GREEN LEAF ENGINEERS PTY LTD Brisbane Office Level 3, The Icon Centre 15 Malt Street Fortitude Valley QLD 4006 Australia T. +61 7 3358 3046 [email protected]Sydney Office Suite 41, Jones Bay Wharf 26-32 Pirrama Road Pyrmont Point NSW 2009 Australia T. +61 2 8096 4482 [email protected]Pacific Office Gemini Place, Suite 15 PO Box 3997 Boroko NCD Papua New Guinea T. +675 323 9709 [email protected]SEISMIC PROTECTION OF STRUCTURES WITH MODERN BASE ISOLATION TECHNOLOGIES A Green Leaf Engineers White Paper by Luis Andrade and John Tuxworth
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Green leaf engineers white paper seismic protection of structures with modern base isolation technol
Esteemed seismic engineer Luis Andrade and senior structural engineer John Tuxworth compare lead-rubber bearing to pendulum bearing base isolations systems within the context of five historical seismic events. The findings are essential reading for those designing buildings like museums and data centres and structures that house other such precious goods.
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A seismologist is of invaluable assistance when selecting applicable time-histories,
however guidance for selecting scaling records can be gleaned from codes, Kelly (5). The
events chosen for consideration in this paper represent several of the major earthquakes
in recorded history, with the 1995 Aigion Earthquake in Greece being of similar
magnitude to the Newcastle earthquake of 1989 (Richter Scale 5.6)
Figure 7 shows maximum response values for each of the earthquake records for roof
acceleration, base shear, inter-storey drift, and isolator displacements.
Maximum roof acceleration is dominated by the 1989 Loma Prieta earthquake record
which yields a value of about 36 m/sec2 for the fixed base structure, while for the
isolated structures is in the order of 8.5 m/sec2 (76% reduction) (see Figure 7(a)).
Maximum isolator base shears are dominated also by the 1989 Loma Prieta earthquake.
A shear of approximately 8900 kN for the fixed base building is reduced to 2800 kN
(68% reduction) and 3300 kN (63% reduction) for LPB and FPS isolators respectively
(see Figure 7(b)).
Maximum Inter-storey drifts for fixed base and isolator cases are again generated by the
1989 Loma Prieta Earthquake, with values of about 129mm for the fixed base structure
and 25mm (81% reduction) and 35mm (73% reduction) for LPB and FPS respectively
(see Figure (c)). The drift ratio derived for Level-1 of the fixed base structure is 4.3%,
about twice the maximum limit of 2% imposed by the UBC 1997. The FPS isolated
structure displays a value of 1.15% which is well under the limit.
Figure 7(d) shows maximum isolator displacements in the order of 473mm and 469mm.
It can be seen in Figure 7(e) that these values are round 215% of the isolator design
displacement of 220 mm, indicating that both isolator systems would fail during the
1989 Loma Prieta Earthquake.
Force-Displacement hysteresis loops for the FPS and LPB isolator (Type A), as subjected
to the 1989 Loma Prieta earthquake record, are provided in Figures 8(a) and 8(b). These
curves follow the mathematical models presented in section 2 of this paper. Elastic and
post-elastic stiffness can be obtained as the slopes of the first two initial segments.
The energy dissipated by each isolator is provided by the area inside each loop cycle.
Effective damping can be calculated using Equations 7 or 11 and compared with the
SEISMIC PROTECTION OF STRUCTURES
WITH MODERN BASE ISOLATION TECHNOLOGIES Page 14
0
5
10
15
20
25
30
35
40
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Acc
eler
ati
on
(m
/sec
/sec
)
Earthquake Record
Roof Acceleration
LBS
FPS
Fixed Base
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Base
Sh
ear
(kN
)
Earthquake Record
Base Shear
LRB
FPS
Fixed Base
0
50
100
150
200
250
300
350
400
450
500
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Iso
lato
r D
isp
lace
men
t (m
m)
Earthquake Record
Isolator Displacement
LRB
FPS
0
20
40
60
80
100
120
140
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Dri
ft (m
m)
Earthquake Record
1st Floor Inter - Story Drift
LRB
FPS
Fixed Base
0
20
40
60
80
100
120
140
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Dri
ft (m
m)
Earthquake Record
1st Floor Inter - Story Drift
LRB
FPS
Fixed Base
assumed design value. Note that there is seemingly an anomaly present in Figure 8 (a),
as maximum ‘-ve’ deflection for the FPS isolator corresponds to a reduction in base
shear. This anomaly was evident only for the Loma Prieta earthquake, and further study
is required to ascertain why this issue occurred.
Finally, time-history results for the Loma Prieta earthquake record are shown in Figure 9.
It can be noticed from Figures 9(a) and 9(b) how the response in time of the isolated
system is considerably less than the fixed base structure, specially between the 10 and
the 15 first seconds of the beginning of the seismic excitation. Figure 9(c) compares the
two types of isolators’ lateral displacements, which appears to be less for the FPS.
Figure (7a) Figure (7b)
Figure (7c) Figure (7d)
SEISMIC PROTECTION OF STRUCTURES
WITH MODERN BASE ISOLATION TECHNOLOGIES Page 15
0%
50%
100%
150%
200%
250%
1940 El Centro
1979 El Centro
1989 Loma Prieta
1994 Northridge
1995 Aigion
Earthquake Record
Time History Displacement / Design Value
LRBFPS
Figure (7e)
Figure 7. Comparison of Results for the 5 earthquake records (a) roof accelerations, (b)
base shear (c) 1st floor inter-story drift, (d) isolator displacement, (e) time history
displacement / design value utilization ratio.
Figure (8a) Figure (8b)
Figure 8. 1989 Loma Prieta Earthquake Record. Force-displacement hysteresis loops for
(a) FPS isolator (b) LPB isolator Type A.
SEISMIC PROTECTION OF STRUCTURES
WITH MODERN BASE ISOLATION TECHNOLOGIES Page 16
Figure 9. Time-history results for 1989 Loma Prieta earthquake record. (a) Roof
acceleration, (b) base shear, (c) isolator displacement.
-40.0
-20.0
0.0
20.0
40.0
0 5 10 15 20 25 30Acc
eler
ati
on
(m
/sec
/sec
)
Time (sec)
Lead Plug Bearing Friction Pendulum System Fixed Base
-9000
-4500
0
4500
9000
0 5 10 15 20 25 30
Base
Sh
ear
(kN
)
Time (sec)
Lead Plug Bearing Friction Pendulum System Fixed Base
-500
-250
0
250
500
0 5 10 15 20 25 30
Iso
lato
r D
isp
lace
men
t
(mm
)
Time (sec)
Lead Plug Bearing Friction Pendulum System
SEISMIC PROTECTION OF STRUCTURES
WITH MODERN BASE ISOLATION TECHNOLOGIES Page 17
CONCLUSIONS & RECOMMENDATIONS
It can be seen that resultant accelerations, base shears and inter-storey drifts were all
effectively reduced by the adoption of Lead-Plug and Friction-Pendulum isolator systems,
resulting in significant improvement in modeled building performance, and a very likely
minimisation of post-event losses. For the ground conditions and sway-frame structural
system adopted, LPB & FPS base isolation would be excellent options to reduce
structural and non-structural damage, and to protect building contents. Both the LPB and
FP systems provided a comparative reduction in roof level accelerations (up to 76%);
however the LPB provided the best reduction in base shear, and inter-storey drift (at first
floor). For the adopted bearing characteristics, the FPS provided greatest control of
isolator displacement — a significant serviceability constraint with respect to boundary
conditions.
Response of the isolated structural framing systems was dominated by the time-history
record of the 1989 Loma Prieta Earthquake. The second highest intensity experienced by
the test structure was due to 1994 Northbridge earthquake. The isolator design
displacement (being a function of the nominated isolator characteristics) of both systems
was exceeded by these earthquakes, indicating alternate properties/sizes would be
required to accommodate higher intensity events.
Further work is recommended to establish applicability of these base-isolation systems
for the common braced-frame structural framing paradigm, and also to confirm suitability
(or lack thereof) for high-rise construction, and or use on deep alluvial soil strata as
evident in Australian centres such as Newcastle.
REFERENCES
1. Wang, Yen-Po, “Fundamentals of Seismic Base Isolation”, International Training programs for Seismic Design of Building Structures.
2. Naeim, F. & Kelly, J. M., “Design of Seismic Isolated Structures: From Theory to Practice”, John Wiley & Sons, Inc. 1999.
3. International Conference of Building Officials, ICBO (1997), “Earthquake Regulations for Seismic-Isolated Structures”, Uniform Building Code, Appendix Chapter 16, Whittier, CA.
4. Mayes, R. & Naeim, F., “Design of Structures with Seismic Isolation”, Earthquake Engineering Handbook, University of Hawaii, CRC Press, 2003.
5. Kelly, T. E., “Base Isolation of Structures Design Guidelines”, Holmes Consulting Group Ltd, July 2001.