12.2 Joint Architecture, CJP versus PJP, and NDE Examination Symbols

Standard Joint Architecture Selection

AWS defines five basic structural joint configurations. Sizing and selecting the proper architecture depends on the primary stress mode, accessibility, fabrication economics, and non-destructive examination (NDE) inspectability.

      BUTT JOINT              CORNER JOINT                T-JOINT
   [Member 1][Member 2]       [Member 1]                  [Member 1]
                              |      |                    |    |    |
                              +------+                    +----+----+
                              [Member 2]                  [Member 2]

              LAP JOINT                             EDGE JOINT
        [Member 1]======                     [Member 1]|
           ======[Member 2]                  [Member 2]|

Joint Architecture Engineering Comparison

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Complete Joint Penetration (CJP) vs. Partial Joint Penetration (PJP)

The choice between Complete Joint Penetration (CJP) and Partial Joint Penetration (PJP) represents one of the most critical structural and economic decisions in welding engineering.

         CJP GROOVE WELD                          PJP GROOVE WELD
    (Full Cross-Section Fused)              (Unwelded Root Land Remains)

     \                      /                \                      /
      \   Sound Weld       /                  \   Sound Weld       /
       \    Metal         /                    \    Metal         /
        =================                       -----------------
                                                | Unwelded Root | <-- Severe Notch
                                                -----------------     (Kt > 3.0)

Comparative Engineering Mechanics

• Complete Joint Penetration (CJP):

  • Definition: A groove weld in which weld metal extends completely through the joint thickness, fully fusing the base metal across the entire cross-section.
  • Mechanical Performance: When executed with matching filler metal, a CJP groove weld transmits the full static tensile, compressive, and shear capacity of the base metal. Its design strength is identical to that of the parent plate ($F_{\text{allow}} = F_{\text{base}}$).
  • Fatigue Resistance: Excellent. When weld reinforcement is ground flush with the plate surface and volumetric NDE confirms absence of planar flaws, a CJP butt joint achieves AISC Fatigue Category B.
  • Fabrication Cost: High. Requires precise fit-up, backing strips or back-gouging to sound metal (e.g., carbon arc gouging), and extensive volumetric non-destructive examination (UT/RT).

• Partial Joint Penetration (PJP):

  • Definition: A groove weld in which weld metal does not extend through the joint thickness, leaving an un-welded root face or root land by design.
  • Mechanical Performance: The effective throat ($E$) is strictly less than the plate thickness ($t$). Capable of transmitting static compression and moderate static shear.
  • Fatigue Resistance: Extremely Poor. The unwelded root face constitutes an intrinsic, pre-existing sharp crack notch. Under cyclic axial tension or out-of-plane bending, high stress concentrations ($K_t > 3.0$) trigger rapid fatigue crack propagation along the unwelded interface. Consequently, PJP groove welds are categorized under severe fatigue classes (AISC Category C' or E) and are strictly prohibited in cyclically loaded tensile tension connections.
  • Fabrication Cost: Low. Eliminates back-gouging, requires less filler metal (often 50% to 70% reduction in weld volume), and significantly lowers residual stresses and angular distortion.

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Nondestructive Examination Symbols (AWS A2.4)

AWS B5.16 Clause 8.3.1 lists NDE symbols as a required Part 3 topic, and AWS A2.4 standardizes them in the same reference-line grammar as weld symbols — which is exactly why they are tested here alongside welding symbols.

Method Letter Designations

The examination method is given by standard letters placed on the reference line:

Combined methods are written with a plus sign (for example MT + UT) when more than one method is required on the same joint.

Placement Grammar

The side conventions mirror the weld symbol exactly:

• Below the reference line (arrow side): examine the arrow side of the joint.
• Above the reference line (other side): examine the other side.
• On both sides, or centred on the reference line with the line broken: examine from both sides.
• Placing the letters centred on the reference line with no arrow-side/other-side distinction indicates the method may be applied from either side.

Additional elements:

• Extent of examination is shown by the length dimension placed to the right of the method letters; with no dimension given, examination is full length.
• Number of examinations is placed in parentheses in the tail — RT (4) calls for four radiographs.
• Partial or spot examination is indicated by a percentage or a length, for example UT 20% or MT 6.
• The tail carries the governing specification, procedure or acceptance standard.
• A radiation direction symbol with its angle may be added to fix the beam orientation for RT.

Combination Weld and NDE Symbols

The same reference line can carry both instructions: the weld symbol on one side and the examination symbol on the other. A single-V groove symbol below the line with RT above it specifies an arrow-side single-V groove weld that is to be radiographed from the other side.

Exam Trap: Reading the NDE side convention backwards. Arrow side is below the reference line for both weld symbols and examination symbols. Candidates who memorize "below equals arrow side" for welds but then invert it for NDE lose the point. The convention is identical; only the content of the symbol changes.

Comprehensive Worked Engineering Example: Deciphering Complex Welding Callouts

Problem Statement

A structural fabrication drawing specifies the following complex welding symbol on a heavy crane column-to-baseplate T-joint connecting a $36\text{ mm}$ thick ASTM A572 Gr 50 web to a $65\text{ mm}$ thick base plate:

               G
             (---)
             16(20)   60°
         ---/-----\----------(100% UT)-----> (Points to vertical web)
            \     / 
             |>        12   100 - 250

(A field weld flag is present at the junction pointing away from the arrow, and a broken arrow line breaks downward, pointing directly at the $36\text{ mm}$ web plate).

Provide the complete engineering interpretation of this symbol, specifying: (a) which plate is beveled and what the preparation dimensions are, (b) the other-side weld details, (c) the finishing and inspection requirements, and (d) the location of fabrication.

Step-by-Step Engineering Solution

Step 1: Identify Arrow Break and Edge Preparation (Arrow Side)

• The symbol on the bottom (Arrow Side) is a Single-Bevel Groove with a bevel angle and dimensions.
• The arrow line is broken and points directly at the $36\text{ mm}$ vertical web. Therefore, the web plate must be beveled; the $65\text{ mm}$ base plate remains square.
• The depth of bevel preparation is $S = 16\text{ mm}$.
• The minimum required effective throat is $E = 20\text{ mm}$ (the parenthesized dimension). Because $E > S$, the welding procedure must achieve $4\text{ mm}$ of deep penetration beyond the bevel root face.
• The included groove angle is $\alpha = 60^\circ$.

Step 2: Identify Other-Side Weld Details

• The symbol on top (Other Side) is a Fillet Weld.
• The leg size is $w = 12\text{ mm}$ (placed to the left of the fillet triangle).
• The weld is an intermittent fillet weld: segment length $L = 100\text{ mm}$, center-to-center pitch $P = 250\text{ mm}$. The unwelded gap between segments is $P - L = 250 - 100 = 150\text{ mm}$.

Step 3: Finishing and Contour Requirements

• Above the other-side fillet symbol sits a flush contour symbol --- topped by the finish designator G.
• Interpretation: The $12\text{ mm}$ intermittent fillet welds on the other side must be ground flush with the base metal surface using mechanical grinding ($G$).

Step 4: Tail References and Fabrication Location

• The field weld flag is attached to the reference line junction. Interpretation: This joint is a field weld to be erected and welded on site, not in the fabrication shop.
• The tail contains the specification "100% UT". Interpretation: The entire length of the groove weld must undergo 100% Ultrasonic Testing in accordance with the governing structural code (AWS D1.1 Clause 8).

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Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Field Disaster Scenario

During the erection of a seismic moment frame for a 40-story commercial tower, the fabricator misinterpreted an unbroken arrow line on a single-bevel groove joint between a $50\text{ mm}$ column flange and a $25\text{ mm}$ beam continuity doubler plate. The detailer failed to include a break in the arrow line, intending for the thinner doubler plate to be beveled. The shop beveled the $50\text{ mm}$ column flange instead. This mistake inverted the bevel volume, quadrupled the required weld metal from $4.2\text{ kg/m}$ to $16.8\text{ kg/m}$, severely overheated the through-thickness Z-direction grain structure of the column, and caused catastrophic lamellar tearing directly beneath the fusion line during shop cooling. The column had to be scrapped, resulting in an eight-week project delay and $420,000 in replacement steel and ultrasonic testing costs.

Common Exam Traps

Exam Trap 1: Fillet Symbol Orientation In AWS A2.4, the perpendicular leg of the fillet right triangle must always be drawn on the left side, regardless of which direction the arrow points. Candidates who draw the perpendicular leg on the right side or mirror it based on arrow direction will lose full credit on Part 3 design questions.

Exam Trap 2: Length and Pitch Order ($L - P$) When sizing intermittent welds, length and pitch are separated by a hyphen to the right of the symbol: $L - P$. Pitch is never the clear distance between welds; it is always the center-to-center spacing! If an exam question specifies a $50\text{ mm}$ weld with an unwelded space of $100\text{ mm}$, the pitch is $P = 50 + 100 = 150\text{ mm}$. The correct callout is $50 - 150$.

Exam Trap 3: Groove Depth ($S$) vs. Effective Throat ($E$) When a symbol reads $10(12)$, $10$ is the prepared depth of bevel ($S$), and $(12)$ is the effective design throat ($E$). If only a single unparenthesized number appears (e.g., $10$), then by definition $S = E = 10\text{ mm}$. If only a single parenthesized number appears (e.g., $(12)$), the engineering drawing dictates the required effective throat, leaving the specific depth of preparation $S$ and root gap to the fabricator's qualified WPS.