Fatigue reliability of concrete wind turbine and bridge elements

The fatigue properties of reinforced and pre-stressed concrete structures are dependent on the fatigue properties of the concrete, the steel reinforcement and the pre-stressing steel. The inhomogeneous and complex structure of concrete, along with large spatial variations, the steel reinforcement and pre-stressing initial defects and the interaction between the steel and concrete, introduce significant uncertainties. These can to some extend be modelled in a similar way as for welded steel structures by using SN-curves and the Miner rule for linear damage accumulation, or alternatively fracture mechanics approaches. In order to obtain a both reliable and cost-competitive design of reinforced and pre-stressed concrete structures, it is important that the individual uncertainties are estimated and taken into account in the design process. This can be obtained by adopting a probabilistic design philosophy where the structure is designed in order to obtain a target reliability level. However, the uncertainty related to each parameter influencing the fatigue strength should be quantified and modelled as stochastic variables in order to estimate the fatigue reliability.
The work conducted in this project will be applied to wind turbine foundations and tower, and the fatigue loading conditions for wind turbines will be modelled considering typical large on- and offshore wind turbines. Further, pre-normative calibration of partial safety factors for application in design standards will be performed as well as considerations of the effect of inspection and monitoring during operation using a life-cycle approach.

Keywords:fatigue, reliability, concrete, bridges, wind turbines.

AAU – Aalborg University (Aalborg, Denmark)

36 months
From 01.12.2016 to 30.11.2019

Aalborg University (Denmark)

PhD director: Prof. John Dalsgaard Sørensen (AAU)

The PhD defence is scheduled for Friday 3 September 2021 in the Copenhagen campus of Aalborg University at 13:00. The defence will be also carried out online via Zoom.
Download the digital invitation

The members of the jury are:

Assessment committee:

  • Associate Prof. Christian Frier, (Chairman), Aalborg University, Aalborg Denmark
  • Prof. Bernt Leira, Norwegian University of Science and Technology (NTNU), Trondheim, Norway
  • Prof. Dimitri Val, Heriot-Watt University (HWU), Edinburgh, The United Kingdom

PhD supervisor

  • Prof. John Dalsgaard Sørensen, Dept. of the Built Environment, Aalborg University, Denmark


  • Prof. Lars Damkilde, Dept. of the Built Environment, Aalborg University

Main aim is to develop a probabilistic framework for reliability assessment of reinforced concrete structures with respect to fatigue. This includes application within the wind turbine industry where reinforced concrete structures are used widely for onshore foundations, but also for new, innovative designs of concrete towers both for onshore and offshore applications. The probabilistic framework will also be applicable for concrete bridges.

The project for ESR9 has the following objectives:

  • Development of a probabilistic framework for reliability assessment of reinforced concrete structures with respect to fatigue.
  • Application for wind turbines: foundations and innovative concrete towers.
  • Application for concrete bridges.
  • Calibration of partial safety factors incl. considerations of the effect of inspections and monitoring.
  • State-of-the-art models for fatigue of concrete and reinforcement.
  • Collection of fatigue data from literature and statistical analyses.
  • Stochastic modelling of fatigue strength and fatigue load for bridge application.
  • Stochastic modelling of fatigue strength and fatigue load for wind turbine application.
  • Reliability analyses.
  • Calibration of partial safety factors.
  • Methodology for probabilistic modelling of fatigue of concrete and reinforcement in concrete structures, incl. fatigue strength and fatigue loads.
  • Reliability analysis and calibration of partial safety factors.
  • Illustrative implementations at real structures: a bridge and a wind turbine.
  • 3 (accepted) peer-reviewed papers.
  • Two presentations at national/international conferences.
  • Presentations at workshops and for potential end-users.
  • Successfully defended PhD thesis.
  • EPFL (Lausanne, Switzerland)
    October 2017 to January 2018
    Modelling of concrete strength subjected to fatigue load
  • BAM (Berlin, Germany)
    August 2018
    Wind turbine tower design
  • COWI (Kongens Lyngby, Denmark)
    March & April 2019
    Structural analysis of wind turbine foundation
  • Madsen H.O., Krenk S., Lind N.C.
    Methods of Structural Safety. Wiley, 1986
  • Toft H.S.
    A Probabilistic Approach to Wind Turbine Fatigue Design
    PhD thesis, Aalborg University, 2010
  • Maljaars J., Steenbergen H.M.G.M. and Vrouwenvelder A.C.W.M.
    Probabilistic model for fatigue crack growth and fracture of welded joints in civil engineering structures
    International Journal of Fatigue, Vol.38, pp. 108-117, 2012
  • Sørensen J.D.
    Reliability-based calibration of fatigue safety factors for offshore wind turbines
    International Journal of Offshore and Polar Engineering, 22(3), 2012, pp. 234–241


  • Mankar A., Sørensen J.D., Velarde J., Kramhøft C.
    Optimization of pre-stressing in gravity based foundation of an offshore wind turbine using reliability framework
    WESC, Wind Energy Science Conference, Cork, Ireland, 17-20 June 2019
  • Zorzi G., Velarde J., Mankar A.
    Reliability analysis of offshore wind turbine foundations under lateral cyclic loading
    WESC, Wind Energy Science Conference, Cork, Ireland, 17-20 June 2019
  • Mankar A., Velarde J., Sørensen J.D.
    Reliability-based design optimisation and inspection planning of wind turbine concrete structures subjected to fatigue
    Submitted to Applied Sciences in April 2020
  • Rastayesh S., Mankar A., Sørensen J.D.
    Comparative investigation of uncertainty analysis with different methodologies on the fatigue data of rebars
    Submitted to International Journal of Reliability, Risk and Safety: Theory and Application, 2018
  • Velarde J., Mankar A., Kramhøft C., Sørensen J.D.
    Uncertainty Modeling and Fatigue Reliability Assessment of Offshore Wind Turbine Concrete Structures
    International Journal of Offshore and Polar Engineering (IJOPE), June 2019

Outreach activities

ESR9 Amol Mankar:



ESR9: Amol Mankar (AAU)



Local academic supervisor and PhD director: Prof. John Dalsgaard Sørensen (AAU)



Industrial co-supervisor: Dr Thierry Yalamas (PHIMECA)