6.Fatigue Stress Amplitude Limits and Calculation Methods for Steel Reinforcement

Value-Taking Principles of Fatigue Stress Amplitude Limits
The fatigue stress amplitude limits of ordinary steel bars and prestressing reinforcement shall be determined by linear interpolation from corresponding tables according to the fatigue stress ratio of steel. This index is applied to the fatigue checking of concrete structural members subjected to repeated loads, to ensure the long-term service safety of structures under cyclic loading.
Note: Δfyf represents the fatigue stress amplitude limit of ordinary steel bars; Δfpyf represents the fatigue stress amplitude limit of prestressing reinforcement. Linear interpolation is used to calculate parameters between adjacent ratio grades in the table, which complies with standard provisions.

Table 1 Fatigue Stress Amplitude Limits of Ordinary Steel Bars (N/mm²)

Note: When longitudinal tensile steel bars are connected by flash butt welding, the fatigue stress amplitude limit of steel bars at the joint shall be taken as 0.8 times the value in the table.

Calculation Method of Fatigue Stress Ratio for Ordinary Steel Bars
The fatigue stress ratio of ordinary steel bars is defined as the ratio of the minimum stress to the maximum stress of steel bars in the same layer during fatigue checking, calculated according to the standard formula. This ratio reflects the stress fluctuation degree of steel bars under cyclic loads: the smaller the ratio, the larger the stress amplitude, and the more unfavorable it is to fatigue performance.
Note: ρsf represents the fatigue stress ratio of ordinary steel bars; σs,minf and σs,maxf refer to the minimum stress and maximum stress of steel bars in the same layer during fatigue checking respectively. The definitions are fully consistent with the referenced standard.

Fatigue Stress Amplitude Parameters for Prestressing Reinforcement
The fatigue stress amplitude limits of prestressing reinforcement are selected from the corresponding table, applicable to steel strands and stress-relieved wires with a characteristic ultimate strength of 1570 N/mm². When the fatigue stress ratio is not less than 0.9, fatigue checking of prestressing reinforcement can be omitted. When sufficient engineering evidence is available, the limits specified in the table can be adjusted appropriately.

Table 2 Fatigue Stress Amplitude Limits of Prestressing Reinforcement (N/mm²)

Notes:

1. When ρpf is not less than 0.9, fatigue checking of prestressing reinforcement is not required.

2. When sufficient evidence is available, the fatigue stress amplitude limits specified in the table can be adjusted appropriately.

Note: ρpf represents the fatigue stress ratio of prestressing reinforcement; fptk represents the characteristic ultimate strength of prestressing reinforcement. The parameters in the table are for Grade 1570 prestressing steel products, conforming to standard provisions.

Calculation Method of Fatigue Stress Ratio for Prestressing Reinforcement
The fatigue stress ratio of prestressing reinforcement is the ratio of the minimum stress to the maximum stress of prestressing tendons in the same layer during fatigue checking, calculated by the corresponding standard formula. This parameter is the core basis for determining the fatigue stress amplitude limit of prestressing reinforcement, and directly affects the result of fatigue checking.
Note: σp,minf and σp,maxf refer to the minimum stress and maximum stress of prestressing tendons in the same layer during fatigue checking respectively. The symbol meanings are consistent with the standard provisions.

Disclaimer

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The data in this article are derived from publicly available industry standards and engineering experience. The viewpoints presented are for reference only. For specific schemes and data in actual engineering projects, adjustments shall be made in combination with the actual conditions of the project.

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