Triangular steel column 1

[agps]

Hello!

I´m testing a FEM of a triangular column with 16.5m height which will hold a publicity screen on the top.

For the triangular column I used on the edges of the triangule tubes D244.4x5.4 and between them steel plates S275 with 10mm thickness. The edges have a distance between them of 1m.

Only submited to wind loads The result of the analysis was ok for the deflections, ok for the tubes but for the steel plates I have a stress value around 1000 N/mm2 (pressure)untill 1 meter from the bottom of fixation.

I tested the same column but instead of steel plates I created a frame work (truss column) and the result was very good.

So, how should I interpretate the stress values of 1000 N/mm2 Comparing with the fy of the steel (275) which is much less? Can I compare this values just like that?

Thank you for any answer.


agps

 

[BAretired]

Why don't you try to tell us precisely what the problem is? Your explanation is not clear. I have a picture in my mind of a triangular column with something at the top (publicity screen, whatever that is). Try to avoid acronyms. Would it kill you to write out what FEM means? Being a structural engineer, I assumed originally that you meant Fixed End Moment, but now I am not so sure. Perhaps you meant Finite Element Model.

And what do you mean by "the bottom of fixation"? I have absolutely no concept of what you mean.

Now, I am not trying to be obtuse, and certainly not critical of your language, but if you want an answer to your problem, I think it is imperative that you state your problem clearly.


BA

 

[corus]

I'd check your plate stresses as those appear to be from a plate fixed at all edges with a uniformly applied pressure. Roark will do for that. If the stresses are correct as they stand then the plates will fail, though not the overall structure. Bending stresses in a plate are proportional to the thickess squared, which suggests you need a plate twice as thick to reduce the stresses by 4. Even so, if the plates are welded then the welds will probably fail due to fatigue under the varying load of the wind.

corus

 

[GregLocock]

BA- could you type your responses in Swedish please, the OP would find that easier to understand. I think most contributors on an FEA forum would know what FEM meant.




Cheers

Greg Locock

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

 

[BAretired]

Ledsen, men jag tror att jag kan ha konsumerats en eller två för många öl innan mitt tidigare svar. Min ödmjuka ursäkter.

Kind Regards,


BA

 

[corus]

Personally I fully support BA's haranguing of the use of acronymns, and also the consumption of one or two beers!

I think the 'bottom of the fixation' means the base of the structure, where it's built in, by the way.

corus

 

[BAretired]

Okay, I think I understand the problem now. I am not sure what is meant by D244.4 x 5.4, so I considered a column whose cross section is an equilateral triangle of 10 mm thick steel plate, each plate being 1.0 m wide.

The height of cantilever is 16.5 m and I assumed a wind pressure of 1 kPa, so the uniform load on the column was taken as 1.0 kN/m. This gave a cantilever moment of 136 kN-m. Didn't know the area of the publicity screen, so I ignored it.

This produced a maximum fiber stress of 23 N/mm^2. How does that sound?


BA

 

[agps]

Dear colleagues,

My apologies for taking so long to answer your questions and thank you all for the effort and consideration in trying to find a valid answer for my thread.

First of all let me clarify the acronym FEM. As you well understood it means Finite Element Model (never thought it will bring so many interpretation doubts).

Second, the "bottom of fixation". It´s the "base of the structure". Thank you Corus.

Regarding the problem itself, I must say that a few hours after posting this thread I found my error.

Corus, thank you for your answer about the column behavior and how to solve it.

BA, D244.4x5.4 refers to a tubular section diameter 244.4 and thickness 5.4 (mm). As you don´t have access to the loads aplied the values that you tested are a little different from mine.
I´ve in round numbers max stress value 280 N/mm2 which forced me to change the steel tensile strenght.

Anyhow, I´ve another issue to put in consideration regarding the same structure.

The moments that I´ve on the base of the structure are much less than the values found on a initial study which I applied the same loads but on a simple cantilever system (one fixed support only with the same height, 16.5m).

On this system I´ve moment values of 600 kN.m and on my computer model, where I´ve 3 supports (the edges of the triangule), I´ve max. moment values of 56 kN.m

What do you think is the reason for such low values in FEA comparing with hand calculation?
Note: The value of the structure self frequency (just due to self-weight) is 1.75 Hz.

Oh! One more thing.

BA, om du bor i Sverige/Stockholm blir jag glad och kan jag träffa dig då kan vi dricka en eller två glas öl tillsammans.
MHV (ooppss!! No acronymns. Med Vänliga Hälsningar!)

agps

PS - What does OP mean Greglocock?

 

[agps]

I forgot to explain that the value 56 kN.m is the sum of the moments of each support joint.

Joint 1 - -17,9 kN.m
Joint 2 - -19,3 kN.m
Joint 3 - -19.3 kN.m

 

[BAretired]

agps,

I don't live there, but if I am ever visiting the area, I will be happy to take you up on your offer.

Oh, yes...OP means original post...in this case that would be you.

Kind regards,

BA

 

[agps]

BA .


And please don´t spend time around my last consideration. I believe I reach what I want crossing information from two different models.

Hej då alla ni

agps

 

[BAretired]

I believe that 56 kN-m is the sum of moments for the three tubular sections individually. If you check the axial force in each tube, you should find that if Joint 1 is in tension then Joints 2 and 3 are in compression and vice versa.

BA

 

[agps]

Exactly!

And the final value for the moment is always there.

agps

 

[BAretired]

agps,

Exakt! Nu är du matlagning med gasar.

All the best,

BA