Buckling length for beam hanging from rods 3

[novembertango88]

Hi all,
I have a beam hanging from rods as shown in the attached sketch.

What do I take for the buckling length?

The load is on the top compression flange so I'm considering it destabilizing, the compression flange has no lateral restraint and no torsional restraint. BS 5950-1 table 13 gives a value of 1.4*L(length)+2*D(depth).

Does this sound right?

Thanks, Nick

 

[WARose]

I am not familiar with "BS 5950-1"....but unless you are absolutely sure it takes into account this is a suspended beam.....I'd forget about it. The normal LTB buckling allowables in most codes are not applicable because the support conditions for suspended cables are not what they are based on.

Several resources that deal with this are:

ASME's BTH-1 (IIRC, they have a allowable bending strength based on this situation)

'Distortion Buckling of Steel Beams', Structural Engineering Report No. 185, University of Alberta Department of Civil Engineering, By: Essa & Kennedy, April 1993.

'Buckling of Suspended I-Beams', ASCE Journal of Structural Engineering, Vol.116, No. 7, July, 1990, by: Dux & Kitiporncha

 

[novembertango88]

Thanks for your advice WARose.

I'm pretty sure it doesn't take account of this case.
I guess this is why lifting beams are usually closed sections.

 

[Lomarandil]

Well, I'm not fluent with BS5950, but I do remember that their equivalent length for beams supported from the bottom without torsional restraint (e.g. stacked cribbing) was only L+2d.

So the equation you mentioned certainly is accounting for the beam being suspended in some way.

Helwig at UT Austin has also done some research (and presented at NASCC, I think) on this topic.

I have also used the Dux and Kitipornchai reference in the past, although my memory was it is complex to apply in general cases.

I don't believe BTH-1 handles this robustly for open sections -- as you mention, closed sections are more common.

----
just call me Lo.

 

[robyengIT]

may be something similar to this

 

[HTURKAK]

The case , 1.4*L(length)+2*D(depth) is valid for partial torsional restraint provided only by pressure of bottom flange onto supports.

Your case ; UDL Destabilising load ( drawn with pink) , since above the shear center of the section .
However, the beam is supported with tension rods .That is, stabilizing effect at supports is provided only with gravity. That is, your case is worse than mentioned case ( which is valid for conditions which some restraint provided at supports).

I will suggest you to use sections not subject to lateral torsional buckling, (i.e. square or rectangular hollow section within the limiting value of LE/ry given in Table 15 .).

 

[novembertango88]

Thanks all.

RobyengIT, that paper is interesting.

You're right HTURKAK. The effective length table is not applicable in this case. Table 15 requires a buckling length still though, do I just use something over the top like 10*L?

I've been wondering why it isn't the same as channel supports on hangers typically used for cable ladders but in that case the cable ladder provides lateral restraint to the channel. In this case I don't have that.

 

[MIStructE_IRE]

I’m wondering to myself if this beam can buckle at all..?

Will the whole trapeze not just swing laterally at that point?

Edit - I guess it can buckle after reviewing robyengIT’s very useful link!

 

[HTURKAK]

You are right...

If this thread for the design purpose rather than theoretical discussion, you may use square hollow section. We know that, LTB is only possible where the beam has a less stiff minor axis (i.e. Ix > Iy).

In your case, the tension elements do not provide lateral restraint to the channel. The lateral restraint only provided with gravity. However, the channel is supported with tension elements from the compression flange . That is, the loading should not be so destabilizing load.
Full lateral restraint is assumed to exist if the frictional or connection to the compression flange of the member is capable of resisting a lateral force of not less than 2.5%.

If you want to use channel, You may use conservatively 1.5*(1.4*L(length)+2*D(depth)).

 

[r13]

I haven't read the paper yet, just to express my doubt on a beam suspended by cables/rods can buckle. Buckle is associated with twisting, can a beam twist between two flexible supports that neither has lateral restrain capability? The cable/rod must be stretched so tight to produce required restrain, how tight then?

 

[WARose]

Good post robyengIT. Note the fact that paper references Dux & Kitiporncha's work as well.

 

[MIStructE_IRE]

We’re on the same page R13. I’m still not fully convinced..

If this is a single trapeze set up as below, and if P represents the Lateral 2.5% force in the Compression flange required to cause buckling.. then what resists this horizontal force to cause buckling in the first place?

 

[r13]

IRE,

Yes, I am curious about that too. However, the paper is still a good read, as it provides tool to pin point the best lifting points along the beam.

 

[rb1957]

that's the trouble when your structure is a mechanism. But lateral buckling due to bending is an issue, no? the lateral stability of the compression cap … it not like there's a lateral force applied, and yes, this mechanism would probably act in odd ways. If there were stiffeners on the web, supporting the cap flange, would that do it ?

another day in paradise, or is paradise one day closer ?

 

[r13]

rb,

Torsional buckling is associated with excessive compression stress and side swing due to rotation. IMO, The side swing is critical only when beam ends are capable of prevent lateral translation. If a beam won't twist, then yielding will proceed buckling.

 

[HTURKAK]

You can express your doubt however,the supporting condition ; hanging of beam with 90 degr tension rods worse than simple supported case.


I will suggest you to look ( Lateral stability of long precast concrete beams for different supporting conditions ; simple support, transport, hanging with 90 degr. for erection, hanging with 45 degr.)

Link

 

[r13]

HTURKAK,

Thanks for allowing me to express my doubt. I've swallowed 3/4 of the paper you provided, and start to look the lifting of precast concrete beam from another perspective/angle - parameters such as unit weight, length...etc, but my doubt on lifting steel beams stays as is. I'll try to find time to read through both papers though.

 

[BAretired]

Suspending from a higher point would improve stability (see below). I think the beam could be considered torsionally braced.

BA

 

[strucbells]

I designed a lot of pipe support trapezes of various sizes early in my career and never worried about this...I suspect a combination of typical shorter span lengths, tight governing pipe deflection limits, and the piping/conduit providing adequate OOP bracing made it a non-issue.

 

[r13]

I think the problem is more pronounced on long span concrete girders than the typical steel beams. The weight been a big factor, that might produce enough lateral restrain by friction (f = µT).