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re: supporting loads (was re: 2x4 joists)
16 oct 1996
paul valentine wrote:
>>why do people want to square the length in bending moment calculations?
>the length is squared in the bending moment formula if the load is
>input in load per unit length along the beam. if the total load on the
>beam is used then the length is not squared.
makes sense to me.
now, i wonder how much weight a 12' beam like this will hold up?
www s www s www
--> www s www s www <--3 1/2" drywall screws
www s www s www
www s www s www
--> www s www s www <--
www s www s www
^ ^
| | __ 2x4
|__ 1/16" steel, or perhaps a u-shaped piece of chicken wire,
hardware cloth, stucco mesh, etc...
harry parker's _simplified design of structural timber_, 2nd edition,
gives this procedure on pages 233-235:
1. assume fs = 20k psi for the steel and fw = 1200 psi for the wood.
2. find the section modulus of the wood sw = bh^2/6 = 9.1875 in^3.
3. find the section modulus of the steel ss = 0.255 in^3.
4. find the distributed load that the wood will carry.
m = wl/8 = 18 in-lb = fwsw, so w = 413 lb.
5. find the distributed load that the steel will carry.
m = wl/8 = 18 in-lb = fsss, so w = 284 lb.
6. add the 2 loads to find the 697 lb max load the flitched beam will carry.
perhaps 2 such collar beams could hold up 2 55 gallon drums full of water,
with a redundant u shaped rope sling around the rafters, and some glazing
to the south, and insulation overhead, and some foil underneath, using a
few stud cavities for the supply airpath, to make a gravity-feed solar
water heater like this:
........ .
drums . drum drum .
polyethylene .g b b . .collar beam. .
film sunspace --> . g foil . . solar air . .
. g polycarbonate . . heater . .
. g <-- single glazing . . . .
..................... ..............................
nick
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