Preferential bending loading direction of a beam

[sfensphan]

Hi All,
I'm trying to think through a problem and was hoping to ping the community for guidance. Suppose you have a beam that's symmetric about 2 axes (like a u channel or a triangular extrusion). Is it better to load the beam in compression on the side with material furthest away or should you load that side in tension? In the case of a U channel, I think that the top of the U should be loaded in tension as it would allow the member to carry more moment before buckling occurs. In the case of a equilateral triangle extrusion, I can't think of a good reason either way. Loading the tip of the triangle in compression would make it more likely to buckle, but it's farther away from the neutral axis. I'm assuming that the Young's modulus is the same in compression and tension. I think that matters but not 100% sure
Any thoughts would be very helpful!

 

[Nidum]

It all comes down to the efficient use of steel and ultimately the cost of the project .

Efficiency here means using the least weight of steel to carry the required loads .

Whilst you can in principle use any type of beam in any orientation to carry a specified load pattern there is usually only a very limited range of types of beam and orientation that will do the job efficiently .

Also when someone uses completely the wrong type and orientation of beam all sorts of secondary problems often arise such as the need to provide stabilising gussets and to provide reinforcement at the anchorages and load points - all of which adds more weight and reduces efficiency further .

Designing steelwork requires structural analysis skills in the designer but just as importantly a good measure of horse sense about what works and what doesn't .

 

[SteamKing]

Hi All,
I'm trying to think through a problem and was hoping to ping the community for guidance. Suppose you have a beam that's symmetric about 2 axes (like a u channel or a triangular extrusion). Is it better to load the beam in compression on the side with material furthest away or should you load that side in tension? In the case of a U channel, I think that the top of the U should be loaded in tension as it would allow the member to carry more moment before buckling occurs. In the case of a equilateral triangle extrusion, I can't think of a good reason either way. Loading the tip of the triangle in compression would make it more likely to buckle, but it's farther away from the neutral axis. I'm assuming that the Young's modulus is the same in compression and tension. I think that matters but not 100% sure
Any thoughts would be very helpful!

I think a U channel has only one axis of symmetry, and that's the one which splits the U down the middle.

If you are concerned about buckling, then using open sections like a U-channel, where the radii of gyration differ greatly depending on the axis of bending, is problematic. IMO, it's much better to use either a closed section or an open section where the material has a better distribution about all of the possible axes on bending. In any event, it's hard to discuss this in a general way; it's much better to analyze an actual structure.

 

[sfensphan]

I think a U channel has only one axis of symmetry, and that's the one which splits the U down the middle.

If you are concerned about buckling, then using open sections like a U-channel, where the radii of gyration differ greatly depending on the axis of bending, is problematic. IMO, it's much better to use either a closed section or an open section where the material has a better distribution about all of the possible axes on bending. In any event, it's hard to discuss this in a general way; it's much better to analyze an actual structure.

Hi All,
Greatly appreciate the replies. I can see that there are a multitude of variables I hadn't consider in my hypothetical.
I had a chance to look at a steel hook on a crane over the weekend and I noticed that the inner part of the was beefier than the outer (cross section of a "T"). By analogy, it would seem that you would want to load the U channel and the triangular extrusion such that the tension passed through the flat. I have to think through the math a little bit more before I post something more rigorous. Thanks again for your thoughts.