The University Of Jordan Faculty of engineering and Technology Civil Engineering Department

Structural Dynamics

Dr.Anis Shatnawi 

Name: Fatima Mustafa Frehat

 Registration number: 8122157

Design of base isolated structure and high density

rubber bearing according to ASCE 7-10

BASE ISOLATION OFSTRUCTURES

Base isolation concept

DESIGN FOR 1G EARTHQUAKE LOADS

The earthquake causes inertia forcesproportional to the product of the building mass and the earthquake ground accelerations. As theground accelerations increases, thestrength of the building, the capacity,must be increased to avoid structuraldamage.

In high seismic zones the accelerationscausing forces in the building mayexceed one or even two times theacceleration due to gravity, g. Designing for this level of strength is

not easy, nor cheap. So most codes allow engineers to useductility to achieve the capacity.

Base isolation concept

A design philosophy focused oncapacity leads to a choice of two evils:1. Continue to increase the elasticstrength. This is expensive andfor buildings leads to higherfloor accelerations. Mitigationof structural damage by furtherstrengthening may cause more damage to the contents than would occur in a building with lessstrength.2. Limit the elastic strength and detail for ductility. This approach accepts damage to structural components, which may not be repairable.

Base isolation takes the opposite approach, it reduce the demand rather than increase thecapacity. We cannot control the earthquake itself but we can modify the demand it makes on the structure by preventing the motions being transmitted from the foundation into the structure above.So, the primary reason to use isolation is to mitigate earthquake effects. Naturally, there is a cost associated with isolation and so it only makes sense to use it when the benefits exceed this cost.And, of course, the cost benefit ratio must be more attractive than that available from alternative measures of providing earthquake resistance.

Base isolation concept:

An effective seismic isolation system shall provide the following main functions:

• Performance under all service loads, vertical and horizontal; shall be as effective as conventional structural bearings.

• Provide enough horizontal flexibility in order to reach the target natural period for the isolated structure.

• Re-centering capabilities after the occurrence of a severe earthquake so that no residual displacements can disrupt the serviceability of the structure.

• Provide an adequate level of energy dissipation in order to control the displacements that could otherwise damage other structural members. 

The base isolation (HDR) causes the natural period of the structure to increase and in the result causes the acceleration of the structure to decrease and the displacement to increase but increased displacement effect only the isolated story which assume flexible and elastic in the result there is a reduction in the lateral force in the super structure.

How does the base isolation actually works ?

STRUCTURE ACCELERATION AND DISPLACEMENT

1) shift the own period of the structure reducing in such a way the seismic response

2) dissipate energy reducing again the response and the displacement

The base isolation utilising HDRB achieves 2 effects:

the stiffness is much higher for small deformations and is reduced for large deformation. This property, is very useful because:

Allows the structure to respond rigidly to low excitations like wind force.

Provides high flexibility for large excitations like earthquakes.

rubber property for the application in the

base isolation of several kind of structures:

The preliminary design of an isolation system can be performed in a very easy way allowing a few simplifying assumptions:

· The base isolators act like perfect spring under the action of the earthquake

· The superstructure is considered as a rigid mass.

Design procedure of base isolated structure:

In that case the structure subject to the dynamic effect of the earthquake can be considered as a system with one degree of freedom and its own period is given by the expression:

The mass of the structure to be isolated must be known. The designer than shall choose the desired own period T of the structure (normally between 2 and 3 seconds).

ITERATIVE PROCEDURE FOR DESIGN

1. At each isolator locations, select a bearing plan size based on vertical load and assume a displacement at the target period and damping.

2. Calculate the effective stiffness, period and equivalent viscous damping at the assumed displacement.

3. From the seismic load parameters, calculate the actual displacement for this stiffness and damping.

4. Calculate revised damping for the actual displacement. Repeat step 3 if necessary.

5. Check and adjust the minimum plan size required to support vertical loads at this displacement if necessary.

The iterative process involves:

dynamic procedure

Equivalent load method

dynamic procedure

1- Equivalent load method

Used under specified condition :

The structure location has S1<0.6g The site class is A,B,C and D The structure height is equal to or less than four stories or

19.8m measured from the base above the isolation system The effective period at maximum displacement Te<3sec The effective period at the design displacement TD>3times

the fixed base structure period

Design procedure of base isolated structure:

The structure has a regular configuration

The effective stiffness of the isolation system at the design displacement > 1/3 of the effective stiffness at 20% of the design displacement of the structure

The isolation system can produce restoring force

The isolation system does not limit MCER to less than the total maximum displacement

Continue

Design step using equivalent force method

1- design displacement

2- design period

3-the element below the isolation system must withstand minimum lateral fore Vb:

4-the structural element a above the isolation system must withstand a minimum lateral force of but Vs should not taken less

than Cs W and not less than The base shear corresponding to the factored design wind load and The lateral seismic force required to fully activate the isolation system multiplied by 1.5

5-Vertical distribution of forces:

6-drift limit The maximum of the story above the isolated system Drift is 0.015 hx

2- dynamic procedure Response Spectrum Procedure Responses are obtained by equivalent linearization

Time History Procedure Earthquake response is obtained by time history analysis

(numerical integration method)using design earthquake ground motion

1-Determine the effective isolation period

2-Determine the effective damping ratio of the isolation system cD

3-Select the thickness of the rubber (tr) and the diameter(d) of each HDR

4-Calculate the horizontal effective stiffness of HDR kD=GA/tr

where G: shear modulus, A: cross section area of the HDR , tr: Thickness of the rubber

5-Calculate the total effective stiffness for the isolated structure Ƹ kDeff

Steps for the design of isolation system (HDR)

6-Calculate the design displacement DTD and check it with DTM

7-If checked end else return the step number 3

Continue

The base isolation techniques are very effective in the seismic resistance since it keep the isolated structure functional and elastic after seismic motion and the rubber in the HDR isolation return the structure to the origin location after some time and these properties is highly important specially in the most critical structures like hospitals which have to be functional after earthquake.

Conclusion:

Example of base isolated structure:

The first building in the USA is the Foothill Communities Law &Justice Center, California isolated using high

damping rubber bearings located below the basement level

The second building application in the USA was the City and CountyBuilding in Salt Lake City, Utah, completed in 1989 with 208 lead-

rubber and 239 natural rubberbearings.

Examples of base isolated structure:

Oakland City Hall, California (retrofit)

This 18-story building with full basement, central rotunda, council chambers, and administration offices was the first high rise government office building in the United States when it was completed in 1914. The original structure of the building is a riveted steel frame with infill masonry walls of brick, granite, and terra cotta, and is supported on a continuous concrete mat foundation. This 1994 retrofit utilized 42 lead rubber bearings and 69 natural rubber bearings