BS7608 Design Life and Stress Calculation

Calculate component Fatigue design life for various fatigue catagories as per BS7608 Standard. Calculated stress input yields a design life. Desired Design life input yeilds an allowable design stress.

BS 7608 is a British Standard code of practice that provides guidelines for the fatigue design and assessment of steel structures. The code is primarily focused on welded steel structures and offers a comprehensive framework for evaluating fatigue life based on stress-life (S-N) curve methods. Here is a summary of the main steps involved in calculating fatigue life using BS 7608:

1. Identify the critical details: Begin by identifying the critical details in the steel structure that are susceptible to fatigue damage. These details usually include welded joints, bolted connections, and other areas subjected to cyclic loads or stress concentrations.

2. Determine the stress range: For each critical detail, calculate the stress range experienced due to cyclic loading. The stress range is the difference between the maximum and minimum stress during a loading cycle. This may involve analyzing the structure under different load combinations or using finite element methods to obtain accurate stress values.

3. Categorize the detail: Based on the geometry, material, and quality of the welded joint or connection, classify the detail according to the fatigue classes defined in BS 7608. The code provides fatigue strength curves (S-N curves) for different detail categories, which represent the stress range's relationship with the number of cycles to failure.

4. Estimate the applied load cycles: Determine the number of load cycles that the structure will experience during its design life. This may involve considering the loading history, environmental conditions, and any other relevant factors that could influence the fatigue life.

5. Determine the fatigue life: Using the S-N curve for the appropriate detail category, find the corresponding fatigue life for the calculated stress range. This value represents the number of cycles the detail can withstand before failure due to fatigue occurs.

6. Compare fatigue life with design life: Compare the estimated fatigue life with the design life of the structure. If the fatigue life is greater than or equal to the design life, the structure is considered safe against fatigue failure. If not, modifications to the design, such as increasing the thickness of the material, improving the quality of the weld, or selecting a more fatigue-resistant detail, may be necessary.

7. Perform a sensitivity analysis (optional): To account for uncertainties in the fatigue assessment, a sensitivity analysis can be performed. This analysis evaluates the effect of variations in parameters such as material properties, stress range, and detail classification on the fatigue life estimation.

In summary, calculating fatigue life using BS 7608 involves identifying critical details, determining the stress range, categorizing the detail, estimating the applied load cycles, and comparing the fatigue life with the design life. The code provides a systematic approach for engineers to assess and ensure the durability and safety of steel structures under cyclic loading conditions.