RISA-2D FAQ
Common support and usage questions.
RISA-2D
Why am I getting instability errors in RISA-2D?
RISA-2D will report an instability error when degrees of freedom in a model have zero or very little stiffness. A common case where this occurs is when all members attached to a node have the end rotation released (e.g., This would happen when trying to model a truss). If there are no external restraints to give the node rotational stiffness, RISA-2D will report an instability error. One way to solve the problem in this case would be to connect ONE member rigidly to the joint. Another way would be to apply a rotational boundary restraint to the node. (i.e., enter a R or F in the rotation field for that node).
What is the difference between Member Releases & Boundary Conditions?
Probably the greatest source of confusion to people doing stuctural modeling is the difference between a Member Release and a Boundary Condition. A Boundary condition describes how a node is attached to the external world around it. The boundary condition says whether or not a NODE can translate or rotate. In contrast, a Member Release describes how a BEAM element is attached to a NODE. The member release says whether or not the beam to node connection will transfer shear, moment, or axial forces. In application, a boundary condition applied to a node would be used to model a fixed or pinned support condition, while a member release applied to a beam element would be used to model a fixed or pinned beam to column connection.
What are the Lb and Lcomp fields on the Hot Rolled Steel Design Parameters screen?
These fields are the unbraced lengths for a member and control the AISC code checking that RISA-2D/3D can perform on AISC hot rolled steel shapes. For a detailed description, see the appropriate section in the program manual. In a nutshell, the Lb values (unbraced length) control the calculation of the allowable axial stress (Fa), while the Lcomp value (unbraced compression flange length) controls the calculation of the allowable bending stress (Fb).
These fields are provided because in many cases the actual unbraced lengths are different than the member lengths in a structural model. For example, a column with K-bracing framing in on one side only would need to be composed of two model members, yet the unbraced length out of the plane of the bracing would be the full column height.
Why do some of my members “flip” over when a R2D model is read into R3D?
The way the local-y axis of a member is defined is different between RISA-2D and RISA-3D. Thus, members in RISA-2D models may get turned over 180 degrees when read in RISA-3D. Any member distributed loads or point loads are automatically adjusted back to their original direction (i.e., A member load that was pointing down which is applied to a member that gets flipped over will still point down). The only noticeable affect this will have is that the sign of the shears shown in RISA-3D for any “flipped” members will be opposite that shown in RISA-2D.
How are the K value approximations calculated by RISA-2D/RISA-3D?
RISA-3D is able to approximate the K values for a member based on that member’s sway condition and end release configuration. The K-factor approximation is based on Table C-C2.1, found on page 5-135 of the 9th Edition ASD code, or page 6-184 of the 3rd Edition LRFD code. A full description of the limitations of the K approximation are given in the Hot Rolled Steel – Design section of the manual.
Why can’t I get an AISC steel unity check for certain members in my model?
The usual things to look for are as follows : Is the shape you using for the member a database shape? RISA only does steel code checks for database shapes and the online shapes. Is the shape for the member in question an Arbitrary database shape? RISA cannot do code checking for sections defined with the Arbitrary shape. For the 3rd Edition LRFD code and 13th Edition ASD/LRFD, you must also do a P-Delta analysis to get code checks.
Why can’t I get a NDS wood unity check for certain members in my model?
RISA only does wood unity checks for sections that are defined in the Wood database. You must also specify the wood species on the Wood tab of the Materials spreadsheet for the section that you want wood checks for. See also the Wood – Design section of the manual for a step by step procedure on how to get wood code checks.
How do I tell if my Plate/Shell Finite Element Results are accurate?
Assessing the accuracy of a finite element (FE) model is not a simple thing because it requires an understanding of how finite elements work, as well as some practical experience in FE modeling. The underlying principle is that the model should be able to accurately represent the deflected shape of the “real” structural element being modeled. As a simple example, a single 4 node quadrilateral finite element cannot be used to model a floor slab loaded out of plane. The reason for this is that the single element cannot accurately represent the true deflected shape of the slab.
A useful rule of thumb is as follows: Build a model and get results. Then submesh the model into smaller elements and compare the new results to the previous results. If the results don’t change by more than 10%, you’re probably close to the correct solution. This rule of thumb is not fool-proof. It is possible to build a bad model to start with and then have the submeshed model still be a bad model. If you’re in doubt about a particular model’s accuracy, you can always send us your model via email and the staff here at RISA will be able to give you some comments on the model.
Why can’t I get a Deflected Plot or a Member Detail Report when I run an Envelope Solution?
When you get results from an envelope solution, the results are often from different load combinations at different locations on the same member. This means that a deflected shape would be very discontinuous and wouldn’t really have any physical meaning. As for the Member Detail report, we may add the ability in a future release for the Member Detail report to show the enveloped forces and unity checks.
Why don’t my Reactions satisfy statics when I run a Response Spectrum Analysis?
The applied loads in a response spectrum analysis (RSA) are the applied masses times the accelerations in the response spectra. The modes of vibration, each of which contain a certain percentage of the mass, are combined using a modal combination method. Unless you use the Dominant Mode feature, any modal combination method will cause the results to be all positive. Because all the results are positive, statics will no longer be satisfied. As a simple example, consider a symmetric portal frame, subjected to a lateral nodal load applied at one of the top corners.
The net upward reactions should be zero and the net lateral reactions should equal the applied force. The vertical deflections of the top two nodes should have equal magnitudes, but opposite signs. Now, if the signs of the vertical deflections were forced to be the same (as would happen in an RSA), you would get a net upward or downward force, instead of having the upward and downward forces in the opposite columns canceling each other out.
Why does the Deflected Plot from a Response Spectrum Analysis look “strange”?
The results from any Response Spectrum Analysis (RSA) will all be positive. This is due to the modal combination methods used to obtain the RSA results. Since the results are all positive, ( instead of being positive and negative ) the deflected plot will be discontinuous at locations where the result would normally have been negative if it had been computed by a regular static analysis. To get a better feel for this, you can compare the deflected shapes of two simple models.
Take a single bay portal frame subjected to a lateral load applied to one of the corners and perform a static analysis. Then subject the same model to dynamic loading via an RSA. If you zoom in on the results for the RSA, you will see that both columns are going UP, instead of one column going up and one going down. You should see a discontinuity at the location where the column in the statics solution would be going down.
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