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Bar Diagram vs Member Verification

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Message 1 of 6
Dirgs
776 Views, 5 Replies

Bar Diagram vs Member Verification

Hello,

 

In a recent analysis I encountered some problems in the results for which I ask your answer about what causes them. There are 2 problems, described as following:

 

  • Strange shear diagram on a bar. As you can see on attach, a concentrated load on middle of a bar, provoke a huge shear peak near the node of load application. That diagram is just for this load case, I can't understand how it may have that inversion. On the 1st picture (diagram amplified) you can see that shear are constant until it reach the middle of the bar, when the weird FZ progress appears;
  • Member verification. Following the same analysis, more worrying is that member verification don't detect maximum shear of the bar. Even dividing the bar, verification don't use FZ at end of the bar.

Barras_Numbers.png

 

Bar_Verification_Fz.png

 

Bar_Verification_My.png

 

Can you explain what is happening?

Thanks,

Dirgs

5 REPLIES 5
Message 2 of 6
Artur.Kosakowski
in reply to: Dirgs

Any chance to look at the actual model? Have you applied a concentrate force at this location? If yes, is it defined as nodal force or bar load and what happens if you use 'the 'other than used' means of defining it?



Artur Kosakowski
Message 3 of 6
Dirgs
in reply to: Artur.Kosakowski

Of course. https://www.yousendit.com/download/UVJoR0lYcVh6NFA1SE1UQw

 

It is defined as bar force. That seems to be the problem. I change it for a node force and it worked correctly.

Nodal_Force.PNG

Dirgs

Message 4 of 6
Artur.Kosakowski
in reply to: Dirgs

The bar force by its nature has to be applied to the bar rather than to the node therefore is 'somewhat shifted' from the bar's end towards the its middle (I have tried to illustrate this situation applying the bar load at 0.99 L). This causes the display of shear force as you noticed it.

The recommended solution is to define concentrated forces at either of bar's ends (x=0L and x=1L) as nodal forces instead.

 

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.

 

 



Artur Kosakowski
Message 5 of 6
Dirgs
in reply to: Artur.Kosakowski

Ok, I understand that for forces at start or end node of a bar the node force method is the correct way of applying the load. The bar force method have some problems when a 0L or 1L load relative position is selected, it causes wrong results, Am I right? 

If so, I think that the possibility of load application as shown before (bar force at extremity) should be restricted in order to eliminate the error presented.

 

For the 2nd problem I pointed out, can you summarize how steel verification gets the forces at the characteristic nodes. Why the peak shear force is "invisible" during the verification?Is it related again with the load application method used?

 

Thanks,

Dirgs

Message 6 of 6
Artur.Kosakowski
in reply to: Dirgs

Ok, I understand that for forces at start or end node of a bar the node force method is the correct way of applying the load. The bar force method have some problems when a 0L or 1L load relative position is selected, it causes wrong results, Am I right? 

If so, I think that the possibility of load application as shown before (bar force at extremity) should be restricted in order to eliminate the error presented. 

 

This way of applying loads has got some 'side effects' yet some people (including myself Smiley Happy) may find it useful knowing its limitation (being aware how to 'read' the results). Imagine that you have several bars meeting in one node from different directions and you would like to apply forces to 'this' node in the directions perpendicular to each of them. Applying them at x=0L or X=1.0L vs. x=0.01L or x=0.99L as you propose has got no sense (still you would be able to define load close to the bar end with the similar effect) and you may actually prefer easier load definition over calculating the direction of the resultant nodal force.

 

For the 2nd problem I pointed out, can you summarize how steel verification gets the forces at the characteristic nodes. Why the peak shear force is "invisible" during the verification?Is it related again with the load application method used?

 

The verification is done slightly further towards the middle of a bar so that the 'limitation' of bar force definition has got no influence on the design process (see the attached picture).

 


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.



Artur Kosakowski

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