AWS D1.8 — Structural Welding Code Seismic Supplement
AWS D1.8 supplements D1.1 for seismic welding applications. It adds demand-critical weld requirements, maximum interpass temperature controls, mandatory CVN toughness testing for filler metals, and enhanced qualification procedures for welds in seismic force-resisting systems designed to AISC 341 and AISC 358 connections.
Key distinction: D1.8 does not replace D1.1 — it supplements it. All D1.1 requirements still apply. D1.8 adds stricter controls for demand-critical welds including a maximum interpass temperature of 550°F and a two-tier CVN toughness system: 20 ft-lbf (27 J) at 0°F baseline per Clause 6.3.4(1), increasing to 40 ft-lbf (54 J) at LAST for cold-service applications per Clause 6.2.2. Preheat values come from D1.1 Table 5.11.
What Is AWS D1.8?
AWS D1.8 supplements D1.1 for welded joints in seismic force-resisting systems. It does not replace D1.1 — it adds requirements for demand-critical welds in buildings designed for earthquake resistance per AISC 341. D1.8 specifies filler metal toughness, interpass temperature limits, and enhanced qualification testing.
AWS D1.8/D1.8M — Structural Welding Code — Seismic Supplement — provides additional welding requirements for members and connections in the seismic force-resisting system (SFRS) of steel building structures. The current edition is AWS D1.8:2021 (4th Edition). D1.8 is not a standalone code — it supplements AWS D1.1 by adding requirements that go beyond standard structural welding practice for joints expected to undergo significant inelastic deformation during design-level earthquakes.
The standard was developed in response to the 1994 Northridge earthquake, which revealed that standard structural welding practices were insufficient for seismic applications. Pre-Northridge moment frame connections experienced brittle fracture in beam flange-to-column groove welds at stress levels well below the expected plastic capacity. Post-Northridge investigations identified several contributing factors including inadequate weld metal toughness, poor access hole geometry, backup bar notch effects, and insufficient quality control. D1.8 addresses all of these factors through enhanced material, fabrication, and inspection requirements.
D1.8 is referenced by AISC 341 (Seismic Provisions for Structural Steel Buildings) and is mandatory for all welding in seismic force-resisting systems in seismic design categories D, E, and F. The connection designs come from AISC 358 (Prequalified Connections for Special and Intermediate Steel Moment Frames) and AISC 341 — D1.8 governs how those connections are welded, qualified, and inspected.
Demand-Critical Welds
D1.8 designates certain welds as demand-critical — welds in connections that must sustain inelastic deformation during a seismic event. Demand-critical welds require CVN toughness-rated filler metals (20 ft-lbs at minus 20 degrees F minimum), maximum 550 degrees F interpass temperature, and enhanced ultrasonic testing acceptance criteria.
The central concept in D1.8 is the demand-critical weld — a weld that must maintain structural integrity through multiple cycles of large inelastic deformation during a seismic event. Demand-critical welds are subjected to the most stringent material, fabrication, and inspection requirements in the AWS D1.x code family.
The engineer of record designates which welds are demand-critical based on the connection design and the expected deformation demands. Common demand-critical welds include:
- Beam flange-to-column CJP groove welds
- In special and intermediate moment frames, the beam flange groove welds to the column flange are the most critical welds in the connection. These welds must transmit the full plastic moment capacity of the beam section through repeated cycles of large rotation. Pre-Northridge failures were concentrated at these welds.
- Link-to-column welds in eccentrically braced frames
- The link beam in an EBF undergoes large shear and/or flexural deformations. The welds connecting the link to the column must sustain these deformations without fracture. Both the flange and web welds at the link-to-column connection are typically demand-critical.
- Column splice welds in plastic hinge regions
- When a column splice falls within an expected plastic hinge zone, the splice welds must be capable of developing the full column section capacity. D1.8 requires CJP groove welds for column splices in these locations, with demand-critical material and inspection requirements.
- Brace connection welds in special concentrically braced frames
- The gusset plate connections in SCBFs must accommodate the expected brace buckling and yielding behavior. The welds connecting the gusset to the beam, column, or brace are often designated demand-critical depending on the connection type and expected deformation mode.
Thermal Controls for Seismic Welding
D1.8 limits interpass temperature to 550 degrees F maximum for demand-critical welds. This prevents excessive heat from degrading toughness in the heat-affected zone. Preheat follows D1.1 Table 5.11 — D1.8 has no separate preheat table. The combination of minimum preheat (D1.1) and maximum interpass (D1.8) defines the thermal window.
D1.8 does not have its own preheat tables. All preheat requirements come from D1.1 Table 5.11. However, D1.8 adds a maximum interpass temperature of 550°F (288°C) for demand-critical welds. This upper limit prevents excessive grain growth in the heat-affected zone that would reduce fracture toughness below the level needed for seismic performance.
The combination of D1.1 minimum preheat and D1.8 maximum interpass creates a controlled thermal window for demand-critical welding. For example, an A992 W36 column with 2-inch-thick flanges requires a minimum preheat of 150°F per D1.1 Table 5.11 and a maximum interpass of 550°F per D1.8. The welder must maintain the weldment within this 150°F to 550°F range throughout the multi-pass welding of the beam flange CJP groove weld.
Temperature monitoring for demand-critical welds is more rigorous than for standard D1.1 welding. The maximum interpass temperature must be verified before depositing each subsequent pass. Contact thermometers are preferred over temperature-indicating crayons for demand-critical applications because they provide a quantitative reading rather than a threshold indication.
Filler Metal Requirements
D1.8 requires filler metals for demand-critical welds to meet CVN toughness of 20 ft-lbs at minus 20 degrees F. This is more restrictive than D1.1, which does not require CVN testing for filler metals. The filler metal manufacturer must certify toughness values. AISC 341 Table A3.1 lists qualifying filler metal classifications.
D1.8 imposes additional filler metal requirements beyond D1.1 for demand-critical welds. The most significant requirement is mandatory Charpy V-notch (CVN) toughness testing. D1.8:2021 uses a two-tier CVN system. The baseline requirement per Clause 6.3.4(1) is 20 ft-lbf (27 J) at 0°F (−18°C) — filler metals listed in Table 6.4 that meet this level are exempted from production lot testing when the Lowest Anticipated Service Temperature (LAST) is 50°F (10°C) or above. When the SFRS is subjected to service temperatures below 50°F, Clause 6.2.2 requires a higher minimum of 40 ft-lbf (54 J) tested at or above the LAST.
The two-tier CVN structure means that projects in moderate climates (LAST ≥ 50°F) can use Table 6.4 filler metals with certified 20 ft-lbf at 0°F without additional lot testing, while cold-climate projects (LAST < 50°F) must use filler metals with lot-certified 40 ft-lbf at the actual service temperature. Standard D1.1 filler metals are not required to meet either toughness level. Fabricators must verify that the specific lot of filler metal purchased for demand-critical welding has been tested and certified to meet the applicable D1.8 toughness tier. Simply specifying the AWS filler metal classification (e.g., E71T-1) is not sufficient — the lot-specific test results must be reviewed.
D1.8 also requires that filler metal manufacturer diffusible hydrogen test results be available for demand-critical applications. Lower hydrogen levels in the weld deposit reduce the risk of hydrogen-assisted cracking in the heat-affected zone and improve the overall toughness of the welded joint.
Procedure and Welder Qualification for Seismic
D1.8 adds supplementary requirements to D1.1 Clause 6 qualification. PQRs for demand-critical welds must include CVN testing of weld metal and HAZ. WPS essential variables include interpass temperature maximum, which is not an essential variable under D1.1 alone. Welders must demonstrate proficiency with the specific joint configurations.
D1.8 modifies D1.1 qualification requirements for demand-critical welding. Supplementary essential variables from D1.1 become essential variables for demand-critical WPS qualification. This means that changes in heat input, preheat reduction, and PWHT conditions that would be permissible under standard D1.1 qualification require re-qualification when the procedure is used for demand-critical welds.
The WPS for demand-critical welds must specify both the minimum preheat (from D1.1 Table 5.11) and the maximum interpass temperature (550°F per D1.8). The procedure qualification test must be performed within these thermal limits. The qualification coupon must also be tested for CVN toughness to verify that the combination of filler metal, base metal, and welding parameters produces a joint with adequate fracture toughness.
D1.8 allows the use of D1.1 prequalified WPS procedures for non-demand-critical welds in the seismic force-resisting system. However, demand-critical welds require qualified (not prequalified) procedures with the additional thermal and toughness requirements. The engineer of record must clearly identify which welds are demand-critical on the contract documents so that the fabricator applies the correct WPS category.
How D1.8 Compares to Other AWS Structural Codes
D1.8 supplements D1.1 for seismic applications — it does not stand alone. D1.5 covers bridges with fracture-critical requirements. Both D1.8 and D1.5 add toughness requirements beyond D1.1. D1.8 uses demand-critical weld classification; D1.5 uses fracture-critical member classification. D1.8 references AISC 341; D1.5 references AASHTO.
D1.8 vs D1.1 (Standard Structural)
D1.1 covers standard structural steel welding without seismic requirements. D1.8 supplements D1.1 by adding demand-critical weld categories, maximum interpass temperature limits, mandatory CVN toughness for filler metals, and stricter qualification requirements. All D1.1 requirements remain in effect — D1.8 only adds to them. A welder qualified under D1.1 must also meet D1.8 requirements when performing demand-critical welds; the D1.1 qualification alone is not sufficient.
D1.8 vs D1.3 (Sheet Steel)
D1.3 covers sheet steel with thickness at or below 3/16 inch. D1.8 does not directly address sheet steel connections, but cold-formed steel framing in seismic regions may involve both standards. D1.8 demand-critical requirements do not apply to sheet steel connections unless specifically designated by the engineer of record, which is uncommon because sheet steel connections are typically designed as flexible elements in the seismic force-resisting system.
D1.8 vs D1.5 (Bridge)
AWS D1.5 Bridge Welding Code has its own fracture-critical member (FCM) provisions that serve a similar purpose to D1.8 demand-critical requirements but for bridge applications rather than building seismic applications. Both D1.8 and D1.5 FCM require enhanced filler metal toughness, stricter thermal controls, and additional inspection. The specific toughness requirements and testing temperatures differ between the two codes because the loading conditions (seismic versus fatigue) and service environments differ.
| Aspect | D1.8 (Seismic) | D1.1 (Structural) |
|---|---|---|
| Scope | Supplements D1.1 for seismic | Structural steel (all loading) |
| Critical weld class | Demand-critical welds | None |
| Interpass max | 550°F for demand-critical | Not code-limited |
| CVN toughness | 20 ft-lbs at −20°F | Not required |
| Own preheat table? | No — uses D1.1 Table 5.11 | Yes — Table 5.11 |
| Connection standard | AISC 341 compliance | D1.1 Clause 4 |
Related Standards Guides
Frequently Asked Questions
No. AWS D1.8 does not have its own preheat tables. It uses the preheat requirements from D1.1 Table 5.11. D1.8 supplements D1.1 by adding additional requirements for demand-critical welds, including a maximum interpass temperature of 550 degrees Fahrenheit and a two-tier CVN toughness system — 20 ft-lbf (27 J) at 0 degrees Fahrenheit baseline per Clause 6.3.4(1), increasing to 40 ft-lbf (54 J) when the Lowest Anticipated Service Temperature is below 50 degrees Fahrenheit per Clause 6.2.2. All base preheat requirements come from D1.1 — D1.8 adds thermal and toughness controls on top.
A demand-critical weld is a weld in the seismic force-resisting system that is expected to undergo significant inelastic strain during a design earthquake. These welds must maintain their structural integrity through multiple cycles of large deformation without fracture. Examples include beam flange-to-column CJP groove welds in special moment frames, link-to-column welds in eccentrically braced frames, and column splice welds in the expected plastic hinge region. Demand-critical welds require filler metals with certified CVN toughness and stricter thermal controls than standard D1.1 welds.
AWS D1.8 limits the maximum interpass temperature to 550 degrees Fahrenheit (288 degrees Celsius) for demand-critical welds. This is more restrictive than standard D1.1 welding, which does not impose a general maximum interpass temperature. The 550 degree limit prevents excessive grain growth in the heat-affected zone that would reduce fracture toughness. For demand-critical welds, maintaining both minimum preheat (from D1.1 Table 5.11) and maximum interpass (from D1.8) creates a controlled thermal window for welding.
AISC 341 (Seismic Provisions for Structural Steel Buildings) is the design standard that defines seismic force-resisting system requirements. AISC 341 references AWS D1.8 for welding requirements in seismic applications. The connection design comes from AISC 341 and the prequalified connections in AISC 358 — these standards define what welds are required and where. D1.8 defines how those welds must be made, qualified, and inspected. Together, AISC 341 plus D1.8 replace standard D1.1 requirements for welds in the seismic force-resisting system.
AWS D1.8:2021 uses a two-tier CVN system. The baseline per Clause 6.3.4(1) is 20 ft-lbf (27 J) at 0 degrees Fahrenheit — filler metals in Table 6.4 meeting this level are exempt from production lot testing when the Lowest Anticipated Service Temperature (LAST) is 50 degrees Fahrenheit or above. For cold-service applications where the LAST is below 50 degrees Fahrenheit, Clause 6.2.2 requires 40 ft-lbf (54 J) tested at or above the LAST, with manufacturer lot certification. Standard D1.1 filler metals may not meet either tier.