Introduction
Designing end beam-column moment connections is a critical task in structural engineering, requiring careful consideration of multiple factors to ensure that the connection can safely carry the applied loads. When following AISC 360 standards, designers must check for several potential failure mechanisms, including flange local bending, web local yielding, web local crippling, web sidesway buckling, and web compression buckling. In this post, we’ll walk through these key factors and considerations in the design process to help ensure robust and efficient connections.
Factors to Consider
Several factors should be considered when designing moment connections or other flanges with concentrated forces:
Load Direction: Tension vs. Compression
One of the primary considerations in moment connection design is the direction of the concentrated force applied to the connection. Whether the load is in tension or compression can significantly impact which checks are critical for the design. For instance, flange local bending only applies when the concentrated force is in tension. Compression loads, on the other hand, require you to pay closer attention to web crippling, sidesway buckling, and compression buckling. Typically, with a moment transferring between a beam and a column, both tension and compression should be checked for a given connection.
Connection Type: Moment vs. Bearing
The type of connection—whether it's designed to transfer moment or simply bear loads—affects the complexity of the design. The web sidesway buckling provisions, equations J10-6 and J10-7, apply only to compressive forces in bearing connections and do not apply to moment connections.
Compression Flange Bracing
Proper bracing of the compression flange applies only for the Web Sidesway Buckling checks with compressive forces acting on bearing connections (and does not apply to moment connections).
Flange Load Application: One vs. Both Flanges
Web compression buckling is a key consideration when loads are applied to both flanges. If forces are applied to both flanges, the web is subject to compression from both sides, increasing the potential for web compression buckling. This could be the case for a continuous beam transferring moment through a column. However, if the load is applied to only one flange, like in the end beam-column connection shown in the example below, web compression buckling does not need to be checked.
Shape Factor: Wide Flange vs. HSS
The shape factor, Qf, impacts both the web local crippling capacity as well as web compression buckling capacity. Qf shall be taken as 1.0 for wide-flange sections and for HSS (connecting surface) in tension, or as given in Table K1.3 for all other HSS conditions (Ref: AISC Spec. J10.3 & J10.5).
Geometry and Material Properties
Geometry, including the depth, flange width, and thickness of the connected members, affects the overall capacity of the connection. Material properties such as yield strength, ultimate strength, and ductility also play a crucial role in determining the performance of the connection under applied loads. Ensuring that the geometry and material properties are compatible with the design loads is essential to prevent issues like excessive deformation or premature failure.
Other Considerations
Beyond the factors discussed above, it is important to consider any eccentricities in load application, environmental conditions (such as corrosion), and fabrication tolerances that might affect the long-term durability and performance of the connection. These may require adjustments to the design or additional detailing to ensure the connection performs as expected under real-world conditions.
Example Problem (Solutions Provided Using CalcBook): Problem Statement:
Step 1: Design Inputs
Step 2: Flange Local Bending
Step 3: Web Local Yielding
Step 4: Web Local Crippling
Step 5: Web Sidesway Buckling
Step 6: Web Compression Buckling
Step 7: Determine Controlling Demand/Capacity Ratio:
Conclusion:
End beam-column moment connections demand careful attention to various factors, from load direction and connection type to compression flange bracing and web buckling checks. By addressing these considerations, engineers can design safe and efficient connections that meet AISC 360 standards. The example above highlights the key steps and checks required for such designs, serving as a practical guide for real-world applications.