8/12/2019 Finite Element Seismic Analysis of a Guyed Mast
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Finite element
seismic analysisof a guyed mast
Matthew Grey
Martin Williams
Tony Blakeborough
First European Conference on Earthquake Engineering and Seismology
Geneva, September 2006
Paper 1189
Structural Dynamics Research Group
Department of Engineering Science
University of Oxford
8/12/2019 Finite Element Seismic Analysis of a Guyed Mast
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Synopsis
Introduction
Key features of guyed masts
Objectives
Modelling
Cable properties Loading
Results
Modal analysis
Seismic response Comparison with static wind analysis
Conclusions
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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
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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
0 500
Mass distribution (kg/m)
Mast
Mast + ancillariesCable catenary modelled by
~80 beam elements
Prestress applied by iterative
procedure of applying
temperature loads
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Cable properties
0
10
20
30
40
0 0.1 0.2 0.3
Displacement (m)
R e a c t i o n ( k N )
SAP2000
Goldberg
Davenport
Sparling
Zero SagEC8
Axial force-displacementcharacteristic of catenary cable
and comparison with theory
Lateral force-displacementcharacteristic 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 speedsof 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
1 2 3 4 5 6Mode:
0.60Period (s): 0.55 0.49 0.46 0.40 0.39
Modes occur in orthogonal pairs
Numerous mast modes in period range of interest
Also numerous cable modes
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Bending moment envelopes
0
100
200
300
400
500
0 30 60 90
Height (m)
B e n d i n g m o m e n
t ( k N m )
3 m/s2
2.5 m/s2
3.5 m/s2
4 m/s2
Wind 23 m/s
Wind 20 m/s
0
100
200
300
400
500
0 30 60 90
Height (m)
B
e n d i n g m o m e n t ( k N m )
3 m/s2
2.5 m/s2
3.5 m/s2
4 m/s2
Wind 23 m/s
Wind 20 m/s
El Centro:
EC8:
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0
50
100
0 30 60 90Height (m)
S h e a r F o r c e
( k N )
0
50
100
0 30 60 90Height (m)
S h e a r F o r c e ( k N )
Shear force envelopes
3 m/s2
2.5 m/s2
3.5 m/s2
4 m/s2
Wind 23 m/s
Wind 20 m/s
3 m/s2
2.5 m/s2
3.5 m/s2
4 m/s2
Wind 23 m/s
Wind 20 m/s
El Centro:
EC8:
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Base forces
0
20
40
60
2 3 4
PGA (m/s2
)
M a s t B a s e S h e
a r ( k N )
100
200
300
2 3 4
PGA (m/s2
)
T o t a l B a s e S h e a r ( k N )
1000
1200
1400
2 3 4
PGA (m/s2)
B a s e A x i a l F o r c e ( k N )
Mast base
shear:
Total base shear
(mast plus cables):
Mast base
axial force:
El CentroEC8ParkfieldWind
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Cable tensions
Cable Wind EC8-2.5 m/s EC8-4 m/s
23 m/s max min max min
A1 211.1 125.0 29.8 166.6 10.9
B1 351.7 168.3 37.5 252.7 21.5
C1 344.9 218.0 60.9 283.3 49.0
El Centro-2.5 m/s2
El Centro-4 m/s2
max min max min
A1 211.1 138.3 13.7 167.7 -2.8
B1 351.7 168.0 29.3 198.0 -1.3
C1 344.9 190.2 82.1 222.2 51.3
Parkfield-2.5 m/s2 Parkfield-4 m/s
2
max min max minA1 211.1 135.1 24.7 166.6 -5.3
B1 351.7 170.3 63.6 209.0 46.5
C1 344.9 176.3 100.6 199.7 79.2
Displacement
F o r c e
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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