Groove Weld — CJP, PJP, Backing Bars, and Run-Off Tabs
A groove weld is the highest-integrity weld type in structural steel, where weld metal fills a prepared groove penetrating into the joint rather than sitting on the surface. Groove welds are specified when the connection must develop the full strength of the connected parts — moment-resisting beam-to-column flange welds, column splices, and tension splices in truss chords.
CJP groove weld strength = φ × Fy × Ag (tension) — the weld is as strong as the plate
PJP groove weld strength = φ × 0.60 × F_EXX × t_e × L_w × 1.5 — limited by penetration depth
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
Complete Joint Penetration (CJP) Welds
A CJP groove weld extends through the entire thickness of the joint, fusing the weld metal to the base metal from the root to the face on both sides. The goal is metallurgical continuity — the welded joint behaves as if the material were never separated.
Strength equals base metal. For CJP groove welds loaded in tension or compression perpendicular to the weld axis, AISC 360 J2.1 states that the available strength is governed by the base metal strength — no weld metal strength check is required. The weld is stronger than the plate because the electrode tensile strength (70 ksi minimum for E70XX) exceeds the base metal yield strength (50 ksi for A992). The failure mode shifts to the base metal remote from the weld.
Tension normal to the effective area uses a base metal check: φRn = φ × Fy × Ag (yielding) and φRn = φ × Fu × Ae (fracture). The φ factors are 0.90 and 0.75 respectively, same as the parent metal — there's no additional weld-derived φ reduction.
Shear on the effective area uses φRn = φ × 0.60 × F_EXX × Ag for shear through the weld metal, but the base metal shear check (φ × 0.60 × Fy × Ag) typically governs because F_EXX > Fy in shear as well.
CJP welds require joint preparation — beveling, V-grooving, or J-grooving — to provide access for the welding electrode to the root. The preparation geometry depends on plate thickness, welding process, and position. AWS D1.1 Table 2.1 provides prequalified joint geometries that have been proven through testing to produce sound welds without procedure qualification testing.
CJP Weld Symbols
The welding symbol for a CJP groove weld omits the size field (no dimension to the left of the groove symbol). The symbol alone implies complete penetration. Supplementary symbols include:
- A backing bar symbol (a rectangle across the reference line) if backing is used
- A spacer symbol (a rectangle between the arrow and reference line) if a spacer is inserted
- A melt-through symbol (a filled black semicircle) on the opposite side if root reinforcement is required
Partial Joint Penetration (PJP) Welds
A PJP groove weld fills only a portion of the joint thickness, leaving some base metal unfused. The unwelded portion is the "root face" — the intentional gap at the root that is not penetrated. PJP welds are specified when full strength is not required, when access to the far side is impossible, or when CJP would require back-gouging (grinding out the root pass from the back side and re-welding — expensive overhead work).
PJP strength per AISC 360 J2.1:
φRn = φ × 0.60 × F_EXX × t_e × L_w × 1.5
The 1.5 multiplier accounts for the fact that PJP welds loaded in tension or compression normal to the effective area develop higher strength through Poisson restraint than fillet welds of equal throat.
The effective throat t_e is the depth of the groove preparation — typically specified on the shop drawing alongside the groove angle and root face dimension. For a 3/4-inch plate with a 45-degree bevel PJP weld penetrating to a depth of 1/2 inch: t_e = 0.50 inch.
PJP limitations:
- Not permitted for tension normal to the weld axis in cyclic (fatigue) service, per AISC 360 Appendix 3
- Minimum effective throat = 1/4 inch for most applications
- The root face (unfused portion) acts as a crack — PJP welds in tension have lower fatigue category than CJP
- Cannot be used for complete-penetration-required connections in seismic moment frames
Backing Bars
Backing bars are steel strips (typically 1/4 × 1 inch flat bar or 1 × 1 inch angle) tack-welded to the back side of a groove joint to support the first weld pass. Without backing, the welder would blow through the open root, depositing weld metal that falls through the gap. The backing bar catches the molten puddle, ensures complete root fusion, and speeds welding by eliminating the need for controlled root pass technique.
Permanent vs. removable backing:
- Permanent backing is left in place. It must be continuous for the full length of the weld, made of weldable steel, and tack-welded at intervals not exceeding 12 inches. For statically loaded structures, permanent backing is acceptable per AISC. For cyclically loaded structures, the backing bar creates a notch at the weld root that reduces fatigue life — it should be removed, the root ground smooth, and the surface inspected.
- Removable backing is pried off after welding (steel backing separates easily because weld metal does not bond well to flat backing surfaces) and the root surface ground flush. Removable backing is required for CJP welds in tension applications per AWS D1.1 Section 5.10.
Run-Off Tabs
Run-off tabs are small rectangular plates (typically 1 × 2 inches) tack-welded to the start and end of a groove weld joint, extending the joint geometry beyond the member edges. The welder starts the arc on the tab, runs through the joint, and stops on the far tab. The tabs absorb the start-and-stop defects:
- Arc start: The first 1-2 inches of a weld pass typically contain porosity (gas entrapped before the shielding gas envelope stabilizes) and incomplete fusion (the base metal hasn't heated to welding temperature). Starting on a tab places these defects outside the finished joint.
- Arc stop: The crater at the end of a weld pass contains shrinkage porosity and low effective throat. The tab absorbs the crater.
After welding, run-off tabs are removed by flame cutting or grinding, and the weld ends are ground flush with the member edges. Run-off tabs are mandatory for CJP groove welds in tension per AWS D1.1. They are optional but recommended for all CJP welds because the alternative — grinding 1-2 inches off each end of the finished weld — is more labor-intensive.
Groove Preparation Types per AWS D1.1
| Joint Type | Symbol | Typical Application | Effort |
|---|---|---|---|
| Square groove (butt) | I I | Thin plates ≤ 3/16 in | Lowest |
| Single V-groove | \ / | Plates 1/4-3/4 in, welded from one side | Moderate |
| Double V-groove | X | Thick plates ≥ 3/4 in, welded from both sides | High (requires flip) |
| Single bevel | / I | Tee joints, corner joints | Moderate |
| Single J-groove | J I | Heavy sections, reduced weld volume vs. bevel | High (machined prep) |
| Single U-groove | U I | Very thick sections, access from one side only | Highest (machined prep) |
The choice of preparation balances weld metal volume (cost of electrode and labor), access (can the welder reach both sides?), and distortion (asymmetric V-grooves cause angular distortion as the weld cools and shrinks; double V-grooves balance the shrinkage). For shop-welded column splices, double V-groove with back-gouging is the standard — the column can be rotated for access to both sides. For field moment connections (beam flange to column), single bevel with backing is standard because access is from one side only.
Frequently Asked Questions
When is CJP required instead of PJP?
CJP is mandatory when: (1) The connection must develop the full strength of the connected part (moment frame beam flanges, tension splices in truss bottom chords), (2) The weld is in tension normal to its axis and subject to fatigue (bridge girders, crane runways), (3) Seismic moment frames per AISC 341 require CJP at beam-to-column flange connections in the protected zone, and (4) The contract documents explicitly call for complete penetration. PJP is acceptable for most compression splices, column base plates (where the column bears directly on the base plate), and stiffener-to-web connections.
Can backing bars be left in place?
Yes, for statically loaded structures, permanent backing is permitted by AISC 360 and AWS D1.1 with no strength reduction. The backing bar acts as a small stiffener at the joint — structurally beneficial. For cyclically or seismically loaded structures, remove the backing bar, back-gouge the root, and apply a reinforcing fillet — the backing bar creates a notch that reduces fatigue life by a factor of 2-4. Some engineers specify permanent backing but require a continuous seal weld along the backing-to-base-metal interface to prevent moisture ingress and corrosion.
What is back-gouging and why is it needed?
Back-gouging is the process of grinding, arc-gouging, or chipping out the root pass of a groove weld from the back side before depositing the final weld passes from that side. It is required for double-sided CJP welds because the root pass (deposited from the first side) inevitably contains slag inclusions and incomplete fusion at the root face. Back-gouging removes this defective material, exposing sound weld metal for the back-side passes to fuse into. Without back-gouging, the root pass defects remain entombed in the finished weld — a latent crack initiation site.
International Code References
- AISC 360-22: Section J2.1 — CJP and PJP groove weld strength. Table J2.5 — Available strength for CJP groove welds.
- AWS D1.1 (2020): Section 2 — Prequalified CJP and PJP groove details (Tables 2.1-2.3). Section 5.10 — Backing and run-off tabs. Section 5.25 — Back-gouging requirements.
- AS 4100: Section 9.7.2 — Butt and compound welds (CJP and PJP). Clause 9.7.2.2 — PJP effective throat.
- EN 1993-1-8: Section 4.7 — Butt welds (full penetration = CJP). PJP butt welds treated as deep penetration fillet welds with a = throat.
Educational reference only. Groove weld design and detailing must be performed per AWS D1.1 and AISC 360 by a licensed Professional Engineer. Welding procedure specifications (WPS) must be qualified per AWS D1.1 Section 4 for all CJP and PJP groove welds.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.