Fatigue reliability of concrete wind turbine and bridge elements

The objective is to develop a probabilistic framework for reliability assessment1 of reinforced concrete structures with respect to fatigue. The focus is on 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 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 approaches3. 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 the present task 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.

Host institution

AAU – Aalborg University, Aalborg, Denmark

ESR9

Amol Mankar

Local academic supervisor

Prof. John Dalsgaard Sørensen (AAU)
jds(at)civil.aau.dk
Tel: +45 9940 8581  

Industrial co-supervisor

Dr. Henrik S. Toft (COWI)
Email:
Tel: +45 56 40 21 35

Dr Thierry Yalamas (PHIMECA)
yalamas@phimeca.com
Tel: +33 (0)4 73 28 93 66

PhD enrolment

Aalborg University
PhD Director: Prof. John Dalsgaard Sørensen

Start date: 1st December 2016
Duration: 36 months

Synopsis

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.

Objectives

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 will be applied to wind turbine foundations and towers, 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.

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.

Tasks and methodology

  • 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.

Results

  • 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.
  • Successfully defended PhD thesis.

Dissemination

  • Three peer reviewed papers.
  • Two presentations at national/international conferences.
  • Presentations at workshops and for potential end-users.

Secondments

    • EPFL (Lausanne, Switzerland)
      August, September & October 2017
      Modelling of concrete strength subjected to fatigue load
    • COWI (Kongens Lyngby, Denmark)
      November & December 2018
      Structural analysis of wind turbine foundation
    • BAM (Berlin, Germany)
      February 2019
      Wind turbine tower design

    Research field

    Reliability, fatigue, concrete bridges and wind turbine

    Keywords

    Fatigue, reliability, concrete bridges, wind turbines

    Key publications

      • Joint Committee on Structural Safety: Probabilistic Model code, 2002, http://www.jcss.byg.dtu.dk/Publications
      • 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. Journal Int. Journal Fatigue, Vol.38, pp. 108-117, 2012.
      • Sorensen, J.D.: Reliability-based calibration of fatigue safety factors for offshore wind turbines. Int. J. of Offshore and Polar Engineering, 22(3), 2012, pp. 234–241.