Tube-Inside-Tube Beams 7

[colar]

Has anyone ever used a smaller cross section HSS member inside another HSS member (i.e. a 2.5 x 2.5 x .125 inside a 3 x 3 x .188) to stiffen/strengthen a beam.

I am considering this becasue of size constraints on the outer envelope of a beam. I realise this is an inefficient use of material, but the size constraints leave little choice.

My main question has to do with the tranverse shear stresses between the mating surfaces of the two beams. There is lots of literature about built up beams, but in this case the two beams share the same neutral axis. Will there be transverse shear between the outside of the 2.5" and the inside of the 3" that will need to be picked up by some plug welds or end welds? Is it just a matter of needing shims between the tubes at the ends so that the normal force can be transmitted and the two tubes are engaged simutaneously?

Look forward to some advice

Colar

 

[par060]

I believe that if the 2 sections are restrained from slipping each section will take a portion of the load relative to it's stiffness regardless if they are fasten together or not.

(total load)(EaIa/(EaIa+EbIb))= portion of load

 

[adlington]

To FinnB

Like most things in life there is either an easy or hard way to do something. The idea of welding two tubes that don't even sit snuggly together is ludicrous. It is unlikely most engineers would consider welding two tubes together as a simple solution. My point regarding longitudinal shear is an attempt to point out that it would be better to design a simple section of plates than worry about how many welds would be required to facilitate composite action of the tubes.

How can you guarantee the integrity of each weld when you can't see what your connecting to, most steel fabricators would be dubious about attempting such an approach.

If a simple more elegant approach exists then use it.

 

[Trussdoc]

Just to throw another idea out.....how about if instead of welding you filled the space between the two tubes with pressure-injected epoxy, such as I've seen used to repair damaged timber and concrete?

 

[flamby]

color,
A long thread....and much was said on tubes..
However tubes are inherently weak in bending and should have been the last choice for your work. Any I-Beam, H-Beam Box section (built up) of similar outer to outer dimensions will give you significantly more moment carrying capacity and will be fairly simple to make, if not available in the market as standard section.
May I ask you what is the reason that you want a tube to be used here? My all-time choice is an I-Beam for all beams.
flame

 

[dpa]

Par060,

If you are talking about two 2x10's side by side with one foot on each board you are right. If you are talking about two 2x10's one on top of the other you may still be right but the deflection is much greater if they are not nailed together. The combined moment of inertia of two 2x10's laid loosely on top of each other is 5.2 in^4 but the moment of inertia of a plank 3" x 9.25" (which is what you would have if you nailed them together)is 20.8 in^4 four times as stiff.

Dave Adkins