[lgalla]

Hey guys good answers.What I was trying to figure out was how to use E/G in a gantry situation with steel for ease of mounting.Appears E/G/steel tube is too heavy for itself.May have to incorporate or mold in steel plates where necessary.
Hollow cross sections of E/G is interesting,but probably difficult to calculate its ultimate strength.Those huge E/G machines we have seen pictures of are probably hollow on the gantry to reduce deflection.Maybee puting a cheap 2"plastic pipe in the centre of the 4"tube and filling around it would help.
What do you think Zumba?
Larry

[jsage]

Brunog,

I was thinking the same thing on the rebar reinforced ceiling beam. How could this really help. However, what is being compensated for is a force in one direction. It's a cure for that one force. The top side of the beam is naturally in compression, causing the low side to be in tension. Over time this unbalanced state causes the unsupported section to creep down.

If the rebar was free floating in a drilled hole it would only add the stiffness of the rebar. However since it is attached to the beam using epoxy it acts as a cable strung tight along the bottom. Since rebar is very resistant to stretch it offsets any creep (if that is right usage). For a small increase in weight it adds a mechanical property that wood wouldn't have high tensile.

The net effect in my IMVHO is that you get sufficient increase in rigidity to offset woods characteristics alone over long spans without having to add column support which would rob space.

Anyway, I'd like to find the original basis of this article. I think the same guy built my garage ; )

[Zumba]

Maybee puting a cheap 2"plastic pipe in the centre of the 4"tube and filling around it would help.
What do you think Zumba?
Larry

Depends on the size of the beam to begin with. If we're talking about the 4" x 4" tube as previously discussed, then the E/G fills up a 3.5x3.5 cavity. Sticking a 2" pipe in the center would only reduce volume by 33% (for a square pipe) or 25% (for a round pipe).

The 4" x 4" x .25" x 48" steel tube weighs about 56 lbs
The 3.5" x 35" x 48" E/G fill weighs about 59 lbs

So we're looking at saving a max of 20 lbs on a 120 lb beam... not much. You can use that cavity for routing cables, however. And that tiny reduction in E/G will hardly affect rigidity.

Bigger beams, however, can definitely benefit much more from a partial E/G fill. I think the key is reduce the steel thickness, e.g. rather than 1/2" steel, get it in 1/4" to cut steel weight in half. Then, do a partial fill with E/G such that it's about 1" thick all around. The net result is a tube of nearly equivalent weight and rigidity of a 1/2" thick steel tube, but with drastically enhanced vibration damping characteristics.