RCS - Fatigue code-check of reinforced concrete section

Introduction

The verification of reinforced concrete sections under fatigue loading is an essential requirement in the design of road bridges, ensuring adequate safety and durability throughout the intended service life. According to EN 1992-2 (Eurocode 2 – Design of Concrete Structures – Part 2: Concrete bridges), the fatigue assessment of reinforcement is carried out in accordance with the provisions of Annex NN, which provides the framework for the code-based check of stress ranges in reinforcing steel.

In the context of road bridges, the most relevant traffic loading model for fatigue design is Fatigue Load Model 3 (FLM3), which represents a standardized single-axle load applied in the traffic lanes. FLM3 is intended to capture the effect of repeated heavy vehicles over the lifetime of the structure and serves as the basis for determining stress ranges in the reinforcement. The calculated stress variations are then evaluated against the allowable values derived from S–N curves provided in the code, ensuring that reinforcement detailing and material performance are adequate to prevent fatigue-induced damage.

The code check according to Annex NN therefore provides a systematic procedure to:

1. Define the relevant fatigue load effects using FLM3.
2. Determine the corresponding stress ranges in reinforcement.
3. Compare these stress ranges with the design resistance obtained from the Eurocode fatigue verification format.

This procedure guarantees that reinforced concrete bridge components possess sufficient fatigue strength under long-term traffic loading, thereby safeguarding structural performance and extending service life with minimal maintenance. 

New project

Begin the code verification process by selecting the code "EN 1992-2:2008-07" along with additional functionalities such as "Fatigue -->Annex NN."

Cross-section

As a first step, review the cross-section definition, go to the "Project data" set the "Design working life" to 100 years and change the "Concrete grade."

Set the cross-section as a standard Rectangle with 800mm width and 400mm depth.

Design member

Begin the essential setup for the upcoming code verification at this stage. 

In the Design Member step, set the following:

• Type of bridge - Road bridge
• Traffic categories - Roads and motorways with two or more lanes with high flow rates or lorries 
• Damage equivalent impact factor 
  •  1.20 (rough surfaces)
• Coefficient λs1 
  •  1.10 (a factor accounting for element type (eg. continuous beam) and takes into account the damaging effect of traffic, depending on the critical length of the influence line or area)
• Coefficient λs2 
  • 1.00 ( a factor that takes into account the traffic volume)
• Coefficient λs3
  • 1.00 ( a factor that takes into account the design life of the bridge)
• Coefficient λs4
  • 1.08 (a factor to be applied when the structural element is loaded by more than one lane)

The "Construction Stage" card is initiated when the fatigue load is applied to the structure, influencing the calculation of the fatigue concrete strength.

Manual calculation illustrating how the inputs are incorporated into the equations.

Internal forces

The subsequent step involves the internal forces for "Max. cyclic load" and "Min. cyclic load." Prior to beginning the input process, it is advisable to understand the appropriate combinations and effects to consider during input. The inputs for internal forces are derived from EN 1992-1-1, Chapter 6.8.3 -Combination of actions.

Reinforcement

Take advantage of the template and set up the longitudinal and shear reinforcement.

Calculation Control & Sections

The objective is to conduct fatigue code verification. Please select Shear and Fatigue checks; all other checks may be deactivated. The shear check is activated to know the resistance of the section in shear for fatigue on concrete in shear. In the section chapter, it is essential to select the section type:

• Factor 1.75 - section is above the support area
• Factor 1.4 - other areas

Results

General overview

The stress induced by Maximum and Minimum cyclic loads determines the stress range for reinforcement and concrete. This constitutes a crucial output for code verification. At first, go to the navigator and select Results -> Fatigue section check. The max. and min. values are concised in the Ribbon.

Fatigue resistance of reinforcement

Fatigue resistance of concrete under compression

Fatigue resistance of concrete under shear

Conclusion

The fatigue verification of reinforced concrete sections in road bridges, performed according to EN 1992-2 Annex NN and using Fatigue Load Model 3 (FLM3), ensures that structural elements can withstand the cumulative effects of repeated traffic loading over their service life. By systematically defining load effects, calculating stress ranges, and comparing them with code-based resistances, the procedure safeguards both reinforcement and concrete against fatigue-induced deterioration.

The practical application of the verification process requires careful setup of project data, cross-section properties, traffic categories, and influence factors (λ1–λ4), followed by accurate definition of cyclic internal forces and reinforcement detailing. With these inputs, the shear and fatigue checks provide reliable results for reinforcement, concrete in compression, and concrete in shear.

Overall, the method provides engineers with a consistent framework to validate durability and long-term performance, ensuring that road bridges remain safe, serviceable, and economical throughout their 100-year design life.

Csatolt letöltések

• VERIFICATION FATIGUE EN 1992-2.7z (7Z, 1,1 MB)