Cantilever Buckling Factor

[Mohamed Maher]

Hi all

I have question please regarding buckling of cantilever column 12 m length and support cantilever beam 14.5 m length with moment connections, my question here how much I should take the k buckling factor for the cantilever column for the below part and top part
..should i calculated the G factor for the point of intersecting between cantilever beam and the column or consider the all height of the column 12x 2.1 normally as the buckling factor of cantilever, Kindly refer to the attache file.

thanks
Regards
Maher

 

[r13]

I could be wrong, but I don't think there posses axial compressive buckling problem.

I am more concerned is - are there at least two cables on the back side of the column? Elevation view couldn't tell.

 

[Mohamed Maher]

It's just one cable..in the out direction I added vl.bracing so i consider out of plan buckling distance between bracing ..do you think only one cable can turn the column from cantilever case to normal column with buckling length 12.5 m

 

[r13]

Think this way, where is the force required to buckle the column came from? Maybe I have missed something.

I couldn't see out of plane/paper brace. All I wanted to point out is you need at least two cables to prevent the column from rotation.

 

[r13]

Since cable connection can't count as a support, as a free end/head column, the suggest K value shall be 2.1 for a plumb column. I have no clue for the inclined column in your case.

 

[Agent666]

Keep in mind that the cables while partially preventing the lateral deflection (depending on their effective stiffness) don't do anything to prevent the rotation of the column.

Neither does the presence of the beam restrain the column as the cantilever isn't attached to any external supports.

Your column has a variable axial load as well if I'm reading the situation right.

Weighing up all of these things means the only way to accurately determine the effect length factor K for determining the axial capacity is to do a buckling analysis to get the buckling load and then use this to determine the design capacity in accordance with your design standard. Don't guess, you'll be wrong.

 

[Mohamed Maher]

What if I have 2 cables for each column..can consider that time by 12.5 m

 

[Mohamed Maher]

@Agent666
I'll make buckling analysis by sap..then I'll take the first mode buckling force ..then equal it by Eular eqn. 3.14E^2I/(KL)^2 ..what is the value of L should be added to this wan.

 

[r13]

Don't take my words for it, but I am tempted to try that, uses L = 6m, with K = 1.2 (no rotation at the beam-column joint). Calculate the buckling stress, and verify it through a buckling analysis. It should be an interesting exercise then. Note, from your layout, I assume the beam has a huge load (maybe through lifting heavy weight), so the backside (long) cable is required to restrain the counter clockwise rotation. It is much appreciated, if you can let us know your finding.

 

[Agent666]

You are after the buckling load, so back calculating K while interesting to understand isn't actually necessary. It really depends on how SAP is providing the output, since the load is fixed, it's kind of arbitrary what length you use as it's just a way of describing the buckling relative to a known length. It's easiest to visualise using the entire column length potentially.

What code are you working to?

 

[Agent666]

Also the euler formula is pi^2 not E^2

 

[Mohamed Maher]

@retired
I'll share the whole 3d model so that it will be more clear what is this structure function and what the cover material for this..this beam support fabric material and it has huge straining actions results as you stated from wind suction and pressure...
I'll make the buckling analysis then share results.

 

[Mohamed Maher]

@Agent666

Yes sorry PI^2
I'm using BS code.

 

[Mohamed Maher]

Hi all

I made SAP model for this case first i add 1 Kn at each column at top (as a new load pattern called Buckling load) , then defined from load case type of analysis to be buckling then define number of modes to be 12 .
then the results that all modes almost affect the cables not the required column.

I don't know how to study the column in such a case . i'm tried to take some print out from the model , kindly refer to the attached file.

Regards

 

[human909]

It seems to me you are out of your depth designing this. Come to think of it, I'd be careful myself getting into that sort of design.

You are likely getting the cables buckling because presumably you don't have the set as tension only members. Or depending on your design you probably want to introduce an initial tension into the cable. That said your structure doesn't look like a particularly stable structure. It is a big cantilever and has very little apart from gravity resisting overturning in the opposite direction. Sure if things are big enough or stiff enough it could work but it doesn't seem thoroughly though through.

 

[r13]

human909 is correct.

Try, put 1 k downward load on the tip of the beam to resemble the operation. I think the result will be quite different. And, did you specify "tension only" for the cables.

 

[Mohamed Maher]

@ human909 & @retired13

I already define the cable for tension only ..but I don't know how can i put initial tension in the cable as pretension force.

The sizes of cross section I think its big for example cantilever beam is I 700/300 x 300 x 6 x 12 and column is I 600 x 300 x 6 x 24

I guess that's enough for that case.

I don't get you want to add 1 Kn on the cantilever beam free edge or at intersection of the cantilever with the column .. for now I need know how to add pretension force in the cable and i'll try putting 1 Kn at beam intersection with the column.

 

[r13]

The load on the beam end will produce tension on both cables, and that where you get force from. Without that load stay on, your system is unstable. Cable can't take compressive forces, that what you have seen by putting a load on top of column. Do a free body diagram, you will understand.

My suggestion is only for your personal practice use. Let someone else to handle the real design, or at least check your work.

 

[Mohamed Maher]

I already check the structure under the applied load and checked its safety by other program called Easy used in the Tensile Structures..but for example I can't define tapered section in easy program so I tried to make another one Sap.

System is stable in Case cross sections is enough and its enough ..even under column buckling length =2.1 x 12.5

However I want to complete my excersie on sap for define pre tension force and get the exact buckling length

 

[r13]

Can't you place joint load to represent pre-tension in the cable? Some program has the commend "pre-tension" though.