Steel Connection Type Determines Whether the Joint Can Transfer Moment - and That Determines the Structural System
A steel structure is only as strong as its connections. The type of connection between beams and columns determines whether the connection can transfer only shear (a simple or "pin" connection) or whether it can also transfer bending moment (a moment connection or "rigid" connection). This distinction drives the structural system: a frame with simple connections has no moment resistance in the frame and must rely on shear walls or braced frames for lateral resistance; a frame with moment connections develops moment frames capable of resisting lateral forces through frame action. The ARE PPD exam tests structural steel connection types as part of the structural systems and construction technology content area.
Simple Connections (Shear Connections)
A simple connection (also called a shear tab connection or single plate connection) is designed to transfer vertical shear forces from the beam to the column, but not to resist bending moment. Simple connections are designed with minimal rotational restraint - the beam end can rotate slightly relative to the column. Common simple connection types: the shear tab (a single plate welded to the column web and bolted to the beam web); the clip angle connection (angles bolted to both the beam web and the column flange or web); and the seated connection (a bracket attached to the column that provides a seat for the beam end). Simple connections are more economical to fabricate than moment connections and are the default connection type for gravity framing when lateral resistance is provided by other means.
Moment Connections
A moment connection is designed to transfer both shear and bending moment between beam and column. To resist moment, the connection must be able to develop tension and compression forces in the beam flanges (where moment stress is highest). Common moment connection types: the fully welded moment connection (beam flanges and web are field-welded to the column); the bolted flange plate moment connection (welded flange plates on the column are field-bolted to the beam flanges); and the end plate moment connection (a plate is shop-welded to the beam end and field-bolted to the column face). Moment connections are significantly more expensive to fabricate and inspect than simple connections; they are used only where moment frame action is required for the structural system.
Shop vs. Field Connections
Steel connections are either made in the fabrication shop (before the steel is shipped to the site) or in the field (during erection). Shop welds are generally higher quality (better controlled conditions, qualified welders, less weather exposure, and closer quality monitoring) than field welds. Field bolting is faster and easier to inspect than field welding. The preferred sequencing: complex welding in the shop; simple bolted connections in the field. Most moment connections today are designed as shop-welded, field-bolted - the complex parts of the connection are shop-fabricated with high quality, and the field work is reduced to bolting pre-fabricated components together.
Key Exam Points
- Simple connection: transfers shear only; allows rotation; more economical; used for gravity framing.
- Moment connection: transfers shear and moment; develops forces in flanges; required for moment frame action.
- Common simple connections: shear tab, clip angle, seated connection.
- Common moment connections: fully welded, bolted flange plate, end plate.
- Shop vs. field: shop welds = higher quality; field bolting = faster; preferred: shop-welded, field-bolted.
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