Difference between Von mises stress with automatic calculation and cartography

[Anonymous]

Dear Robot users,

I am beginner with robot software, and I found differences between results of Von Mises stress.

Actually I have launched automatic calculation with regard to NF EN1993-1:2005/NA:2007 and maximum ratio for the beam 166 at combination 42 is 1.16. This ratio corresponds to verification of Von Mises stress. The material used is S355, so fy=355MPa and as my understanding the result of stress should be 1.16x355=412MPa

But when I display the von mises stress on the beam 166 and at combination 42, the maximum Von mises result is 309MPa (see figure below). So the ratio corresponds to 0.87.

I don't understand why there is as many high difference between the both calculation.

Could you please help me or give me any tips?

PS: Model is attached for information.

Thanks in advance,

Best regards,

Frédéric

[mustafahesenow]

Hi @Anonymous

I did check the model you send me and 

I think there is really problem in steel design according this code for Von mises check to be corrected 

@Artur.Kosakowski

[Anonymous]

Hi Mustafa,

Thanks very much for your answer,

What would you like to mean? Do you think that I did a mistake in the definition of steel in Robot software? Which option should I integrate ?

The steel that I used is S355JR, with yield stress=355 MPa.

Thanks very much,

Best regards,

Frédéric

[mustafahesenow]

Hi,

Your work was very good and I  think there is no mistake at all and there is problem in RSA need to be fixed .

[Artur.Kosakowski]

Hi @mustafahesenow and @Anonymous

I'm sorry but I'm not sure if I understand where you think the incorrect value is:

Actually I have launched automatic calculation with regard to NF EN1993-1:2005/NA:2007 and maximum ratio for the beam 166 at combination 42 is 1.16. This ratio corresponds to verification of Von Mises stress. The material used is S355, so fy=355MPa and as my understanding the result of stress should be 1.16x355=412MPa

Reading the above my understanding is that 412 MPa is OK and the value in the steel design is correct too

but when I read

I think there is really problem in steel design according this code for Von mises check to be corrected 

I assume that the value from the stress analysis is correct 309 MPa.

My initial assumption is that in the steel design Robot  adds shear stress due to torsional moments from both the corner and the middle of the wall at the same time and therefore the value is too large but could you clarify what you meant please? Thank you.

[mustafahesenow]

HI @Artur.Kosakowski

You are partially correct in this when I exclude the torsion the factor become smaller but instead of become 0.87 it came 1.02 

Could you please explain why this .

[Artur.Kosakowski]

Hi @mustafahesenow and @Anonymous

I will need some help from the development team to investigate what actually happens. I do my best to update you as soon as possible but I'll not be able to meet with them earlier than in 3 weeks 

[mustafahesenow]

Hi @Artur.Kosakowski

Noted thanks.

[Anonymous]

Hi @Artur.Kosakowski,

Thanks for your feedback.

Hi @mustafahesenow,

Thanks to revive the post.

Bestregards,

Frédéric

[Artur.Kosakowski]

Hi @Anonymous and @mustafahesenow

The results of the investigation of this situation. The difference is due to the fact that the code checking contrary to stress analysis include the fact that this is class 4 section for MZ in the stress calculation process. Following EN 1993-1-1 provisions non effective part of the wall is disregarded and new set of 'equivalent' section properties are calculated for 'shifted' position of the neutral axis which results in non symmetrical Sig,Mz stress distribution. I'm attaching the picture from EN 1993-1-5 oshowing how it works for My moment below:

What we get in effects are different values of stresses on the opposite sides of the profile:

As you can see in the part under compression there is an increase of SigMZ comparing to what it would be for the section as it is (without the ineffective part being disregarded) which would be SigMz = 157 MPa. This is why teh ratio increased from 0.97 to 1.16.

As the reference for the correctness of such approach we may look at the paragraph 6.1.6 of EN 1993-1-3:

I hope this helps.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski,

Thanks very much for your very detailed answer.

I have other questions:

1) In the picture here below I would like to know where this table come from and also the value of SigMz=157MPa  (sorry I don't have acces to Robot from my current computer so I can not checked from my previous model).

2) The Von Mises stress from finite element calculation is 309MPa and the yield stress is 355MPa so the ratio corresponds to 0.87. I would like to understand where come from the ratio 0.97 that you mentionned on the picture here above.

3) So If I could do a synthesis, the section of the beam 166 should be changed to be consistent with Eurocode but It is also accepted with FE calculation (as Von mises stress is lower than yield stress). It should be clearly mentionned (or explained with tutorial) that it could be have some differences between the both calculations and advise the engineers in the selection of results.

Thanks very much for your answer.

Best regards,

Frédéric

[Artur.Kosakowski · Accepted solution]

Hi

1) In the picture here below I would like to know where this table come from

This is the detailed results table from the code checking verification.

and also the value of SigMz=157MPa  

The maximal value of von Mises stress is obtained in the point A

For the outer corner:

SigN = N/Ax = 14804/16544 = 0.89 MPa

SigMz = Mz/Wel,z = 256866000/1636455 = 156,96 MPa

SigMy = My/Wy  = 17328500/3174514.35 = 5,46 MPa

For the point A stress from MZ:

Sig_Mz = 156,96*242/250 = 151,9 MPa

Tau,y = 127,43 MPa (from Qy)

Tau,ty = 43,93 MPa (from torsional moment)

SIG =sqrt( (SigN + SigMy + Sig_Mz)^2 + 3*(Tauy +Tauty)^2) = sqrt(158,25^2 + 3*171,31^2) = 336,28 MPa

2) The Von Mises stress from finite element calculation is 309MPa and the yield stress is 355MPa so the ratio corresponds to 0.87. I would like to understand where come from the ratio 0.97 that you mentionned on the picture here above.

SIG/fy = 336,28/355 = 0,947 (I'm sorry for the 0.97 typo error)

The steel design module assumes the average value of  Tauy 

and constant value of stress from torsion which is also the simplified approach:

3) So If I could do a synthesis, the section of the beam 166 should be changed to be consistent with Eurocode but It is also accepted with FE calculation (as Von mises stress is lower than yield stress). It should be clearly mentionned (or explained with tutorial) that it could be have some differences between the both calculations and advise the engineers in the selection of results.

I hope that now you have all required information to decide which way to go.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski,

Thanks very much for your very detailed answer, that allow me to accept the beam size 😉

Best regards,

Frédéric