When modeling plate elements alongside member elements in RISA, users may notice that member forces—such as bending moments or axial loads—are lower than expected. This behavior is often the result of how loads are shared between plates and members based on their relative stiffness. If the distribution of forces doesn't align with your design expectations, it may be due to unintended interaction between these elements. Accurately capturing structural behavior requires not only the right modeling approach, but also a clear understanding of how plates and members interact within the analysis. In this article, we’ll explore common scenarios where this issue can arise, explain why it happens, and provide practical tips and workarounds to ensure your results match your design intent.
Why Model Plate Elements in the First Place?
Before diving into the interaction between elements, it’s worth asking: Do I need to model plate elements at all? In many cases, member design and stability can be accurately represented using simplified area or line loads—and plates might not be necessary.
However, there are valid reasons to model plates, including:
The model is unstable and needs plate elements for diaphragm action.
Plates are part of the lateral load path.
You want a more realistic load distribution, instead of relying on tributary areas.
You need to analyze the forces and behavior of the plate elements themselves.
When plate elements are added to your model, they can share gravity and axial load with the members depending on their relative stiffness. This can lead to unexpected reductions in member forces. Let’s look at a few examples that highlight this behavior.
Consider a simple bay of steel beams with an area load:
Without plates: Expected bending moment
With plates (concrete slab modeled): Moment in beam drops to 6.7 k-ft
→ 46% reduction due to plate sharing load.
Is this reduction not aligned with your intent?
If you were expecting and intending for the beam to carry the full load (12.5 k-ft), consider:
Removing the plates and applying area loads instead.
Using the “Plane Stress” option on plate properties. This removes out-of-plane stiffness (no longer resists bending), but maintains in-plane stiffness (still acts as a diaphragm).
⚠️ Tip: When using “Plane Stress,” you may see large out-of-plane deflections. To counteract this:
Use area loads instead of plate surface loads.
Set the plate material density to zero, and account for self-weight separately.
See screenshot below showing the difference in strong axis bending between the center beam when no plates are used, when plates are used, and when plates with Plane Stress checked are used.
A similar setup using wood beams and plywood:
This example is very similar to the previous example, except that the difference in stiffness between the wood beam vs thin plywood is much greater.
Without plates: Moment = 1.25 k-ft
With plates (plywood modeled): Moment drops slightly to 1.19 k-ft
→ 5% reduction due to plywood’s lower stiffness.
Even though this reduction is small, it shows that relative stiffness matters. As before, you can:
Skip plate modeling.
Use the “Plane Stress” option to eliminate unintended stiffness contributions.
See screenshot below showing the difference in strong axis bending between the center beam when no plates are used, when plates are used, and when plates with Plane Stress checked are used.
Consider steel trusses with area load applied:
Without plates: Top chord axial force = 6.1 k
With plates (20-gauge deck): Top chord force drops to 3.9 k → 36% reduction
Bottom chord remains unaffected since plates are only on top.
This reduction is due to axial stiffness in the plates.
Workarounds:
Remove plates and use area loads.
Note: “Plane Stress” won’t help here—it affects out-of-plane stiffness (bending), not in-plane axial stiffness.
Try using an Orthotropic Plate to reduce stiffness parallel to the truss chord direction.
Model horizontal braces or dummy elements instead of plates. In one test, this reduced chord force to 5.1 k (still a 16% reduction).
See screenshot below showing the difference in axial force between the center top chord when no plates are used, when plates are used, and when h-braces are used instead of plates.
The same behavior is observed with wood trusses and plywood sheathing—similar trends. See screenshot below showing the difference in axial force between the center top chord when no plates are used, when plates are used, and when h-braces are used instead of plates.
The interaction between plate and member elements can significantly affect analysis results. Here’s how to maintain control:
Review member forces closely when plates are included.
Model both ways (with and without plates) if you need to compare effects or extract plate results separately from member design.
Use hand calculations for expected bending moments or chord forces to benchmark your model behavior.
With a careful approach and clear understanding of your design goals, you can get the most out of your RISA models—while avoiding unintended load sharing that could compromise design accuracy.