replacing a steel beam with a composite one?

[northerncat]

i have a need to replace a piece of 75*50*3mm rectangular steel tube and i was thinking of making a glass foam composite beam, would anybody know a roughly what thickness of foam and how much glass id need?
thanks in advance for any help
sean

[Homefront]

Sounds like a problem for an engineer... also much information about the existing tube is lacking, such as alloy, wall thickness, application, etc.

[gonzo]

If it is now welded, how are you going to fasten it?

[baeckmo]

What the limiting factor, ultimate strength or deflection/stiffness?

[Petros]

I can design you a beam, but more important than the dimensions of the old beam is how it is loaded. Is it in compression, if so how long it is? Or is it in bending, or a combination of the two? Or pure bending, any point loads, or evenly distributed loads?

The properties of square steel beams and composite beams area very different, to design a beam you have to design for each of bending, shear , buckling, compression, etc. for the properties of each of the materials. It is possible to back calculate the strength of the steel beam from the dimensions, but I would need to know how it is loaded. For example if it is deflection limited, than the deflection properties of the composite is what determines the size and thickness of the replacement beam, and bending and shear strength would be check to make sure they are within acceptable limits.

If the beam is in a complex loading condition, than a lot more detailed information is required. Sometimes the end mounting conditions is what determines the size and thickness of the beam as well. So it is not an easy question to answer.

I can give you a composite beam size with the same bending strength, with the same compression strength, and with the same shear strength of the steel beam, but each would be a different size from the other. One might way overkill the design if sizing for the wrong characteristic.

email me off the list if you want to share pictures of the installation, and the type of loading that is on the steel beam. my email is petros98223(at)gmail.com

[northerncat]

looking for similar deflection/stiffness to the above mentioned 75*50*3mm

[EuroCanal]

If the beam is attached at both ends, you will need to take account of the different coefficient of thermal expansion. GRP usually has a higher linear CTE than steel. The GRP beam will shrink in winter and expand in summer compared to adjacent steelwork. The difference is roughly of the order of 1mm per m per 10 deg C change. You need to check the product you are using - there's a lot of variation in GRP CTEs. You can use slotted holes if attaching with bolts, taking account of the calculated movement over the expected temperature range.

Originally Posted by northerncat

looking for similar deflection/stiffness to the above mentioned 75*50*3mm

In other words, you don´t like to provide hard data.

But expect serious advice.

[northerncat]

Originally Posted by apex1

In other words, you don´t like to provide hard data.

But expect serious advice.

not at all i just thought it might be a bit simpler (like maybe someone had made one before) than it is, just a case of saying these are the relative strengths of this piece of box section you need x diameter of foam and x weight of bi/tri whatever cloth,
it supports an unknown load in the center across 700, its 4m long and sits on 2 pillars, i dont have any photos, it is coastal and the steel has died hence thinking an epoxy composite beam might last a little longer.

[Petros]

Northerncat,

Across 700 what? 4m long on 2 pillars? does that mean it is simple supported beam, or are the ends rotation fixed? Does it have a point load in the middle, or is it distributed loading along the span? Does it carry any torsion loads? It is a beam that connects too hulls on a catamaran, with a mast load in the center? What is it?

Simplest would be replace it with a similar steel beam, have it galvanized before you install it if you want to it last longer. A composite beam may not last any longer if it is exposed to sunlight or weathering.

Composite beams are not homogeneous like most metals (where the properties are usually the same in all directions), you can build-up uni- or bi-directional cloth in any direction and in any amount with a composite beam, so it is a complicated design question and not just a matter of saying a FRP beam of this size and thickness will be the same strength.

[gonzo]

The attachment points would also have to be re-engineered. Steel is much harder than a composite, so the load doesn't need to be spread as much. A steel beam is you best bet. It will save you a lot of time and money.

[northerncat]

fair enough i didnt realise it was so tricky, just kinda assumed that you could take say 100*50mm of foam wrap it in 3-4 layers of biax and wallah

sean