001 conference paper - supplementary damping - a new concept in earthquake resistant buildings

8/12/2019 001 Conference Paper - Supplementary Damping - A New Concept in Earthquake Resistant Buildings

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Supplementary Damping - A New Concept in Earthquake ResistantBuildings

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1 !ntroduction

A major earthquake is the most extreme condition that a building may be required to survive during its lifetime.

Incase buildings are unable to survive this natures might then the price to be paid can be colossal in terms of

loss to lives and property. To survive this natures fury safely and surely also poses the greatest challenge to the

architects and structural engineers. Hoever the modern day computational poer and the technological

advances in the field of seismic protection have made the solution once considered un!surmountable a reality.

"onsiderable testing both in the field and the laboratory coupled ith quality research ork has helped increase

our understanding of ho buildings behave and respond during earthquakes and other intense motions. This

has led to neer approaches and methodologies toards designing safer structures.

" Background

The heightened seismic activity in and around the Indian sub!continent and the recent quakes in #ashmir and the

lo intensity temblors in $elhi% &ujarat and eastern India are a constant reminder that e are living in an active

seismic belt. Aareness levels are groing and people today are becoming increasingly savvy about the seismic

components of the buildings they live and ork in. Also over the years the expectations of consumers has

increased manifold% today many expect and demand that their building's be designed to the highest possible

standards. A decade ago most ere satisfied ith and anted to prevent a total building collapse% today many

are demanding buildings that are safe to stay and ork in immediately after an quake. Important public buildings

like hospitals and emergency command centers are being designed for full functionality even during a major

earthquake.

# E$$ect o$ Earthquake on Built En%ironment(hen an earthquake strikes it supplies a great amount of sudden energy to buildings and structures. The only

ay this energy can be absorbed by the building is by causing some damage. The damage could be classified

into to kind% structural members and non!structural. The non!structural components are the indo panes'

brick infill alls' tiles' false ceiling etc. and this type of damage does not threaten the structural integrity of the

building% hoever% structural damage to columns% beams% shear alls and floor slabs is also caused by the

cracking of concrete and elongation'yielding of steel. )ffectively all energy absorbed is associated ith some

form of damage. (hen the damage in the structural members crosses a threshold level hich can also be said

to be the capacity of that building the building ould collapse. *resently the designers are aiming to absorb all of

the seismic energy through controlled yielding of steel and cracking of concrete so that the threshold danger

level is not exceeded. The aim is to use the full capacity of the structure so as to prevent a total collapse. )ven if

the building does not collapse% the yielding of steel and cracks in concrete may cause the structure to be so

badly damaged that the building ould be unusable and subsequently condemned.

& 'ighrise Buildings*opulation explosion has made highrises the order of the day as it is the only logical solution and ay ofaccommodating the groing population ithin the boundaries of the cities. It is needless to emphasise that tallbuildings are prone to larger movements and damage than lo rise structures during earthquakes and as thenumber of people occupying a highrise at any given time is far greater so also the risk of collateral damage. Apartfrom ensuring structural safety during earthquakes highrises are giving the engineers another cause of concerni.e. mitigation of ind induced vibrations that cause occupant discomfort. )xcessive floor accelerations hich arecaused by relatively frequent strong ind motions can render a building unserviceable for reasons of occupantdiscomfort. Humans can perceive accelerations greater than one hundredth of that of acceleration due to gravity.
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This effect is more pronounced in tall slender buildings and for the building to qualify for serviceability thedynamic response of the structure to ind induced vibrations needs to be reduced. To overcome these effectsthe common approach is that of increasing the stiffness of the lateral load resisting members. This has to majordisadvantages hich can lead to further complications and occasionally degradation in building design+

,a- Increased structural costs due to additional steel and concrete for making the building stiffer and,b- Increased stiffness ould imply that the building no ould attract higher seismic forces than before.

Thus the vulnerability of the building for seismic loading becomes higher hich is a cause of concern.

( Building )roperties A$$ecting the *ateral *oad Resisting Structural System)arthquakes induce very intense lateral motion and thereby subject the columns and shear alls of buildings toexcessive forces. (hen there is a situation that lateral load resisting members cannot handle these forces theydamage and this can lead to a total building collapse. To prevent this kind of damage it is extremely important toreduce the dynamic lateral response of the building. This dynamic lateral response has three main governingfactors to include mass% stiffness and damping and based on these three contributing factors there exist threeseparate methodologies to control the dynamic response. )ach of the approaches mentioned belo uses one ofthe three structural properties of the building hoever in practice the designer may choose a hybrid solution andtarget to or sometimes even all three approaches simultaneously.

+ass centric approach. &reater the building mass% higher ill be the seismic demand. Thus one of the ays to

improve seismic performance is to reduce the overall building eight. This can be achieved either by reducingbuilding height or by reducing the eight of non!structural members like infill alls. Hoever for very tall buildingsreduction in mass ould also imply that ind induced vibrations may generate acceleration levels that mayrender a building unserviceable due to occupant discomfort.

Sti$$ness centric approach tiffness can be added to a building by adding shear alls% jacketing of beams andcolumns% addition of ne columns% addition of steel braces etc. Higher the stiffness of a building higher is going tobe the forces acting on it. Hoever a building requires some basic minimum stiffness for its lateral load resistingmechanism to be effective/ hoever excessive use of shear alls ill attract larger earthquake forces that cancause failure. This is because a stiff structure ould no have to deal ith much higher forces than if it asmoderately stiff.

Damping centric approach. $amping is a property of a structure by virtue of hich it can absorb'dissipate

induced energy ithout undergoing structural damage. In other ords incase a building has higher dampinglevels it ill say less thereby implying that it ill damage less. Also higher the damping loer are theearthquake forces as most of the energy is absorbed as a result of the damping property.

, !ntrinsic Damping in Rein$orced Concrete Structures0"" structures are considered to possess 12 inherent damping hereas steel structures are believed to have32 damping. Hoever actual site measurements have shon that intrinsic damping of buildings is far morecomplicated and variable than the generic figures of 3 and 12. $amping reduces as height increases and alsothe damping levels greatly differ from one building to another. 4or building upto 15 meters in height the measuredintrinsic damping as seen to vary from 6 to 12 hereas for very tall structures greater than 355 meters in heightthe intrinsic damping as just 5.1 to 62. (hat is of greater concern is that this intrinsic damping cannot beaccurately knon or calculated at the design stage. The only ay to tell the correct damping is by physical testingand measurements after the building is constructed. This uncertainty in the damping levels can prove fatal under

seismic conditions. To prove the case in point if in actual the damping is 62% here as the designer has designedthe building assuming 12 damping then the structure so designed ill not be able to perform to the expectedstandards in the event of an earthquake. This emphasises the thought process that the designers should assumea conservative damping value hile designing else it is almost certain that even ith computer aided analysisand design the buildings designed ould be unsafe.

Supplementary Damping in BuildingsAdditional engineered and accurate damping can be very easily added to buildings by installing certainmechanical devices called dampers. $ampers can provide damping upto 31!752 of the critical% thereby ensuringthat the building ill perform very ell in seismic conditions as also strong inds in case of very tall buildings.$ampers act as shock absorbers and energy dissipaters during any type of motion and thus prevent the building


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from damage. 8y using dampers the designer is able to overcome the uncertainties of lo intrinsic damping andthis helps in predicting the dynamic response accurately. 8y adding additional damping the stiffness and buildingmass can also be reduced thereby ensuring that the building is no subjected to loer seismic forces. Theadvantages of additional damping is reduced building say thus preventing damage to structural and non!structural components% reduced design forces as much of the energy is dissipated by the dampers and theuncertainty in the level of intrinsic damping is overcome through engineered supplementary damping.

upplementary damping is also the most efficient and cost effective ay to achieve energy dissipation in

buildings. This ould inadvertently mean decreasing the energy dissipation demand on the structural

components i.e. beams'columns'slabs thereby increasing the survivability of the building structure. $ampers are

mechanical devices that look some hat like huge shock absorbers and their function is to absorb and dissipate

the energy supplied by the ground movement during an earthquake so that the building remains unharmed.

(henever the building is in motion during an earthquake tremor or excessive inds% dampers help in restricting

the building from saying excessively and thereby preventing structural damage. The energy absorbed by

dampers gets converted into heat hich is then dissipated harmlessly into the atmosphere. $ampers thus ork to

absorb earthquake shocks ensuring that the structural members i.e. beam and columns remain unharmed. There

are four types of dampers i.e. 9iscoelastic% 4riction% :etallic ;ield and 4luid 9iscous.

,a- Traditional 9iscoelastic dampers are stacked plates separated by inert polymer materials. They have

proved to be problematic over a varying temperature range and have not achieved much success in practicalapplications due to the somehat undesirable added spring effect of these devices. There are no

manufacturers that manufacture purely 9iscoelastic damper.

,b- 4riction dampers consist of sliding steel plates and ork on the principal that hen to metal surfaces slide%

friction heat is produced and energy gets dissipated. These types of dampers are susceptible to corrosion and

cold elding hich has a direct effect on the yielding threshold. There are also some associated maintenance

problems.

,c- :etallic dampers consist of multiple steel plates hich yield hen a threshold force is reached. In other

ords these dampers become active only after a trigger force is crossed. As the metal yields% it dissipates

energy. These dampers are required to be replaced after every seismic event.


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001 conference paper - supplementary damping - a new concept in earthquake resistant buildings

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