Calculating the steel equivalent for a concrete filled steel pipe ?

[Awre]

I am trying to calculate the effect of filling a steel pipe pile with concrete to control deflection due to "lateral load". The concept, which I am following, is to calculate the equivalent steel pipe pile section that represents the composite section.

I reviewed the post, which has discussed the process of this calculation at:

http://www.eng-tips.com/viewthread.cfm?qid=280751

According to the post, the equivalent section can be calculated based on the transposed moment of inertia as:

I(transposed) = I(pipe) + I(concrete)*Ec/Es
I use the “I (transposed)” to back calculate the pipe wall thickness by fixing the diameter then calculate the deflection.

My question is how different if I use similar concept based on "Area" instead as the followingin:
A(transposed) = A(pipe) + A(concrete)*Ec/Es
The equivalent area will be used to back calculate the wall thickness as above.

Which formula is the right for the purpose since both will give different results?

I realize the inertia uses the diameter to the fourth power compared to the area that uses the diameter squared, which is clear. However, which method is right?

Thanks

 

[FixedEarth]

Two things come to mind to simplify your analysis. Since allowables stresses are controlled by the weakest material & the product E x I advantage is also for Concrete, would it not work to just use the concrete diameter and proceed with your lateral load deflection using the concrete EI?

 

[JStephen]

If you're considering bending, you'd use the moment of inertia. If you're considering axial compression, you'd use the area.

 

[jrw501]

This is a pretty simple question and like most people here I'd suggest speaking with a mentor, but I'm sure a lot of things I'd ask would be considered simple by most people on this forum.

Unless you also have axial load, you will primarily be concerned with inertia (your first equation) for controlling bending stress and deflection. If you also have axial load as well you will have a combination equation including both moment and axial load and then also will need to check deflection. If the span is short, you may have shear concerns.

 

[SAIL3]

A few questions come to mind.
How does one ensure the integrity of the conc?
How does one address the potential cracks in the conc.?
Intuitively, I might use it for axial load but would have doubts in using the combined section for bending.

 

[rb1957]

could you use the concrete to support the lateral load, and the steel tube to support the bending stresses ?

if you've tried this a need to have the concrete carrying some of the bending (from lateral load) ... wouldn't you need to add rebar ? or are you asking the concrete to be effective in compression only ??

 

[Splitrings]

The AISC LRFD 3rd Edition covers concrete filled HSS members. Allowable moments are listed at the bottom of the tables 4-12 through 4-15. Specification Section I3 covers the design of these members for flexure.

 

[racookpe1978]

Please correct me: Concrete has poor resistance to tension - which will be the dominant load for a bending stress, right?

So how will filling a hollow column (HSS or tube) increase the bending resistance very much? Seems that only the little bit of "compression" improvement is in the half-section of the vertical beam on the load side of the beam - and that resistance only is present until the concrete separates from the steel and begins crumbling.

 

[WillisV]

@ Splitrings. Nope - though what you say would make sense, the values listed for flexural strengths of composite HSS sections in both the 3rd LRFD and 13th Edition were actually the noncomposite strength for the bare steel only. The new 14th Ed. manual does have actual composite strengths though.

 

[Splitrings]

@ WillisV. Thanks for correcting me on that. I thought I had run through the calculations to convince myself it was for the composite section before, but apparently not.