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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

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
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
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
objectives
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
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
structural model of a mast
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

cable properties
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

loading
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
linear mode shapes
Linear mode shapes
  • Modes occur in orthogonal pairs
  • Numerous mast modes in period range of interest
  • Also numerous cable modes
bending moment envelopes
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

shear force envelopes
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

base forces
Base forces

Mast base shear:

Total base shear (mast plus cables):

Mast base axial force:

conclusions
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
other ongoing work
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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