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First European Conference on Earthquake Engineering and Seismology Geneva, September 2006 Paper 1189. Finite element seismic analysis of a guyed mast. Matthew Grey Martin Williams Tony Blakeborough. Structural Dynamics Research Group Department of Engineering Science

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Finite element seismic analysis of a guyed mast l.jpg

First European Conference on Earthquake Engineering and Seismology

Geneva, September 2006

Paper 1189

Finite element seismic analysis of a guyed mast

Matthew Grey

Martin Williams

Tony Blakeborough

Structural Dynamics Research Group

Department of Engineering Science

University of Oxford


Synopsis l.jpg

Synopsis

  • Introduction

    • Key features of guyed masts

    • Objectives

  • Modelling

    • Cable properties

    • Loading

  • Results

    • Modal analysis

    • Seismic response

    • Comparison with static wind analysis

  • Conclusions


Key features of guyed masts l.jpg

Key features of guyed masts

  • Support broadcasting equipment at 100 – 600 m above ground

  • Slender lattice structure supported by inclined, prestressed cables

  • Cable supports may be 400 m from base of mast

  • Mass of ancillaries is significant

  • Seismic loading normally assumed less onerous than wind


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Objectives

  • Assess magnitude and distribution of forces developed under seismic loading

  • Compare forces due to seismic and design wind events

  • Identify trends and indicators for use in preliminary design

  • Evaluate effects of asynchronous ground motions

  • Assess significance of vertical seismic motions

  • Assess suitability of linear response spectrum analysis


Modelling l.jpg

Modelling

  • Four guyed masts with heights up to 314 m analysed using SAP2000

  • This paper focuses on the shortest mast – 99.88 m

  • Mast data supplied by Flint and Neill Partnership, UK, masts designed according to BS8100

  • Analysed under:

    • indicative wind load using the equivalent static patch load method

    • non-linear time-history analysis under earthquakes of varying magnitudes


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Structural model of a mast

Mast lattice modelled by equivalent beam elements

Cable catenary modelled by ~80 beam elements

Prestress applied by iterative procedure of applying temperature loads


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Cable properties

Axial force-displacement characteristic of catenary cable and comparison with theory

Lateral force-displacement characteristic of a stay cluster

Cables in this case are prestressed to approx. 90% of max stiffness


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Loading

  • Wind loading – BS8100 patch load method – wind speeds of 20, 23 and 28 m/s

  • Earthquake records scaled to PGA of 2.5 – 4.0 m/s2

    • El Centro 1940

    • Parkfield 1966

    • Artificial accelerogram compatible with EC8 type 1 spectrum, ground type C

  • 3D motion used

  • Non-linear time history analysis using Newmark’s method


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Linear mode shapes

  • Modes occur in orthogonal pairs

  • Numerous mast modes in period range of interest

  • Also numerous cable modes


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Bending moment envelopes

El Centro:

Wind 23 m/s

4 m/s2

3.5 m/s2

3 m/s2

2.5 m/s2

Wind 20 m/s

EC8:

Wind 23 m/s

4 m/s2

3.5 m/s2

3 m/s2

2.5 m/s2

Wind 20 m/s


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Shear force envelopes

El Centro:

Wind 23 m/s

4 m/s2

3.5 m/s2

3 m/s2

2.5 m/s2

Wind 20 m/s

EC8:

Wind 23 m/s

4 m/s2

3.5 m/s2

3 m/s2

2.5 m/s2

Wind 20 m/s


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Base forces

Mast base shear:

Total base shear (mast plus cables):

Mast base axial force:


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Cable tensions


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Conclusions

  • Mass of mast ancillaries has a significant effect on dynamic response

  • In spite of the non-linearities present, mast behaviour under seismic loads shows broadly linear trends with PGA

  • With PGA of 4 m/s2 mast bending response approaches and at some points exceeds that under design wind load of 23 m/s

  • Mast shear and cable tension remain below values due to design wind moment

  • Earthquake loading may be more onerous than wind in areas of high seismicity and/or low design wind speed


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Other/ongoing work

  • Development of simple formulae giving preliminary estimates of natural period and key response parameters

  • Assessment of applicability of linear response spectrum analysis approach

  • Effect of asynchronous ground motions between mast and cable support points

  • Importance of vertical ground motion for overall seismic response


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