Preventing Failure Webinar: The Importance of Buckling Checks in Steel Connections

As Bao indicated, buckling checks are crucial for the success of our designs. In supporting IDEA StatiCa users over the years, I often see slender elements included in designs. This makes buckling analysis an essential consideration. As engineers designing with steel, we are highly tuned to the need for checking buckling in member and frame design.

A well-known example of buckling failure is the collapse of a bridge during construction in Canada, likely due to inadequate temporary bracing. This illustrates lateral torsional buckling failure, which, while commonly associated with members, can also occur in connections if not properly addressed.

Another significant example is the 2007 I-35W Minnesota Bridge collapse, largely attributed to inadequate gusset plates. Some reports suggest these plates were too thin, leading to buckling failures or possibly shear and ultimate failures. This disaster resulted in multiple deaths and injuries, highlighting the importance of buckling analysis in design.

IDEA StatiCa’s Approach to Buckling Analysis

IDEA StatiCa is a dedicated finite element (FE) application for steel connection design. Its primary advantage is its ability to handle general geometry and loading, unlike traditional empirical methods, which only address specific connection types (e.g., rigid or simple connections). The software’s first-principles approach allows for a generalised and rigorous buckling check.

In IDEA StatiCa, plates and sections are checked using first principles, while bolts and welds are verified against design codes. Users selecting Australia will see checks to AS 4100, while other regions can access Eurocode, American, and Canadian standards.

Today, I’ll focus on the first-principles approach. There are many other aspects of IDEA StatiCa, such as integration with other applications and comprehensive solution capabilities, but those topics are for another time.

Example: Buckling Analysis in a Knee-Type Connection

Let’s look at an example: a knee-type connection. If I had a sloped beam, this could be part of a portal frame, but in this case, it’s a horizontal floor beam. I’ve added:

• The members
• The connection details
• One load case (though you can add as many as needed)

After running the calculation, we see that the connection passes the initial checks—no red warnings in the summary. However, we don’t yet know if this connection is truly safe. The buckling analysis still needs to be performed.

Understanding the Buckling Load Factor

One of the key parameters in buckling analysis is the buckling load factor (BLF), sometimes called the critical factor (α_CR). This factor relates the applied design load to the elastic buckling capacity. The buckling load factor should ideally be:

• ≥ 3 for most connections
• ≥ 4 for two-sided stiffeners
• ≥ 15 for gusset plate-connected members

Because IDEA StatiCa focuses on elastic buckling, the actual safety margin is slightly conservative. The engineer must review and interpret buckling results to ensure stability.

Practical Example in IDEA StatiCa

Running a buckling analysis in IDEA StatiCa involves:

1. Selecting buckling analysis from the Check menu
2. Reviewing buckling shapes and load factors
3. Adjusting the model or stiffness where necessary

If a buckling mode appears at a factor < 3, we may need to increase stiffener thickness, adjust weld sizes, or modify load paths.