Welded Joints

Welded joints create a permanent material bond and can produce light, rigid structures. Design must address the weld metal, heat-affected zone, connected parts, residual stress, distortion, imperfections, and fatigue.

Butt and fillet welds

A full-penetration butt weld is normally designed through the effective section. A fillet weld is specified by its leg size \(k\); for an equal-leg weld the effective throat is approximately

$$a\approx0.7k.$$

For total effective weld length \(l_{eff}\), the effective area is \(A_w=al_{eff}\).

Loads

For a centrally loaded weld group, average shear is

$$\tau=rac{F}{A_w}.$$

An eccentric load adds a moment. In an elastic weld-group model, direct and moment-induced stresses are combined vectorially. The most distant point of the group is usually critical.

Design considerations

• avoid abrupt changes of stiffness and intersecting welds;
• provide access, a stable welding position, and an inspectable joint;
• distinguish weld leg \(k\) from effective throat \(a\);
• use the applicable code interaction rule for combined normal and shear stress;
• for cyclic loading, improve weld-toe geometry and reduce defects because fatigue is governed by detail rather than static yield alone.

Inspection may include visual, penetrant, magnetic-particle, ultrasonic, or radiographic methods according to defect type and required quality level.

Effective dimensions

For a fillet weld, the leg size \(k\) is the distance from the root to the toe measured along the plate surface. The effective throat \(a\) is the shortest design distance from the root to the weld face; for an equal-leg ideal weld, \(a\approx0.7k\). Effective length excludes portions where the required section is not fully developed.

Weld-group analysis

A force through the centroid produces nominal stress from \(F/A_w\). An in-plane moment is distributed according to the polar property of the weld group, while an out-of-plane moment produces a nonuniform normal stress. The direct and moment components are combined at critical points using the applicable design-code rule.

Connected-part checks

A strong weld does not guarantee a strong joint. Check yielding and rupture of plates, local bending, tear-out, lamellar tearing, and buckling. Loads should enter the joint gradually; intermittent welds and end returns are used only where permitted and where corrosion and fatigue requirements allow.

Heat effects and distortion

Nonuniform heating and cooling cause shrinkage, residual stress, and angular or longitudinal distortion. Balanced weld placement, suitable sequence, restrained heat input, presets, and post-weld straightening may be required. Highly restrained thick joints also need attention to hydrogen-assisted cracking and preheat.

Fatigue design

Under cyclic loading, nominal strength of weld metal is often less important than detail category, stress range, weld-toe profile, penetration, defects, and residual stress. Avoid load-carrying attachments at highly stressed regions and grind or improve toes only under a controlled procedure.

Calculation sequence

1. Choose joint type and establish the force path.
2. Define weld leg \(k\), effective throat \(a\), and effective length.
3. Find the weld-group centroid and section properties.
4. Calculate stresses from force and moments and apply the interaction rule.
5. Check connected parts, fatigue, distortion, and brittle-fracture risk.
6. Specify welding process, sequence, acceptance level, and inspection.