When creating a structural model in STAAD.Pro, defining the correct support condition is one of the most important modeling steps. Supports determine how the structure is restrained and directly influence the reactions, displacements, internal forces and overall stability of the analytical model.
The STAAD.Pro SUPPORT command is used to define these boundary conditions at joints. Depending on the structural situation, you can define FIXED, PINNED, FIXED BUT, spring-supported and other specialized support conditions.
A support is not simply a graphical symbol at the bottom of a column. In the analytical model, it represents constraints on the joint's six degrees of freedom (DOF):
Translation in global X —
FXTranslation in global Y —
FYTranslation in global Z —
FZRotation about global X —
MXRotation about global Y —
MYRotation about global Z —
MZ
Understanding these six degrees of freedom is the key to understanding the STAAD.Pro SUPPORT command.
What is the STAAD.Pro SUPPORT Command?
The SUPPORTS command defines support conditions at specified joints in a STAAD.Pro model.
A simplified form of the command is:
SUPPORTS
joint-list PINNED
joint-list FIXED
joint-list FIXED BUT release-spec [spring-spec]The STAAD.Pro technical reference describes PINNED, FIXED, and FIXED BUT support specifications and provides options for releasing individual degrees of freedom or assigning spring constants. (SEFindia)
For example:
SUPPORTS
1 3 5 PINNED
2 4 6 FIXEDHere, joints 1, 3 and 5 receive pinned supports, while joints 2, 4 and 6 receive fixed supports.
The exact support you should use depends on the physical connection and restraint you are trying to represent, not simply on whether a support "looks like" a particular symbol in a drawing.
Understanding the Six Degrees of Freedom
Before looking at individual STAAD.Pro support types, understand the six DOF.
| DOF | Meaning | Type |
|---|---|---|
FX | Translation along global X | Force |
FY | Translation along global Y | Force |
FZ | Translation along global Z | Force |
MX | Rotation about global X | Moment |
MY | Rotation about global Y | Moment |
MZ | Rotation about global Z | Moment |
In a typical STAAD.Pro model, the global Y-axis is commonly used as the vertical direction, although this depends on how the model was created.
Therefore, in a conventional model:
FYcontrols vertical translation.FXandFZcontrol horizontal translations.MX,MY, andMZcontrol rotations.
The important point is that FX/FY/FZ are translational restraints, while MX/MY/MZ are rotational restraints.
1. PINNED Support in STAAD.Pro
A PINNED support restrains translation but releases rotation.
Conceptually:
| DOF | Condition |
|---|---|
| FX | Restrained |
| FY | Restrained |
| FZ | Restrained |
| MX | Free |
| MY | Free |
| MZ | Free |
The STAAD.Pro technical reference describes PINNED as a support with translational restraints and no rotational restraints; consequently, it does not provide moment-carrying capacity. (SEFindia)
STAAD.Pro syntax
SUPPORTS
1 2 3 PINNEDThis assigns pinned supports to joints 1, 2 and 3.
What does a pinned support mean?
In a simplified structural model, a pinned support can resist:
Horizontal force
Vertical force
But not support moments
This makes it useful when the real structural connection is intended to behave as a hinge or when a structural idealization requires rotational freedom.
However, don't automatically use PINNED simply because a connection is called a "hinge" in architectural or construction terminology. The analytical idealization should correspond to the structural behavior being modeled.
2. FIXED Support in STAAD.Pro
A FIXED support restrains all six degrees of freedom.
Conceptually:
| DOF | Condition |
|---|---|
| FX | Restrained |
| FY | Restrained |
| FZ | Restrained |
| MX | Restrained |
| MY | Restrained |
| MZ | Restrained |
Syntax
SUPPORTS
1 2 3 FIXEDThe technical reference defines a FIXED support as having both translational and rotational restraints. (SEFindia)
A fixed support therefore allows the structure to develop reactions in:
FX
FY
FZ
MX
MY
MZ
When is FIXED appropriate?
Use a fixed support when your structural idealization requires the joint to have no translation and no rotation relative to the global coordinate system.
But there is an important modeling warning:
A physical column base should not automatically be modeled as FIXED just because it is connected to a foundation.
Whether the base behaves as pinned, fixed, semi-rigid or something between these conditions depends on the actual connection, foundation stiffness and design assumptions.
3. FIXED BUT Support
This is one of the most useful STAAD.Pro support types.
FIXED BUT starts from a fixed support and allows you to release selected degrees of freedom.
For example:
SUPPORTS
5 6 FIXED BUT MX MY MZThis restrains translation in X, Y and Z while releasing rotation about all three axes.
That is equivalent to the usual idealized pinned condition.
The STAAD.Pro technical documentation describes FIXED BUT as allowing individual global directions to be released and also allowing spring constants to be assigned. (SEFindia)
4. STAAD.Pro FIXED BUT Syntax
The general idea is:
FIXED BUT release-specThe release specification can contain:
FX
FY
FZ
MX
MY
MZFor example:
SUPPORTS
10 FIXED BUT FXThis means the joint is fixed in all DOF except translation in global X.
Similarly:
SUPPORTS
10 FIXED BUT MZreleases rotation about the global Z-axis while keeping the other DOF restrained.
5. Modeling a Roller Support
One of the most useful applications of FIXED BUT is modeling a roller-type support.
Suppose the support is:
Free to move in X
Restrained in Y
Restrained in Z
Restrained against rotation
You could represent that idealization as:
SUPPORTS
10 FIXED BUT FXHere FX is released, while the remaining DOF remain restrained.
A technical-reference example similarly shows how FIXED BUT can be used to release selected translational and rotational degrees of freedom. (SEFindia)
Important
The direction of the roller must correspond to your global coordinate system.
If the roller is intended to move along global Z instead, the release would be:
FIXED BUT FZSo don't copy a support definition without first checking your model's global axes.
6. FIXED BUT with Spring Support
FIXED BUT becomes even more powerful when spring stiffness is introduced.
STAAD.Pro allows spring constants such as:
KFX
KFY
KFZ
KMX
KMY
KMZThese correspond to spring stiffness in the respective translational or rotational DOF. (SEFindia)
For example:
SUPPORTS
20 FIXED BUT MZ KFY 125.0This means the support has:
Fixed behavior in the other restrained DOF
Released
MZA vertical spring stiffness
KFYof 125 in the current model units
The technical reference provides examples of this type of support definition. (SEFindia)
7. Understanding KFX, KFY, KFZ, KMX, KMY and KMZ
These parameters define spring stiffness.
| Parameter | Spring direction |
|---|---|
KFX | Translation along global X |
KFY | Translation along global Y |
KFZ | Translation along global Z |
KMX | Rotation about global X |
KMY | Rotation about global Y |
KMZ | Rotation about global Z |
For example:
KFY 500defines a vertical translational spring if global Y is your vertical direction.
Units matter
Spring constants must be entered using the current units of the STAAD.Pro model. The technical reference specifically notes that spring constants are provided in the current units. (studylib.net)
This is an easy place to make a serious modeling error.
A spring value that is correct in one unit system can be completely wrong if entered into a model using another unit system.
8. Example: Vertical Spring Support
Suppose a column base is represented using a vertical spring.
You could use:
SUPPORTS
15 FIXED BUT MX MY MZ KFY 1000The interpretation is:
MX,MY,MZare releasedOther translational DOF remain restrained
Vertical stiffness is represented by
KFY = 1000
This type of idealization can be useful when the foundation-soil system is being represented through an equivalent spring rather than a perfectly rigid support.
However, the spring stiffness should come from an appropriate structural/geotechnical basis.
Do not choose a spring value just because it makes the model converge.
9. FIXED vs PINNED vs FIXED BUT
Here's the quick comparison.
| Support | Translation | Rotation |
|---|---|---|
| FIXED | Restrained | Restrained |
| PINNED | Restrained | Free |
| FIXED BUT | User-defined | User-defined |
| Spring | Based on stiffness | Based on stiffness |
This is the easiest way to remember the concept:
FIXED = everything restrained
PINNED = translation restrained, rotation free
FIXED BUT = choose what to release or spring
10. Practical Example of a STAAD.Pro SUPPORT Command
Consider a simple structure with:
Joint 1: fixed base
Joint 2: pinned base
Joint 3: roller-like support
The support commands could be:
SUPPORTS
1 FIXED
2 PINNED
3 FIXED BUT FXInterpretation:
Joint 1
1 FIXEDAll six DOF are restrained.
Joint 2
2 PINNEDAll three translations are restrained, while rotations are released.
Joint 3
3 FIXED BUT FXGlobal X translation is released while the remaining DOF remain restrained.
11. Multiple Joints Can Share One Support Definition
STAAD.Pro allows multiple joints to be included in the joint list.
For example:
SUPPORTS
1 TO 8 PINNEDassigns pinned support conditions to joints 1 through 8.
You can also combine joint numbers:
SUPPORTS
1 3 5 7 PINNEDThis is useful for larger models because you don't have to repeat the same support definition for every node.
12. Support Conditions and Global Axes
This is one of the most important points when creating FIXED BUT supports.
The FX, FY, FZ, MX, MY, and MZ specifications refer to the global support directions for normal global support definitions. The STAAD.Pro technical reference distinguishes these from inclined-support axes when inclined supports are used. (SKYSCRAPERS)
Therefore, before defining a roller or partial restraint:
Check the global X direction.
Check the global Y direction.
Check the global Z direction.
Determine which physical movement should be permitted.
Release the corresponding DOF.
Example
If the physical roller is intended to move horizontally along global X:
FIXED BUT FXIf it is intended to move along global Z:
FIXED BUT FZThe support definition must follow the model's coordinate system, not the orientation of the drawing on your screen.
13. FIXED vs ENFORCED Support
STAAD.Pro also provides ENFORCED and ENFORCED BUT support types.
These should not simply be treated as alternative names for FIXED.
Bentley's technical support explains that for FIXED and FIXED BUT, fully restrained DOF are treated as known zero displacements and are excluded from the stiffness-matrix unknowns. For ENFORCED and ENFORCED BUT, STAAD represents restrained DOF using very high stiffness springs, allowing their displacements to be calculated. (Bentley Systems)
This distinction becomes particularly relevant when dealing with support displacement loading and models containing plates or solids.
For a normal beginner model, you may encounter FIXED much more often, but engineers working with more advanced models should understand why ENFORCED exists.
14. Common STAAD.Pro SUPPORT Command Mistakes
Mistake 1 — Making every column base FIXED
This is one of the most common modeling shortcuts.
If the actual structural system does not provide full rotational restraint, using FIXED may make the analytical model significantly stiffer than the real structure.
The correct support condition should be based on the structural idealization and connection behavior.
Mistake 2 — Using PINNED everywhere
The opposite mistake is also possible.
Making every base pinned may remove rotational restraint that actually exists in the structural system.
The support condition must represent the intended behavior.
Mistake 3 — Confusing FX with MX
Remember:
F = force/translation
M = moment/rotation
Therefore:
FXmeans release translation in X.
Whereas:
MXmeans release rotation about X.
They are completely different DOF.
Mistake 4 — Ignoring units for springs
If you use:
KFY 1000you must know what the 1000 means in the current unit system.
Never copy a spring constant from another model without checking its units.
Mistake 5 — Using a spring to artificially stabilize an unstable model
A spring should represent a physical or analytical stiffness.
It should not be added simply because STAAD.Pro reports instability.
If your model is unstable, first investigate:
Missing supports
Incorrect connectivity
Releases
Member orientation
Duplicate/missing joints
Mechanisms
Incorrect support directions
A spring is not a substitute for correcting a bad structural model.
STAAD.Pro SUPPORT Command Quick Reference
| Command | Meaning |
|---|---|
PINNED | Translation restrained, rotation released |
FIXED | All six DOF restrained |
FIXED BUT FX | X translation released |
FIXED BUT FY | Y translation released |
FIXED BUT FZ | Z translation released |
FIXED BUT MX | X rotation released |
FIXED BUT MY | Y rotation released |
FIXED BUT MZ | Z rotation released |
KFX | X translational spring |
KFY | Y translational spring |
KFZ | Z translational spring |
KMX | X rotational spring |
KMY | Y rotational spring |
KMZ | Z rotational spring |
Frequently Asked Questions
What is the SUPPORT command in STAAD.Pro?
The SUPPORTS command defines boundary conditions at joints, including pinned, fixed, partially released and spring-supported conditions.
What is the difference between FIXED and PINNED in STAAD.Pro?
A FIXED support restrains all six degrees of freedom. A PINNED support restrains the three translations but allows rotation. (SEFindia)
What is FIXED BUT in STAAD.Pro?
FIXED BUT allows selected degrees of freedom to be released from an otherwise fixed support. It can also be used with spring stiffness values. (SEFindia)
How do I define a roller support in STAAD.Pro?
A roller-type support can be modeled using FIXED BUT by releasing the translational direction in which the support is allowed to move. For example, FIXED BUT FX releases movement in global X.
What are KFX and KFY in STAAD.Pro?
KFX and KFY define translational spring stiffness in the global X and Y directions respectively. Similar parameters exist for Z translation and rotational springs. (SEFindia)
Can I use FIXED BUT for a soil spring?
Yes, FIXED BUT can be combined with spring stiffness definitions. However, the stiffness should be based on an appropriate engineering model rather than selected arbitrarily.
Final Takeaway
The STAAD.Pro SUPPORT command is fundamentally about defining what a joint is allowed to do.
Instead of memorizing commands, think in terms of the six degrees of freedom:
TRANSLATION
FX FY FZ
ROTATION
MX MY MZThen choose the support condition that represents the structural behavior.
FIXED
↓
Nothing moves or rotates
PINNED
↓
Translations restrained
Rotations free
FIXED BUT
↓
You choose what is released
SPRING
↓
Movement/rotation resisted according to stiffnessThe most important rule is simple:
Do not select a STAAD.Pro support condition because it makes the analysis pass or produces a favorable result. Select it because it represents the structural behavior you are trying to model.
That distinction separates a model that merely runs successfully from one that is actually a useful representation of the structure.
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