[walter]

You must keep in mind that the beam will deflect under its own weight. Hollow centers are a good idea.

Steel tubing..?

[ger21]

Here's an article about stiffening a wood beam using epoxy and steel rebar. Seems like it has some relevance.
http://www.epoxyworks.com/17/beam.html

[lgalla]

Walter
Here is a good question for the engineering members.
If you fill say for example an A-36 4"X4"X1/4" tube withE/G,Will it be stiffer or will the added weight cause more deflection.
Larry

[walter]

If you fill say for example an 4" x 4" x 1/4" tube with E/G....

How about encapsulating the tube in E/G? (hollow center like Zumba suggested)


EDIT:
Check this out>

[jsage]

It's sounds to me there are some trade-offs. Dampening and structural. Length and design.

Not to throw another wrench but complementary structural qualities of the composite versus dissimilarity. More of a percieved conclusion than a tested one but if the reinforcing doesn't complement the base material then it doesn't really work.

At first glance I would wonder if rebar would help in reinforcing the wood. However since it is encapsulated in epoxy with microfillers then instead of acting as merely as rebar it keeps the wood from warping by tension rather than stiffness.

So now I think I understand why rebar helps. It complements through tensile? strength where concrete would fail quickly and E/G would be effected at somewhat higher level of continuous stresses on long spans.

[Zumba]

Walter
Here is a good question for the engineering members.
If you fill say for example an A-36 4"X4"X1/4" tube withE/G,Will it be stiffer or will the added weight cause more deflection.
Larry

Yes and yes.

It will be stiffer in that it will be less prone to deflection under cutting forces.

However, it will also deflect more while the machine is at rest, under gravitational forces. But we're talking tenths at the most. A 4"x4"x.25" x 48"L steel tube deflects .00024" under its own weight in a horizontal position (load at the center of the beam). A 3.5" x 3.5" x 48" solid E/G bar deflects .00033" under its own weight. Add the two together and you end up with something in the middle.

However, the important thing here is the 48"L. In the beam deflection equations, the length is cubed. So double your length, and you increase your deflection by 8 times. The weight of the E/G becomes an issue at this point.

So let's say you want a 16 foot beam. You can't use a 4" x 4" cross section for this. Without center support, this sucker will deflect over SIXTY FOUR TIMES as much as the 4-foot beam. That's around 0.019. That's with NO LOAD!

EDIT: actually that's an underestimate... I increased the length but didn't take into account the 4x weight increase! So we're talking more like .080.

So to fight the gravitational force, you'll have to increase the height of your beam. 8" high will decrease deflection to about 0.010 (and I again, I haven't taken into account the the added weight of the E/G). Still not great. I'd recommend adding at least one center support, preferably more. Either that, or go real big with your tubes.

And don't fill up completely with E/G unless you have to!

[ger21]

Larry, can I ask you a question...

How would you stiffen the 16' beam?

Why do you need a 16' unsupported beam? Can you fabricate it as a truss like structure. Possibly fill it with your EG as individual pieces, but allow it to be bolted and bonded together after filling, if that makes it easier.

[martinw]

How about encapsulating the tube in E/G? (hollow center like Zumba suggested)


EDIT:
Check this out>

Dear Walter,

With hollow core precast RC slabs (the type used for floors in buildings), they create hollow cylindrical ( or rectangular) voids that run horizontally, and are centred about the neutral axis of the slab/beam. The general idea is to remove concrete that is doing very little structurally ie the concrete that is close to the neutral axis.

I would guess that the use of steel hollow sections as the formwork for creating these voids would be a waste of steel because the steel itself would be close to the neutral axis. It might be better to use cardboard rather than steel.

IMVVHO

Best wishes

Martin

[walter]

Thanks, I find these posts very helpful.

Gerry,
re: Beam supports

My concern is about the supported beam.. If your garage concrete floor moves 0.5" the whole structure will move with it?

[ger21]

If your garage concrete floor moves 0.5" the whole structure will move with it?

Whenever possible, it's a good idea to have machines on a single slab, and not span any concrete joints. But that's probably not an option with a 16' span. I doubt a floor in a heated garage would move much though.

[brunog]

Here's an article about stiffening a wood beam using epoxy and steel rebar. Seems like it has some relevance.
http://www.epoxyworks.com/17/beam.html

Ger,
This is the perfect example that you must not believe everything that is on the internet. This guy actually weakened a 2 x 12 to then reinforce it with rebar and epoxy. I seriously doubt what was the actual gain in load bearing strength of the whole process, epoxying the 2x 12s on the existing beam is ok however, but IMVHO the results would have been as good by using threaded rods nuts and flat washers to secure everything in place, at a lower cost, in less time, and with a lot less effort.

You can kill flies with a canon, but you don't need a canon to kill flies. (shoot them with a laser beam instead) LOL

Have a nice day

Bruno

[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 ; )

[martinw]

This guy actually weakened a 2 x 12 to then reinforce it with rebar and epoxy. I seriously doubt what was the actual gain in load bearing strength of the whole process

Bruno,

I'm pretty sure there is a gain in strength, because the bottom of the beam acts in tension, and both epoxy and steel are stronger in tension than timber.

I agree completely that the effort of going down this route may well not be worth the aggravation. The article did mention one brand of epoxy quite a lot. I wonder why.

Best wishes

Martin

[jsage]

Guegon brothers make West Epoxy. West Epoxy is often used for boats. Geugon sp? does get into structural applications.

I have a few of their practical guides.

[ger21]

The article did mention one brand of epoxy quite a lot. I wonder why.

The article was in a magazine (Epoxyworks) that West System puts out. It's free, you can sign up for it at their website.

[Geof]

Bruno,

I'm pretty sure there is a gain in strength, because the bottom of the beam acts in tension, and both epoxy and steel are stronger in tension than timber.
.....

Almost certainly a gain in stiffness and strength. It reminds me how wooden ladders were made light and strong. A small groove was cut on the lower edge and a length of wire run from top to bottom and secured at the ends; identical in principle to the bean repair.

[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.

[martinw]

The article was in a magazine (Epoxyworks) that West System puts out. It's free, you can sign up for it at their website.

Dear ger21,

I guess what I was gently suggesting is that if you are a manufacturer of epoxy, it is in your financial interest to suggest wacky uses for the product.

I really should be less of a cynic.

Best wishes

Martin

[jsage]

Martinw,

No doubt. Same product new market. At the same time ; ), I think it crystallized an engineering component I was missing through overgeneralizing. Why would rebar be useful in high aspect pieces. I believe they also revolutionized or at least popularized epoxy boat building where failure can definitely suck.

For practical purposes if it minimizes creep, then it should help maintain truer beams.

I agree why didn't you just use the right materials in the first place. Saw an interesting show this morning that showed that lumber is now grown in half the time and is not as strong 50 yrs versus 100. They have countered this and smaller lumber with I beam joist construction and laminated dimensional beams.

My recurring concern is once you go beyond medium size then it gets a bit trickier. If you expand this rebar solution in four pieces and tie to together as walter (aka mr. smith) demonstrated then maybe you have solved problem.

My apologies for being cynical on e/p alone. It just seemed to me it is not altogether different from mdf, smaller pieces tied together with glue. Epoxy has has an elongation of 5% or 7%, 7% I think. How would that effect tolerances spanned longer distances with loading. How can you reduce size and weight and the corresponding problems. I like what you can do with epoxy but don't necessarily like working with it.

Anyhow that was one unresolved issue for me. So, although the support solution is a bit unconventional I think it has useful implications. I think Gugeon is less hyped than most, but of course it is a business ; )

jsage