RISA Tips and Tricks for Structural Engineers

Posted by Megan Zupan on 04.07.17

Structural engineering design software can be an invaluable resource to perform efficient, engineered solutions of both simple and complex-framed structures. It can also be a source of much frustration if you lose sight of your precise data entry.

You've spent hours carefully inputting your structure, all the individual nodes, members, pinned or fixed connection types, getting the orientation of each member correct, checking the unbraced lengths, the k values.  You've assigned the boundary supports, checked the code solutions, and double checked that the Y axis is vertical.  You've assigned your loads, then applied your loads, and then created your load combinations.   

Now you're finally ready to go.  It's time to run the analysis.  You click solve.  You select the solution choice.  Right away, error messages!

Users familiar with RISA have felt that sinking feeling when this pops up:

Or this:

Or you run the model, receive no error messages, but discover no analysis was performed.

What should you do?  Here are some suggestions:

 

Instability Detected

Error 2006: Instability Detected!

“In the process of solving the model, RISA-3D detected instability.  This means there is the potential for an unlimited displacement.  Check your model. If you are unable to find the source of the instability it may be because of a setting in the program. Open Tools>Application Settings, uncheck the “Lock isolated ROTATIONAL instabilities without notification?” and then solve your model and you may see the problem.” 1

For a structural frame system, modifying the boundary condition supports can resolve the instabilities.  If pins are chosen for all support points, there are (6) joint aspects for the model to solve and it becomes unstable.  In reality though, for a column with a pinned base, we can hold the Y rotation by setting the drop-down box to “reaction” as we do not expect to see this rotation in the field.  

Now, check that the reactions solved by RISA are what you are expecting. View the reactions directly on the screen by selecting “Show the Reactions” on the Model Display Options tab.  

The results now indicate zero moment at the base, which is what you want for this pinned base situation.

 

P-Delta:

“When a model is loaded, it deflects. The deflections in the members of the model may induce secondary moments due to the fact that the ends of the member may no longer be co-linear in the deflected position. These secondary effects, for members (not plates), can be accurately approximated through the use of P-Delta analysis. This type of analysis is called “P-Delta” because the magnitude of the secondary moment is equal to “P”, the axial force in the member, times “Delta”, the distance one end of the member is offset from the other end.” 1

“The actual modeling of these secondary moments is done through the calculation of secondary shears (shown as V in the diagram):

These shear forces are applied at the member ends.  For a 3D model, this P-Delta calculation is done for the member's local y and local z directions.  The solution sequence is as follows:

  1. Solve the model with original applied loads
  2. Calculate the shears (V's) for every member in the model
  3. Add these the shears (V's) to the original loads and re-solve
  4. Compare the displacements for this new solution to those obtained from the previous solution. If they fall within the convergence tolerance the solution has converged.  If not, return to step 2 and repeat.

If the P-Delta process is diverging dramatically, it will be stopped before numerical problems develop and an error will be displayed.  If this error is displayed, the P-Delta displacements have reached a level where they are more than 1,000 times greater than the maximum original displacements. If this happens with your model, the model may be unstable under the given loads, or there may be local instabilities present.” 1

So what does that mean for your model?

Well, the first step is to check your code selection.  AISC 14th will require you to specify either Yes or Compression.  If you leave this blank and try to solve your model using AISC 14th, it will not perform calculations on your members.

If you have properly specified the PDelta, maybe you run the model and you get an error message saying “PDelta diverging”.  If this happens, run the model with PDelta turned off and check for huge deflections or instabilities, indicating improper modeling. 

You can also turn off P Delta in the Application Settings in the Tools menu: 

 

Additional tips to make your life easier:

Segment:

Type in “S” or “segment” in the member information.  Designating a member as a segment will automatically calculate the unbraced length of a long girder with beams framing into it.  Remember to specify the member is physical.

Self-weight calculation:

A (-1) in the direction of gravity will automatically calculate the self-weight of your structure.

Distribution of an area load:

Check the Area Load Mesh – default is 144sqin which may lead to a distortion of the area load calculation.  

 

 

 

 

References:

Welcome to the RISA-3D Version 15 Help File; RISA Technologies, Inc., Copyright 2016.

 

RISA-3D is a proprietary structural engineering and design software package developed and licensed by RISA Technologies, Inc., 26632 Towne Centre Drive, Suite 210, Foothill Ranch, CA 92610, www.risa.com. This product, along with all other engineering software packages, have proven to be powerful tools to aid in the development of structural engineered solutions. Use of all engineering software shall be performed by qualified, licensed professional engineers with the proper knowledge, training, and experience to responsibly execute the necessary requirements specific to their projects. Neither the author of this blog, nor HWH AEP Inc. make any warranties, either expressly or implied, as to the accuracy of results or proper usage of RISA-3D.  

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