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Finite element seismic analysis of a guyed mast

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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

University of Oxford

- Introduction
- Key features of guyed masts
- Objectives

- Modelling
- Cable properties
- Loading

- Results
- Modal analysis
- Seismic response
- Comparison with static wind analysis

- Conclusions

- 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

- 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

- 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

Mast lattice modelled by equivalent beam elements

Cable catenary modelled by ~80 beam elements

Prestress applied by iterative procedure of applying temperature loads

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

- 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

- Modes occur in orthogonal pairs
- Numerous mast modes in period range of interest
- Also numerous cable modes

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

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

Mast base shear:

Total base shear (mast plus cables):

Mast base axial force:

- 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

- 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