Fatigue and Progressive-Collapse Concepts
Key Takeaways
- Fatigue demand is governed primarily by repeated stress range and number of cycles, not simply by the largest algebraic stress
- Connection geometry, weld profile, holes, attachments, and fabrication details determine fatigue category and crack sensitivity
- A constant dead-load stress may shift mean stress without changing the calculated range between the same two cyclic extremes
- Progressive-collapse robustness limits disproportionate spread after local damage through continuity, redundancy, ductility, and viable alternate load paths
- No single progressive-collapse procedure governs every structure; use the applicable 2026 references, occupancy criteria, and project-specific requirements
Fatigue and Progressive-Collapse Concepts
Two different questions: Fatigue asks whether repeated stress fluctuations can initiate and grow a crack. Progressive-collapse assessment asks whether local damage can spread into failure that is disproportionate to the initiating event. Do not use one check as a substitute for the other.
Fatigue Starts With Stress Range
For one repeated cycle, calculate the signed extreme stresses at the same detail:
Δf = f_max - f_min.
If stress reverses from compression to tension, preserve signs before subtracting. A cycle from -5 ksi to +7 ksi has a 12-ksi range, not 2 ksi. Peak stress still matters for yielding and other limit states, but fatigue provisions commonly organize demand around stress range, number of repetitions, and detail category.
| Input | What it represents | Frequent error |
|---|---|---|
f_max, f_min | Extremes at the same fatigue-sensitive point | Comparing stresses from different locations |
Δf | Fluctuation driving repeated crack growth | Using only ` |
n | Expected cycles at a given range | Counting years instead of load events |
| Detail category | Geometry/fabrication sensitivity | Selecting by steel yield strength alone |
Fatigue cracks tend to initiate at stress concentrations: weld toes and terminations, abrupt section changes, bolt holes, attachments, copes, and poor surface transitions. A higher-strength base metal does not automatically improve a fatigue-sensitive geometry. The applicable AASHTO or AISC detail description, loading direction, and fabrication condition control classification.
Worked Stress-Range and Cycle Screen
At a welded detail, constant dead load produces +8 ksi. A repeated vehicle event adds a cyclic stress varying from -6 ksi to +10 ksi. The total extremes are
f_min = 8 - 6 = +2 ksi
and
f_max = 8 + 10 = +18 ksi.
Therefore,
Δf = 18 - 2 = 16 ksi.
The constant 8-ksi component changes both total extremes but cancels from their difference. It cannot be ignored in a separate maximum-stress or strength check.
Suppose the event occurs 250 times per day for 40 years. The expected count is
n = 250(365)(40) = 3,650,000 cycles.
If the problem provides a 12-ksi permissible range for the identified category at that cycle count, the range ratio is
16/12 = 1.33,
so this illustrative screen does not pass. The 12-ksi value is problem-given, not a universal threshold. On a 2026 exam, retrieve the correct relationship from AASHTO LRFD 8th edition with its identified May 2018 errata or the AISC Steel Construction Manual, 15th edition, as applicable.
When several ranges occur, group cycles consistently with the required method. If a problem supplies fatigue lives N_i, a cumulative-damage check may use Σ(n_i/N_i) against its stated criterion. Do not average high and low ranges before verifying that the method permits it; a few large cycles can be disproportionately important.
A Fatigue Workflow
- Identify the exact member location and potential crack plane.
- Determine the current-code detail category from geometry, connection, and fabrication.
- Calculate signed stress extremes from the same load event and point.
- Find
Δf; separate constant stress from the fluctuating component. - Count cycles over the stated life and group variable amplitudes as required.
- Compare demand with the category/cycle rule in the controlling 2026 reference.
- Check other limit states independently and consider inspection access where required.
Progressive Collapse and Robustness
Progressive collapse is the spread of local damage through a structure, producing an extent of failure disproportionate to the initiating damage. Robustness is the ability to tolerate a local loss or abnormal condition without that disproportionate spread. Relevant attributes include:
- continuity: connections and ties can transfer forces after the original path changes;
- redundancy: more than one credible path can carry critical actions;
- ductility: components and connections can rotate or deform while retaining useful resistance;
- compartmentalization: deliberate boundaries can limit the region affected;
- local resistance: selected critical elements may be designed for specified abnormal actions.
Redundancy is not merely counting members. Two nominal paths that rely on one brittle connection or the same vulnerable support are not independent. Ductile members do not create robustness if their connections fracture before rotations develop. Load reversal can also place tension where a gravity connection was detailed primarily for compression.
Alternate-Path Reasoning
When an applicable authority or project criterion requires a local-damage scenario, use this reasoning sequence:
- Define the removed or impaired component and damage extent exactly as required.
- Redraw the structure after damage; do not leave the lost reaction or member in the model.
- Trace redistributed gravity and lateral actions through surviving members, connections, diaphragms, and foundations.
- Apply the prescribed load combination, dynamic treatment, deformation limits, and acceptance criteria.
- Check connection rotation, force reversal, stability, second-order effects, and whether supports can develop the alternate path.
- Compare the predicted damaged region with the permitted extent and document weak links.
This is a framework, not a universal design equation. Requirements vary with structure, occupancy, risk, jurisdiction, and governing documents. For a 2026 PE Civil: Structural problem, use the supplied IBC 2018, AISC 15th, AASHTO 8th, and any explicit problem criteria; do not import a later edition or invent a single removal scenario for every building and bridge.
Final Distinction
A detail can have adequate monotonic strength yet poor fatigue performance, and a member can pass fatigue while the system lacks a viable alternate path. Keep the scales separate: fatigue follows local stress fluctuation through millions of events; robustness follows system redistribution after defined local damage.
In the worked fatigue example, what stress range acts at the welded detail during each vehicle event?
How many cycles result from 250 repeated events per day over a stated 40-year period using 365 days per year?
Which statement best describes a defensible progressive-collapse robustness assessment?