Spooky season is here with ghastly ghosts, glowing jack-o’-lanterns, and wicked witches around every corner. While Halloween scares can be fun, unexpected surprises in a structural model are a different story.
This spooky season, keep an eye out for a few frighteningly common modeling issues in RISA-3D. Catching these modeling concerns early can help you create models that are a treat to analyze, not a trick.
Does it ever feel like there’s a ghost in your model?
You run your analysis and discover a LOCKED reaction in the results. Don’t be scared, these reactions are not supernatural. They are created by RISA-3D when the model is not adequately supported for a specific degree of freedom.
Locked nodes provide temporary restraint to help stabilize the model during analysis, but they should not be treated as the final solution. Because these added restraints do not report the load removed from the model, they can obscure the intended load path. Investigate and resolve the underlying instability so you can confirm that loads are transferring through the structure as expected.
For a closer look at common sources of RISA-3D instability warnings and ways to troubleshoot LOCKED reactions, read The Ultimate Guide to Instability Warnings in RISA-3D:
Black cats might be a sign of bad luck if you’re superstitious, but twisted, distorted, or overlapping plates are a definite sign of poor plate modeling.
Plate geometry, element size, and mesh quality affect how RISA-3D calculates plate behavior. Plates with sharp angles, excessive distortion, or overlapping regions can produce less reliable finite element results. When you encounter problematic geometry, redraw or subdivide the plates to create a cleaner, more consistent mesh.
A little model cleanup can help you avoid analysis results that are more frightening than helpful.
Running an analysis only to find that nearly every member is failing can be scarier than the haunted house next door.
Before assuming the design is doomed, review the members’ unbraced lengths. Unbraced length can significantly affect member capacity, especially for compression and flexural design checks. By default, RISA-3D conservatively assumes that the unbraced length is the full member length unless you manually define a different value.
Member unbraced lengths can be updated based on bracing points in the framing and the applicable construction type. A quick review of these values may turn a design nightmare into a much more manageable result.
Learn how member unbraced length affects RISA-3D member design and capacity calculations.
Like the spiderwebs in the attic, successful plate design depends on how the plates are connected.
In RISA-3D, plate elements transfer load through their corner nodes. When applying point loads or interacting with adjacent elements, the plate nodes must align for the load to transfer as intended. If nodes do not connect properly, the model may not reflect the expected load path.
Learn more about RISA-3D node connectivity, plate to member interaction, and plate load transfer in the articles below.
Dressing up in costume to hide your identity is part of the fun of Halloween. Hidden load paths, however, are much less entertaining.
Incorrect or misassigned boundary conditions can unintentionally remove load from the model or transfer it in a way you did not intend. This is especially important at upper levels, where wall panels and members must transfer loads correctly to the supporting elements below.
Review boundary conditions carefully to confirm that walls, diaphragms, and members are behaving as intended. Pay particular attention to vertical load transfer between levels, as well as the distinction between boundary conditions and member end releases.
Explore these resources for a closer look at RISA-3D wall-panel boundary conditions, member end releases, and member load transfer behavior.
A few quick modeling checks can help keep scary analysis behavior from creeping into your RISA-3D model. Review support conditions, member unbraced lengths, node connectivity, plate geometry, and load transfer assumptions before relying on your results.
That way, the only things haunting your October are ghosts, goblins, and maybe a few leftover pieces of candy.