Does AWS D1.1 apply to power transmission tower fabrication?

Yes, when the tower is fabricated from structural steel and the contract documents reference AWS D1.1. Utility owners and tower manufacturers often write project specifications that reference AWS D1.1:2025 as the governing welding standard. Some owners also maintain their own supplemental welding requirements layered on top of the code.

What steel grades are typically used in lattice transmission towers?

Angle and plate sections are commonly A36 and A572 Grade 50. Weathering-steel towers use A588 or A709 Grade 50W. High-strength members on long-span towers may use A572 Grade 65 or A913. All of these are prequalified base metals under AWS D1.1:2025 Table 5.6 except A572 Grade 65, which requires a tested WPS.

What positions must welders be qualified in for field-erected transmission tower welding?

Field welders on tower structures typically require 3G (vertical groove) and 4G (overhead groove) qualifications, or the all-position 6G pipe qualification. In-shop fabrication of base assemblies and splice plates can be done with 1G/2G or 1F/2F qualifications if the work is positioned for flat or horizontal access.

Is preheat required for A572 Grade 50 gusset plate welds on a transmission tower?

Preheat requirements depend on member thickness and carbon equivalent of the specific heat. For A572 Grade 50 members up to about 3/4 in at ambient temperature with low-hydrogen filler metal, minimum preheat is typically not required under AWS D1.1:2025. Thicker members and higher-CE heats require preheat per the applicable table. The WPS must specify preheat requirements.

Oct 25, 20268 min read

Transmission Tower and Lattice Structure WPS Under AWS D1.1

WPS requirements for welding power transmission towers, telecom lattice structures, and steel poles under AWS D1.1:2025 — base metals, joint types, field welding, and CWI inspection guidance.

WPS AWS D1.1 Structural Steel Lattice Structure Field Welding CWI Transmission Tower

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

Lattice steel transmission towers, telecom structures, and steel poles are critical infrastructure — and their structural welds are subject to the same AWS D1.1 requirements that govern any other structural steel fabrication. Whether the tower is shop-assembled and transported to site or field-welded section by section, the fabricator and erector need documented WPS, qualified welders, and a CWI inspection program.

This article covers the WPS and inspection requirements for welding lattice steel transmission towers, telecom lattice structures, and steel monopoles under AWS D1.1:2025 — including base metal selection, joint types, preheat, field welding positions, and what the CWI needs to verify.

Code Jurisdiction for Tower Welding

AWS D1.1:2025 is the governing welding standard when the project specifications reference it. Utilities, tower manufacturers, and EPC contractors routinely cite AWS D1.1 as the baseline welding code for lattice steel transmission structures.

Some utility owners maintain supplemental welding specifications that add requirements on top of AWS D1.1 — specific filler metal grades, additional NDE beyond the code minimum, or stricter preheat requirements for cold-climate installations. The project-specific welding specification always takes precedence over the code baseline when it is more restrictive.

For transmission towers designed by PE-stamped drawings under AISC specifications, the AISC Chapter N quality control provisions also apply. When AISC certification is required for the fabricator, the welding quality control plan must conform to AISC Chapter N and AWS D1.1 together. See AISC 360 structural weld QC and AWS D1.1 for how these requirements interact.

Rule library based on AWS D1.1:2025; verify against your governing edition.

Common Base Metals in Lattice Tower Fabrication

A36: The workhorse of light lattice-angle towers. Group I prequalified base metal under AWS D1.1:2025 Table 5.6. Low-carbon equivalent — preheat generally not required for moderate thickness at ambient temperature.

A572 Grade 50: The dominant structural steel for medium and heavy lattice towers. Group II prequalified base metal per Table 5.6. The higher strength allows lighter sections than A36 for the same design load, reducing tower weight.

A588 and A709 Grade 50W: Weathering steel grades used in corrosion-resistant towers that are left unpainted. Both are Group II prequalified base metals. When welding weathering steel, the filler metal must also provide weathering-steel matching chemistry — ER70S-G or E8018-W type electrodes are typically required; standard ER70S-6 or E7018 provides adequate strength but not the weathering chemistry match.

A572 Grade 65: Higher-strength steel used in long-span or heavily loaded tower members. Grade 65 is not a prequalified base metal under Table 5.6 and requires a tested WPS supported by a PQR. See A572 Grade 65 structural steel WPS for the qualification path.

A913 Grade 65 or 70: Quenched and self-tempered high-performance steel used in large monopole shafts and heavy base plates. Requires a tested WPS and careful control of heat input to avoid HAZ degradation. Maximum interpass temperature is critical for A913.

Joint Types and Prequalified Details

Lattice tower fabrication involves a mix of fillet welds, partial joint penetration (PJP) groove welds, and occasional complete joint penetration (CJP) groove welds:

Gusset plate-to-chord angle connections. These are the most numerous joints in any lattice tower — fillet welds on both sides of the gusset plate connecting bracing angles and chord angles at panel points. Single-sided fillet welds are sometimes used on the accessible side only; the WPS must address single-pass vs. multi-pass and the accessible weld side.

Base plate-to-leg connections. The leg angle or leg tube frames into a base plate assembly. The connection is typically a CJP groove weld with backing, or a large fillet weld, depending on the design. CJP base plate connections require full qualification — prequalified groove details per Clause 5, Figure 5.1 if the geometry matches, or a tested WPS with a PQR if not.

Splice plate connections. Tower sections are often field-spliced using bolted splice plates with shop-welded attachment tabs. The tab welds in the shop are fillet welds and fall under a prequalified fillet WPS.

Pole shaft seam welds. Steel monopoles are fabricated from plate rolled into a conical or tapered cylinder. The longitudinal seam weld is a CJP groove weld, typically made by SAW in the shop. The WPS must address SAW essential variables under Table 6.6 and the prequalified SAW requirements in AWS D1.1.

Flange ring welds. Monopoles and sectional poles use bolted flange connections with continuous fillet welds or CJP groove welds between the flange ring and the pole shaft. These are high-fatigue details and may require additional NDE.

Preheat for Tower Base Metals

For A36 and A572 Grade 50 in member thicknesses common to lattice tower angles (up to about 1 in), preheat is typically not required at ambient temperature when using low-hydrogen filler metals. At thicknesses above about 3/4 in for A572 Grade 50, or when the carbon equivalent of the specific heat is elevated, minimum preheat applies per the applicable AWS D1.1:2025 table.

Cold-climate tower installations require particular attention: ambient temperature below 32°F [0°C] mandates minimum 70°F [20°C] preheat on all structural steel welds regardless of thickness. Wind and moisture at elevated installation sites can accelerate cooling rates — temporary wind shields and preheat monitoring are practical requirements that the CWI and field foreman must coordinate. See cold weather welding requirements under AWS D1.1 for preheat maintenance protocols in cold conditions.

For weathering steel (A588, A709 50W), preheat requirements follow the same logic as for A572 — based on thickness and carbon equivalent — with the added requirement that the filler metal chemistry must match the weathering-steel base metal if the weld is to provide the same corrosion resistance as the surrounding parent metal.

Field Welding Positions and Welder Qualification

Shop fabrication of lattice tower sub-assemblies is done in controlled conditions on welding positioners. The shop WPS typically covers 1F/2F fillet and 1G/2G groove positions for sub-assembly work, moving to 3F/3G for vertical welds on larger leg assemblies.

Field erection is a different story. Tower sections are lifted by crane and welded in place — overhead, vertical, and inclined positions are unavoidable. Field welders on transmission tower erection typically require:

3G (vertical groove) qualification
4G (overhead groove) qualification
Or the 6G (fixed pipe, inclined) qualification that covers all positions in groove welds

For fillet welds in the field, the equivalent is 3F (vertical fillet) and 4F (overhead fillet). A welder holding a current 3G groove qualification also covers 3F fillet welds for the applicable process. Check the WPS and the project specification for the specific position coverage required.

Welder continuity is a live issue for long tower projects — a welder who has not made a production weld with the qualified process for six months must requalify per AWS D1.1 Clause 6.4.1. Field crews rotating between projects need active qualification tracking. See AWS D1.1 welder continuity six-month rule for the lapse and requalification requirements.

Essential Variables: What Changes Require a New WPS

Under AWS D1.1:2025 Table 6.6, the following changes to a tested WPS require requalification with a new PQR for the common processes used in tower fabrication (SMAW, GMAW, FCAW-G):

Change in base metal P-number group (e.g., from Group I to Group II)
Change in filler metal F-number classification group
Significant change in heat input (for CVN-qualified WPS)
Change in welding position (e.g., adding overhead when the WPS only covers flat/horizontal)
Addition of preheat or postweld heat treatment requirements

For a prequalified WPS, any change that takes the WPS outside the prequalified limits of AWS D1.1 Clause 5 means you either need a tested WPS or must adjust the variables back within prequalified bounds. Changes to non-essential variables (travel speed within a range, minor preheat increases) do not require requalification but must be documented on a revised WPS.

NDE Requirements for Tower Welds

Visual inspection is required for all structural welds. Additional NDE requirements come from three sources:

Engineering specification: CJP base plate welds and pole seam welds in the primary load path typically require UT or RT per the tower manufacturer's quality plan.

Code requirements for cyclically loaded structures: Transmission towers carry cyclic wind and ice loads that qualify as cyclic loading under AWS D1.1. The governing specification should identify which welds in the primary load path require MT or PT for surface-crack detection.

Seismic design requirements: In seismic design categories where towers are designed for ductile behavior, the demand-critical weld requirements of AWS D1.8 may layer additional CVN filler metal requirements and NDE onto the base AWS D1.1 program.

For the CWI establishing an inspection program for tower fabrication, see NDE method selection for structural welds for the comparison of inspection methods and their applicability to different joint geometries.

WPS Documentation Checklist for Tower Projects

A complete WPS package for a transmission tower project should include:

Shop prequalified WPS covering fillet welds on gusset plate and splice connections (A36 and A572 Grade 50; SMAW E7018, FCAW-G E71T-1, or GMAW ER70S-6)
Shop tested WPS for CJP base plate and pole seam welds (SAW or FCAW-G; PQR required)
Field WPS covering vertical and overhead fillet and groove positions for the same processes
Weathering steel WPS if A588 or A709 50W is used, with matching filler metal designation
High-strength WPS with PQR for any A572 Grade 65 or A913 members

The AWS D1.1:2025 Annex J Form J-1 provides the standard WPS format. The PQR test report for each tested WPS accompanies it in the project quality file. The project owner typically requires WPS submittals for review and acceptance before fabrication begins — see WPS submittal package for EOR review for what to include in the submittal.

Managing WPS documents, welder qualifications, and inspection records across multiple tower crews and field sites benefits from a centralized digital platform. See how wpswelding.com supports multi-project WPS management →

FAQ

Frequently asked.

Does AWS D1.1 apply to power transmission tower fabrication?: Yes, when the tower is fabricated from structural steel and the contract documents reference AWS D1.1. Utility owners and tower manufacturers often write project specifications that reference AWS D1.1:2025 as the governing welding standard. Some owners also maintain their own supplemental welding requirements layered on top of the code.
What steel grades are typically used in lattice transmission towers?: Angle and plate sections are commonly A36 and A572 Grade 50. Weathering-steel towers use A588 or A709 Grade 50W. High-strength members on long-span towers may use A572 Grade 65 or A913. All of these are prequalified base metals under AWS D1.1:2025 Table 5.6 except A572 Grade 65, which requires a tested WPS.
What positions must welders be qualified in for field-erected transmission tower welding?: Field welders on tower structures typically require 3G (vertical groove) and 4G (overhead groove) qualifications, or the all-position 6G pipe qualification. In-shop fabrication of base assemblies and splice plates can be done with 1G/2G or 1F/2F qualifications if the work is positioned for flat or horizontal access.
Is preheat required for A572 Grade 50 gusset plate welds on a transmission tower?: Preheat requirements depend on member thickness and carbon equivalent of the specific heat. For A572 Grade 50 members up to about 3/4 in at ambient temperature with low-hydrogen filler metal, minimum preheat is typically not required under AWS D1.1:2025. Thicker members and higher-CE heats require preheat per the applicable table. The WPS must specify preheat requirements.