Module 9: Steel Connections and Base Plates - Theory & Concepts

Module 9: Steel Connections and Base Plates

Steel Connections

Connections form the critical junctions in a structural steel framework. Design must ensure that forces transfer safely between members (e.g., beam-to-column, tension bracing, base plates) without localized failure (e.g., bearing, block shear, bolt rupture, weld tearing).

Bolted Connections

Bearing-Type vs. Slip-Critical Connections

Structural bolted connections are classified into two main categories based on how they transfer load:

Bearing-Type Connections: The bolts are installed to a snug-tight condition. When the load is applied, the connected plates slip slightly until they bear directly against the bolt shank. The load is transferred through the shearing resistance of the bolt and the bearing resistance of the plates against the bolt. These are common, economical, and used when slight slip is acceptable.
Slip-Critical Connections: High-strength bolts are highly pre-tensioned during installation, clamping the connected plates tightly together. The load is transferred purely through the static friction between the faying surfaces. Slip is entirely prevented under service loads. These are mandatory for oversized holes, slotted holes parallel to the load, and joints subject to fatigue, impact, or stress reversal (e.g., seismic forces).

Checklist

Evaluating the shear strength of individual bolts (for Bearing-Type):

$$
R_n = F_{nv} A_b
$$

Checklist

Evaluating the bearing strength of the connected material. This ensures the steel plate doesn't tear out (edge distance failure) or crush against the bolt shank:

$$
R_n = 1.2 L_c t F_u \\le 2.4 d t F_u
$$

Checklist

Bearing-Type Joints: Designed assuming bolts will slip and bear directly against the holes. The connection relies on the shear resistance of the bolts and the bearing capacity of the steel plates. Threads may be included (N) or excluded (X) from the shear plane, affecting the nominal shear stress ( $F_{nv}$ ). Standard bolt holes ( $d_h = d_b + 1/16"$ ) are typically used.
Slip-Critical Joints: Designed with high-strength bolts (e.g., A325, A490) tightened to a specific minimum tension ( $T_b$ ). Friction between the faying surfaces transfers the load, preventing any slip before the service load is reached. Required for connections subjected to fatigue, vibration, or stress reversal (e.g., seismic forces).

$$
R_n = \\mu D_u h_f T_b N_s
$$

Hole Considerations

Hole types include Standard, Oversized (used in slip-critical joints to facilitate erection), Short-Slotted, and Long-Slotted. Minimum edge distances must strictly adhere to NSCP/AISC tables based on bolt diameter to prevent tear-out.

Key Takeaways

Bolted connections transfer loads either through direct bearing (bearing-type) or clamping friction (slip-critical).
The capacity of a bearing-type connection is governed by the lesser of bolt shear strength or the bearing strength of the connected plates.
Slip-critical connections rely on pre-tensioned bolts and the coefficient of friction ( $\mu$ ) between the faying surfaces.

Prying Action in Tension Connections

Amplified Tension Forces

When bolted connections are subjected to pure tension (e.g., hanger connections or T-stubs), the flexibility of the connected parts (like the flange of a T-stub) can induce additional tension in the bolts known as prying action.

As the T-stub is pulled, the flange bends outward. The tips of the flange pry against the supporting surface, acting like a lever.
This lever action creates an additional prying force ( $q$ ) that adds directly to the external applied tension ( $T$ ), severely increasing the total load the bolt must carry ( $T_{total} = T + q$ ).
Mitigation: Designing the connection with thicker plates (making them stiffer and less likely to bend) drastically reduces prying forces. The AISC manual provides specific procedures to ensure the plate thickness is sufficient to limit prying action to acceptable levels.

Welded Connections

Fillet Welds

Fillet welds are the most common type of weld in structural steel, used to join members at angles (e.g., overlapping plates, T-joints). Their capacity is based on the effective throat area resisting shear forces along their entire effective length.

Checklist

The effective throat ( $t_e$ ) is the shortest distance from the root to the face of a standard 45-degree fillet weld.

$$
t_e = w \sin(45^\circ) \approx 0.707 w
$$

Checklist

Nominal shear strength of the weld metal ( $F_{nw}$ ):

$$
F_{nw} = 0.60 F_{EXX}
$$

Checklist

Nominal shear strength of the base metal:

$$
F_{BM} = 0.60 F_{y} \\quad \\text{or} \\quad 0.60 F_{u}
$$

Checklist

Limitations: Minimum and maximum weld sizes dictated by plate thickness ( $t_p$ ) to ensure proper fusion and prevent over-welding. Maximum weld size for plates $\lt 1/4"$ : $t_p$ . For plates $\ge 1/4"$ : $t_p - 1/16"$ . The effective length of a straight fillet weld must not be less than four times the nominal size, or else the size must not exceed one-quarter of the length.

Key Takeaways

Fillet welds are the most common structural weld.
Their strength depends on the effective throat area ( $t_e \approx 0.707w$ ) and the strength of the weld electrode ( $F_{EXX}$ ).
The strength of a fillet weld is governed by the effective throat area, which is typically the shortest path through the weld root.

Eccentric Connections

In many situations (like bracket connections), the applied load does not pass through the center of gravity of the fastener group (bolts or welds). This eccentricity induces both a direct shear force and a twisting moment (torsion) on the connection.

Analysis Methods for Eccentricity

Elastic Method: A conservative, traditional approach. It assumes the connection plate is perfectly rigid and the fasteners act elastically. The direct shear is distributed equally among the bolts. The torsional moment is distributed proportional to each bolt's distance from the centroid of the group. The two shear vectors are then resolved using vector addition to find the maximum loaded bolt.
Instantaneous Center of Rotation (IC) Method: A more accurate, modern approach based on ultimate strength. It recognizes that the fastener group actually rotates about an instantaneous center (not the geometric centroid) under eccentric loading. It accounts for the non-linear load-deformation relationship of bolts or welds, resulting in significantly higher, more realistic calculated capacities.

Design of Axially Loaded Column Base Plates

Column Base Plates

Base plates distribute the concentrated, high-stress axial loads from a steel column over a larger area of the underlying concrete footing. This prevents the concrete from crushing (exceeding its bearing strength,

f_c'

Checklist

Determining the required base plate area ( $A_1$ ) based on the allowable bearing pressure on the concrete:

$$
f_p = 0.85 f_c' \\sqrt{\\frac{A_2}{A_1}} \\le 1.7 f_c'
$$

Checklist

Optimizing the base plate dimensions ( $N$ and $B$ ) to minimize the thickness ( $t_p$ ) by making the cantilevered distances roughly equal.
Calculating the required base plate thickness based on bending moments induced by the bearing pressure acting as a uniform upward load on the cantilevered portions of the plate (lever arms $m$ , $n$ , and $\lambda n'$ ). The maximum of these three moment arms is termed ' $l$ '.
The minimum required thickness:

$$
t_{p,req} = l \\sqrt{\\frac{2P_u}{0.9 F_y B N}}
$$

Moment-Resisting Base Plates

When a column is part of a rigid moment frame, its base plate must resist not only the axial load (

P

) but also a bending moment (

M

). This creates a much more complex stress distribution on the foundation.

Types of Structural Welds

Beyond standard fillet welds, there are other methods of permanently fusing steel elements.

Weld Categories

Fillet Welds: The most common structural weld. They have a triangular cross-section and are used to join two surfaces at approximately right angles (e.g., lap joints, T-joints). Their strength is based on the Effective Throat—the shortest distance from the root to the face of the weld (typically $0.707 \times \text{leg size}$ ).
Groove Welds: Primarily used for butt joints where plates are aligned in the same plane. Complete Joint Penetration (CJP) groove welds develop the full strength of the base metal. Partial Joint Penetration (PJP) groove welds are used when full strength is not required or when welding from only one side is possible.
Plug and Slot Welds: Used in lap joints to transmit shear forces or prevent buckling of overlapping parts. They involve cutting a hole or slot in one plate and filling it with weld metal to fuse it to the underlying plate.

Weld Symbols

In structural drawings, standard AWS (American Welding Society) symbols dictate exactly how a connection should be welded. A standard symbol includes a reference line pointing to the joint, indicating the weld type (e.g., a triangle for a fillet weld), its size (leg dimension), its length, and whether it is performed on the "arrow side" or "other side" of the joint. Special flags indicate if the weld must be performed entirely in the field (on-site).

Key Takeaways

Eccentricity ( $e$ ): The bending moment is mathematically converted into an equivalent eccentricity of the axial load ( $e = M / P$ ).
Small Eccentricity (Compression over full plate): If the moment is relatively small ( $e \le L/6$ , where $L$ is the plate length), the entire base plate remains in compression. The pressure distribution is trapezoidal but entirely downward.
Large Eccentricity (Tension required): If the moment is large ( $e > L/6$ ), the resultant axial load falls outside the middle third of the plate. This causes the concrete on one side to go into heavy compression, while the opposite side tries to lift off the foundation. This uplift must be resisted entirely by the tension capacity of the anchor rods embedded in the concrete. The base plate thickness must then be designed for the upward bending moment from the concrete and the downward pulling moment from the anchor rods.

Key Takeaways

Bolted connections are categorized as either bearing-type (relying on bolt shear and plate bearing) or slip-critical (relying on friction).
The design strength of a fillet weld is governed by the shear capacity across its effective throat ( $0.707w$ ).
Column base plates must provide sufficient area ( $A_1$ ) to prevent concrete crushing and sufficient thickness ( $t_{p,req}$ ) to resist bending moments induced by the foundation pressure on the cantilevered overhangs.

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