Welded connections

Welding joins steel by melting the parts and a filler metal together so that they solidify as one piece. It makes compact, stiff connections with no holes, it can join members at any angle, and it is the natural way to attach stiffeners, base plates and connection plates in the shop. Field welding is slower and needs good conditions and inspection, so the usual practice in North America is to weld in the shop and bolt in the field. This lesson introduces welding processes and weld types, then designs fillet welds for the truss chord and for the monorail bracket.

Welding processes

All arc welding processes strike an electric arc between an electrode and the work, melting both, and shield the molten pool from the air, which would make the weld porous and brittle.

• Shielded metal arc welding (SMAW), or stick welding, uses a coated electrode whose coating produces the shielding gas and slag. It is simple and portable, and common for field work and repairs.
• Flux-cored arc welding (FCAW) feeds a tubular wire filled with flux continuously, with or without an external shielding gas. It is fast and is the most common process in structural fabrication shops and for field welding.
• Gas metal arc welding (GMAW) feeds a solid wire with a gas shield. It is clean and fast but sensitive to wind.
• Submerged arc welding (SAW) buries the arc under a granular flux and is used for long, heavy shop welds such as the flange-to-web welds of plate girders.

In the United States, welding of structural steel follows AWS D1.1, Structural Welding Code — Steel. Prequalified joint details and procedures may be used without testing; others must be qualified by test, and welders must be qualified for the processes and positions they use. The filler metal is matched to the steel; E70 electrodes, with a tensile strength \(F_{EXX} = 70\) ksi, match A992 and A572 Grade 50 steel.

Types of welds

Fillet welds have a roughly triangular cross-section and join surfaces at an angle, usually 90 degrees, in lap, tee and corner joints. They need no edge preparation, so they are the cheapest and most common welds, perhaps 80 percent of all structural welding. A fillet weld is specified by its leg size, and its strength comes from the throat, the shortest distance from the root to the face, which is 0.707 times the leg for an equal-leg weld.

Groove welds fill a gap or a beveled groove between the parts. A complete-joint-penetration (CJP) groove weld fuses the full thickness and, with matching filler metal, is as strong as the base metal, so no calculation of weld strength is needed. Partial-joint-penetration (PJP) welds fuse only part of the thickness and are designed for their effective throat. Groove welds need edge preparation and often backing bars and ultrasonic testing, so they cost much more than fillet welds. Plug and slot welds fill holes in one part to join it to another and are used mainly to stitch plates together.

Welds are shown on drawings with standard welding symbols: an arrow to the joint, a reference line, a symbol for the weld type below the line for the arrow side or above it for the other side, and the size, length and pitch.

Strength of fillet welds

Tests show that fillet welds loaded transversely, with the load perpendicular to the weld axis, are about 50 percent stronger than welds loaded longitudinally, along their axis, though less ductile. Both standards include this directional effect.

AISC 360 gives the design strength of a fillet weld, per unit length, with the directional increase:

where \(w\) is the leg size and \(\theta\) is the angle between the load and the weld axis. When longitudinal and transverse welds share a load, the directional increase is limited, because the stiffer transverse welds fracture before the longitudinal welds reach their strength. For E70 electrodes loaded longitudinally, each sixteenth of an inch of leg is worth \(0.75(0.60)(70)(0.707)/16 = 1.392\) kips per inch, so a 1/4 in. fillet gives 5.57 kips/in. The base metal is checked for shear yielding (\(\phi = 1.00\), \(0.6F_y\)) and shear rupture (\(\phi = 0.75\), \(0.6F_u\)) along the weld.

Size and length limits

A weld that is too small for thick parts cools too quickly and may crack, so AISC 360 Table J2.4 sets minimum fillet sizes based on the thinner part joined: 1/8 in. for material up to 1/4 in., 3/16 in. over 1/4 to 1/2 in., 1/4 in. over 1/2 to 3/4 in., and 5/16 in. beyond. Along the edge of a plate, the maximum leg is the plate thickness less 1/16 in. for plates 1/4 in. or thicker, so the edge is not melted away. The effective length of a fillet weld should be at least four times its size, and end returns around corners help resist prying and fatigue.

The largest single-pass fillet weld is about 8 mm (5/16 in.). Larger welds need several passes and cost much more, so two small welds on both sides of a plate usually beat one large weld.

Welding the truss chord

In lesson 3 net fracture or block shear at the bolt holes governed the bottom chord. Owen asks the fabricator whether the chord angles could be shop-welded to the gussets, with the truss shipped in two halves and spliced with bolts at midspan. Welding removes the holes from most of the chord, so gross yielding would govern at the welded ends.

The weld along the heel of an angle and the weld along the toe are at different distances from the centroid of the angle. If the two welds have equal length, the connection is eccentric and the heel weld is overloaded. A balanced connection sizes each weld so that the resultant of the weld forces passes through the centroid of the angle: the heel weld takes the fraction \((b - \bar x)/b\) of the force and the toe weld takes \(\bar x/b\).

For single angles and tees in light trusses, many fabricators simply use equal welds and accept the small eccentricity; the standards allow this for statically loaded members. Balancing matters more for members under fatigue loading.

Eccentrically loaded weld groups

Weld groups loaded eccentrically are analyzed like bolt groups. In the elastic method, the welds are treated as lines of unit width. The direct shear is spread uniformly over the total length, and the moment produces a force per unit length proportional to the distance from the centroid of the group, using the polar moment of inertia of the lines:

The instantaneous center method, using measured load-deformation curves for welds at different angles, gives higher capacities, and handbook tables list coefficients for common weld shapes such as C-shapes and L-shapes.

The program reproduces both weld designs and confirms the directional increase for a transverse weld.

import math
# Fillet welds, AISC 360 Section J2: E70 electrodes, phi = 0.75
phi, FEXX = 0.75, 70.0

def weld_per_in(w, theta_deg=0.0):
    th = math.radians(theta_deg)
    return phi * 0.60 * FEXX * (1.0 + 0.50 * math.sin(th)**1.5) * 0.707 * w     # kips/in.

def base_per_in(t, Fy, Fu):
    return min(0.75 * 0.6 * Fu * t, 1.00 * 0.6 * Fy * t)                      # shear rupture / yielding

w = 0.25
qw, qb = weld_per_in(w), base_per_in(0.375, 36.0, 58.0)
print(f"1/4 in. fillet, longitudinal: weld metal {qw:.2f} kips/in., base metal (3/8 in. angle) {qb:.2f} kips/in.")
print(f"Transverse weld is {weld_per_in(w, 90)/qw:.2f} times as strong; {qw/4:.3f} kips/in. per sixteenth")

# Truss chord: each L4x4x3/8 carries half of 132 kips, welded to the gusset along heel and toe
T = 132.0 / 2
x_bar, b = 1.14, 4.0
L_total = T / min(qw, qb)
L_heel = L_total * (b - x_bar) / b
L_toe = L_total * x_bar / b
print(f"Weld length per angle = {L_total:.1f} in.: heel {L_heel:.1f} in., toe {L_toe:.1f} in. (balanced)")
l = (8.5 + 3.5) / 2
U = 1 - x_bar / l
print(f"Shear lag with 8.5 and 3.5 in. welds: l = {l:.1f} in., U = {U:.2f}, rupture phiPn = {0.75*58*U*5.72:.0f} kips")

# Eccentric weld group: bracket plate welded to a column flange with two vertical welds
P, e = 13.5, 12.0
Lw, xg = 10.0, 3.0
A = 2 * Lw
Ip = 2 * (Lw**3 / 12 + Lw * xg**2)
M = P * e
fy = P / A + M * xg / Ip
fx = M * (Lw / 2) / Ip
f = math.hypot(fx, fy)
print(f"Elastic method: Ip = {Ip:.1f} in3, largest force = {f:.2f} kips/in.")
D16 = f / (qw / 4)
print(f"Required fillet size = {D16:.1f} sixteenths -> use 3/16 in., the minimum for the 0.435 in. column flange")

1/4 in. fillet, longitudinal: weld metal 5.57 kips/in., base metal (3/8 in. angle) 8.10 kips/in.
Transverse weld is 1.50 times as strong; 1.392 kips/in. per sixteenth
Weld length per angle = 11.9 in.: heel 8.5 in., toe 3.4 in. (balanced)
Shear lag with 8.5 and 3.5 in. welds: l = 6.0 in., U = 0.81, rupture phiPn = 202 kips
Elastic method: Ip = 346.7 in3, largest force = 3.13 kips/in.
Required fillet size = 2.2 sixteenths -> use 3/16 in., the minimum for the 0.435 in. column flange

Quality, inspection and cost

Weld quality depends on the procedure, the welder and the conditions. Common defects include porosity, slag inclusions, lack of fusion, undercut at the toes, and cracks, especially hydrogen-induced cracking in thick or high-carbon steel welded without preheat. Visual inspection by a qualified inspector checks every weld; magnetic particle testing finds surface cracks; ultrasonic testing checks the interior of groove welds. CJP welds in tension in moment connections are usually tested ultrasonically.

Welding also distorts. Weld metal shrinks as it cools, pulling the parts toward the weld and leaving residual stresses. Good detailing balances welds on both sides of a member, limits weld size and sequence, and avoids heavy welds in the field. When choosing between bolting and welding, consider the whole cost: shop welding of small attachments is cheap, field welding is expensive, and a detail that lets the field crew bolt is usually preferred.