Flexural tube buckling

[ugandabob]

Hi, I'm working with hollow tubes for poles (cantilever bending). I'm trying to estimate where the tube will buckle - I'm currently using diameter from the base as an estimate, but I'm having difficulty confirming this anywhere.

 

[MikeHalloran]

Why do you care _where_ it buckles?



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Well, it'd be a good place to put a reinforcement.

What are the tube dimensions?

Interesting problem.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ugandabob]

The tube is made of GFRP. Outer diameter 220mm, inner 210mm; 3.66m (12ft) tall. Most literature provides the buckling load/stress, but we're curious about the location.

 

[rb1957]

Bruhn has an analysis for a tube loaded in bending (section C8.6) with some general design rules (1.3* axial compression. he also lists a "recent" study in the Journal of Applied Mechanics, March 1961 (an article by Siede and Weingarten).

good luck

 

[rb1957]

sorry misread the thread ...

it'll buckle at the section with the highest compression stress ... i'd look at the section with the highest moment, probably at the base (since you say "tall", i'm picturing a flag pole (or mast) with a side load creating bending).

no apologies for stating the obvious.

 

[Ron]

ugandabob...yes, as rb1957 stated, check the maximum bending moment location (at the base in this case). Don't forget though, that buckling will occur on 1/2 the area, not the whole area since your buckling will be related to bending, not axial loading (that is assuming you have no axial load and that bending prevails...if you have axial load, add the half-area influence).

 

[AggieYank]

I disagree rb1957. It won't necessarily buckle at the point with the highest compressive stress.

For a cantilever pole/column, assuming a lateral load, the compressive stress in the compression side/flange will be highest at the base. This is not where it will buckle, as the tube is connected to the base at that point. It will buckle at some distance from the base, maybe between one diameter from the base and 5 diameters from the base, as the original poster said. Buckling occurs at some combination between compressive force and out of plane stiffness of the element in question. The buckling in this case will be lateral torsional buckling.

 

[rb1957]

true enough, the fixity of the end support will add to the strength of the section, depending enormously on how it is attached ... a close fitting socket, a riveted/bolted connection.

but if i was doing a hand calc i'd use the maximum moment to keep something in my back pocket (other than my wallet). this conservativism is only a few percentage points (< 10%, L = 3660 mm, dia = 210mm), and modelling the real world stiffness would be more trouble than it's worth (IMHO).

and with less than 1 minute's thought on the problem, i don't see where the torsion's coming from ...

 

[Galambos]

im not sure a round pole will exhibit LTB, but maybe local buckling.

 

[drawoh]

I am working on a composite boom that hangs out sideways, i.e. cantilevered from a base at one end. I am assuming that my primary failure would be something compressive at the bottom side at the supported end.

Does fibre reinforced plastic buckle the same way metal does? Are the calculations meaningful?

JHG

 

[Ron]

Buckling of fiber reinforced composites can be quite complex (as can other stress distributions in FRC). It does not buckle in the same manner as steel or other metals in that there is more localized crushing involved as well as interlayer shear of the the lay-up, depending on how it was fabricated (spun-cast, molded, etc.).

Thinner materials tend to buckle a bit more similarly to metals,

 

[AggieYank]

rb1957, it isn't so much that there is torsion on the member, but that there is a tendency for the compression face of the beam to "kick out", or to fail in what is known as lateral torsional buckling, as I'm sure you know. I'm not sure if LTB applies to square tubes, or if it only applies when the depth exceeds the width, etc. I'm a bit busy, or I'd spend time looking through the AISC manual.

I have no expertise or experience with plastic. Typically plastic is ductile, so I'd guess it would buckle out of plane before crushing, but I really don't know. Sorry I'm of no help to the original poster.

 

[haynewp]

LTB occurs when the minor axis inertia is less than the major axis inertia when bending about the major axis. There is no LTB for square tubes.

 

[rb1957]

nor round tubes, as in this case ...

 

[kxa]

LTB occurs in open sections like channels, if I remember correctly.

 

[MNLiaison]

Mil-HDBK-17 Volume 3 Section 5.7 describes buckling and crippling of composite laminated plates. It is dependent upon the [D] matrix of the laminate which can be varied to get the results one is looking for. Mil-HDBK-17 is available for free at the ASSIST website.

http://assist.daps.dla.mil/quicksearch/

There is also NASA document NASA-97-TP3659 which is free at the NASA technical report server.

Regards
MNLiaison

 

[dik]

Ron: with thin sections, CFRP's tend to be more orthotropic and as noted, analysis/modelling can be extremely difficult.

Dik

 

[haynewp]

kxa; I understand it does not control for rectangular tubes bent about the major axis for the HSS sizes that are available (and it is therefore not addressed by AISC, but not an impossibility). I don't know about built-up box sections bent about the major axis where the depth is very large compared to the width with very long lengths. It also can occur for rectangular bars.


When things are steep, remember to stay level-headed.
-Horace