Modelling and analysis of base isolated structures

MODELLING AND ANALYSIS OF BASE ISOLATED
STRUCTURES THROUGH IPERSPACE MAX
Copyright  Soft.Lab srl
D.M. 14/01/2008 (Italian Technical Construction Regulation)
Phd Ing. Stefano Ciaramella
Technical Consultant R&D
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For more informations you
can call:
ing. Francesco Ambrosio
Sales Director
salesdirector@soft.lab.it
Tel: +39 – 3318559813

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Seismic isolation
The approach to the earthquake-resistant construction problem:
CAPACITY  DEMAND
where:
 the demand depends on the seismic event, which generates inertial forces in the
structure. These forces are equal to the product of the masses of the structure
and the accelerations due to the vibration induced by the event itself.
 the capacity depends on the strength and on the non-linear deformability of the
structure.
Seismic Isolation: is an alternative design approach that acts on demand drastically
limiting the accelerations

a) increase of the fundamental period of
the building to bring it in the field of
lower responses to accelerations
b) limitation of the maximum horizontal
force transmitted
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a) Increase of the period (and dissipation) b) Limitation of the force (and dissipation)
Seismic isolation strategy
Model of a base isolated building

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Superstructure Substructure
Isolation Interface
Seismic isolation system

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Benefits of seismic isolation
 Economically acceptable and convenient structures
 Drastic reduction of the story drift which allow to create structures that do not
suffer damage for devastating earthquakes
 High protection of structural content
 The people in the building have a minor perception of the seismic event
 Great savings for repairs after high intensity earthquakes
 If the building has strategic importance the earthquakes does not cause the
interruption of the service.

Definition of the characteristics of the isolating system:
 Stiffness
 Dissipative capacity
Identification of the period-damping couple (Tis, esi).
Compared to the configuration of fixed-based structure (FB), this approach
determines a better balancing between a satisfactory reduction of the seismic effects
and horizontal displacement of the superstructure.
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System pre-dimensioning
Case Configuration T 
1 Structure (FB) 0.47 sec 5%
2 Structure (BI) 1.50 sec 10%
3/4
1 0.47secfbT C H  
fixed-based structure (FB)
base-isolated structure (BI)

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System pre-dimensioning
2 iso
is
esi
M
T
K

2
2
esi iso
is
K M
T
 
  
 
 ,iso e is esiF M S T 
 
 
2
,
,
2
iso e is esi is
dc e is esi
esi
M S T T
d S T
K



 
   
 
 Horizontal equivalent stiffness of the
isolating system:
 Equivalent period of the isolating
system:
 Resultant of horizontal forces applied
to the isolated system:
 Displacement of the stiffness centre
of the isolating system:

Palette Widget
Property Widget
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15

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Response Spectrums
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Period [sec]

Acceleration Displacement Response Spectrum
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Case Configuration
T
[sec]

[%]
ddc
[mm]
2 Structure (BI) 1.50 10% 156
3 Structure (BI) 2.00 10% 218
4 Structure (BI) 2.50 10% 280
5 Structure (BI) 1.50 15% 135
6 Structure (BI) 2.00 15% 189
7 Structure (BI) 2.50 15% 242
 
2
,
2
dc e
T
d S T 

 
  
 
Stiffness Centre Displacement

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Response Spectrums
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Elastic Spectrum
Structure
Project Spectrum
Structure (FB)
Period [sec]

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Case Configuration
T
[sec]

[%]
Shear Force
[KN]
1 Structure (FB) 0.47 5% 1550
2 Structure (BI) 1.50 10% 1260
3 Structure (BI) 2.00 10% 960
4 Structure (BI) 2.50 10% 740
5 Structure (BI) 1.50 15% 1100
6 Structure (BI) 2.00 15% 770
7 Structure (BI) 2.50 15% 630
 ,
600
eF M S T
M t
 

Shear force at the bottom of the superstructure

Case Configuration
T
[sec]
Kesi
[KN/m]
ki
[KN/m]
2-5 Structure (BI) 1.50 13861 770.0
3-6 Structure (BI) 2.00 7896 438.7
4-7 Structure (BI) 2.50 5053 280.7
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2
2
790
esi iso
is
iso
K M
T
M t
 
  
 

Horizontal stiffness
pillarsofnKk esii / 

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Case Configuration
T
[sec]

[%]
ddc
[mm]
Shear Force
[KN]
Kesi
[KN/m]
ki
[KN/m]
1 Structure (FB) 0.47 5% - 1550 - -
2 Structure (BI) 1.50 10% 156 1260 13861 770.0
3 Structure (BI) 2.00 10% 218 960 7896 438.7
4 Structure (BI) 2.50 10% 280 740 5053 280.7
5 Structure (BI) 1.50 15% 135 1100 13861 770.0
6 Structure (BI) 2.00 15% 189 770 7896 438.7
7 Structure (BI) 2.50 15% 242 630 5053 280.7
Seismic Effects: 50% reduction compared to the FB configuration
Summary of the results

Palette Widget
Property Widget
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d =189 mm + 30% =246 mm
Ko = 0.439 kN/mm
Preliminary Analysis

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1. Go to the section Isolatori
(“Isolator”) in the widget Elementi
e click on Nuovo (“New”).
2. Insert the code for
the new isolator.
3. In the property widget
(“Proprietà”) through the section
Generici, insert the vertical and
horizontal stiffness taken from the
catalogue.
Adding an isolating element to the program library

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Inserting isolators in the model of the structure
1. Selecting one or more pillars in
the substructure.
2. Click on Crea (“Create”)  Isolatore sui
selezionati (“selected isolators”)
3. Choose the isolator type, define its high and confirm (√)

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Structural analysis: fixed-based structure
1st mode
2nd mode
3rd mode

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T = 0.47 secPreliminary Analysis

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F = 155000 daNPreliminary Analysis

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 Use of isolation devices “FIP INDUSTIALE” series SI-S 400/125
 Reduction of the elastic spectrum for T  0,8 Tis = 1.6 sec
 Assumes  = esi = 15% for T  0,8 Tis and  = 5% for T < 0,8 Tis
Structural analysis: base-isolated structure

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For the ultimate limit state verification, the needed resistance of structural elements of the
superstructure can be met by considering the seismic effects reduced by the factor of
1/q=0.6667, where q=1.5 is the structure factor.

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T = 2.0 secPreliminary Analysis

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F = 77000 daNPreliminary Analysis

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The following figure shows the deformation of the structure due to a seismic event aligned with the x-axis.
The isolator maximum horizontal
displacement is d = 221 mm, not far from
our preliminary prediction (246 mm) and
however under the limit of the isolator (250
mm).

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Limit State Verification
 Ultimate Limit State Verification
 Damage Limit State Verification

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Ultimate Limit State Verification
The superstructure and substructure should be designed with reference to construction
details related to the non seismic zone (Geometric and Reinforcement Limitations)

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Ultimate Limit State Verification

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Damage Limite State Verification
For the superstructure, the verification must be
carried out controlling that the story drift,
obtained from the analysis, is under the 2/3 of
the Damage Limite State limits of conventional
structures.
This verification is carried out by setting k(*h) =
0.005x2/3 = 0.00333333 into the “Impalcati”
section of the property widget and finally
checking the results.

 However, it remains to be performed the verification for the parts involved in the
non-dissipative function. These should remain in the elastic range even under the
conditions of maximum stress, according to the rules relating to the materials they
are made. For this verification, also a safety factor (≥1.5) have to be taken into
account.
 For the replacement of isolators, the lifting by hydraulic jacks could be required.
Therefore it is necessary to evaluate the dimensions of the concrete squat above
the isolation interface and calculate an additional bottom reinforcement.
 In order to prevent or reduce traction in the seismic isolation devices, the vertical
load design "V“, due to seismic actions, should be compressive or zero (V ≥ 0).
In the case that V < 0, the modulus of the tensile stress should be minor both of
2G and 1 Mpa into the isolators (G is the shear modulus).
 For further examinations regarding these issues, the reader can refer to the
specific publications available.
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Further Verifications
Copyright  Soft.Lab srl http://www.soft.lab.it – salesdirector@soft.lab.it
For more informations you can call: ing. Francesco Ambrosio
Sales Director
salesdirector@soft.lab.it Tel: +39 – 3318559813

Modelling and analysis of base isolated structures

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