The leg size ($a$) is the physical length of the weld legs contacting the base plates. The effective throat ($t_e$) is the shortest distance from the joint root to the weld face. For a standard $90^\circ$ joint, $t_e = a \cdot \cos 45^\circ = 0.707 \cdot a$. Structural strength is calculated solely based on throat thickness ($t_e$), as this represents the weakest plane prone to shear failure.

When starting and stopping an arc weld, the welder leaves shallow craters at both ends of the run. These end craters contain voids, surface depressions, and slag inclusions that lack structural integrity. Design standards require deducting one leg size ($a$) from each weld end, meaning the effective length $L_e = L - 2a$ per line. Stresses are calculated using this reduced effective length.

Shop welding is performed in controlled, windless factory environments, using rotating positioners that allow optimal flat-downhand welding. Field welding is performed on site, exposing the process to wind, humidity, variable preheats, and restricted worker access (overhead or vertical welds). For example, IS 800:2007 applies a safety factor $\gamma_{mw} = 1.25$ for shop welds and a more conservative $\gamma_{mw} = 1.50$ for field welds to account for this variation.

Thick steel plates act as huge heat sinks, rapidly drawing heat away from the welding zone. This rapid cooling rate causes the Heat Affected Zone (HAZ) to transform into a hard, brittle martensitic structure, which is highly susceptible to hydrogen-induced underbead cracking. Preheating the plates to $100^\circ\text{C}-150^\circ\text{C}$ slows the cooling rate, allowing hydrogen to diffuse out safely and producing a softer, more ductile microstructure.

When fusion faces are not perpendicular ($90^\circ$), the effective throat changes. Standards modify the multiplier factor $K$ based on the face angle $\theta$ to locate the shortest path through the weld. For angles between $60^\circ$ and $90^\circ$, $K=0.707$. As the angle increases (obtuse joints), the throat becomes shallower, requiring a smaller multiplier (e.g. $K=0.50$ at $120^\circ$) to keep calculations conservative and safe.

Transverse fillet welds (welds oriented perpendicular to the direction of load) exhibit more uniform stress distributions across the throat and do not experience shear lag. In contrast, longitudinal fillet welds (welds parallel to the load direction) experience concentrated shear stress at the weld ends, leading to localized progressive failure. AISC 360 recognizes this by allowing a 50% increase in allowable stress for transverse welds ($1.0 + 0.5\sin^{1.5}\theta$).

The Heat Affected Zone (HAZ) is the band of base metal immediately adjacent to the weld pool that has not been melted, but has had its microstructure and properties modified by the intense welding heat. The HAZ is critical because it represents a structural discontinuity where grain growth, tempering, or hardening occurs, frequently reducing ductility and local toughness. In many alloy steels, the HAZ is the location most prone to brittle failure and stress corrosion cracking.

A Complete Joint Penetration (CJP) butt weld extends through the entire thickness of the joint, completely fusing the interface. A CJP weld has a design strength equal to the base metal, eliminating local notch effects at the root. A Partial Joint Penetration (PJP) weld leaves the root unfused. PJPs are cheaper and easier to fabricate but have reduced strength and act as built-in stress concentrators, which limits their use in cyclic fatigue or high-pressure applications.

The von Mises yield criterion predicts failure under multiaxial loading by calculating a single equivalent tensile stress $\sigma_{eq}$. For weld check zones, the interaction between normal stress $\sigma$ (axial + bending) and shear stress $\tau$ (direct + torsional) is calculated as: $\sigma_{eq} = \sqrt{\sigma^2 + 3\tau^2}$. The weld is considered safe if $\sigma_{eq} \le F_{w,allow}$, where $F_{w,allow}$ is the code-specified allowable design stress.

Notch sensitivity is the measure of how a material's fatigue strength is reduced by geometric discontinuities (notches). Welds are inherently notch-sensitive due to the abrupt change in profile at the weld toe. Designers reduce notch sensitivity by: (1) grinding the weld toe profile to a smooth curve, (2) executing smooth buttering layers, (3) specifying full penetration joints, and (4) avoiding placement of weld joints in high-tension regions prone to cyclic stresses.