STAAD PLANE - what are the assumptions

Question:

I have modeled a continuous beam. The support at the left end is PINNED. The bending moment diagram shows a moment at the support. See the figure.

Please explain the reason for this.

Answer:

This generally occurs if you have

• chosen PLANE as the structure type when you created the model
• applied loads that have a component along the global Z direction

The expression STAAD PLANE is a way to instruct STAAD that the structure spans in the global XY plane only. The structure does not have any components that span along global Z, and does not deform along global Z.

To enforce this, for such models, the Z-translation, X-rotation and Y-rotation at every node becomes restrained automatically during the analysis. Thus, during the analysis, a support with the specification

FIXED BUT FX FY MZ

gets added to every node in the model. This support is in addition to the supports that you explicitly declare in your input.

So, if a load is applied that has a force component along global Z, or a moment component about the global X or Y axes that causes the structure to rotate about the global X or Y axes is applied, the afore-mentioned support will prevent that. When a support resists a load, that leads to a reaction forming there.  In other words, an out-of-plane load is considered to be incompatible with a PLANE type of structure.

That is why, in the output file, you will see a warning message that an out-of-plane load has been applied on a PLANE structure.

In your model, check whether you have applied any load using instructions such as

MEMBER LOAD
member-list UNI GZ load-intensity

or

JOINT LOAD
joint-list GZ load-value

Since the rotation about Y is considered restrained even at PINNED supports, such loads will cause a moment to develop there as is evident in the figure in the question above.

In a PLANE frame, a “force” type of load should be applied only along GX or GY, and a moment should be applied only about “GZ”.

While it is true that there can be structures that span the global XZ plane or the global YZ plane, those do not qualify as the PLANE type in STAAD.Pro.

If you wish to apply a load along GZ, you need to change the structure type to “STAAD SPACE”.