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Active Contributor
LieberSteve4929
Posts: 26
Registered: ‎11-01-2010

L-shaped concrete member incorrect stress calculated.

333 Views, 6 Replies
03-26-2012 06:58 AM

In the help section dealing with section properties the diagram showing Vpz, Vz and Vy Vpy are reversed.  I checked the values and agree with reported properties however they are associated with the wrong axis.  This error is carried further by incorrectly calculating the stress MC/I with the incorrectly associated C dimension.  The other issue is that the principle axis is not used to calculate the stress for this L-shape.  Since the help section is not associated directly with this shaped member I also wonder if the incorrect C values is also used commonly though out the program?

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Product Support
Posts: 4,440
Registered: ‎04-26-2010

Re: L-shaped concrete member incorrect stress calculated.

03-26-2012 07:37 AM in reply to: LieberSteve4929

Yes you are right.

These concrete section properties are not caclulated in principal axes.

You can use steel sections instead applyuing concrete material to them later on. Keep in mind that such sections can not designed in RC modules.



Rafal Gaweda
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Active Contributor
LieberSteve4929
Posts: 26
Registered: ‎11-01-2010

Re: L-shaped concrete member incorrect stress calculated.

03-26-2012 07:57 AM in reply to: Rafal.Gaweda

It is also incorrectly calculating the stress on the unrotated property values using the reported properties in the program for Iy, Iz.  Calculates stress about the Iy axis using distances from extreme edges from NA with the wrong values.  Gather that values are stored in wrong variables or in what ever storage structure you are using.  i.e. Vy is correct value to use for the Vz value for the section when calculated stress about the Iy axis.

 

The values for Vy and Vz are switched in the report and in the calculations.

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Active Contributor
LieberSteve4929
Posts: 26
Registered: ‎11-01-2010

Re: L-shaped concrete member incorrect stress calculated.

03-26-2012 01:22 PM in reply to: LieberSteve4929

To help I have attached a PDF that might explain the issue.

 

It was mentioned to use a steel section for the L shape.  Unless I am missing something this would have to be a user created section using section definition from the tool bar as there is no parametric shape to choose from that would make life easy.

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Product Support
Posts: 4,440
Registered: ‎04-26-2010

Re: L-shaped concrete member incorrect stress calculated.

03-27-2012 01:13 AM in reply to: LieberSteve4929

Wrong local axes  - colors of arrows always coresponds to GCS sign in left bottom corner : red Z, green Y

 

axes.jpg

 

Stress results depend also on moment sign:

positive moment : sigmamax = My/Iy*vz

negative moment: sigmamax = My/Iy*vpz

 

 

Keep in mind robot sign convention for bars : negative axial force \ stresses  = tension, positive = compression

 

moments.jpg

 



Rafal Gaweda
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Active Contributor
LieberSteve4929
Posts: 26
Registered: ‎11-01-2010

Re: L-shaped concrete member incorrect stress calculated.

03-27-2012 05:47 AM in reply to: LieberSteve4929

Attached is the robot file, the green or Y axis is along the 56" leg, the Iy=41688 in output shows that it is associated with the 56" leg being horizontal and the 32" leg vertical, this agrees with my seperate calculation of the section properties.  Per your diagram Iy would be used with Vz and Vpz which together sums to 56, not the 32 correctly associated with Iy for distance from NA to extremes.  Don't see I am interpreting incorrectly local axis.  Also Iz is inherently larger so that would tend to validate it is associated with the strong axis in this case bending about Z axis as shown in my sketch.

 

As a check I did run a simple rectangle 8" x 32" and the table showed the correct values of Vy and Vz so believe it is an issue with this type of section only.

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Product Support
Posts: 4,440
Registered: ‎04-26-2010

Re: L-shaped concrete member incorrect stress calculated.

03-27-2012 07:32 AM in reply to: LieberSteve4929

My apologies.

I have checked stress calculations, not data for calculations although you had mentioned it so clearly.

We will correct it.


Create this section in Section Builder module and use it in 3D model.




Rafal Gaweda
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