Crash Course to Piping Stress | Module 3
Load Combinations per ASME Code
If you followed Module 2 of this series, you already know that piping systems face three categories of loads — primary, secondary, and occasional — and that these loads generate stress inside the pipe wall. But in real-world engineering, loads are never checked one at a time. They're checked in specific combinations, because a pipe experiences weight, pressure, temperature, and (occasionally) wind or seismic forces all at once, in different mixes depending on what condition it's operating under.
In this post, we'll walk through the four standard load combinations defined by ASME B31.3 — one of the most widely used codes in piping stress analysis — and see how CAESAR II, the industry-standard piping stress software, automatically builds these combinations in practice.
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Why Load Combinations Matter
A pipe doesn't experience its own weight on one day and internal pressure on another — every load acts at the same time, in a mix that depends on the system's operating condition. A pipe undergoing a pressure test behaves differently than one running hot in normal operation, which behaves differently again during a rare event like an earthquake.
This is exactly the gap ASME B31.3 fills. It defines precisely which loads should be combined for each realistic scenario, and which code paragraph governs the allowable stress for that specific combination. These combinations can be summarized in four standard cases, each of which we'll break down below.
The Four ASME Load Cases
Each case combines a different mix of loads, checks different stress components, and calculates allowable stress differently. Here's a quick preview before we go deeper:
- Case 1 — Hoop Case: maximum pressure only
- Case 2 — Sustained Case: operating pressure + system weight
- Case 3 — Expansion Case: maximum expected temperature difference
- Case 4 — Occasional Case: weight + pressure + an occasional load like wind or earthquake
Case 1: Hoop Case
The system is checked against maximum pressure only — normally around 1.5 times the normal operating pressure. Think of this as a stress-test scenario: what happens if pressure spikes well above what the system typically sees? The only stress component checked here is hoop stress — the stress wrapping around the pipe's circumference, similar to the metal hoops holding together a wooden barrel, which tries to split the pipe into two halves.
This combination falls under the primary load category, since it's driven directly by pressure rather than displacement. The allowable stress for this case comes from ASME B31.3, paragraph 304.1.2.
Case 2: Sustained Case
This case reflects everyday, steady-state operation. The system is checked against the normal operating pressure — not the inflated 1.5x figure from the Hoop Case — in addition to the system's weight. The stress components checked are longitudinal stress and shear stress. This combination also falls under the primary load category, and its allowable stress comes from paragraph 302.3.5.
Case 3: Expansion Case
Here's where secondary loads enter the picture. The system is checked against the maximum expected temperature difference — how much the pipe heats up from its cold, installed condition to its hottest operating condition. The stress components checked are again longitudinal and shear stress. This combination falls under the secondary load category, since it's driven by thermal displacement rather than direct force. Interestingly, the allowable stress is calculated from the same paragraph as the Sustained Case — 302.3.5 — though the resulting allowable value differs because secondary loads are treated differently under the code.
Case 4: Occasional Case
The system is checked against the combined effect of system weight and internal pressure — the same everyday loads as the Sustained Case — with an additional occasional load layered on top, such as wind or an earthquake. This represents a rare, worst-case, short-duration scenario. The stress components checked are longitudinal and shear stress, this combination falls under the occasional load category, and its allowable stress comes from paragraph 302.3.6.
The Four Cases at a Glance
| Case | Loads Checked | Stress Checked | Category | Code Reference |
|---|---|---|---|---|
| 1. Hoop | Max. pressure (~1.5× operating) | Hoop stress | Primary | §304.1.2 |
| 2. Sustained | Operating pressure + weight | Longitudinal + shear | Primary | §302.3.5 |
| 3. Expansion | Max. temperature difference | Longitudinal + shear | Secondary | §302.3.5 |
| 4. Occasional | Weight + pressure + wind/quake | Longitudinal + shear | Occasional | §302.3.6 |
Rule of thumb: never mix the allowable stress from one case with the loads from another. Each case has its own matched pair of loads and allowable stress — treat them as a set.
From Code to Software: How CAESAR II Builds These Combinations
Understanding the code requirements is one thing — applying them efficiently is another. CAESAR II, the piping stress software most engineers use day to day, doesn't require you to manually build each combination by hand. Instead, it automatically generates load combinations that satisfy ASME code requirements, based on the load components defined in your model: weight, pressure, temperature, and any occasional loads.
It's worth noting that the exact number of cases CAESAR II generates depends on how many temperature and pressure conditions are defined for a given system. The example below illustrates the underlying methodology using a simple, representative case set — not necessarily the exact count you'll see on every project.
Sample Auto-Generated Load Cases (L1–L5)
| Case | Name | Composition | Notes |
|---|---|---|---|
| L1 | Hydrotest | Weight + Hydrostatic pressure. (W+HP) | Pre-service pressure test condition |
| L2 | Operating | Weight + Operating pressure +Operating Temperature. (W+P1+T1) | Not an ASME check case — added by CAESAR II to obtain maximum support forces |
| L3 | Sustained | Weight + Operating pressure. (W+P1) | Matches the ASME Sustained Case |
| L4 | Expansion | L2 − L3 (isolates temperature difference) | Matches the ASME Expansion Case |
| L5 | Occasional | Sustained + Wind/Seismic | Matches the ASME Occasional Case |
Case L4 deserves a closer look: it's calculated simply as Case L2 minus Case L3. Since L2 includes the temperature effect and L3 doesn't, subtracting the two isolates just the thermal expansion contribution — exactly what the ASME Expansion Case is meant to evaluate.
Keep in mind this is just one sample sequence. The actual number of load cases CAESAR II generates will vary depending on how many temperature and pressure cases are defined in a specific model. What matters isn't memorizing L1 through L5 exactly — it's understanding the methodology CAESAR II uses to translate ASME code requirements into a working set of load cases.
Key Takeaways
- ASME B31.3 defines four standard load combinations: Hoop, Sustained, Expansion, and Occasional — each representing a different real-world scenario a pipe might face.
- Each case checks specific stress components against an allowable stress from its own code paragraph — §304.1.2, §302.3.5, or §302.3.6.
- CAESAR II automates this process, generating cases like L1 through L5 based on the number of temperature and pressure cases defined in the model.
- The Expansion case is typically derived as Operating minus Sustained, isolating the thermal effect on its own.
Frequently Asked Questions
Why can't engineers just check each load separately instead of combining them?
Because a pipe experiences all its loads simultaneously, not in isolation. Combining them into representative scenarios — like normal operation, pressure testing, or a seismic event — gives a realistic picture of what the pipe actually faces at any given moment.
Why do the Sustained and Expansion cases reference the same code paragraph?
Both draw their allowable stress calculation from ASME B31.3 paragraph 302.3.5, but the resulting allowable stress differs between the two, since primary loads (sustained) and secondary loads (expansion) are treated differently under the code.
Is the L1–L5 case sequence always the same in CAESAR II?
No. The number and composition of auto-generated cases depends on how many temperature and pressure conditions are defined for a specific piping system, so the sequence shown here is just a representative example.
What's covered in the next module?
Module 4 shifts focus to the different types of pipe supports used in piping systems — the physical hardware that carries all the loads discussed in this post.