What is the best way to evaluate fatigue performance in steel connections?

Fatigue is a common cause of failure in steel connections, especially in structures that are subjected to cyclic loading, such as bridges, cranes, and offshore platforms. Fatigue performance depends on many factors, such as the geometry, stress range, load history, weld quality, and environmental conditions of the connection. Therefore, it is important to evaluate fatigue performance in steel connections using reliable and accurate methods. In this article, you will learn about some of the best ways to evaluate fatigue performance in steel connections, based on the latest steel design codes and standards.

1 Nominal stress method

The nominal stress method is the most widely used method to evaluate fatigue performance in steel connections. It is based on the concept of a nominal stress range, which is the difference between the maximum and minimum stresses at a critical location of the connection. The nominal stress range is compared with a fatigue strength curve, which is derived from experimental data and represents the relationship between the stress range and the number of cycles to failure. The fatigue strength curve depends on the fatigue category of the connection, which is determined by its geometry and detail. The nominal stress method is simple and conservative, but it has some limitations, such as ignoring the effects of stress concentration, residual stress, and load sequence.

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Muhammad Farhad Ahmed

Electrical Installer | City & Guilds, BS 7671:2018
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The best approach to evaluate fatigue performance in steel connections involves a combination of laboratory fatigue testing, finite element analysis (FEA) for stress simulation, reference to industry standards, historical data, and non-destructive testing (NDT) methods like ultrasonic testing. Consideration of environmental factors and implementing structural health monitoring systems further enhance evaluation accuracy, ensuring the connection's durability and safety.

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2 Hot spot stress method

The hot spot stress method is an alternative method to evaluate fatigue performance in steel connections. It is based on the concept of a hot spot stress, which is the peak stress at the root of a potential crack in the connection. The hot spot stress is calculated using finite element analysis or extrapolation from measured strains. The hot spot stress range is compared with a hot spot fatigue strength curve, which is also derived from experimental data and represents the relationship between the hot spot stress range and the number of cycles to failure. The hot spot fatigue strength curve depends on the thickness and stress gradient of the connection. The hot spot stress method is more accurate and realistic than the nominal stress method, but it requires more advanced tools and expertise.

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3 Fracture mechanics method

The fracture mechanics method is a more sophisticated method to evaluate fatigue performance in steel connections. It is based on the concept of a crack growth rate, which is the rate at which a crack propagates in the connection under cyclic loading. The crack growth rate is calculated using fracture mechanics principles and parameters, such as the stress intensity factor, the threshold stress intensity factor, and the Paris law. The fracture mechanics method can predict the remaining life and the critical crack size of the connection, as well as the effects of crack closure, variable amplitude loading, and corrosion. The fracture mechanics method is the most comprehensive and reliable method, but it requires more detailed information and assumptions about the connection.

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4 Comparison and recommendation

The three methods described above have different advantages and disadvantages in evaluating fatigue performance in steel connections. The nominal stress method is the easiest and most conservative method, but it may overestimate or underestimate the fatigue life of the connection. The hot spot stress method is the most balanced and widely accepted method, but it may not capture some complex phenomena that affect fatigue performance. The fracture mechanics method is the most accurate and realistic method, but it may be too complex and costly for some applications. Therefore, the best way to evaluate fatigue performance in steel connections depends on the specific situation and the available resources. However, as a general recommendation, the hot spot stress method is preferred for most cases, as it is consistent with the latest steel design codes and standards, such as Eurocode 3, AISC 360, and ISO 19902.

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Enderson Cruz R.

☀️Top Laboratory Techniques Voice I Oilfield Formulator I Drilling Fluids Formulator I R&D Lab I Chemicals Synthesis I Resins I Demulsifiers I Chemicals Products Development I EOR
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Each method for evaluating fatigue performance in steel connections indeed has its own set of advantages and disadvantages. The nominal stress method, as you mentioned, is relatively straightforward and conservative, making it easier to apply. The structural stress method, on the other hand, considers the local stress conditions at the connection detail, providing a more accurate representation of the actual stress experienced by the material. Lastly, the hot-spot stress method focuses on identifying critical locations within the connection where stress concentrations are most severe. While this method offers detailed insight into specific high-stress areas, it can be challenging to determine these hot-spot locations accurately.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Bokang Sithole Pr.Eng, MBA

Professional Engineer | Candidate Pr.CPM | MBA | MSc Eng (Civil-Water) | BSc Bioresources Eng | PGD Project Management | Btech Civil Eng - Water | Golden Key Member | CMSAICE | SAICE Durban FLP Chairperson
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Evaluate fatigue performance in steel connections using analytical methods, complemented by experimental testing to validate predictions and assess actual behavior. Refer to industry standards for guidance on fatigue design, ensuring structural integrity and durability.

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What is the best way to evaluate fatigue performance in steel connections?

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1 Nominal stress method

2 Hot spot stress method

3 Fracture mechanics method

4 Comparison and recommendation

5 Here’s what else to consider

Construction Management

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What is the best way to evaluate fatigue performance in steel connections?

1

2

3

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5

1 Nominal stress method

2 Hot spot stress method

3 Fracture mechanics method

4 Comparison and recommendation

5 Here’s what else to consider

Construction Management

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