Modelling and analysis of base isolated structures

MODELLING AND ANALYSIS OF BASE ISOLATED
STRUCTURES THROUGH IPERSPACE MAX
D.M. 14/01/2008 (Italian Technical Construction Regulation)

Phd Ing. Stefano Ciaramella
Technical Consultant R&D

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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


Seismic isolation strategy

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

Model of a base isolated building

a) Increase of the period (and dissipation) b) Limitation of the force (and dissipation)

Seismic isolation system

Isolation Interface

Superstructure Substructure

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.


System pre-dimensioning
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.

Case Configuration T 
1 Structure (FB) 0.47 sec 5% T fb  C1  H 3/4  0.47sec
2 Structure (BI) 1.50 sec 10%
3 Structure (BI) 2.00 sec 10% fixed-based structure (FB)
4 Structure (BI) 2.50 sec 10% base-isolated structure (BI)

System pre-dimensioning

 Equivalent period of the isolating M iso
Tis  2
system: K esi
2
 Horizontal equivalent stiffness of the  2 
isolating system: K esi     M iso
 Tis 
 Resultant of horizontal forces applied
to the isolated system: F  M iso Se Tis , esi 

M S T ,    T 
2
 Displacement of the stiffness centre
d dc  iso e is esi   is  Se Tis , esi 
of the isolating system: Kesi  2 

Palette Widget

Property Widget


Response Spectrums

Period [sec]


Acceleration Displacement Response Spectrum


Stiffness Centre Displacement
2
 T 
d dc    Se  T ,  
 2 

T  ddc
Case Configuration
[sec] [%] [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


Response Spectrums

Elastic Spectrum
Structure

Project Spectrum
Structure (FB)

Period [sec]


Shear force at the bottom of the superstructure

F  M  Se  T ,  
M  600 t

T  Shear Force
Case Configuration
[sec] [%] [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


Horizontal stiffness

2
 2 
K esi     M iso
 Tis  ki  Kesi / n of pillars
M iso  790 t

T Kesi ki
Case Configuration
[sec] [KN/m] [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


Summary of the results

T  ddc Shear Force Kesi ki
Case Configuration
[sec] [%] [mm] [KN] [KN/m] [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


Palette Widget

Property Widget

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


Adding an isolating element to the program library

1. Go to the section Isolatori
(“Isolator”) in the widget Elementi 3. In the property widget
e click on Nuovo (“New”). (“Proprietà”) through the section
Generici, insert the vertical and
horizontal stiffness taken from the
catalogue.

2. Insert the code for
the new isolator.


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 (√)


Structural analysis: fixed-based structure

2nd mode

1st mode

3rd mode


Preliminary Analysis T = 0.47 sec


Preliminary Analysis F = 155000 daN


Structural analysis: base-isolated structure

 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

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.



Preliminary Analysis T = 2.0 sec



Preliminary Analysis F = 77000 daN


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).


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


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)


Ultimate Limit State Verification


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.


Further Verifications
 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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Modelling and analysis of base isolated structures

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