21.1 Welded Steel Connections
Key Takeaways
- A welded connection must transfer force through the weld metal, heat-affected and connected base metal, and the supporting member; the weld is only one link.
- For a standard equal-leg 90-degree fillet weld, effective throat is `0.707` times leg size, while effective length follows the AISC 15th-edition geometry rules rather than gross drawn length.
- Complete-joint-penetration and partial-joint-penetration groove welds have different effective areas, access, qualification, and applicable strength checks.
- An eccentric weld group carries direct force plus moment-induced shear whose magnitude and direction vary with location; vector combination governs the critical point.
- LRFD weld demand must be compared with LRFD design strength and ASD demand with allowable strength, including connected-material yielding, rupture, and block shear checks.
A weld transfers force only when its geometry, metal, connected parts, and load path all work together. A strong weld deposited on a thin plate can simply move failure into the plate. For July 2026, use the April 2024 PE Civil: Structural specification, current PE Civil Reference Handbook, and the AISC Steel Construction Manual 15th edition. Do not import the April 2027 reference set.
Read the Connection Before Calculating
Start with the force path and weld symbol. Identify the connected parts, force direction, weld type, leg or groove dimensions, length, continuity, access, and whether demand is concentric or eccentric. Then select LRFD or ASD and keep demand and resistance on that basis. A weld drawn in elevation may occur on one side, both sides, intermittently, or all around; counting nonexistent segments is a common error.
Fillet welds have an approximately triangular cross-section. For an equal-leg weld joining surfaces at 90 degrees, the effective throat is
t_e = 0.707w
where w is fillet leg size. Effective weld area is throat times effective length, not leg size times visible length. AISC rules address minimum and maximum size, effective length, end returns, terminations, and long end-loaded joints. If a problem states effective length, do not subtract an end correction again.
Groove welds can be complete-joint-penetration or partial-joint-penetration. A CJP groove weld extends through the joint thickness as qualified and can develop different base-metal behavior than a PJP weld with a specified effective throat. Groove angle, root opening, backing, access, welding position, and procedure affect execution. “Full-length groove weld” does not by itself prove CJP. Use the symbol and effective-throat provisions.
Worked Concentric Fillet-Weld Strength
Two effective 6.0 in-long fillet welds, one on each side of a plate, transfer concentric shear. Each weld has leg size w = 1/4 in and uses an E70 electrode. The problem directs the AISC LRFD fillet-weld model F_nw = 0.60F_EXX, with φ = 0.75, and states the full 6.0 in is effective. Find weld-metal design strength.
Total effective length:
L_e = 2(6.0) = 12.0 in
Effective throat:
t_e = 0.707(0.25) = 0.1768 in
Effective weld area:
A_we = t_eL_e = (0.1768)(12.0) = 2.121 in^2
Nominal weld strength:
R_n = (0.60)(70 ksi)(2.121 in^2) = 89.1 kips
LRFD design strength:
φR_n = (0.75)(89.1) = 66.8 kips
If factored shear is 60.0 kips, weld-metal utilization is 60.0/66.8 = 0.898, so that stated limit state passes. If the problem separately gives connected-plate design rupture strength of 55 kips, the connection still fails because the base material governs. Never stop after the first passing resistance.
The calculation assumes force distribution appropriate to the stated concentric pair and the problem-provided AISC model. Weld directionality provisions, load angle, effective length, and electrode compatibility must follow the actual question.
Eccentric Weld Groups
When load misses the weld-group centroid, replace it at the centroid with the same force plus torque T = Pe. Direct shear is distributed through effective weld area or length as the adopted elastic method specifies. Moment-induced shear at a weld point is tangent to the radius from the group centroid and proportional to radius:
q_T = Tr/J_w
where J_w is the weld group's polar property on the same line- or area-based convention used for q_T. Direct and torsional shear are vectors; at one point they reinforce and at another they oppose. Adding only magnitudes everywhere can misidentify the critical location.
Use this sequence:
- Locate the effective weld-group centroid.
- Resolve forces and calculate moments about that centroid.
- Compute group length, area, and directional properties consistently.
- Evaluate direct and moment-induced components at candidate extreme points.
- Combine vectors and compare the largest resultant with compatible resistance per unit length or stress.
- Check deformation or instantaneous-center behavior if the problem requires that method.
For combined shear and tension, use the AISC interaction and directional provisions identified by the question; do not invent a scalar sum.
Base Metal and Detail Checks
The connected plate, gusset, flange, web, or HSS face may govern through gross yielding, net-section rupture, block shear, local bending, shear yielding, tear-out, crippling, or punching-type behavior. At an HSS wall, connection flexibility and local limit states can control before weld metal. An eccentric weld can also bend the connected element. Check the supporting member and the connection between it and the next component.
Weld quality depends on compatible base and filler metal, qualified procedure, fit-up, access, preheat or heat control where required, and inspection. Longer or larger welds are not automatically better: excess heat and restraint can increase distortion or residual stress, and an oversized weld can be difficult to place soundly. Intermittent welds may be prohibited in certain exposure or load conditions and must meet length and spacing rules.
Final Audit
Label every dimension as leg, throat, actual length, or effective length. Preserve kip-in versus kip-ft in eccentricity. Then verify weld metal, base metal, block shear, local supporting-member behavior, and force transfer beyond the connection. The governing strength is the smallest compatible design or allowable resistance, not the most convenient weld calculation.
When weld groups join parts of different thickness, also verify through-thickness demand, weld access, and whether force entering through one plane creates secondary eccentricity. Connection geometry controls both structural demand and practical inspectability.
Two 6 in effective fillet welds have 1/4 in legs. What is their total effective throat area using t_e = 0.707w?
How should direct shear and torque-induced shear be combined at a point in an elastic eccentric weld group?
A weld-metal LRFD design strength is 66.8 kips, but the connected plate's compatible design rupture strength is 55 kips. What is the connection strength for these two checked modes?