EARTHQUAKE RESISTANT
STRUCTURES
EARTHQUAKE RESISTANT
OR
EARTHQUAKE
PROOF ?
• Complete protection against earthquakes of all sizes not economically feasible
• Strength criteria alone can not be
the basis of design of structures
IS CODES
IS 4326: 1993 Earthquake Resistant Design and construction of Buildings
IS 13827:1993 Improving Earthquake Resistance of Earthen Buildings
IS 13828:1993 Improving Earthquake Resistance of Low Strength Masonry Buildings
IS 13920:1993 Ductile Detailing of Reinforced Concrete Structures subjected Seismic Forces
IS 1893 (PART 1): 2002 Criteria for Design of
Earthquake Resistant Structures (General Provisions and Buildings)
• General Considerations
1. Symmetry 2. Regularity
3. Separation of Blocks 4. Simplicity
5. Enclosed Area
6. Separate Building for Different
Functions
A& B C D & E
b5 0 230 mm 450 mm
(b1+b2+b3)/l1 (b6+b7)/l2
One st. bldg 0.60 0.55 0.5
Two st bldg 0.50 0.46 0.42
3 0r 4 st bldg 0.42 0.37 0.33
Pier Width b4 340 mm 450 mm 560 mm
h3 (min) 600 mm 600 mm 600 mm
b8 (max) 900 mm 900 mm 900 mm
Basis of Basic Approach for Earthquake Resistant Design
• Lateral Strength
• Deformability
• Ductility Capacity
of structure with limited damage but no collapse
IS 13920:1993 Ductile Detailing of Reinforced Concrete Structures subjected Seismic
Forces
Covers requirements of
• Lateral strength Design
• Detailing of monolithic reinforced concrete buildings
To give
• Toughness
• Ductility
Ductility
• One of the most important factors affecting seismic performance of structure
• Inelastic Material Behavior
• Detailing of Reinforcement in a manner to avoid brittle failure
• To induce Ductility, allow steel to yield in controlled manner
Impact of Ductility
• Serves as shock Absorber
• Reduces the transmitted forces to the sustainable level
Ductility
• Capability of a material, structural component, or entire structure to undergo deformation after its initial yield without any significant reduction in yield strength
• Measured in terms of Ductility Ratio (DR) or Ductility Factor (DF)
DR = Maximum deformation without loss of initial yielding resistance / Initial yield deformation
Factors Affecting Ductility
• Ductility increases linearly with an increase in the shear strength carried by concrete
for small values of compressive stress (0≤ σa ≤1 Mpa)
• With the increase of ultimate stain of
concrete , ductility factor increases. Thus confining of concrete increases the
ductility.
• An increase in yield strength of steel with all other variables constant decreases
ductility
• Ductility increases with the increase in concrete strength
• Lateral reinforcement improves ductility by preventing shear failure and restraining the compression steel against buckling.
• Shear failure occurs at a smaller deflection than the flexural failure hence absorbs much less energy. Members should be designed and detailed by providing web reinforcement so that their strength in shear exceeds the strength in flexure. Thus Ductility increases as the stirrups in the specimen increases.
• Bond failure and anchorage failure are sudden and brittle. Special attention must be given in detailing to prevent them occurring in structure.
Ductile Detailing Considerations (IS 13920:1993)
• Anchorage and splices of longitudinal reinforcement
• Spacing, anchorage and splices of lateral reinforcement
• Joint of members
General Specifications
• Design and construction of RCC buildings shall be governed by provisions of IS 456 : 2000 except as modified by the provisions of IS 13920 : 1993
• For all buildings which are more than 3 stories in height the minimum grade of concrete shall be M20
• Steel reinforcement of grade FE 415 or less shall be used
• Joint of members
Flexural Members
• The member shall preferably have a width to depth ratio of more than 0.3
• Width of member shall not be less than 200mm
• The depth of member shall preferably be not more than one-fourth of the clear span
Longitudinal Reinforcement
• The top as well as bottom reinforcement
shall consist of at last two bars consist of at least throughout the member length
• The tension steel ratio on any face , at any section , shall not be less than
• The maximum steel ratio on any face , at any section , shall not shall not exceed
) /
( 24 .
min 0 fck fy
P
025 .
max 0 P
• The positive moment steel at a joint face
must be at least equal to half of the negative steel moment at that face
• The steel provided at each of the top and bottom face of the member at any section along the length shall be at least equal to
one-fourth of the maximum negative moment steel provided at the at the face of either
joint
• Anchorage of beam bars in an external joint
Lap splice in beam
• Longitudinal bars shall be spliced only if hoops are provided over the entire splice length at spacing not exceeding 150 mm
Lap splice in beam
• Lap splice shall not be provided within a joint
• Not more than 50% of bars shall be spliced at one section
Web Reinforcement
A vertical hoop with 1350 hook or a U strip with 1350 hook
•Web Reinforcement
• Minimum diameter of bar forming hook shall be 6 mm
• In beams with clear span exceeding 5m, minimum bar diameter shall be 8mm
• Spacing of hoops
Columns and Frame Members Subjected to Bending and Axial Load
General Specifications
• Minimum dimension of member not less than 200mm
• In frames having beams with c/c span exceeding 5m or column unsupported length exceeding 4m, shortest dimension of column not less than 300mm
• The ratio of shortest x-sectional dimension to the perpendicular dimension shall preferably not less than 0.4
Transverse
Reinforcement
• Spacing of hoops shall not exceed half the least
lateral dimension of the column
Column and Joint Detailing
CL 7.4.1
L0 shall not be less than
(i) Larger lateral dimension of the member
(ii)1/6 of clear span of the member
(iii) 450mm CL 8.1
The spacing of hoops shall not exceed 150mm.
• CL 7.2.1 Lap splices shall be provided only in the central half of the member length. It should be proportioned as a tension splice. Hoops shall be
provided over the entire splice length at spacing not exceeding 150 mm centre to centre. Not more than 50 per cent of the bars shall be spliced at one section.
• CL 7.3.3 The spacing of hoops shall not exceed half the least lateral dimension of the column, except where
special confining reinforcement is provided.
• CL 7.4.2 When a column terminates into a footing or mat, special confining reinforcement shall extend at least 300 mm into the footing or mat
CL8.1
The special confining reinforcement as required at the end of column shall be provided through the joint as well, unless the joint is confined as specified by 8.2.
CL 8.2
A joint which has beams framing into all vertical faces of it and where each beam width is at least 3/4 of the column width, may be provided with half the special confining reinforcement required at the end of the column. The spacing of hoops shall not exceed 150 mm.
SHEAR WALL
Shear Wall
• The thickness of any part of the wall preferably be not less than 150mm
• Reinforcement in longitudinal and transverse directions. Minimum reinforcement ratio shall be 0.0025 of gross area in each direction.
Uniform distribution of reinforcement across the x-section of wall
• Dia of bars in any part shall not exceed 1/10 of thickness of that part
• Max spacing of reinforcement shall not exceed the smaller of lw/5, 3tw and 450 mm
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Seismic retrofitting:
Conventional (Seismic) methods :
The strengthening in the buildings can be achieved by additional bracings, shear walls, wall panels, foundations etc. to the building such that the lateral stiffness of the structure is increased. (resistance of str is increased) (Fig.1 (a,b,c,d))
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Fig.(1a) Additional Foundations Fig.(1b) Additional Shear Wall
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Fig.(1c) Jacketing (Col.) Fig.(1d) Additional Column fig.1(a,b,c,d) Source: (“methods of seismic retrofitting”
web.mit.edu/istgroup/ist/documents/earthquake/Part5.
pdf ))
Aseismic (Force mitigation) methods:
These methods involves provision of some devices which mitigate the EQ forces coming to the str. These devices can be of two types:
i) Passive : visco -elastic dampers, tuned mass dampers, base isolators, friction
dampers etc.
ii) Active : active tendons, actuators, active bracing systems, active variable stiffness
system etc. (semi active: MR dampers etc.)
MUSE 11B
Seismic Retrofit
Frames can be used to strengthen older
concrete buildings
Unconventional Earthquake Protective Systems
Base Isolation is the most common System
Earthquake Protective
Systems Passive
Protective Systems
Tuned Mass Damper
Energy Dissipation
Base Isolation
Hybrid Protective
Systems
Active Isolation
Semi-Active Isolation
Semi-Active Mass Damping
Active Protective
Systems
Active Mass Damping
Active Bracing
Adaptive Control
Classification of Control Methods
Active/Feedback control:
• External source of power drives actuators (i.e., provides input voltage) .
• Voltages required are computed by controller using certain algorithms with inputs from sensors.
• Sensors measure motion (strains, displ, vel, accl.)
• Actuators apply forces to structure, thereby adding or dissipating energy.
• Examples of sensors are acceleromters, strain gauges.
• Examples of actuators are tendons, solenoids,
piezoelectric stacks, active mass dampers (AMD).
• Destabilization possible.
• External power may not be available during earthquake.
Classification of Control Methods
Passive control:
• No external power required.
• Passive control device (TMD, Base Isolator) imparts forces that are developed directly as a result of motion of structure (i.e., no actuator involved).
• Total energy (structure + passive device) cannot increase, hence inherently stable.
• Relatively inexpensive.
• Reliable during earthquake
• Not as effective as active, hybrid, semi-active control.
Classification of Control Methods
Hybrid control:
• Uses active & passive devices.
• Advantages of both active and passive systems are present and their limitations are reduced.
• Essentially an active control system
• Examples: viscous damping with AMD, base isolation with actuators, TMD+AMD).
Classification of Control Methods
Semi-active control:
• Uses devices where input power requirements are orders of magnitude less than fully active devices. In fact in some cases battery power is sufficient.
• These devices usually don’t add energy to the system, hence stability ensured.
• These devices can be viewed as controllable passive devices (eg., Magneto-Rheological Fluid damper
where voltage input applied to change viscosity
depending on motion measured by sensors, variable orifice damper, controllable friction devices, variable stiffness devices).
• Structure mounted on a suitably flexible base such that the high frequency component of ground motion is
filtered out and the fundamental vibration period is
lengthened. This results in deformation in the isolation system only, thus keeping the structure above almost rigid. However, if the earthquake excitation contains a
major component of this fundamental period, there will be large sidesway (albeit almost rigid) motions.
• San Fransisco city hall (retrofitted, 530 rubber bearings), International terminal at SF airport (267 Friction
pendulum sliding bearings).
• Not suitable for tall slender buildings (subject to high wind loads). For these auxiliary dampers (viscous,
viscoleastic) are deployed (eg. WTC).
Passive control: Base isolation
BASE ISOLATION
• Base isolators reduce structural
responses/earthquake forces in two ways:
• By deflecting the seismic energy.
• By absorbing the seismic energy.
• Isolators are stiff in the vertical direction and flexible in the horizontal direction; lateral
deformation is much more than inter story deformation.
• Isolators are placed between the columns and the foundation.
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It is a system that may be defined as a flexible or sliding interface
positioned between a structure and its foundation, for the purpose of
decoupling the horizontal motions of the ground from the horizontal
motions of the structure, thereby
reducing earthquake damage to the
structure and its contents.
Principle of Base Isolation
The fundamental principle of base
isolation is to modify the response
of the building so that the ground
can move below the building without
transmitting these motions into the
building
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Base Isolated Building
Principle
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Effects of Base Isolation System
17
Building with Seismic Isolation
Conventional Building
・ Improvement for Safety of Building
・ Keep for Function of Building
・ Preservation for Property
Base Isolation in Buildings
Original Structure Isolated Structure
Isolation at foundation level
Base Isolation in Buildings
Isolator Components Between the Foundation and Superstructure
An Isolation Interface is formed
Base Isolation in
Buildings
Base Isolation in
Buildings
Types of Isolators : -
1.SLIDING SYSTEMS
A layer with a defined coefficient
of friction will limit the
accelerations to this value and the
forces which can be transmitted
will also be limited to the
coefficient of friction times the
weight.
1.SLIDING SYSTEMS
1.SLIDING SYSTEMS
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.
1.SLIDING SYSTEMS
2.ELASTOMERIC (RUBBER) BEARINGS
Elastomeric bearings are formed of horizontal layers of natural or synthetic rubber in thin layers bonded between steel plates. The steel plates prevent the rubber layers from bulging and so the bearing is able to support higher vertical loads with only small deformations.
Under a lateral load the bearing is
flexible.
2.ELASTOMERIC (RUBBER)
BEARINGS
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.
Eg : Lead Rubber Pad
Eg Lead Rubber Pad
Elastomeric Isolators
– Lead Core Rubber Bearings
3.SPRINGS
3.SPRINGS
3.SPRINGS
There are some proprietary devices based on steel springs but they are not widely used and their most likely application is for machinery isolation. The main drawback with springs is that most are flexible in both the vertical and the lateral directions. The vertical flexibility will allow a pitching mode of response to occur.
Springs alone have little damping and will move
excessively under service loads.
4. Roller and Ball Bearings
Rolling devices include cylindrical rollers and ball races. As for springs, they are most commonly used for machinery applications.
Depending on the material of the roller or ball bearing the resistance to movement may be sufficient to resist services loads and may generate damping.
Rolling devices include cylindrical rollers and ball races. As for springs, they are
most commonly
used for machinery applications.
Depending on the material of the roller or
ball bearing the
resistance to movement may be sufficient
to resist services loads and may generate
damping.
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To TOP
Principle of Base Isolation System
Base Isolation System is like a Suspension System of Motorcar
Soft spring + Oil Damper
Damper Laminated rubber bearing
2
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To Back
Laminated rubber bearing
Soft in Horizontal direction Hard in Vertical direction
Rubber Sheet Steel Plate
Base Isolation System
3
77
To Back
Base Isolation System
Laminated rubber bearing Rubber block
Rubber block
Laminated rubber bearing 1. Vertical direction
2. Horizontal direction
Principle of Laminated rubber bearing
Rubber block
4
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To TOP
Steel damper
Lead damper
Base Isolation System Damper
Oil damper
Friction damper with Coned disc springs
5
Base Isolation System
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Example of Base-isolation Building
Base Isolated Research Building Base Isolation Device
Location : Tokyo
Architect : Obayashi Corporation Structure : RC ; B1,5F
Total floor area : 1,624 ｍ2 Date of completion : August 1986
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