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Assigning Proper Fixities

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How should I set the fixities in a model that represents a truss with pinned joints?

 

When working with truss models with pinned joints, consider setting the following Joint Boundary Restraints:

 

Joint

X Translation

Y Translation

Z Rotation

Notes

Typical

Unrestrained

Unrestrained

Restrained

Although rotation at the joints is restrained, moment transfer is prevented by member end releases.

Support 1

Restrained

Restrained

Restrained

Support 2

Unrestrained

Restrained

Restrained

 

Even though the support joints are modeled as Restrained against rotation, all members framing into those joints will have their Local z Rotational degree of freedom released at both ends, so no moment will be delivered to the supports from the members.

 

There must be at least two "support joints" to create global stability of the entire truss. Typically, we see two support joints with Y translational restraints and one support joint with X translational restraint, but many conditions are possible.

 

Next, consider the following settings the Member End Releases:

 

Member Ends

Local x Translation

Local y Translation

Local z Rotation

Notes

Typical

Unreleased

Unreleased

Released

Members are hinged at the joints.

 

 

 

How should I set the fixities in my moment frame model with fixed bases?

 

When working with portal frames with moment connected joints and fixed bases, consider setting the following Joint Boundary Restraints:

 

Joint

X Translation

Y Translation

Z Rotation

Notes

Typical

Unrestrained

Unrestrained

Unrestrained

Allows joints to freely move in X and Y and rotate about Z.

Supports

Restrained

Restrained

Restrained

Fixed base.

 

Consider the following settings the Member End Releases:

 

Member Ends

Local x Translation

Local y Translation

Local z Rotation

Notes

Typical

Unreleased

Unreleased

Unreleased

Members are continuous through the joints.

 

 

 

How should I set the fixities in my moment frame model with pinned bases?

 

When working with portal frames with moment connected joints and pinned bases, consider setting the following Joint Boundary Restraints:

 

Joint

X Translation

Y Translation

Z Rotation

Notes

Typical

Unrestrained

Unrestrained

Unrestrained

Allows joints to freely move in X and Y and rotate about Z.

Supports

Restrained

Restrained

Unrestrained

Pinned base.

 

Consider the following settings the Member End Releases:

 

Member Ends

Local x Translation

Local y Translation

Local z Rotation

Notes

Typical

Unreleased

Unreleased

Unreleased

Members are continuous through the joints.

 

 

 

How should I set the fixities in a model that includes a mix of some pin-connected members and some continuous members?

 

When working with models with a mix of fixity conditions, consider setting the following Joint Boundary Restraints:

 

Joint

X Translation

Y Translation

Z Rotation

Notes

Typical

Unrestrained

Unrestrained

Unrestrained

Allows joints to freely move in X and Y and rotate about Z.

 

Consider the following settings the Member End Releases:

 

Member Ends

Local x Translation

Local y Translation

Local z Rotation

Notes

Typical

Unreleased

Unreleased

Released

Members are hinged at the joints.

Continuous

Unreleased

Unreleased

Unreleased

Members are continuous through the joints.

 

The next step is to achieve stability. Review the model and check for any joints where the joint boundary restraints are Unrestrained – Unrestrained – Unrestrained. These joints must be rigidly connected to at least one member end with an Unreleased Local z Rotational fixity.

 

If any such nodes are found, they must be stabilized by ensuring that at least one member end connecting to the joint has an Unreleased Local z Rotational fixity.

NOTE: Be cautious during this process. If more than one member end at a joint is Unreleased for Local z Rotation, continuity forms at that location, and the joint no longer behaves as fully pinned. Only one member end must be rigidly connected to the joint to ensure stability while preserving the fully hinged behavior.

 

Finally, you must focus on the support nodes.  If pinned supports are desired, consider setting the following Joint Boundary Restraints:

 

Joint

# of member ends at support with

Local z Rotation = Unreleased

X Translation

Y Translation

Z Rotation

Pinned Supports

0

Restrained

Restrained

Restrained

1+

Restrained

Restrained

Unrestrained

 

If fixed supports are desired, consider setting the following Joint Boundary Restraints:

 

Joint

X Translation

Y Translation

Z Rotation

Fixed

Supports

Restrained

Restrained

Restrained

 

Just make sure that the appropriate member ends are modeled with Unreleased Local z Rotational fixities in order for those members to be able to transfer moment to the fixed bases.

 

 

 

How can I model partially fixed bases in a frame model?

 

At present there is no explicit method of defining a base with partial moment fixity, but it can still be modeled with a little creativity. Consider modeling something like the concept shown below:

 

clip0003

 

In this model, members A and E are fictitious members that are only there for the purpose of providing partial moment fixity at the bottoms of members B and D. With this concept, it would be possible to experiment with the section and the member length for members A and E to achieve the desired rotational stiffness at joints 1 and 4.