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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The Software…

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The Software…


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The Software…



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The Software…



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The Software…



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The Software…



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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



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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



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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.



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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%






3/41 0.47secfbT C H

fixed-based structure (FB)

base-isolated structure (BI)



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System pre-dimensioning

2 isois

esi

MT

K

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esi isois

K MT

,iso e is esiF M S T 2,

,2

iso e is esi isdc e is esi

esi

M S T Td 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:



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Palette Widget

Property Widget

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Response Spectrums

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Period [sec]



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Acceleration Displacement Response Spectrum

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[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

,2dc e

Td 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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[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

,

600eF M S T

M t

Shear force at the bottom of the superstructure



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[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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790

esi isois

iso

K MT

M t

Horizontal stiffness

pillarsofnKk esii /



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[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



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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

ふさIsolatorざぶ in the widget Elementi

e click on Nuovo (さNewざ).

2. Insert the code for

the new isolator.

3. In the property widget

ふさProprietàざぶ 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 ふさCreateざぶ Isolatore sui

selezionati ふさselected isolatorsざぶ

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 sec Preliminary Analysis

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F = 155000 daN Preliminary Analysis

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Use of isolation devices さFIP INDU“TIALEざ 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 sec Preliminary Analysis

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F = 77000 daN Preliminary 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



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Ultimate Limit State Verification

Damage Limit State Verification

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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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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 さImpalcatiざ

section of the property widget and finally

checking the results.



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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 ふ≥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さ, 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

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modelling and analysis of base isolated structures

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