Equivalent moment of Inertia and Young's modulus of concrete filled pipe pile 1

[moe333]

Hi, I'm not a structural engineer, but I'm doing a rough check on buckling of a pile that is supporting axial load only for a wharf. There is an unsuppoted length between the top of wharf and mudline. I'm using Euler's formula and need to know equivalent moment of inertia of a concrete filled pipe pile. It also has a reinforcing bar in the concrete, but I am assuming that is not significant to bending resistance. I also need the equivalent Young's modulus. Any assistance is appreciated.

 

[TLHS]

It's going to be less awful working out a combined EI than getting them independently

CSA S16-09 works out EI of the composite section as being equal to the below (Clause 18.2.2). It's interesting that it gives you more credit for transient loads. I'd need to do some research to figure out where this came from. I suspect it's because the concrete creeps.

EI_S+(0.6*E_C*I_C)/(1+(C_fs/C_f))

E - Young's modulus of steel
I_S - Moment of intertia of steel portion
E_C - Young's modulus of concrete section (E_C=4500*sqrt(f_c) with E_C and f_c both in MPA)
I_C - Moment of inertia of concrete portion
C_fs - Sustained axial load on member
C_f - Total axial load on member


You could go to first principles, but I bet whatever code you have in your jurisdiction has requirements for this kind of member. Euler buckling assuming a fully composite section may end up being less conservative than your local code, which may have provisions for how much composite action you're allowed to assume for concrete filled members.

 

[TLHS]

With that last paragraph, I mean that CSA S16-09 has a series of equations that actually define the code axial capacity of a concrete filled pipe, including buckling behaviours. I'm assuming something similar is in ACI or AISC in the US.

The CSA formulations have a bunch of inherent assumptions in them, including the way they calculate the combined EI value, and your code might have different ones. Both will definitely give different results than a Euler buckling analysis. It sounds like this might get some lateral deflection as well, just from what I've seen on this type of structure... In which case you might not get a proper picture of the strength of the structure without a second order analysis that accounts for P-Delta effects and the bending capacity of the member.

Anyway, if you're trying to get a feel, euler buckling might give you some ballpark numbers. There is a potential for it to be reasonably far off, though, depending on what else is happening and the assumptions that get used.