AWS Joint Design & Preparation 2 — Questions and Answers

Question 1: What is the primary advantage of a double-V groove joint over a single-V groove joint for welding thick plates?

Reduces total weld metal volume and distortion by welding from both sides (Correct answer)
Eliminates the need for root pass inspection
Requires no back gouging
Allows welding only from one side

Correct answer: Reduces total weld metal volume and distortion by welding from both sides

A double-V groove allows deposition of approximately half the weld metal of a single-V groove for the same plate thickness, reducing filler consumption, heat input, and angular distortion by balancing shrinkage forces from both sides.

For thick plates (typically above 20 mm), a single-V groove requires a large included angle and results in a trapezoidal weld cross-section with high weld metal volume at the top. A double-V groove splits this volume by welding from both sides: the total included groove angle is divided between the two sides, reducing the total weld metal required by approximately 50% compared to a single V-groove. This significantly reduces heat input, angular distortion, and filler material cost. The main disadvantage is that the plate must be accessible from both sides and typically requires back gouging of the root before welding the second side.

Question 2: Per AWS D1.1, what is the maximum allowable root face (land) dimension for a prequalified single-V groove weld joint?

3 mm (⅛ in.) (Correct answer)
6 mm (¼ in.)
0 mm (no land allowed)
13 mm (½ in.)

Correct answer: 3 mm (⅛ in.)

AWS D1.1 prequalified joint details specify a maximum root face (land) of 3 mm (⅛ in.) for single-V groove joints to ensure adequate root penetration while controlling burn-through.

AWS D1.1 Annex A prequalified joint details specify dimensional requirements for groove welds. For a single-V groove joint, the root face (the flat surface at the root of the joint preparation) has a typical specified dimension of 0–3 mm (0–⅛ in.). A zero root face (feather edge) maximizes penetration access but increases burn-through risk. A root face of up to 3 mm (⅛ in.) provides better support for the root pass without requiring a backing bar. Exceeding 3 mm typically requires qualification through a PQR rather than using prequalified status.

Question 3: What is the function of a backing strip (backing bar) in a groove weld joint?

Support the root pass to achieve complete joint penetration without requiring a welder to weld in difficult root access positions (Correct answer)
Provide additional strength to the weld joint
Act as a heat sink to reduce distortion
Replace the need for root face preparation

Correct answer: Support the root pass to achieve complete joint penetration without requiring a welder to weld in difficult root access positions

A backing strip supports the molten root pass, enabling full penetration to be achieved without the precise control required for open-root welding, particularly in restricted-access or single-sided weld situations.

A backing strip (typically the same or compatible material as the base metal) is placed on the reverse side of the joint root before welding begins. It supports the root pass melt pool, preventing burn-through and allowing the welder to achieve complete joint penetration (CJP) without back-gouging or having to weld from the reverse side. Backing bars per AWS D1.1 may be either temporary (removed after welding and the root face ground smooth) or permanent (fused into the joint as part of the weld). Permanent steel backing must be continuously fused to the base metal and is typically not removed unless the design requires access to the root for inspection.

Question 4: In joint design, what is the purpose of a weld access hole (rat hole) cut at the end of a beam web at a moment connection?

Provide access for the welder to make a complete penetration weld at the beam flange without depositing weld metal in areas of high stress triaxiality (Correct answer)
Reduce the weight of the beam at connections
Allow drainage of water from hollow sections
Increase the effective throat of the flange weld

Correct answer: Provide access for the welder to make a complete penetration weld at the beam flange without depositing weld metal in areas of high stress triaxiality

Weld access holes remove the web material adjacent to the flange-to-column connection, eliminating the triaxial stress state at the web-flange junction that is prone to lamellar tearing and weld cracking in seismically loaded moment frames.

Weld access holes (also called cope holes or rat holes) are cut in the beam web at moment connections to allow the CJP flange groove weld to be made continuously across the full width of the flange without interruption or weld intersection at the web. Without the access hole, the web would intersect the flange weld zone, creating a triaxial stress concentration and potential for lamellar tearing in the web. AISC and AWS D1.8 (Seismic Supplement) specify the geometry of weld access holes (radius, length, and smoothness) for seismic moment frames to minimize stress concentration and improve connection ductility.

Question 5: What is the minimum base metal thickness for which a fillet weld is permitted as the sole means of joining two plates per AWS D1.1 prequalified requirements?

There is no minimum thickness for fillet welds, but minimum size requirements are based on the thicker member (Correct answer)
3 mm (⅛ in.) minimum base metal thickness
6 mm (¼ in.) minimum
Fillet welds are permitted on any thickness regardless of size constraints

Correct answer: There is no minimum thickness for fillet welds, but minimum size requirements are based on the thicker member

AWS D1.1 does not prohibit fillet welds based on base metal thickness per se, but Table 5.8 specifies minimum fillet weld sizes based on the thicker part being joined to ensure adequate fusion and avoid rapid quenching of the root area.

AWS D1.1 Table 5.8 specifies minimum fillet weld sizes as a function of the thickness of the thicker part joined, ranging from 3 mm (⅛ in.) for base metal up to 6 mm (¼ in.) thick, up to 10 mm (⅜ in.) for base metal over 38 mm (1½ in.) thick. This ensures that the fillet weld deposit is large enough relative to the base metal mass to slow the cooling rate sufficiently and avoid hydrogen cracking in the HAZ. These requirements do not prohibit fillet welds on thin metal but do establish sizing minimums that must be met regardless of design load requirements.

Question 6: What is the effective throat of a 45° equal leg fillet weld with a 10 mm (⅜ in.) leg size?

7 mm (0.707 × leg size) (Correct answer)
10 mm (equal to the leg size)
14 mm (1.414 × leg size)
5 mm (0.5 × leg size)

Correct answer: 7 mm (0.707 × leg size)

The effective throat of a fillet weld is the theoretical throat = leg size × sin 45° = leg size × 0.707. For a 10 mm leg, the effective throat = 10 × 0.707 = 7.07 mm.

For an equal-leg fillet weld with 90° joint angle, the cross-section is a right isosceles triangle. The theoretical throat (effective throat without considering weld convexity) is the altitude from the right angle to the hypotenuse: T_eff = leg × sin(45°) = leg × 0.707. For a 10 mm leg, T_eff = 7.07 mm. This effective throat is used in structural calculations of fillet weld shear capacity. Weld design codes (AWS D1.1, AISC) base allowable fillet weld load capacity on the effective throat, not the leg size. Convex fillet welds may have a slightly larger actual throat, but the theoretical (effective) throat is used in standard calculations.

AWS Joint Design & Preparation 2

What is the primary advantage of a double-V groove joint over a single-V groove joint for welding thick plates?