Understanding Moment Connections in Steel Structures
In steel frame construction, moment connections play a crucial role in transferring both shear and moment forces between connected members. These connections are categorized as fully restrained (Type FR) or partially restrained (Type PR), each with distinct behaviors. Understanding the different types of moment connections and their limit states is essential for designing safe and efficient steel structures.
Types of Moment Connections
Even though they are referred to as moment connections, these connections are designed to transfer both shear and moment forces between the connected structural elements. Moment connections are classified as fully restrained (Type FR) or partially restrained (Type PR), based on their behavior. Fully restrained connections maintain zero relative rotation between members while transferring moment, whereas partially restrained connections allow for predictable relative rotation alongside moment transfer. Simple shear connections, on the other hand, rotate freely without transferring any moment. Real connection behavior may deviate from the ideal, as demonstrated by the moment-rotation curves in Fig 1.
5 Commonly Used Moment Connections
Let’s examine five moment connections frequently utilized for connecting beams to the strong axis of columns as shown in Fig 2:
- Direct-Welded Flange Connection: This connection involves field-welding the flanges to the supporting member using complete joint penetration groove welds. The web connection usually comprises a single plate welded to the column and bolted to the beam. The direct-welded flange connection is a straightforward and efficient method, requiring minimal components.
- Welded Flange Plate Connection: In this type of connection, flange forces are transferred to corresponding plates through bolt shear or weld shear. The plate forces are then transmitted to the supporting member through welds. Proper sizing of the flange plate connectors (bolts or welds) ensures effective force transfer at the plate-flange interface.
- Bolted Flange Plate Connection: Similar to the previous connection, the flange forces are transferred through bolts and plates. Bolted flange plate connections offer an alternative when an all-bolted connection is required. Although slightly more complex, they provide a solution for specific design needs.
- Bolted Tee Connection: The bolted tee connection is employed when a bolted connection to the supporting member is necessary. The beam flange connection follows a similar approach to the flange plate connections, while the connection to the support resembles the tension connection.
- Extended End Plate Connection: The extended end plate connection takes multiple forms, but the underlying principle involves fully welding an end plate to the end of the beam and subsequently bolting it to the support. To accommodate moment reversal, the end plate must extend beyond both the top and bottom flanges.
Limit States in Moment Connections
- Bolts: Limit states for bolts include shear rupture, tension, shear-tension interaction, and bearing and tear-out.
- Welds: Weld limit states encompass tension rupture and shear rupture.
- Plates: Plate limit states involve compression buckling, tension yielding, tension rupture, shear yielding, shear rupture, and block shear.
- Beam: The beam side of the connection is subject to limit states such as flexure of reduced section, shear yield, and shear rupture.
- Column: The impact of the connection on the column includes limit states such as flange local bending, web local yielding, web local crippling, web compression buckling, and web panel zone shear.
Engineers must analyze connection behavior and its impact on the overall structure, considering relevant limit states to ensure structural integrity. By selecting appropriate moment connections and considering limit states, engineers can build resilient steel structures capable of withstanding diverse loads and forces.
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