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arailos
Posts: 29
Registered: ‎01-26-2010
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STEEL DESIGN - USER SECTION - Different Section properties database-analysis

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07-05-2012 11:07 AM

I might be repeating myself and I'm sure it's annoying for everyone who've seen my similar post before but I cannot understand and it's a serious matter for me.

 

As you can see in the attached picture the Wpl values that are calculated in the section defined by four TRON 101x4 sections in the section builder are different from the Wpl values taken into account for structure analysis....

 

In my previous post RG_Adsk told me that:

 

No, no "factor". Closed = known formula is used to calculate such properties instead of taking them from database

 

Could anyone elaborate this......

Which is the known formula?

How can I calculate it by hand?By the way the Wpl values in the section database have been corroborated by hand.

 

It seems that the shape type defined when saving the section in the database plays important role, but what is it?

 

Thanks for your time and effort. Really appreciate that...

 

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Rafal.Gaweda
Posts: 5,061
Registered: ‎04-26-2010

Re: Different Section properties database-analysis

07-06-2012 12:31 AM in reply to: arailos

 

No, no "factor". Closed = known formula is used to calculate such properties instead of taking them from database

 

Could anyone elaborate this......

Which is the known formula?

 

Formula for SHAPE_TYPE = 36 section , circular hollow section ("single tube")

 

Wply = Wplz = 4.0/3.0*(R*R*R-Ri*Ri*Ri);

 

where

R – ext radius

Ri – int radius

 

If you save section to database with "?" symbol (SHAPE_TYPE = 0) the database values will be considered.

 

shapetype0.jpg



Rafal Gaweda
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Artur.Kosakowski
Posts: 4,774
Registered: ‎12-17-2010

Re: Different Section properties database-analysis

07-06-2012 01:07 AM in reply to: arailos

I believe this has already been explained on the forum but I think it would be useful to write this again.

 

When you create a user defined section and this section is to be saved to the user section database you need to answer yourself a question: am I going to use this section in the steel design module?

- if no, you just do not have to care about the selection of the section type as this is irrelevant for calculation of the model itself

- if yes, then you have to be aware that Robot itself is not able to classify sections based on their shapes (channels, angles etc.) and this has to be done manually.

This decision influences section class (slender, semi-compact etc.) classification which is determined by wall height to thickness ratio that you provide after selecting the symbol type as well as the use of the verification formulas from the selected design code (including e.g. Wpl calculation). In case you select "?" (generic cross section; you may for better understanding of the situation assume this is a solid rectangle) all cross sectional properties are taken from the section database but as the section type is undetermined the code checking does not include such effects as LTB or section slenderness.

 

Hope this helps.



Artur Kosakowski
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Active Contributor
arailos
Posts: 29
Registered: ‎01-26-2010

Re: Different Section properties database-analysis

07-09-2012 01:53 AM in reply to: Artur.Kosakowski

Thanks a lot guys...This was crystal clear now.....

 

Which is in your opinion the best way to model such a section. Use d and t values when saving the section in the database as SHAPE_TYPE = 36, such as the Wpl values that would be calculated in design would be equivalent to the prefered values of the defined section, or save it as ?_TYPE and have no LTB and slenderness checking?

 

Is Wpl or respectively Wel (according to section class) the only value that is calculated by the program during steel design?

 

I am asking because I've seen in this particular example (where section is class 1) that the value Ix (which was changed manually) is used as it's in the database...

 

 

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Artur.Kosakowski
Posts: 4,774
Registered: ‎12-17-2010

Re: Different Section properties database-analysis

07-09-2012 02:16 AM in reply to: arailos

Assuming that you are about to calculate (verify) this section by hand what is be the verification procedure you would follow? In other words would you verify it as a tube, a box or just checked the stresses? Other?



Artur Kosakowski
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arailos
Posts: 29
Registered: ‎01-26-2010

Re: Different Section properties database-analysis

07-09-2012 02:28 AM in reply to: Artur.Kosakowski

To be honest I'm not sure how I would treat such a section if I was to calculate it by hand...

I assume I would consider it as a tube section meaning that I would classify it according to the class of the element it's consisted of, but I would perform LTB analysis according to the loading.

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Artur.Kosakowski
Posts: 4,774
Registered: ‎12-17-2010

Re: Different Section properties database-analysis

07-09-2012 02:49 AM in reply to: arailos

In such case select the tube section. Typing the dimensions you  may consider to enter h equal to the horizontal size of the entire compound section. The other possible approach is define the section (in the model) as 4 separate bars (each with the 'standard' tubular cross section) connected at certain spacing along their lengths with rigid links that cause them to work together. The idea is to distribute load to each of them and then design them following the standard code procedure for tubes.



Artur Kosakowski
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Active Contributor
arailos
Posts: 29
Registered: ‎01-26-2010

Re: Different Section properties database-analysis

07-09-2012 03:34 AM in reply to: Artur.Kosakowski

If I choose to save it as a tube and type as h the horizontal size of the compound section which should be the thickness then?

The second suggestion might be the most feasible but which is the most appropriate distance between the rigid links?Let's say it doesn't matter...

Would I have to brake the columns at this distance?

How would I specify the buckling lenghts....?

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Artur.Kosakowski
Posts: 4,774
Registered: ‎12-17-2010

Re: Different Section properties database-analysis

07-09-2012 03:47 AM in reply to: arailos
As I wrote the key is to answer yourself a question how would you checked such section by hand. Knowing the answer you can make the decision how to assign parameters. 

 

 


 

If I choose to save it as a tube and type as h the horizontal size of the compound section which should be the thickness then?

 

I think there are two options: 1. Set is as for a single tube. 2. Set is in such a way that the section class is as you would like it to be.

 

The second suggestion might be the most feasible but which is the most appropriate distance between the rigid links?Let's say it doesn't matter...

 

This depends on the way the tubes are connected together. If they are welded let's say each 3 m then it would be 3 m. If you have continuous welds than more often.

 

Would I have to brake the columns at this distance?

No. All you need is to have nodes along the bars.

 

How would I specify the buckling lenghts....?

Again this depends on the boundary conditions of this profile in the model. You can try to define them in the same way as you would do for the compound profile.



I'm very sorry but I'm not in the position to tell what would be the best - all I'm trying to do is to provide you with the list of the possible solutions I can think of. Perhaps the users of the forum have also faced this sort of the situation and can shear their experience with you?



Artur Kosakowski
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Active Contributor
arailos
Posts: 29
Registered: ‎01-26-2010

Re: Different Section properties database-analysis

07-09-2012 03:52 AM in reply to: Artur.Kosakowski

Thanks a lot Artur..I justed wanted your opinion and possible solutions for me to choose....Exactly what you did.Trully appreciate your help...

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