward a petition from a Spanish architect who wants to know if his idea has been tried, and if it worked.

He is planning to build a Load-bearing SB home. The roof will have two “waters” (what is that called in english?)- creating a triangular gap on both sides of the roof.

He wants to try to put the main beam (Spanish architecture doesn’t use triangular rafters, but a main roof beam, with the other beams resting on it) directly on the bales. His idea is to put a wooden panel on top of each row that is above the roof-plate, until reaching the main beam- using very well compressed bales.

Has anyone tried this?

Here is a drawing of the idea:

<hastial Alejandro.jpeg>
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