1. 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

2. Synopsis Introduction Modelling Results Conclusions
Key features of guyed masts
Objectives
Modelling
Cable properties
Loading
Results
Modal analysis
Seismic response
Comparison with static wind analysis
Conclusions

3. 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

4. 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

5. Modelling
Four guyed masts with heights up to 314 m analysed using SAP2000
This paper focuses on the shortest mast – 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

6. 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

7. 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

8. 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

9. Linear mode shapes Modes occur in orthogonal pairs
Numerous mast modes in period range of interest
Also numerous cable modes

10. 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

11. Shear force envelopes El Centro: EC8: Wind 23 m/s 4 m/s2 3.5 m/s2

12. Base forces Mast base shear: Total base shear (mast plus cables):
Mast base axial force:

13. Cable tensions

14. 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

15. 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