Structural welding codes: AWS D1.1 and its family

Most of what Hartwell welds is structural steel: the Riverside pedestrian bridge, the base frames for the Coldwater skids, platforms, pipe racks and the occasional farm repair. In the United States the rules for that work come from the AWS D1 family of structural welding codes, led by AWS D1.1 for steel. D1.1 sets requirements for the design of welded connections, the qualification of procedures and welders, fabrication, inspection and acceptance, and for stud welding, tubular structures and repair. Unlike the Canadian system, the code does not certify the company; it places duties on the contractor, and other programs, such as AISC certification, judge the company. Elena's second class is a tour of D1.1, with the Riverside job open beside it.

What a structural welding code covers

Every structural welding code answers the same set of questions, in its own order and with its own numbers. Which steels and filler metals may be used, and how are they matched? How must welded connections be designed and detailed, including the sizes, lengths and effective throats of welds? How must a welding procedure be shown to work, either by using prequalified details or by testing? How must welders and operators be qualified, and how long does a qualification last? What workmanship is required in fit-up, preheat, sequence and finish? Who inspects, how, and against what acceptance criteria? How are defects repaired? The answers differ between codes in detail, but a welder who knows the questions can find the answers in any of them.

AWS D1.1, Structural Welding Code — Steel, covers welding of carbon and low-alloy steel structures by the common arc processes. Its 2025 edition is arranged in eleven clauses: general requirements, normative references, terms and definitions, design of welded connections, prequalification of procedures, qualification, fabrication, inspection, stud welding, tubular structures, and strengthening and repair, followed by mandatory and informative annexes. It separates requirements for statically loaded structures from those for cyclically loaded ones, where fatigue governs and the rules on details, profiles and inspection are stricter.

How the design standards call up the welding code

The engineer designing a steel building in the United States works to the AISC Specification for Structural Steel Buildings, which the International Building Code adopts, and a highway bridge engineer works to the AASHTO specifications. AISC calls up AWS D1.1 for structural welding, with its own additions, and bridges use AASHTO/AWS D1.5, the Bridge Welding Code, which is stricter than D1.1 on materials, procedures and inspection, especially for fracture-critical members. The chain from the law to the welder is unbroken: state or city law, building code, steel design specification, welding code. When the specification for the Riverside bridge says “welding to AASHTO/AWS D1.5,” it is taking the bridge path rather than the building path.

Bridges also show why a code family exists. D1.5 was written jointly by AASHTO and AWS because bridges carry repeated traffic loads for decades, and a crack that would not matter in a building can grow in a bridge girder. D1.5 limits the steels and filler metals that may be used, requires procedures to be qualified by test except in narrow cases, adds Charpy toughness testing, and has a fracture control plan for members whose failure would bring the bridge down. Hartwell's Riverside girders were welded to D1.5, which is why Elena qualified the flange splice procedure by test in the gas metal and flux-cored course.

Statically and cyclically loaded structures

A platform that carries people and equipment, a building frame or a pipe rack is mostly statically loaded: its loads change slowly and not many times. A crane runway, a bridge, a conveyor gallery or a vibrating screen frame is cyclically loaded: its stresses go up and down thousands or millions of times. Fatigue cracks start at small notches, so the code is stricter for cyclic structures. It limits undercut and porosity more tightly, requires smoother weld profiles at the toes, restricts certain details such as intermittent fillets and backing left in place across the stress, and usually calls for more non-destructive testing. The drawing or the specification tells the shop which category applies, and the inspector uses the matching acceptance criteria.

Lucia shows Devon two drawings side by side. The Coldwater base frame is statically loaded, so a fillet weld with an undercut of 1/32 in along part of its length can be accepted under the statically loaded criteria. The Riverside floor beams are cyclically loaded, and the same undercut perpendicular to the main stress would be rejected and repaired. “Same weld, same welder, different answer,” she says. “Always check which table you are in before you measure.”

Prequalified and qualified procedures

A structural code gives the shop two ways to show that a welding procedure works. The first is prequalification: if the joint geometry, process, filler, preheat and other variables are all within the limits in D1.1's prequalification clause and tables, the procedure is accepted without testing, because the code's writers have already proved those combinations sound. The second is qualification by test under the qualification clause, recorded on a procedure qualification record. Either way the procedure must be written down as a welding procedure specification, and the engineer may ask to review it. Lesson 4 covers these documents in detail.

Prequalification has limits that matter on the shop floor. Some processes and transfer modes are not prequalified at all, such as short-circuit gas metal arc welding, because they can leave lack of fusion that the code's writers did not want to accept without testing. Prequalified joints must match the code's figures for root opening, bevel angle and root face within the listed tolerances, and the fit-up tolerances still apply in the shop. A joint that is fitted outside them is no longer prequalified until it is corrected, which is why Lucia's fit-up check matters even on simple joints.

Fillet weld sizes and effective throat

The structural codes set minimum fillet weld sizes based on the thickness of the thicker part joined, because a small weld on thick plate cools quickly and can crack. They also set maximum sizes along edges, so that the toe does not melt away the plate edge. The strength of a fillet comes from its effective throat, the shortest distance from the root to the face of the weld, which for an equal-leg fillet on a right-angle joint is about 0.707 times the leg. A weld that is undersize over part of its length is allowed only within the limits in the acceptance criteria, and a fillet in a skewed joint has a different throat for the same leg.

# Minimum fillet size check (values from a table like the code's; check the edition the job names)
import math
thick_unit = 'in'
# (largest thickness of the thicker part, minimum fillet leg)
table = [(0.25, 0.125), (0.5, 0.1875), (0.75, 0.25), (99, 0.3125)]
joints = [('stiffener to web S-14', 1.0, 0.1875), ('base frame gusset', 0.5, 0.25), ('handrail post', 0.3125, 0.1875), ('lifting lug', 1.25, 0.375)]
for name, t_thick, leg in joints:
    need = next(m for limit, m in table if t_thick <= limit)
    throat = leg * math.cos(math.radians(45))
    status = 'ok' if leg >= need else 'UNDERSIZE: raise an RFI or use the code alternative'
    print(f"{name:<26} thicker part {t_thick:>6} {thick_unit}  leg {leg:>6}  min {need:>6}  throat {throat:.2f}  {status}")

stiffener to web S-14      thicker part    1.0 in  leg 0.1875  min 0.3125  throat 0.13  UNDERSIZE: raise an RFI or use the code alternative
base frame gusset          thicker part    0.5 in  leg   0.25  min 0.1875  throat 0.18  ok
handrail post              thicker part 0.3125 in  leg 0.1875  min 0.1875  throat 0.13  ok
lifting lug                thicker part   1.25 in  leg  0.375  min 0.3125  throat 0.27  ok

The table in the program has the shape of the code's minimum fillet size table: the minimum leg steps up with the thickness of the thicker part. The stiffener on sheet S-14 fails the check, which is the request for information from lesson 1. The real table and its notes, including any allowance when enough preheat is used, are in the edition of the code the job names.

Workmanship, fit-up and preheat

The fabrication part of a structural code is where welders spend most of their working lives. It sets rules for preparing base metal, removing mill scale and moisture, assembling joints within tolerance, tack welding, preheat and interpass temperature, heat input on quenched and tempered steels, weld sequence to control distortion, run-off tabs, backing, peening, and the profiles of finished welds. Many of these rules appeared in earlier courses as Hartwell practice; in this course Devon learns that they are code requirements, which means an inspector can reject work that ignores them even if the weld looks good.

Preheat is a good example. The code gives a table of minimum preheat and interpass temperatures by steel group and thickness for prequalified procedures, with an alternative method based on hardness or hydrogen control. A weld made on 1-1/2 in plate without preheat may look perfect and still be rejected, because the inspector cannot see hydrogen cracking that may appear a day later. Lucia's temperature-indicating crayons and contact thermometer are inspection tools, and the preheat she records is part of the evidence that the weld was made to the procedure.

Inspection and acceptance in the structural code

D1.1 separates fabrication or erection inspection, done by the contractor, from verification inspection, done for the owner or the engineer. The contractor's inspector must be qualified, and the code lists acceptable bases, including certification as an AWS Certified Welding Inspector under AWS QC1 or as a Level 2 or 3 inspector under CSA W178.2, or engineering training and experience. The inspection clause gives visual acceptance criteria in Table 8.1 and acceptance criteria for radiography and ultrasonics, and the contract decides how much non-destructive testing is done.

Inspection under a structural code is not only about the finished weld. The inspector checks that materials match the drawings and their mill certificates, that procedures are available at the work station and followed, that welders are qualified for the process and position they are using, that equipment works, and that preheat, interpass temperatures and sequence are as the procedure says. A finished weld that passes visual inspection but was made by an unqualified welder, or with the wrong filler, is still nonconforming.

Repairs and alterations

Structural codes also say how defects are repaired: the defect is removed by grinding, gouging or machining until sound metal is reached, the excavation is checked, often by magnetic particle testing, and the area is rewelded to an approved procedure and inspected again. Repairs to members that are already in service, strengthening of old structures and welding on steels of unknown composition are covered in their own clause or annex and usually need the engineer's approval. Hartwell's quality manual adds its own rule: every repair beyond simple grinding gets a repair record that names the defect, the method of removal, the procedure used, the welder and the inspection after repair.

Studs, tubes and other special work

Structural codes also carry rules for work that falls outside ordinary plate joints. D1.1 has a clause for arc stud welding, used for shear connectors on bridge girders and composite floors, with its own qualification and production testing, including bend or torque tests on the first studs of each shift and after any change in settings. It has a clause for tubular structures, where the load passes through a curved wall and the joint details differ. Steels such as weathering grades and quenched and tempered plate have limits on heat input and filler metal. When Hartwell welds studs to the Riverside girders, the first two studs each morning are bent to 30 degrees and must not crack at the weld before production starts, and Lucia records the result.

Hartwell's structural documents

By the end of the class Devon has a list. For the Riverside bridge, Hartwell works to AASHTO/AWS D1.5 for welding, with the project specification adding ultrasonic testing of all flange splices and Charpy requirements for the filler metal. For the Coldwater base frames and the platform work it works to D1.1. Its procedures are written as welding procedure specifications backed by procedure qualification records where testing was needed, its welders hold current qualifications for the processes and positions they weld, and Lucia inspects as an AWS Certified Welding Inspector. The owner's inspector for the bridge comes from an engineering firm hired by the city.