Welded Connections

Analysis and design of welded connections, covering fillet, groove, plug, and slot welds.
Welding provides continuous connections by melting the base metal and adding a filler metal to form a single, homogenous structural element. It allows for rigid joints, complex geometries, and highly efficient load transfer without the reduction in cross-sectional net area typical of bolted connections. However, welding requires skilled labor, stringent quality control, and is highly sensitive to environmental conditions (wind, temperature).

Welding Processes

The methods used to deposit weld metal in structural steel construction.
  • Shielded Metal Arc Welding (SMAW): The classic "stick welding." Uses a consumable electrode coated in flux. Versatile, portable, excellent for field work, but slow and leaves slag that must be chipped off.
  • Gas Metal Arc Welding (GMAW): Also known as MIG. Uses a continuous wire feed and an external shielding gas (like argon/CO2) instead of flux. Fast, clean, but sensitive to wind. Best for shop fabrication.
  • Flux-Cored Arc Welding (FCAW): A hybrid. Uses a continuous tubular wire filled with flux. Extremely fast deposition rates. The standard process for heavy structural shop and field welding today.
  • Submerged Arc Welding (SAW): The arc is buried under a mound of granular flux. Extremely high deposition rates and deep penetration, but limited to flat or horizontal positions. Ideal for long, continuous welds like plate girders.

Joint Types

The configuration of the structural members being connected.
Structural steel can be joined in several distinct ways depending on the geometry of the connection:
  • Butt Joint: Two plates are aligned in the same plane and joined edge-to-edge. Typically uses groove welds.
  • Lap Joint: Two plates overlap and are joined at their edges. Always uses fillet welds or plug/slot welds.
  • Tee Joint: One plate is placed perpendicular to another to form a "T" shape. Uses either fillet or groove welds.
  • Corner Joint: Two plates meet at their edges to form a right angle.
  • Edge Joint: The edges of two parallel plates are joined together.

Weld Types

The cross-sectional shape and function of the deposited weld metal.

Fillet Welds

  • The most common structural weld, accounting for ~80% of all structural connections.
  • Deposited in the interior corner formed by two plates meeting at an angle (typically 90∘90^\circ).
  • Assumed to have a right-triangular cross-section.
  • Size (ww): The length of the leg of the largest inscribed right triangle.
  • Effective Throat (tet_e): The shortest distance from the root to the face of the theoretical triangle (te=wcos⁡45∘=0.707wt_e = w \cos 45^\circ = 0.707w). The strength of a fillet weld is entirely based on shear stress across this minimum throat area, regardless of the direction of the applied load.

Groove Welds

  • Used primarily for butt joints where plates are aligned, or tee/corner joints requiring high strength. The edges of the plates are often prepared (beveled or V-grooved) to allow the weld to penetrate deeply.
  • Complete Joint Penetration (CJP): The weld metal fully penetrates the entire thickness of the base metal. If a "matching" filler metal is used (e.g., E70 for A36/A992 steel), the CJP weld is considered equal to or stronger than the base metal. No complex calculations are needed; you simply design the base metal member.
  • Partial Joint Penetration (PJP): The weld only penetrates partway through the thickness. Designed similarly to fillet welds based on an effective throat area (te<tplatet_e < t_{plate}), which depends on the groove angle and welding process.

Plug and Slot Welds

  • Used to connect overlapping plates (lap joints) where there is insufficient edge length for a fillet weld, or to prevent buckling of thin outer plates.
  • A round hole (plug) or elongated hole (slot) is punched in the top plate and filled with weld metal to fuse with the bottom plate.
  • Strength is calculated based on the shear capacity of the nominal area of the hole (AwA_{w}).

Electrode Classification

Specifying the strength of the filler metal.
Electrodes are classified by their minimum ultimate tensile strength.
  • E70XX: FEXX=70F_{EXX} = 70 ksi minimum tensile strength. (The standard "matching" electrode for A36 plates and A992 W-shapes).
  • E60XX: FEXX=60F_{EXX} = 60 ksi. (Rarely used in modern structural work).
  • E80XX, E90XX: High-strength electrodes for specialized high-strength low-alloy steels.

Design Strength (Fillet Welds)

Calculating the capacity of the most common connection.
The design strength of a fillet weld is always based on the shear strength of its minimum effective throat area, even if the load is applied in tension or compression relative to the weld axis.

Weld Design Strength

Calculates the nominal capacity of a welded connection.

$$ R_n = F_{nw} A_{we} $$
  • ϕ=0.75\phi = 0.75 (LRFD).
  • The 0.600.60 factor converts tensile strength to shear strength (von Mises criterion).

Simplified Formula (E70XX)

For standard E70XX electrodes loaded parallel to the weld axis:
Fnw=0.60×70 ksi=42 ksiF_{nw} = 0.60 \times 70 \text{ ksi} = 42 \text{ ksi}te=0.707wt_e = 0.707 wRn=42×0.707w×L≈29.69wLR_n = 42 \times 0.707 w \times L \approx 29.69 w LϕRn=0.75×29.69wL≈22.27wL\phi R_n = 0.75 \times 29.69 w L \approx 22.27 w L
Rule of Thumb: The LRFD design strength is approximately 1.3921.392 kips per inch of length per 1/161/16 inch of weld leg size. (e.g., A 1/4" fillet weld (w=4/16w=4/16) carries 4×1.392=5.574 \times 1.392 = 5.57 kips/in).

Weld Inspection and Defects

Ensuring the quality and integrity of welded joints.
Unlike bolted connections, which can often be visually inspected for proper installation (e.g., checking for the correct number of bolts and using a torque wrench), internal weld quality is impossible to verify by eye alone.

Common Weld Defects

  • Porosity: Small gas pockets or voids trapped inside the weld metal, usually caused by poor shielding gas coverage or moisture on the electrodes.
  • Slag Inclusions: Solid non-metallic material (flux) trapped within the weld metal or between the weld and base metal. Common in multipass SMAW or FCAW welding if the slag is not properly chipped away between passes.
  • Undercut: A groove melted into the base metal adjacent to the toe of the weld and left unfilled by weld metal. This creates a severe stress concentration.
  • Lack of Fusion / Incomplete Penetration: The weld metal fails to completely fuse with the base metal or fails to reach the required depth in a groove weld. This drastically reduces the strength of the joint.

Non-Destructive Testing (NDT)

To detect these defects, various NDT methods are employed:
  • Visual Inspection (VT): The first line of defense. Checks for proper size, profile, and surface defects like severe undercut or surface porosity.
  • Dye Penetrant Testing (PT) & Magnetic Particle Testing (MT): Used to detect microscopic surface-breaking cracks or defects.
  • Ultrasonic Testing (UT): Uses high-frequency sound waves to detect internal flaws (lack of fusion, deep slag inclusions). It is the primary method for inspecting Complete Joint Penetration (CJP) groove welds.
  • Radiographic Testing (RT): X-raying the weld to produce an image of internal defects. Highly accurate but expensive and hazardous.

Directional Strength Increase

Fillet welds loaded transversely (perpendicular to their longitudinal axis) are significantly stronger ( 50%~50\% stronger) than welds loaded longitudinally (parallel to their axis). AISC allows an increase factor: (1.0+0.50sin⁡1.5θ)(1.0 + 0.50 \sin^{1.5}\theta), where θ\theta is the angle between the weld axis and the load direction. However, transverse welds are less ductile.

Base Metal Checks

The connection is only as strong as its weakest link. Usually, the weld metal is stronger than the base metal, but shear failure of the connected plates adjacent to the weld must be checked independently.
  • Base Metal Shear Yielding: ϕ=1.00\phi = 1.00, Rn=0.60FyAgvR_n = 0.60 F_y A_{gv} (Gross area subject to shear, L×tplateL \times t_{plate}).
  • Base Metal Shear Rupture: ϕ=0.75\phi = 0.75, Rn=0.60FuAnvR_n = 0.60 F_u A_{nv} (Net area, which equals gross area for welds since there are no holes).

Balancing of Welds

Aligning the connection's resistance with the applied force.
When connecting unsymmetrical members (like single angles connected by one leg) subjected to axial forces, the centroid of the member does not align with the center of a symmetrical weld group. This eccentricity induces unwanted bending moments in the connection.
To "balance" the weld, the designer proportions the lengths of the longitudinal fillet welds on the heel and toe of the angle so that the centroid of the weld group perfectly matches the centroid of the angle. This requires setting up a static equilibrium equation (ΣM=0\Sigma M = 0) about one of the weld lines to solve for the required lengths L1L_1 and L2L_2.

Eccentric Weld Groups

Welds subjected to combined shear and torsion/bending.
When an external load is applied eccentrically to a weld group (e.g., a bracket welded to a column flange supporting an offset load), the weld must resist both direct shear (P/LP/L) and a twisting moment (PePe).

Instantaneous Center of Rotation (ICR) Method

The preferred, exact method in the AISC Manual for analyzing eccentric weld groups. It is highly non-linear. It assumes the weld group rotates rigidly around an "instantaneous center." The deformation of each weld segment is proportional to its distance from the ICR.
Because the equations are extremely complex to solve by hand, designers rely entirely on the tabulated coefficients (CC) in AISC Manual Part 8. The capacity is simply Rn=CC1DlR_n = C C_1 D l, where CC is the coefficient from the table based on geometry, C1C_1 adjusts for electrode strength, DD is the weld size in sixteenths, and ll is the length.

Elastic Vector Method

A simplified, conservative, traditional approach. It assumes the load translates to the centroid of the weld group, causing uniform direct shear (fv=P/Awef_v = P/A_{we}), plus a twisting moment that causes torsional shear stresses proportional to the distance from the centroid (ft=Mr/Ipf_t = M r / I_p). These vectors are added geometrically at the most critical point to find the maximum stress.

Minimum and Maximum Sizes

Constructability limits for fillet welds.

Minimum Size (wminw_{min})

Depends solely on the thickness of the thicker part joined. A small weld on a massive, thick plate will cool so rapidly (due to the plate acting as a heat sink) that the weld metal becomes brittle and cracks, or fails to fuse properly.
  • tthicker≤1/4→wmin=1/8t_{thicker} \le 1/4 \to w_{min} = 1/8 in.
  • 1/4<tthicker≤1/2→wmin=3/161/4 < t_{thicker} \le 1/2 \to w_{min} = 3/16 in.
  • 1/2<tthicker≤3/4→wmin=1/41/2 < t_{thicker} \le 3/4 \to w_{min} = 1/4 in.

Maximum Size (wmaxw_{max})

Depends on the thickness of the thinner part, specifically when welding along the edge of a plate. If the weld is too large, the heat will melt away the edge of the plate, reducing its effective thickness.
  • Along edge of material <1/4< 1/4 in thick: wmax=tw_{max} = t.
  • Along edge of material ≥1/4\ge 1/4 in thick: wmax=t−1/16w_{max} = t - 1/16 in. (The 1/16" gap ensures the inspector can verify the corner wasn't melted off).

Fillet Weld Strength

Weld Design Strength (φR_n)0.0 kips

Status

OVERSTRESSED

DCR = Infinity

Note: Base metal yielding and rupture limit states must also be checked independently.
Key Takeaways
  • The strength of a fillet weld is universally controlled by shear stress across its effective throat (0.707w×L0.707 w \times L), regardless of load direction.
  • Welds connecting unsymmetrical members (like single angles) often need to be "balanced" (unequal lengths) to align the resistance centroid with the load centroid, avoiding bending moments.
  • E70XX electrodes (FEXX=70F_{EXX} = 70 ksi) are standard for welding A36 and A992 structural steel.
  • Weld size is strictly limited by maximum and minimum requirements based on the connected materials' thicknesses to ensure proper heat distribution, prevent rapid cooling (cracking), and prevent melting the base metal edges.
  • Base metal shear capacity (yielding and rupture) must always be checked alongside weld metal capacity.
  • Eccentrically loaded weld groups are complex to analyze by hand; engineers rely on the Instantaneous Center of Rotation (ICR) tables in the AISC Manual.
PreviousBolted Connections - Examples & Applications
Quiz Me
NextWelded Connections - Examples & Applications