Transcript 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

Introduction



Modelling



Cable properties
Loading
Results




Key features of guyed masts
Objectives
Modal analysis
Seismic response
Comparison with static wind analysis
Conclusions
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

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

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
Mass distribution (kg/m)
Mast lattice modelled by
equivalent beam elements
Mast
Cable catenary modelled by
~80 beam elements
Mast + ancillaries
Prestress applied by iterative
procedure of applying
temperature loads
0
500
Cable properties
Axial force-displacement
characteristic of catenary cable
and comparison with theory
SAP2000
Reaction (kN)
40
Goldberg
30
Lateral force-displacement
characteristic of a stay cluster
Davenport
Sparling
Zero Sag
EC8
20
10
0
0
0.1
0.2
0.3
Displacement (m)
Cables in this case are prestressed
to approx. 90% of max stiffness
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

Modes occur in orthogonal pairs

Numerous mast modes in period range of interest

Also numerous cable modes
Mode:
Period (s):
1
2
3
4
5
6
0.60
0.55
0.49
0.46
0.40
0.39
Bending moment envelopes
Bending moment (kNm)
El Centro:
Wind 23 m/s
500
4 m/s2
400
3.5 m/s2
3 m/s2
2.5 m/s2
300
200
100
0
0
30
60
Height (m)
Bending moment (kNm)
EC8:
Wind 20 m/s90
Wind 23 m/s
500
4 m/s2
400
3.5 m/s2
3 m/s2
2.5 m/s2
300
200
100
0
0
30
60
Height (m)
Wind 20 m/s90
Shear force envelopes
Shear Force (kN)
El Centro:
Wind 23 m/s
100
4 m/s2
3.5 m/s2
3 m/s2
2.5 m/s2
50
0
0
30
Height (m)
60
Shear Force (kN)
EC8:
90
Wind 20 m/s
Wind 23 m/s
4 m/s2
100
3.5 m/s2
3 m/s2
2.5 m/s2
50
0
0
30
Height (m)
60
90
Wind 20 m/s
Base forces
60
40
20
0
2
3
4
PGA (m/s 2)
El Centro
EC8
Parkfield
Wind
Mast base
axial force:
Base Axial Force (kN)
Total base shear
(mast plus cables):
Total Base Shear (kN)
Mast Base Shear (kN)
Mast base
shear:
300
200
100
2
3
PGA (m/s 2)
4
1400
1200
1000
2
3
PGA (m/s 2)
4
Cable
A1
B1
C1
A1
B1
C1
A1
B1
C1
Wind
23 m/s
211.1
351.7
344.9
EC8-2.5 m/s2
max
min
125.0
29.8
168.3
37.5
218.0
60.9
EC8-4 m/s2
max
min
166.6
10.9
252.7
21.5
283.3
49.0
211.1
351.7
344.9
El Centro-2.5 m/s2
max
min
138.3
13.7
168.0
29.3
190.2
82.1
El Centro-4 m/s2
max
min
167.7
-2.8
198.0
-1.3
222.2
51.3
211.1
351.7
344.9
Parkfield-2.5 m/s2
max
min
135.1
24.7
170.3
63.6
176.3
100.6
Parkfield-4 m/s2
max
min
166.6
-5.3
209.0
46.5
199.7
79.2
Force
Cable tensions
Displacement
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

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