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4D Journal of Technology and Science

@4D Crossconnect.com, Inc, 2012

www.4dinternationaljournal.com

Vol.1, Issue1, 2013

STUDY OF BUILDINGS DUCTILE FOR SEISMIC

PERFORMANCE

J.venkateswara Rao1

ABSTRACT

One of the major developments in seismic design over the past 10 years has been increased

emphasis on limit states design, now generally termed Performance Based Engineering. Three

techniques the capacity spectrum approach, the N2 method and direct displacement-based design have now matured to the stage where seismic assessment of existing structures or design

of new structures can be carried out to ensure that particular deformation-based criteria are met.

Following the worldwide recognized expectation and ideal aim to provide a modern society with

high seismic performance structures, able to sustain a design level earthquake with limited or

negligible damage, emerging solutions have been developed for high-performance, still cost-

effective, seismic resisting systems, based on adequate combination of traditional materials and

available technology. In this paper, an overview of recent developments and on-going research

on precast concrete buildings with jointed ductile connections, relying on the use of unbonded

post-tensioned tendons with self-centering capabilities, is given. A critical discussion on the

conceptual behavior, design criteria and modeling aspects is carried out along with an update on

current trends in major international seismic code provisions to incorporate these emerging

systems. The solution to further confirmation of the easy constructability and speed of erection

of the overall system has been taken into consideration.

INTRODUCTION

Seismic performance defines a structure's ability to sustain its main functions, such as

its safety and serviceability, at and after a particular earthquake exposure. A structure is,

normally, considered safe if it does not endanger the lives and well-being of those in or around it

by partially or completely collapsing. A structure may be considered serviceable if it is able to

fulfill its operational functions for which it was designed.Basic concepts of the earthquake

engineering, implemented in the major building codes, assume that a building should survive a

rare, very severe earthquake by sustaining significant damage but without globally collapsing

. On the other hand, it should remain operational for more frequent, but less severe seismic

events.Engineers need to know the quantified level of the actual or anticipated seismic

1 Can be reached at drjvrao2007@yahoo.com

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performance associated with the direct damage to an individual building subject to a specified

ground shaking. Such an assessment may be performed either experimentally or analytically.

Experimental evaluations are expensive tests that are typically done by placing a (scaled) model

of the structure on a shake-table that simulates the earth shaking and observing its

behavior.[6]

Such kinds of experiments were first performed more than a century ago.Still only

recently has it become possible to perform 1:1 scale testing on full structures.

Due to the costly nature of such tests, they tend to be used mainly for understanding the seismic

behavior of structures, validating models and verifying analysis methods. Thus, once properly

validated, computational models and numerical procedures tend to carry the major burden for the

seismic performance assessment of structures.

The structural overstrength factor, which can be determined from analytical studies, depends on

structural redundancy, story drift limitations, multiple load combinations, strain hardening,

participation of nonstructural elements, and other parameters. Although the ductility factor of an

individual structural member can be determined experimentally. There is no general agreement within the profession of how the concept of ductility factor should be applied at the structural

system level.( ChiaMing Uang, Associate Member, ASCE)

Several classes of buildings, which are representative of typical buildings based on year of

construction and brittle pre-Northridge connections, were designed in accordance with the 1973,

1985, and 1994 UBC provisions. Then, the frame analysis models were developed including the

effects of brittle connections, panel zone deformation, and interior gravity frames. Based on the

drift demands and capacities calculated using each set of 20 SAC ground motions representing

2/50 and 50/50 hazard levels, a performance prediction and evaluation procedure based on the

reliability framework is presented. Confidence levels that existing buildings will exceed the

predefined performance level for different hazard levels are calculated. The pre-Northridge design and construction represented by the old building codes and brittle connections force the

buildings to experience large seismic demand and result in a low confidence level in achieving

the desired performance levels(Khak Lee and Douglas A. Foutch)

In explores the current building code seismic design requirements for typical buildings in

regions of moderate seismic hazard. In particular, the costs and benefits of various levels of

ductile connection detailing requirements are reviewed for steel buildings constructed in the

northeastern United States. The design of lateral force resisting systems for seismic forces has

been required in the northeast states for over 20 years. However, recent building codes have

introduced special ductile detailing requirements that substantially increase the cost of building

connections and lateral force resisting system framing. In general, these increased costs for

improved ductility allow for the proportioning of members based on assumptions of higher levels

of inelasticity, and thus lower member forces, in the response of the structure. In regions of high

seismicity, the benefits of such ductile detailing are clear as they improve building performance

and allow for economical design of structures for more severe earthquake effects. In regions of moderate seismicity, the cost of these details can be very high in comparison to conventional

details. Consequently, many engineers in the northeastern United States have questioned the

value of design for inelastic response in non-essential facilities.( Daniel P. Batt, P.E., M.ASCE; and David J. Odeh, P.E., M.ASCE )

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Analysis

If two bars of same length and same cross-sectional area one made of ductile material and another of a brittle material. And a pull is applied on both bars until they break, then we notice

that the ductile bar elongates by a large amount before it breaks, while the brittle bar breaks

suddenly on reaching its maximum strength at a relative small elongation. Amongst the materials

used in building construction, steel is ductile, while masonry and concrete are brittle.

Comparison of Brittle and Ductile Building materials

The correct building components need to be made ductile. The failure of columns can affect the

stability of building, but failure of a beam causes localized effect. Therefore, it is better to make

beams to be ductile weak links then columns. This method of designing RC buildings is called

the strong-column weak-beam design method. Special design provisions from IS: 13920-

1993 for RC structures ensures that adequate ductility is provided in the members where damage

is expected.

Quality Control in Construction-The capacity design concept in earthquake resistant design of

buildings will fail if the strengths of the brittle links fall below their minimum assured values.

The strength of brittle construction materials, like masonry and concrete is highly sensitive to the

quality of construction materials. Workmanship, supervision, and construction methods.

Similarly, special care is needed in construction to ensure that the elements meant to be ductile

are indeed provided with features that give adequate ductility. Thus, strict adherence to

prescribed standards, of construction materials and processes is essential in assuring an

earthquake resistant building. Regular testing of materials to laboratories, periodic training of

workmen at professional training houses, and on-site evaluation of the technical work are

elements of good quality control.

it is easiest to see the principle at work by referring directly to the most widely used of these

advanced techniques, known as base isolation. A base isolated structure is supported by a series

of bearing pads, which are placed between the buildings and building foundation.

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Base Isolation Technique

The concept of base isolation is explained through an example building resting on frictionless

rollers. When the ground shakes, the rollers freely roll, but the building above does not move.

Thus, no force is transferred to the building due to the shaking of the ground; simply, the

building does not experience the earthquake.

Now, if the same building is rested on the flexible pads that offer resistance against lateral

movements (fig 1b), then some effect of the ground shaking will be transferred to the building

above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a

few times smaller than that experienced by the building built directly on ground, namely a fixed

base building (fig 1c). The flexible pads are called base-isolators, whereas the structures

protected by means of these devices are called base-isolated buildings. The main feature of the

base isolation technology is that it introduces flexibility in the structure.

As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely

flexible. The isolators are often designed, to absorb energy and thus add damping to the system.

This helps in further reducing the seismic response of the building. Many of the base isolators

look like large rubber pads, although there are other types that are based on sliding of one part of

the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to

medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on

soft soil are not suitable for base isolation.

Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber

bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of

the solid lead plug. On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the

vertical direction, but flexible in the horizontal direction.

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Working of Isolation Base-To get a basic idea of how base isolation works, first examine the

above diagram. This shows an earthquake acting on base isolated building and a conventional,

fixed-base, building. As a result of an earthquake, the ground beneath each building begins to

move. Each building responds with movement which tends towards the right. The buildings

displacement in the direction opposite the ground motion is actually due to inertia. The inertia

forces acting on a building are the most important of all those generated during an earthquake.

In addition to displacing towards right, the un-isolated building is also shown to be changing its

shape from a rectangle to a parallelogram. We say that the building is deforming. The primary

cause of earthquake damage to buildings is the deformation which the building undergoes as a

result of the inertial forces upon it.

Response of Base Isolated Buildings-The base-isolated building retains its original, rectangular

shape. The base isolated building itself escapes the deformation and damage-which implies that

the inertial forces acting on the base isolated building have been reduced. Experiments and

observations of base-isolated buildings in earthquakes to as little as of the acceleration of

comparable fixed-base buildings.

Acceleration is decreased because the base isolation system lengthens a buildings period of

vibration, the time it takes for a building to rock back and forth and then back again. And in

general, structures with longer periods of vibration tend to reduce acceleration, while those with

shorter periods tend to increase or amplify acceleration.

Spherical Sliding Base Isolation

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Spherical Sliding Base Isolation

Spherical sliding isolation systems are another type of base isolation. The building is supported

by bearing pads that have a curved surface and low friction. During an earthquake the building is

free to slide on the bearings. Since the bearings have a curved surface, the building slides both

horizontally and vertically. The forces needed to move the building upwards limits the horizontal

or lateral forces which would otherwise cause building deformations. Also by adjusting the

radius of the bearings curved surface, this property can be used to design bearings that also

lengthen the buildings period of vibration.

Popular Earthquake Resistant Techniques-Conventional seismic design attempts to make

buildings that do not collapse under strong earthquake shaking, but may sustain damage to non-

structural elements (like glass facades) and to some structural members in the building. This may

render the building non-functional after the earthquake, which may be problematic in some

structures, like hospitals, which need to remain functional in the aftermath of earthquake. Special

techniques are required to design buildings such that they remain practically undamaged even in

a severe earthquake. Buildings with such improved seismic performance usually cost more than

the normal buildings do.

Two basic technologies are used to protect buildings from damaging earthquake effects. These

are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is to detach

(isolate) the building from the ground in such a way that earthquake motions are not transmitted

up through the building or at least greatly reduced. Seismic dampers are special devices

introduced in the buildings to absorb the energy provided by the ground motion to the building

(much like the way shock absorbers in motor vehicles absorb due to undulations of the road.

New Breed of Energy Dissipation Devices-The innovative methods for control of seismic

vibrations such as frictional and other types of damping devices are important integral part of

seismic isolation systems as they severe as a barrier against the penetration of seismic energy

into the structure. In this concept, the dampers suppress the response of the isolated building

relative to its base.

The novel friction damper device consists of three steel plates rotating against each other in

opposite directions. The steel plates are separated by two shims of friction pad material

producing friction with steel plates.

When an external force excites a frame structure the girder starts to displace horizontally due to

this force. The damper will follow the motion and the central plate because of the tensile forces

in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite

way. The damper dissipates energy by means of friction between the sliding surfaces.

The latest Friction-ViscoElastic Damper Device (F-VEDD) combines the advantages of pure

frictional and viscoelastic mechanisms of energy dissipation. This new product consists

of friction pads and viscoelastic polymer pads separated by steel plates. A prestressed bolt in

combination with disk springs and hardened washers is used for maintaining the required

clamping force on the interfaces as in original FDD concept.

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In recent years significant progress has been made on the analytical side of active control for

civil engineering structures. Also a few models explains as shown that there is great promise in

the technology and that one may expect to see in the foreseeable future several

dynamic Dynamic Intelligent Buildings the term itself seems to have been joined by the

Kajima Corporation in Japan. In one of their pamphlet the concept of Active control had been

explained in every simple manner and it is worth quoting here.

People standing in swaying train or bus try to maintain balance by unintentionally bracing their

legs or by relaying on the mussels of their spine and stomach. By providing a similar function to

a building it can dampen immensely the vibrations when confronted with an earthquake. This is

the concept of Dynamic Intelligent Building (DIB).

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The philosophy of the past conventional a seismic structure is to respond passively to an

earthquake. In contrast in the DIB which we propose the building itself functions actively against

earthquakes and attempts to control the vibrations. The sensor distributed inside and outside of

the building transmits information to the computer installed in the building which can make

analyses and judgment, and as if the buildings possess intelligence pertaining to the earthquake

amends its own structural characteristics minutes by minute.

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Active Control System-The basic configuration of an active control system is schematically

shown in figure. The system consists of three basic elements:

1. Sensors to measure external excitation and/or structural response.

2. Computer hardware and software to compute control forces on the basis of observed

excitation and/or structural response.

3. Actuators to provide the necessary control forces.

Thus in active system has to necessarily have an external energy input to drive the actuators. On

the other hand passive systems do not required external energy and their efficiency depends on

tunings of system to expected excitation and structural behavior. As a result, the passive systems

are effective only for the modes of the vibrations for which these are tuned. Thus the advantage

of an active system lies in its much wider range of applicability since the control forces are

worked out on the basis of actual excitation and structural behavior. In the active system when

only external excitation is measured system is said to be in open-looped. However when the

structural response is used as input, the system is in closed loop control. In certain instances the

excitation and response both are used and it is termed as open-closed loop control.

Active-tuned Mass Dampers (TMD) -these are in passive mode have been used in a umber of

structures as mentioned earlier. Hence active TMD is a natural extension. In this system 1% of

the total building mass is directly excited by an actuator with no spring and dash pot. The system

has been termed as Active Mass Driver (AMD). The experiments indicated that the building

vibrations are reduced about 25% by the use of AMD.

Tendon Control-Various analytical studies have been done using tendons for active control. At

low excitations, even with the active control system off, the tendon will act in passive modes by

resisting deformations in the structures though resulting tension in the tendon. At higher

excitations one may switch over to Active mode where an actuator applies the required tension in

tendons.

Other Methods-The liquid sloshing during earthquakes has assumed significance importance in

view of over flow of petroleum products from storage tank in post earthquakes. One of the

important consideration with sloshing is that is associated with a very low damping. The wave

height was controlled through force applied to the side wall by a hydraulic actuator. The active

control successfully reduced wave heights to the level of 6% of those without control, for

harmonic excitations at sloshing frequency. For earthquake type excitation the wave heights

were reduced to 19% level.

Conventional approach to earthquake resistant design of buildings depends upon providing the

building with strength, stiffness and inelastic deformation capacity. But the new techniques like

Energy Dissipation and Active Control Devices are a lot more efficient and better.

CO

NCLUSION AND SUGGESTIONS

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Presently an intensive examination of the implications of performance limit states to seismic

design of structure. In the current study, most suggested design procedures require the addition

of a displacement, or damage, check to an essentially force-based design procedure. These

approaches have the advantage of retaining familiar design steps, and have been implemented in

some design procedures for many years. An alternative approach based on designing to achieve a

specified strain or drift performance level under a specified seismic intensity has been developed.

This is very simple to apply and should result in uniform levels of seismic risk. Significant

differences in seismic performance can be expected from structures designed to this approach

when compared with conventional force-based/displacement-check approaches. In particular,

design for inclusion of foundation compliance, for non-standard hysteretic characteristics, and

for variation in seismic intensity are treated in a rational manner not feasible with current

procedures. Although not specifically considered in this paper, due to space limitations, the

rational approach to torsional response of ductile structures developed by Paulay (Paulay 1997)

can readily be incorporated in direct displacement-based design.

It is to be noted that the key merits of the design method are simplicity and rationality. The

method has already achieved some acceptance, being advocated by the Structural Engineering

Association of California for the 1999 Blue Book and as an alternative seismic design procedure for the draft Australia/New Zealand seismic design code.

REFERENCES

(1) F Knoll - 1993 www.nrcresearchpress.com/

(2)Uang, C. (1991). Establishing R (or Rw) and Cd Factors for Building Seismic Provisions. J. Struct. Eng., 117(1), 1928. doi: 10.1061/(ASCE)0733-9445(1991)117:1(19)

(3)T Paulay, MJN Priestly - 2009

(4)S Pampanin - Journal of Advanced Concrete Technology, 2005 - J-STAGE

(5)CA Goulet, CB Haselton - Earthquake , 2007 - Wiley Online Library

(6)H Sezen, AS Whittaker, KJ Elwood, KM Mosalam - Engineering Structures, 2003 Elsevier

(7)Kī hak Lee, DA Foutch - Journal of Structural Engineering, 2002 - ascelibrary.org

((8JM Bracci, SK Kunnath, AM Reinhorn - Journal of Structural , 1997 - ascelibrary.org