Classification of Joint Stiffness

The role of Joint Stiffness in Structural Integrity

In steel design, the behavior of connections often defines how forces travel, how frames deform, and ultimately how safe and economical a structure becomes.

Yet often times our designs still rely on simplified assumptions: a joint is either “rigid” or “pinned.” This simplification can potentially be uneconomical or unsafe that risk overlooking critical structural behavior.

• Rigid assumptions overestimate moment transfer, often resulting in overdesign.
• Pinned assumptions ignore rotational stiffness, potentially overestimating deflections and redistributing internal forces incorrectly.
• Semi-rigid behavior is common in most structure but is frequently bypassed in favor of conservatism or for design work to be done quickly.  

Ignoring the actual stiffness behavior of joints leads to unexpected deflections, misaligned force paths, over-design, or worse unsafe joint performance. If we want to design structures for performance, safety, and efficiency, as well as save resources/expenses on big structures or you have critical members in your building that is very costly to procure so you need to know the precise deflection, then to be precise, a stiffness classification should not be optional but, should be essential.

What Would be the Implications if you Don't Classify Joint Stiffness?

Why should we bother with connection stiffness and what would be the implications if you don't classify the stiffness of the joints?

• Inaccurate structural behavior in global analysis that affects the moment distribution and under-estimation of structure deformation.
• Inadequate detailing and design of joints (e.g. Connections may have been designed to resist zero moment-pinned; when in fact, they attract moment due to partial rigidity.
• Overdesign or worse under-design of structural members due to inaccurate moment distribution.
• Stability implication especially to seismic and wind dominated regions.
• Vulnerability to disproportionate collapse if joints are semi-rigid and misclassifying it as rigid or pinned may cause you to miss potential alternate load path.
• Misleading vibration and dynamic response if joints are modelled perfectly rigid or pinned when in reality it is semi-rigid then natural frequency of structure is mispredicted. (Example: Take an office floor, if rigid connections, you'll predict a stiffer floor and higher natural frequency resulting less vibration. If semi-rigid, it will flex more, lowering its natural frequency into the range where footsteps can excite it.) 
• Inaccurate or unconservative fire resistance prediction if designed as rigid as likely most connections' loose its stiffness and strength faster than members during fire.
• And the list goes on...

In short, modeling assumption must be consistent with connection provided; otherwise, you risk non-compliance. Not classifying the stiffness of the joints can ripple into stability and durability performance.

The Eurocode and AISC provide frameworks for categorizing joint stiffness—but implementing this in real design practice requires more than theory. 

How the Standards Classify Connection Stiffness

The Eurocode (EN 1993-1-8) and AISC (360) defines three types of connections:

1. Rigid or Fully Restrained
2. Semi-Rigid or Partially Restrained
3. Pinned or Simple

In Eurocode (EN 1993-1-8 Cl. 5.2.2), classification is based on the initial rotational stiffness, S_j,ini of the connection relative to the beam's stiffness.

Where:

• S_j,ini = Initial rotational stiffness of the connection
• E = Young's modulus of Elasticity
• I_b = Theoretical length of analyzed beam
• L_b = Beam's span
• E*I_b = Flexural stiffness of analyzed beam

In AISC 360-16 Commentary B3.4, classification is based on the secant stiffness, K_s measured at the service load level.

Where:

• K_s = secant stiffness measured at service level
• E = Young's modulus of Elasticity
• I = Theoretical length of analyzed beam
• L = Beam's span
• E*I = Flexural stiffness of analyzed beam

While the values for both of these standards appeared to be different, these figures are derived from the moment–rotation (M–θ) curve, which represents how much rotation a joint experience under increasing moment.

To evaluate these accurately, engineers must:

• Design the joint geometry (plates, bolts, welds),
• Simulate the moment–rotation curve,
• Compare it against each code thresholds,
• Then decide whether the joint is rigid, semi-rigid, or pinned.

This process, while conceptually simple, is tedious and prone to error especially if you have multiple types of connections - unless you have the right tool. This is where IDEA StatiCa steps forward, by enabling engineers to move from assumptions to a verified and code-compliant design.

Approach to Stiffness Classification in Connection application

While IDEA StatiCa uses the same stiffness analysis approach for both AISC and Eurocode even if these standards differ in methodology, it uses code-independent stiffness classification that generates the full moment–rotation (M–θ) behavior of the connection, primarily for practical and computational consistency. This approach aligns not just for both standards Eurocode and AISC, but also for most of the national standards worldwide.

Information on other national standards can also be found from the articles listed below:

• Joint Classification according to Eurocode (EN)
• Steel Joint Classification according to American Standard (AISC)
• Steel Joint Classification according to Canadian Standard (CSA)
• Joint Classification according to Australian Standard (AS)
• Classification according to stiffness for Indian standard (IS)
• Classification according to stiffness for Hong Kong Code (HKG)
• Joint classification according to Chinese standard (GB)
• Joint classification according to Russian standards (SP)

How is Classification of Joint Being Done in IDEA StatiCa?

You can run stiffness analysis in IDEA StatiCa Connection application. For a much quicker workflow, you can make use of your global FEA or CAD models to export your connections through BIM Link. This webinar provides insightful discussion on the stiffness analysis of steel connections. Or try from one of our available tutorials. 

To carry-out stiffness analysis, you would need to select from the drop-down list the analysis type in connection application to stiffness.

Once the geometry of the members is created and operations are established, you would need to specify which member is the analyzed member and set a theoretical length for My, Mz. 

The theoretical length has a behavior similar to the buckling length. These are geometric distances between supports for the purpose of the bending of the element.

So, what load effects do we need to apply? 

The recommended approach is to use only the moment since the code contains information on the stiffness classification under bending moment. 

But why do we also consider load effect of moment + shear in stiffness analysis? 

According to the validations, the results produced by the simultaneous application of shear and moment are the closest to the experimental results. However, if you analyze for moment + shear and there are hundreds of possible combinations, this may also lead to hundreds of rotational springs just for the single node which is impractical for design.

Watch this webinar about stiffness classification for much deeper understanding.

After running the analysis, you will obtain the results on the classification of your joints which could either be Rigid, Semi-rigid or Pinned. 

And most of all, all the important parameters are displayed in a table for ease of interpretation.

You can then make use of rotational stiffness to input into your global FEA analytical model. With this, your model is aligned with the actual behavior of the structure.

Conclusion

IDEA StatiCa Connection application streamlines the stiffness classification process and automatically assigns stiffness class and extracting precise rotational stiffness parameters. By integrating these parameters into your global FEA analytical models, you align your design with the true structural behavior of the joints ensuring more accurate load distribution, realistic deformation predictions, and reliable safety checks. This avoids unsafe assumptions and eliminates unnecessary overdesign, ultimately delivering structures that are both safe and economical.