Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME STP-PT-034-2011 — Alignment of Sustained Load Stress Indices in the ASME B31 Code
📊 ASME STP-PT-034-2011 is a technical research report prepared by George Antaki of Becht Engineering Company, Inc., under the sponsorship of ASME Pressure Technologies Codes and Standards and the ASME Standards Technology, LLC (ASME ST-LLC). The report systematically reviews the sustained-load stress equations and stress indices used across the ASME B31 code series — B31.1, B31.3, B31.4, B31.8, B31.7, and the piping provisions of ASME Boiler and Pressure Vessel Code Section III (NC/ND-3600 for Class 2 and 3 piping) — compares their features, and explores options for aligning them into a consistent, non-redundant set of rules. It is a key reference for understanding how the various piping codes treat sustained stresses and where their margins differ.
💡 In the ASME B31 world, “sustained loads” are generally understood to mean pressure and weight — the loads that act on the piping continuously. For buried pipe, the weight of the soil cover is also a sustained load. Sustained loads are one of the three load categories used in pipe stress analysis, alongside occasional and thermal loads.
1. Overview: Load Categories and the Status of Stress Checks 🧭
The ASME B31 codes do not use the “primary/secondary stress” terminology of the pressure vessel codes. Instead, external loads are classified into three categories:
- Sustained loads: pressure and weight; soil cover for buried pipe.
- Occasional loads: wind, earthquake, and pressure transients such as waterhammer.
- Thermal loads: expansion, contraction, and through-thickness thermal gradients.
Each load category has its own dedicated design equation. One of the central tasks of this report is to collect and compare the sustained-load stress equations in B31.1 (Power Piping), B31.3 (Process Piping), B31.4 (Liquid Transportation Systems), B31.8 (Gas Transmission and Distribution Piping), B31.7 (Nuclear Power Piping), and ASME III NC/ND-3600, and to identify where the equations and their stress indices diverge.
2. Technical Comparison of Sustained Stress Equations 📐
Sustained-stress evaluation generally combines the stress from internal pressure with the stress from sustained bending moments, the latter being amplified by a stress intensification factor or a stress index. The report highlights several important differences among the codes:
- The meaning of the stress intensification factor i: The SIF used by the B31 codes is an empirical factor derived from extensive fatigue testing of pipe components; it captures the combined effect of local stress concentration on fatigue endurance and static strength. Comparisons of experimental and theoretical peak stresses for unpressurized pipe bends (see Table 2 of the report) show that i is not automatically the right multiplier when applied to sustained-load evaluation.
- The ASME III stress index B: Indices such as B1 (primary membrane) and B2 (primary bending) rest on the theory of primary stress classification and are physically different from the fatigue-derived i.
- Equation form differences: For example, the B31.3 sustained stress equation applies a 0.75 factor to the bending term with the index i, while other codes use different forms and different allowable-stress bases (with coefficients such as αSh and βSh), giving noticeably different margins from one code to another.
In B31.3, for example, the sustained stress equation typically takes the form SL = PD/(4t) + 0.75·i·(MA/Z), where P is the internal pressure, D the outside diameter, t the wall thickness, i the in-plane/out-of-plane stress intensification factor, MA the sustained bending moment, and Z the section modulus. Note the specific way in which i is applied.
⚠️ The SIF i and the stress index B both “amplify” stress, but they come from different physical foundations: i is an empirical summary of fatigue test data, while B is rooted in primary-stress classification. Mixing the two systems in one sustained-load check can produce significantly different margins for the same line, which is exactly the problem this report addresses.
3. Options and Recommendations Proposed by the Report 📋
Based on its review of the existing equations and data, the report presents three options with a discussion of advantages and disadvantages for each:
| Option | Main Content | Advantages | Disadvantages |
|---|---|---|---|
| Option 1: Continue as is | Each B31 code keeps its existing sustained-stress equation and i factors | No code changes; accumulated engineering experience is preserved | Inter-code differences persist indefinitely |
| Option 2: Consistent equations with B indices | Rewrite sustained-load equations in a unified form based on ASME III indices B1/B2 | Clear physical basis; consistent with vessel and nuclear evaluation logic | Needs substantial validation data; large scope of change |
| Option 3: Adopt B31.3 Case 178 | Use the modified sustained-stress equation already published in Code Case 178 | Field-proven basis; smoother transition | Applicability constraints of the Case must be confirmed |
After weighing the three options, the report offers a preferred direction for sustained loads, while acknowledging that further input from the B31 committees, test data, and service-failure experience is needed. Appendices A through E reproduce the complete sustained-stress equations of B31.1 (2007), B31.3 (2004), B31.4, B31.8, and ASME III NC/ND-3600, giving engineers a side-by-side reference without having to open each code volume.
💡 The appendices are arguably the most valuable part of the report: five sets of sustained-stress equations compiled in one place make it easy to compare formula forms and allowable-stress conventions at a glance.
4. Engineering Practice Essentials 🛠️
- 🔍 Classify loads first: before running any stress analysis, sort loads into sustained, occasional, and thermal, and apply only the equation that corresponds to each category.
- 🔧 Check the direction of i: in-plane and out-of-plane stress intensification factors differ; obtain the correct value for elbows, tees, and reducers from the relevant code table or formula rather than reusing a number from another code.
- 📊 Mind the allowable-stress basis: different codes use different allowable stresses (Sh, Sy) with different safety margins; when switching codes, adopt the full set of values of the chosen code.
- 📋 Watch Code Cases: B31.3 Case 178 and its successors hint at the direction the committees are taking; if a Code Case is used, verify its approval status and scope before applying it.
5. Frequently Asked Questions ❓
❓ Q: What exactly are “sustained loads,” and why don’t the codes define them?
A: None of the B31 codes explicitly defines the term, but in practice it means pressure and weight — loads that act continuously. Buried pipe adds soil cover. The equations implicitly define the content of each category.
A: None of the B31 codes explicitly defines the term, but in practice it means pressure and weight — loads that act continuously. Buried pipe adds soil cover. The equations implicitly define the content of each category.
❓ Q: What is the difference between the stress intensification factor i and the stress index B?
A: i is an empirical factor fitted to fatigue test data that reflects the combined influence of local stress concentration on fatigue and strength; B indices derive from primary-stress classification theory and are used mainly for nuclear piping in ASME III. They are not interchangeable.
A: i is an empirical factor fitted to fatigue test data that reflects the combined influence of local stress concentration on fatigue and strength; B indices derive from primary-stress classification theory and are used mainly for nuclear piping in ASME III. They are not interchangeable.
❓ Q: Does this report change engineering design directly?
A: No — it is a methodology study that informs the B31 committees, not a code revision itself. Designers should track future code revisions on stress indices and margins.
A: No — it is a methodology study that informs the B31 committees, not a code revision itself. Designers should track future code revisions on stress indices and margins.
❓ Q: How can we avoid cross-code inconsistencies in multi-standard projects?
A: Pick one code and stay with it: use its equations, allowable stresses, stress indices, and acceptance criteria as a complete package without combining elements from other codes, and record the edition used in the calculation report.
A: Pick one code and stay with it: use its equations, allowable stresses, stress indices, and acceptance criteria as a complete package without combining elements from other codes, and record the edition used in the calculation report.
