Telescopic boom buckling 1

[bc75]

Hello All,
Using this forum after long time, i am looking for some guidance with regards buckling of the a telescopic boom, the telescopic boom i am looking at will be similar to the telescopic crane boom but not sure how do you go about calculating the buckling load since there is change in cross section.

Any pointers like design code etc.

I did a search in internet there are lots of research articles and methodology adopted but my thinking is there must be a guidance with regards to calculate buckling involving different cross sections for checking the global buckling of the whole boom.

Thank you in advance.

Thanks
bc

 

[3DDave]

Each section gets evaluated for buckling. There isn't a global buckling calculation to be done.

 

[bc75]

3DDave,
Thank you answering the query, yes your answer corroborates my finding.

Thanks
bc

 

[CANPRO]

Each section gets evaluated for buckling. There isn't a global buckling calculation to be done.

Really? That doesn't sound right to me. The global buckling capacity is going to be somewhere between the value if the boom was all the larger section and if the boom was all the smaller section. How would each section get evaluated individually, what unbraced length would you use? As a rough check you could assume the entire boom is the smaller section and if that meets your needs then you saved yourself a whole lot of math. If you need a more accurate answer you're going back to first principles. Last time I did this I used Theory of Elastic Stability by Timoshenko and Gere as a reference. In my section edition, section 2.14 covers this exact condition.

If you're looking for an online reference I just googled "buckling varying cross-section" and there are many resources there to help you with this.

 

[SWComposites]

This is something that a FEM is well suited for. Build a shell model and run a buckling analysis. And put a large factory of safety on the eigenvalue results as geometric imperfections may have a significant effect.

 

[HTURKAK]

Telescopic boom buckling load can be computed applying differential equations . In case of two stepped boom, hand calculation could be OK but multiple-stepped booms should be analyzed with a software.( ANSYS etc)

I will suggest you to look ,STABILITY OF STRUCTURES Principles and Applications (By C.H. YOO,S. C. LEE )



Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[3DDave]

CANPRO,

Is there some other telescopic boom that is not individual sections with severe discontinuities where one ends and the next begins? The ability to buckle will depend on the extension amount and angle and load. That seems unlikely to be represented by a single number.

It's a boom, not a column. It's in bending, as well as compression; at least as I understand the original question.

 

[CANPRO]

3DDave, I don't understand your question. OP didn't say how this is loaded, he said it is similar to a crane boom. I imagine there to be some combination of bending and axial loading. Either way, to properly evaluate this you absolutely need to do a global analysis with consideration for the varying cross-section. I agree that there is no single number answer here unless there is a single configuration being analyzed. Each configuration will have a buckling value which will need to be determined based on the varying cross-sections in that configuration. Maybe I misunderstood what you meant in your original reply, but to say "There isn't a global buckling calculation to be done" I believe is fundamentally incorrect.

 

[canwesteng]

This seems fairly poorly suited for a shell model in FEM, since global buckling isn't concerned with the shell itself, only the stiffness of the boom, and local buckling of the shell is dominated by imperfection magnitude which is not captured in the FEM. A simple line model can get you global elastic buckling capacity, which can be plugged into AISC equations. You should check by hand the tendency of the boom to buckle locally.

 

[JStephen]

If there is any "slop" in the junctions of the telescoping sections, that would invalidate a normal stepped-column approach.
The assumption of "small" deflections for a typical crane boom is another issue, as that deflection can be "not small".

 

[ThomasH]

and local buckling of the shell is dominated by imperfection magnitude which is not captured in the FEM.

If you include the imperfections in the shell model and use non-linear analysis that should work.

I am not saying that this is well suited for FEM, but it can be solved with FEM.

 

[3DDave]

Less of a column and more of a multi-segment beam.