Connections and Base Plates

When a steel column bears directly on a concrete foundation, the steel's yield strength ($F_y = 50$ ksi) is significantly higher than the concrete's compressive strength ($f'_c \approx 3$ to $5$ ksi). A thick steel base plate is welded to the bottom of the column to increase the bearing area ($A_1$), ensuring the actual bearing pressure on the concrete remains below its allowable limit.

Base Plate Design Concepts:

• Bearing Area ($A_1$): The physical plan area of the steel base plate ($B \times N$). This is the area directly loaded by the column.
• Concrete Support Area ($A_2$): The maximum area of the portion of the supporting concrete surface (e.g., the pier or footing) that is geometrically similar to and concentric with the loaded area $A_1$. This is crucial for confinement effects.
• Bearing Limit State: The concrete must safely support the required axial compressive load ($P_u$). This dictates the minimum required area ($A_1$).
• Flexural Limit State: The base plate acts as an inverted cantilever beam projecting outward beyond the stiff column flanges and web. The uniform upward pressure from the concrete tries to bend the plate upward. The plate must be thick enough ($t_p$) to resist bending yielding ($M_u \le \phi M_p$).

The design bearing strength of the concrete foundation ($\phi_c P_p$) depends heavily on the confinement provided by the surrounding concrete.

Case 1: Plate covers the full area of the concrete support ($A_1 = A_2$) This occurs when the base plate exactly matches the size of a concrete pier. There is no confinement.

Case 2: Plate covers less than the full area of the concrete support ($A_2 > A_1$) This is the most common case (a base plate on a large footing). The surrounding unloaded concrete confines the loaded concrete directly beneath the plate, significantly increasing its crushing strength. The confinement multiplier $\sqrt{A_2/A_1}$ is capped at 2.0 (hence the $1.7 f'_c A_1$ upper limit).

Once the required area ($A_1 = B \times N$) is determined, the required thickness ($t_p$) is calculated by treating the projecting portions of the base plate as cantilever beams. The critical bending planes occur at the faces of the column flanges and web.

This is derived by equating the required bending moment per unit width ($M_u = \frac{q l^2}{2}$, where $q = \frac{P_u}{BN}$) to the flexural design strength of a rectangular plate ($\phi M_p = \phi F_y \frac{1 \times t_p^2}{4}$).

While anchor rods can resist some shear, they are primarily designed for tension (uplift) and erection stability. When massive shear forces exist at the column base (e.g., at the base of a braced frame or a shear wall), relying solely on anchor rods is often insufficient or problematic due to bending in the rods.

Friction The primary and most efficient mechanism for transferring shear is friction between the bottom of the base plate and the grout pad/concrete footing. The friction force is $V_f = \mu P_u$, where $\mu$ is the coefficient of friction (typically 0.40 for steel on concrete). If the minimum sustained axial load ($P_u$) is large enough, friction alone may suffice.

Shear Lugs If friction and anchor rods are insufficient, a shear lug must be utilized. A shear lug is a thick steel plate or piece of structural shape welded perpendicularly to the bottom of the base plate. It is embedded into a grout pocket formed into the concrete footing.

This allows massive shear forces to be transferred directly into the foundation via direct bearing of the lug face against the confined grout and concrete. The lug must be designed for bending and shear, and the concrete must be checked for bearing failure and breakout.

Anchor rods (historically called anchor bolts) are cast into the concrete foundation before the steel is erected. They are used to resist net uplift (tension) from wind, to resist overturning moments, to provide some shear resistance, and, crucially, to provide stability during erection before the rest of the frame is tied together. OSHA legally mandates a minimum of four anchor rods per column base for erection safety.

Anchor Rod Failure Modes (ACI 318 Chapter 17) While the steel rod itself is designed per AISC, the connection's capacity is almost always governed by the concrete's capacity to hold the rod. The design of cast-in anchors is dictated by ACI 318 (Building Code Requirements for Structural Concrete).

• Steel Tension Breakout: The steel rod fractures under pure tensile load. $\phi = 0.75$.
• Concrete Breakout in Tension: A massive, shallow cone of concrete pulls out of the foundation. This is the most common failure mode for shallow anchors or anchors near an edge. It depends heavily on the effective embedment depth ($h_{ef}$) and edge distances.
• Pullout / Blowout: The anchor head crushes the concrete locally and pulls straight out, or the concrete cover blows out laterally.
• Steel Shear Failure: The rod fractures under lateral shear force (often governed by bending if a grout pad is present).
• Concrete Edge Breakout in Shear: The lateral shear force pushes a chunk of concrete off the edge of the footing. This is critical for columns located at the perimeter of a slab or foundation wall.

Prying Action in Base Plates: If a base plate is subjected to an overturning moment, it pries against the concrete. This prying action increases the tensile force in the anchor rods significantly beyond the nominal uplift, and the base plate thickness must be increased to mitigate it.

The AISC Specification classifies structural connections based on their stiffness, specifically their ability to transfer bending moments while maintaining the original angle between the connected members. This is quantified by a moment-rotation ($M-\theta$) curve.

• Simple Connections (Shear Connections): Designed to transfer shear forces only. They are highly flexible and assumed to allow the beam end to rotate freely under load without transferring any significant bending moment to the column. Example: A standard double-angle web connection.
• Fully Restrained (FR) Moment Connections: Designed to transfer both shear and the full bending moment. They are rigid enough that the angle between the intersecting members remains virtually unchanged under load (zero relative rotation). Example: A connection where the beam flanges are Complete Joint Penetration (CJP) welded directly to the column flange.
• Partially Restrained (PR) Moment Connections: Fall between Simple and FR connections. They transfer some moment but also experience significant rotation. The structural analysis of the frame must explicitly account for the non-linear flexibility (the specific $M-\theta$ curve) of the PR connection, making it much more complex to design. Example: A connection using thick top and bottom flange angles bolted to the column.

Unlike simple (shear) connections, which allow rotation and transfer only vertical shear force (e.g., a single-plate shear tab), moment connections deliberately restrict rotation, forcing the transfer of massive bending moments. They are essential for rigid frame structures resisting lateral wind or seismic loads without diagonal bracing.

Design Considerations for Welded Flange FR Connections In a typical Fully Restrained (FR) connection, the beam web is bolted (or welded) to the column flange to resist the shear force ($V_u$). The beam flanges are Complete Joint Penetration (CJP) welded directly to the column flange to resist the entire bending moment ($M_u$).

This bending moment is resolved into a massive force couple: tension ($T$) in one flange and compression ($C$) in the other.