456A Blueprint Reading, Weld Symbols & Joint Preparation 2 — Questions and Answers
Question 1: What is the difference between 'effective throat' and 'actual throat' of a fillet weld?
- Actual throat is the distance from the root to the face including convexity; effective throat is the shortest distance from the root to the weld face (hypotenuse), which is used for strength calculations (Correct answer)
- Effective throat includes the weld reinforcement; actual throat does not
- They are the same for equal-leg fillet welds
- Actual throat is the leg length; effective throat is the average of both legs
Correct answer: Actual throat is the distance from the root to the face including convexity; effective throat is the shortest distance from the root to the weld face (hypotenuse), which is used for strength calculations
For an equal-leg fillet weld with leg size 'w', the effective (theoretical) throat = w × 0.707. It is the shortest distance from the root to the theoretical face of the weld and is used for stress calculations. The actual throat includes any weld convexity above the theoretical face. Codes use the effective throat for design purposes as convexity is not considered reliably load-bearing.
Question 2: On a structural drawing, a fillet weld symbol shows '8' to the left and '150-300' to the right of the fillet symbol. What does this specify?
- 8 mm leg fillet welds, 150 mm long, spaced 300 mm apart (intermittent welds) (Correct answer)
- 8 mm throat, 150 mm wide, 300 mm from the edge
- 8 mm root opening, 150 mm and 300 mm leg sizes
- 8 mm reinforcement, weld from 150 mm to 300 mm along the joint
Correct answer: 8 mm leg fillet welds, 150 mm long, spaced 300 mm apart (intermittent welds)
In standard weld symbol notation: the number to the left of the symbol is the weld size (8 mm leg), and the number(s) to the right indicate weld length and pitch. '150-300' means weld segments 150 mm long at 300 mm pitch (centre-to-centre spacing). This is an intermittent fillet weld specification — commonly used for non-critical connections to reduce heat input and cost.
Question 3: What does a double-sided V-groove symbol (X-groove) indicate on a drawing?
- Full penetration groove weld to be made from both sides of the joint (Correct answer)
- Fillet welds on both sides of the joint
- A weld that crosses over itself in an X pattern for extra strength
- Two separate single V-grooves at perpendicular joints
Correct answer: Full penetration groove weld to be made from both sides of the joint
The double-V groove (X-groove) symbol shows V-groove symbols on both sides of the reference line, indicating that the joint is to be prepared (bevelled) and welded from both sides. This is used for thick plate where welding from one side would require an excessively wide groove, resulting in excessive weld metal and distortion. It provides full penetration with less total weld metal than a single V.
Question 4: What is a 'backing symbol' (rectangle) on a groove weld symbol and what does it specify?
- A backing bar or backing material is to be used at the root of the joint to support the root pass and ensure full penetration (Correct answer)
- The weld is to be made against a backing plate that remains permanently in the joint
- A temporary backing must be removed after welding and the root back-gouged
- The weld face must be flush to a backing level
Correct answer: A backing bar or backing material is to be used at the root of the joint to support the root pass and ensure full penetration
The backing symbol (a rectangle below the reference line, on the other side from the weld symbol) indicates a backing bar or ring is required. In most structural applications, a CSA/ASTM steel backing bar is permanently fused into the joint. On process piping and pressure vessels, a removable ceramic or copper backing may be used. The presence or absence of backing affects the required root opening and edge preparation.
Question 5: What does the 'contour symbol' on a weld symbol specify?
- The required finished profile of the weld face — flat, convex, or concave — and whether finishing is required (Correct answer)
- The orientation of the weld relative to the base metal surface
- The width-to-depth ratio of the weld bead
- The inspection method required for the weld surface
Correct answer: The required finished profile of the weld face — flat, convex, or concave — and whether finishing is required
Contour symbols are placed above or below the weld symbol to specify the desired weld face profile: a straight line (flush/flat), a convex arc, or a concave arc. A letter below the contour symbol specifies the finishing method: G (grinding), M (machining), C (chipping), R (rolling), or U (unspecified). These specifications are important for fatigue-critical joints where notches at the weld toe must be minimized.
Question 6: On a drawing, a fillet weld symbol has a '(E)' notation. What does this indicate?
- The effective throat dimension is specified (rather than the leg size) (Correct answer)
- The weld is to be made by an experienced welder only
- The electrode type is specified separately in the welding schedule
- Equal-leg fillet weld with both legs equal to the specified dimension
Correct answer: The effective throat dimension is specified (rather than the leg size)
When the letter 'E' appears before or with a fillet weld size, it indicates that the dimension given is the effective throat rather than the leg length. This is sometimes used when a specific throat dimension is required for stress calculations and the engineer specifies it directly rather than converting to a leg size. The fabricator must determine the required leg size from the effective throat (leg = throat ÷ 0.707).
456A Blueprint Reading, Weld Symbols & Joint Preparation 2
What is the difference between 'effective throat' and 'actual throat' of a fillet weld?