Steel tube column filled with concrete, what load does it resist? 2

[sacem1]

I have used a leftover rolled a36 plate steel tube 15" diameter x 15' long as a column to attach a 15' jib crane swing arm with a 3 MTon winch. Plate thickness 3/16", column is embedded into a 3' foundation so 12' are actual column height. I filled the column with concrete (210 kg/cm2 resistance) and have used it ever since with no problem at all, no deflection is observable when loaded.

I really never calculated the real load that the column can resist, now I have fitted a 14" swing arm to a second exact column and think of fitting a 5 ton winch but I would like to do some resistance calcs first, searched in my books and not found a single design of steel tube filled with concrete can any one show me what would be an equivalent load or a calc method for this design.

It is definitly very easy to manufacture this setup and cost wise it should be very competitive.

Please any guidelines would be appreciated.

SACEM1

 

[JStephen]

Simplest method would be to treat it as an "empty" steel tube.

One effect of the concrete would hopefully be to avoid local buckling of the cylinder, which might increase the strength considerably from that effect alone. Actually being able to prove that it couldn't buckle would be a different matter, though.

You could also treat it as a composite steel-concrete beam- basically assuming the concrete doesn't carry any tensile stresses. With that much steel, the concrete may not have too much effect, anyway.

 

[dbuzz]

These papers may be interest:
- O'Shea MD and Bridge RQ. Behaviour of circular thin-walled steel tubes filled with very high stength concrete. Aust J of Struct Engrg, IEAust, 1999;2(1):19-30.
- Bridge RQ and O'Shea MD. Local buckling and confinement in axially loaded steel tubes filled with normal and high stength concrete. Aust J of Struct Engrg, IEAust, 1999;2(2&3):123-134.
- Vrcelj Z, Uy B. Behaviouir and design of steel dqaure hollow sections filled with high stength concrete. Aust J of Struct Engrg, IEAust, 2002;3(3):153-170.

 

[desertfox]

Hi sacem1

If you know the Modulus of Elasticity of the concrete you can reduce your problem to one of an equivalent section of either steel or concrete. However in order to do this we have to assume or know that the interface between the materials are such that they both under go the same bending strains.
Assuming this is the case the bending can be worked out as follows:-

M= Ms + Mc

M= (Es*Is}/R + (Ec*Ic}/R

BUT Es/Ec = m (the modular ratio)


therefore M = ((Is * m)+ Ic)*(Ec/R)


as Ec/R = Fc/y

then M = ((Is*m)+Ic)*Fc/y

Where:- M = bending moment (resultant)

Ms = bending moment in the steel

Mc = bending moment in concrete

R = radius of curvature of beam

Ec, Es = elastic modulii of concrete and steel

Ic,Is = second moment of area of concrete and steel

Fc = stress in concrete

y = distance from the neutral axis of the beam.


further more as well as a bending load your column also withstands a direct load of whatever mass your lifting and therefore can be treated as a compound bar in compression.
The treatment of which can be found in most text books and uses the modular ratio as I have here for the bending case.


best regards

desertfox

 

[sacem1]

Thanks desertfox for your time and patience to put everything so clearly.

So if I use the steel in the outside of the concrete I can also use the equivalent formula, I was not sure it would work like that.

The adhesion between steel tube and concrete is enhaced by a lot of steel scrap pieces we weld in the inside of the tube to do just that adhere the concrete, plus having such a big concrete mass with out any steel in it made me a little nervous so we always just locate inside about 4 pieces of 1/2" steel rebars located transversally by the same scrap bars welded inside, always keeping in mind that they don't create voids where concrete would not fill them.

To dbuzz: I'm trying to locate those papers but they seem to be the exact situation I've been descriving.

Thanks

SACEM1

 

[desertfox]

Hi sacem1

Your welcome, if you need anymore info for the direct load case then let me know.
You may already be aware that you have to sum the tensile
stresses and compressive stresses on each side of the column to get the resultant stress.

regards

desertfox