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Does structural metal tubing (e.g., either as HSS, or in space frames, or even motorcycle/bicycle frames) ever come with any small metal rods or columns inside of the tubing itself?

While it might add some weight (and/or manufacturing complexity), could it also significantly improve the rigidity and load-bearing ability of the tubing?

*Edit: a more "biological" visualization of this could be similar to that of a bird bone's hollow structure (see this image), but with possibly far fewer crossbeams (to save on weight), perhaps just one or two small "spokes" or crossbeams every now and then?

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Structural metal tubing is normally used to take bending, axial and shear loads, i.e. to globally transfer load from one place to another. It also has to resist local loads, i.e. if there is a concentrated force or impact force in a particular location, the member has to not buckle locally. In other words: bending shear and axial and taken by (more or less) the whole cross-section; local loads are resisted by parts of the cross section, and they are then transferred into the whole cross section.

Bending and axial loads travel down the outside walls of a hollow member. Internal bracing won't make any difference to this.

Shear force and local loads travel across the cross-section. Internal bracing could help with this.

In my experience as a structural engineer, the critical factors governing design are normally bending and axial force. Sometimes shear is a controlling factor in cross-section selection. I can't think of a single instance when local loading has been of any importance to a structural hollow section. (NB I work in bridges. Local impact may be of more importance in other disciplines.)

Therefore, when you're using "standard" structural hollow sections, I find it unlikely that internal bracing would be beneficial. And the cost of adding it would make it uneconomically viable.


There is, however, one aspect in my experience where internal bracing is required. That's when fabricating a steel box girder. These use relatively thin plates spaced a long distance apart, and in order to avoid buckling they need to be braced, internally, at regular distances.

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  • $\begingroup$ Interesting, perhaps if local and shear loads are an issue, then would metal tubing not even be used in the first place? Or, if it would be, then it would simply exist as just a part of a larger frame design for load-bearing purposes? $\endgroup$ – ManRow Feb 13 '18 at 9:32
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    $\begingroup$ Hollow sections are good at carrying shear, compared to an I-beam. If you have a structure which is experiencing high shear, hollow sections would be the sensible choice. As for high local loads - they're not that common, but I can't see that hollow sections would be much worse at resisting them than other section types. $\endgroup$ – AndyT Feb 13 '18 at 9:48
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No, you make the tubular strong enough to take the loads.

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I ride dirt bikes as a hobby and the frame of the bike is a mix of tubes, patch stiffener plates welded over it, reinforcing sleeves and what not. It is designed to be light and tough and take a lot of punishment! Here is the frame of Suzuki RMz450, a very agile and powerful dirt bike:

RMZ450 2012 Frame.

But in design of tubular trusses I have never had to be concerned with local buckling or concentrated stress mid length of member.

Because they are designed mostly to have a joint at every possible load or support point! Basically they undergo only tension or compression stresses.

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