High Rise Buildings in Earthquakes
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Underlying PhysicsNewtons Second LawF = mawhere m = mass of building a = acceleration of groundAnimation from www.exploratorium.edu/faultline/ engineering/engineering5.htmlWhat do the physics tell us about the magnitude of the forces that different types of buildings feel during an earthquake?
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What is really happening?F is known as an inertial force, Engineering representation of earthquake force
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Period and FrequencyFrequency (f) = number of complete cycles of vibration per secondPeriod (T) = time needed to complete one full cycle of vibration
T = 1 / f
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Idealized Model of Buildingsmaller kbigger mincrease building period
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Natural Period of BuildingsEach building has its own natural period (frequency)slower shaking
Building HeightTypical Natural PeriodNatural Frequency2 story0.2 seconds5 cycles/sec5 story0.5 seconds2 cycles/sec10 story1.0 seconds?20 story2.0 seconds?30 story3.0 seconds?
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ResonanceResonance = frequency content of the ground motion is close to building's natural frequencytends to increase or amplify building responsebuilding suffers the greatest damage from ground motion at a frequency close or equal to its own natural frequency
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Learn from Earthquake
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Soil FailureLiquefaction
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Foundation FailureWeak soil conditions
Weak foundation
Soil structure interaction
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Proximity to Other Buildings - PoundingBuildings repeatedly hit each other
Can cause collapse of frame buildings
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Soft First StoryOccurs when first story much less stiff than stories aboveTypical damage collapse of first story
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Unreinforced Masonry (URM)Built of heavy masonry walls with no reinforcingmissing anchorage to floors & roof
floors, roofs and internal partitions are usually of wood
older construction no longer built
Typical damageWalls collapse and then roof (floors)
Parapets fall from roof
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What affects building performance / damage?
Shape (configuration) of building: Square or rectangular usually perform better than L, T, U, H, +, O, or a combination of these.
Construction material: steel, concrete, wood, brick. Concrete is the most widely used construction material in the world. Ductile materials perform better than brittle ones. Ductile materials include steel and aluminum. Brittle materials include brick, stone and unstrengthened concrete.
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What affects building performance / damage?Load resisting system
Height of the building: (i.e. natural frequency)
Previous earthquake damage
Intended function of the building (e.g. hospital, fire station, office building)
Proximity to other buildings
Soil beneath the building
Magnitude and duration of the earthquake
Direction and frequency of shaking
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Key Factor in Building Performance
Good connections
Transfer loads from structural elements into foundation and then to ground
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Building Systems: FramesFrame built up of beams and columnsSteelConcrete
Resists lateral load by bending of beams and columns
Provides lots of open interior space
Flexible buildingsF
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Building Systems: Braced FrameBraces used to resist lateral loadssteel or concrete
Damage can occur when braces buckle
Stiffer than pure frame
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Building Systems: Shear Wallswall elements designed to take vertical as well as in-plane horizontal (lateral) forces Concrete buildingsWood buildingsMasonry buildings
resist lateral forces by shear deformation
stiffer buildingsFShear Deformation
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Building Systems: Shear WallsLarge openings in shear walls a much smaller area to transfer shear resulting large stresses cause cracking/failureFCracking around openings
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Tilt-up ConstructionShear wall load resisting systemQuick and inexpensive to buildWarehouses (Costco), industrial parksTypical damageWalls fall outward, then roof collapsesMUSE 11B
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Seismic RetrofitMUSE 11BFrames can be used to strengthen older concrete buildings
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Earthquake Protective Systems
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Base Isolated Buildingsseries of bearing pads placed between the building and its foundation
acceleration of the building is reduced = less damage
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Base Isolation in BuildingsOriginal StructureIsolated StructureIsolation at foundation level
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Base Isolation in BuildingsIsolator Components Between the Foundation and SuperstructureAn Isolation Interface is formed
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Base Isolation in Buildings
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Base Isolation in Buildings
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How exactly does Base Isolation Work?Large deformation potential
Large drift on the Isolation InterfaceExhibit nonlinear behaviorLengthening of the Structures Period and increased Damping that result in a large scale decrease of the Seismic Response
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Common Types of Isolation Systems (Isolators)
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Elastomeric Isolators Lead Core Rubber Bearings
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Rubber bearings
layers of rubber with thin steel plates between them, and a thick steel plate on the top and bottom.
very stiff and strong for vertical load, so that they can carry the weight of the building.
designed to be much weaker for horizontal loads, so that they can move sideways during an earthquake. *
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Friction Pendulum Bearings
two horizontal steel plates that can slide over each other because of their shape and an additional articulated slider.
very stiff and strong for vertical load*
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Sliding Isolators Friction Pendulum SystemSuperstructureFoundation
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Friction Pendulum System
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Dampers Absorb partial energy going into the building from the shaking ground during an earthquake.
Reduce the energy available for shaking the building.building deforms less, so the chance of damage is reduced.
*
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Metallic Dampers
Usually made from steel.
They are designed to deform so much when the building vibrates during an earthquake that they cannot return to their original shape.
inelastic deformation, and it uses some of the earthquake energy which goes into building.
*X - Plate Metallic Damper
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Friction Dampers
Designed to have moving parts slide over each other during a strong earthquake. Friction uses part of energy from the earthquake that goes into the building. *
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Set of steel plates, with slotted holes in them, and they are bolted together.
the plates can slide over each other creating friction.
specially treated to increase the friction between them.
*
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Viscous Fluid dampersSuspension system: a mechanical system of springs and shock absorbers that connect the wheels and axles to the chassis of a wheeled vehicle
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Viscous fluid dampers similar to shock absorbers in automobilesOil suspensionGas suspension
*
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The Function of suspension system:The job of a car suspension are:- to carry the static weight of the vehicle - to maximize the friction between the tires and the road surface, - to provide steering stability with good handling (minimize body roll)- to ensure the comfort of the passengers (ability to smooth out a bumpy road).
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Viscous fluid dampersinstalled as part of a building's bracing system using single diagonals. sways to and fro, the piston is forced in and out of the cylinder.
*
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Active ControlControl system to keep stresses / strains / displ. / accel. (called outputs) at certain locations below specified bounds (peak, rms) when disturbances (wind, earthquake) below specified bound are applied.
Designer decides choice of on comfort (accelerations) safety (stresses).
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Active Control
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Active control with TMD
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Active control with TMD
Fig. 8: AMD on Kyobashi Seiwa building
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Active controlFirst full scale application of active control to a building was done on Kyobashi Seiwa building (Japan) in 1989 (Fig. 7,8). Two AMDs were used. Primary one weighs 4t and damps transverse motion. Secondary one weighs 1t and damps torsional motion.Can also use Magnetorheological fluid dampers (semi-active), active tendons, etc. (Fig. 9, 10, 11)
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Orifice based Semi-Active Suspension
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Orifice based Semi-Active Suspension
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MR fluid based Semi-Active Suspension
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MR Fluidoil and varying percentages of ferrous particles (20-50 microns in diameter) that have been coated with an anti-coagulant material.Varying the magnetic field strength variation in viscosity
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Active VS Semi-ActiveSemi-Active:Lower implementation costLower power consumptionEasier to controlSimpler designEasy to install
Disadvantage: damper c
