[Strawbale] Box Beams (was re: Bales on edge and insulated structures)

[RT]

Derek Roff wrote:

> disadvantages of your three sided column approach. One is that a three  
> sided box column will be substantiallyless strong than a four sided one.


> <rikkinitzkin at earthlink.net> wrote:

>>  Box Beams  ... vertically as a structure work very easily
>>  I only   cover one side with OSB (9mm),
>>  a maximum of   2,5 meters between posts (which is the length of the  
>> OSB).

(for those who may have missed the original messages from which the above  
quotes were taken, see:
http://amper.ped.muni.cz/pipermail/strawbale/2010-October/002153.html  )

Just to clarify, "box beams" make optimal use of material by placing most  
of the lumber at a maximum distance from the neutral axis of the member  
cross section thereby maximising that section's moment of inertia and  
hence, its stiffness and bending resistance. Same principle as that behind  
steel "I" beams, open-webbed steel joists, drinking straws, plant stems  
etc)

(attempt at an ASCII sketch depicting cross section of a box beam since I  
don't know if the EuroSB list software can accept graphics )

		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
		web                    web
		web                    web
		web                    web
                 web                    web
		web                    web
		web                    web
		web                    web
		web                    web
		web                    web
		web                    web
                 web                    web
		web                    web
		web                    web
		web                    web
		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
		web FlangeFlangeFlange web
				

Since the depth of the box beam in this application (ie column in a bale  
wall) is limited by the breadth of the bale wall, in order to increase the  
strength of the box beam, the breadth of the flange material would need to  
be increased. ie from a 38 x 89 member (ie a 2x4) to a 38 x 140 member (ie  
a 2x6)
		

In order to provide the lateral support necessary to prevent the flanges  
of the box beam from buckling under load, the OSB or plywood webs  
connecting the two flanges need to be applied to both edges of the flanges.

The webs may not need to be continuous for the entire length of the column  
(will depend upon the specifics of the loading situtation).  ie. It may be  
possible to only have webs extending through the bale wall at every other  
course so as to not break a running bond pattern. Or single webs may be  
placed on alternate sides over the length of the flanges, again depending  
upon the specifics of the building.)

It should be obvious then, that in order to have box beams with OSB webs  
applied to only one edge of the flange, the flange would need to be  
rotated 90 degrees from that shown in the ASCII sketch above and in so  
doing reduces the amount of effective material in that flange (and hence  
the beam's strength) by ~ a half or more.

						*

One of the beauties of using box beams is that once can build clear-span  
beams of almost unlimited length on site.
ie The span is not a function of the length of the OSB webs (2440 mm) as  
is implied by the quoted material above).

That is to say, there is no reason that a box beam roof bearing assembly  
couldn't be made 12 metres (40 feet) long with supports provided only at  
the ends of the beam. (The beam would need to be over a metre deep though).


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  c a >
manually winnow the chaff from my edress if you hit "reply"
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