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2023-10-18

# Member Stiffness Modifications

A member stiffness modification offers various options to affect the stiffness of members.

Info

The stiffness modification is only taken into account for the structural analysis, not for the design checks in the design add-ons.

### Modification Type

Various types are available for selection in the list. The other sections of the dialog box match the modification type.

### Multiplier Factors

You can adjust the stiffness in two ways using the "Multiplier Factors":

• Total stiffnesses: All elements of the stiffness matrix are uniformly multiplied by the factor "k".
• Partial stiffnesses, weights, and masses: The elements of the stiffness matrix and the weight can be modified by specific factors.

The image New Member Stiffness Modification shows how the torsional stiffness GIt is reduced to 10%.

Tip

The FAQ 5554 describes how you can use a member stiffness modification to deactivate the self-weight of a member in the model.

### Settings

For US and Canadian standards, there are some modification options to apply stiffness factors in compliance with the standards.

#### ACI 318-19 / CSA A23.3

Select the "Component Type" from the list:

• Columns
• Beam

The factors for determining bending stiffness and axial stiffness are set according to the standard specifications.

#### AISC 360-16 / CSA S16

Define the parameters to "Determine τb" and the "Design method". When determining internal forces and moments, it is necessary to consider the factor τb for all members whose bending stiffness has a share in the stability of the structure. This coefficient depends on the axial force in the member: The larger the axial force, the larger τb is.

If τb is determined iteratively according to AISC, you have to specify the design method—LRFD or ASD. Then, RSTAB calculates the factor according to Equation (C2‑2a) or (C2‑2b) in several steps until convergence is reached. Independent of the factor τb, the reduction factor 0.8 is applied to all members for bending and axial stiffnesses, as required in AISC 360‑10.

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