Code Compliance & Stress Evaluation

Once the global displacements have been solved and the local element forces extracted, the engine routes these forces through the selected Design Code module.

This section details how the engine applies Stress Intensification Factors (SIFs) and calculates the Code Stress Ratios for standard plant piping and fully restrained pipelines.

3.1 Stress Intensification Factors (SIFs)

Piping components like elbows, tees, and reducers exhibit localized stress concentrations. The engine applies user-defined or ASME B31J derived SIFs to the extracted bending and torsional moments.

The engine extracts three primary SIFs for every node:

• $i_{in}$: In-plane bending SIF
• $i_{out}$: Out-of-plane bending SIF
• $i_{tor}$: Torsional SIF

Axial SIF Logic: By default, the axial SIF ($i_{ax}$) is taken as $1.0$. However, if the solver detects that a node belongs to a TEE component, the axial SIF is automatically set to equal the out-of-plane SIF ($i_{ax} = i_{out}$), capturing the intensified stress at the branch connection.

Effective Properties

All stress calculations utilize the effective pipe thickness ($t_{eff}$), which removes the user-defined mill tolerance percentage and corrosion allowance from the nominal wall thickness.

3.2 ASME B31.3 & B31.1 (Plant Piping)

The engine contains dedicated routers for Process Piping (B31.3) and Power Piping (B31.1), as they handle torsion and sustained SIFs differently.

ASME B31.3 (Process Piping)

For B31.3, the intensified bending ($S_b$), torsional ($S_t$), and axial ($S_a$) stresses are calculated as:

$S_b = \frac{\sqrt{(i_{in} M_{in})^2 + (i_{out} M_{out})^2}}{Z}$

$S_t = \frac{i_{tor} M_{tor}}{2Z}$

$S_a = \frac{i_{ax} |F_{ax}|}{A_{eff}}$

The Expansion Stress ($S_E$) and Sustained Stress ($S_L$) are evaluated as:

• $S_E = \sqrt{(S_a + S_b)^2 + (4 S_t^2)}$
• $S_L = S_p + S_b + S_a$ (Where $S_p$ is longitudinal pressure stress)

ASME B31.1 (Power Piping)

B31.1 diverges from B31.3 in several critical ways which the engine automatically enforces:

1. Torsion SIF: B31.1 generally ignores SIFs on torsion. The engine forces $i_{tor} = 1.0$.
2. Expansion Stress: Axial stress ($S_a$) is decoupled from the expansion evaluation: $S_E = \sqrt{S_b^2 + 4 S_t^2}$.
3. Sustained SIF: The sustained stress calculation applies a distinct multiplier of $0.75 \times i_{max}$ (with a minimum floor of $1.0$) to the resultant bending moment.

Occasional Allowable Multipliers: For short-duration events (Wind/Seismic), the basic allowable stress is multiplied by $1.33$ for B31.3, and $1.20$ for B31.1.

3.3 AS 4041 (Australian Pressure Piping)

If the Australian AS 4041 code is selected, the engine routes the local forces through a regional compliance check.

• Bending Stress ($S_b$): Calculated identically to B31.3.
• Torsional Stress ($S_t$): AS 4041 strictly enforces a Torsional SIF of $1.0$ for standard geometries, overriding any user inputs.
• Axial Stress ($S_a$): Calculated using the $i_{ax}$ logic for Tees.

The resulting Expansion Stress is formulated without the axial component ($S_E = \sqrt{S_b^2 + 4 S_t^2}$), while the Sustained Stress includes it ($S_L = S_p + S_b + S_a$). Occasional loads apply a $1.33$ multiplier to the base allowable.

3.4 Pipeline Codes (AS 2885 / ASME B31.8)

Standard plant piping codes evaluate flexibility. Buried pipelines, however, are essentially locked in place by soil.

The engine contains an intelligent Buried Pipe Router. If the solver detects that an element has an assigned soil_id (meaning it is buried), it bypasses the standard B31.3/B31.1 unrestrained allowables and switches to the Fully Restrained Pipe logic dictated by codes like AS 2885 and ASME B31.8.

Instead of evaluating against a standard fatigue allowable ($S_h$), the stresses are evaluated directly against the material's Specified Minimum Yield Strength (SMYS).

• Operating Allowable: $0.90 \times SMYS$
• Occasional Allowable: $1.0 \times SMYS$

This ensures that long cross-country pipelines are correctly evaluated for yielding under extreme soil confinement, rather than being falsely failed by plant piping fatigue rules.