guidelines for earthquake resistant design
TRANSCRIPT
GUIDELINES FOR EARTHQUAKE RESISTANT DESIGN
Dr. G. P. ChandradharaProfessor of Civil EngineeringS. J. College of EngineeringMysore- 570 006
E mail : [email protected]: 094482 46425
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General Guidelines for Planning
Building and its Structure Should Have a Uniform and Continuous Distribution of
Mass, Stiffness, Strength and Ductility
Problem in Extreme Direction Structural Problem Remedial Measures
Extreme height/depth ratio
High overturning forces, Large drift causing non-structural damage, foundation stability
Revise proportion or special structural system
Extreme plan area Built-up large diaphragms forces Subdivide building by seismic joints
Extreme length depth ratio Built up of large lateral forces in perimeter, large differences in resistance of two axesExperience greater variations in ground movement and soil conditions
Subdivide building by seismic joints
Building Configuration:
Problems & Solution
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Problem of Vertical Layout
Structural Problem Remedial Measures
Vertical set back& reversesetbacks
Stress concentration atnotch, different periodsfor different parts ofbuilding, highdiaphragms forces totransfer at setback
Special structuralsystems, carefuldynamic analysis
Soft storeyframe
Causes abrupt changes ofstiffness at point ofdiscontinuity
Add bracing, Addcolumns, braced
Variation incolumn stiffness
Causes abrupt changes ofstiffness, much higherforces in stiffer columns
Redesign structuralsystem to balancestiffnesses
Building Configuration: Problems & Solution
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Design of Soft storey frame
Special arrangements are necessary to increase the strength and stiffness
1. Dynamic analysis is carried out by considering stiffness and mass distribution and inelastic deformation of the members
OR1. Columns and Beams of soft storey are to be designed for 2.5 times
storey shear and moments calculated under seismic loads.
2. Shear walls are provided in both the parallel direction away from the cente of the building and need to be designed for 1.5 times the lateral storey shear.
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Guidelines for Planning
Arrangement of Columns
Good arrangement of columns Poor arrangement of columns
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Guidelines for Planning
Rotation about the axis
Building is strong about YY axis and weak
About XX axis
Provide columns to make
it strong about XX axis
x
x
x
Y
Y
x
Y
Redundancy in Building
Column/Beam
Walls
Simple Connection
RigidConnection
One load pathTwo load paths
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POUNDING
Maintain a good distance Between adjacent tallBuildings to avoid Collision during oscillation
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Whip Effect of ApartmentBhuj 2001
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Problems of Adjacency
Structural Problem Remedial Measures
Buildingseparation(Pounding)
Possibility of poundingdependent on buildingperiod, height, drift,distance
Ensure adequateseparation, assumingopposite buildingvibrations
Building Configuration: Problems & Solution
Short Column Behavior
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Short Column Behavior
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Brittle shear failure of columns andbeams
Buckling of longitudinal bars inbeam - columns due to inadequatespacing or lack of transversestirrups
Shear failure of columns whichwere shortened by the supportingeffect of non – structural elements
Brittle failure in corner columnscaused by torsion and biaxialbending effects
Shear Failure in Captive Column
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The diagonal cracks and shear failures in the short columns of a multi-storey car park almost caused collapse (Northridge, California 1994)
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opening
column
Solution:
1. Add ties at closer spacing. Preferably spiral ties.
2. Provide masonry walls on either side equal to twice the opening sizes by reducing the openings.
3. The best solution is to avoid the opening so that no captive column is created.
CAPTIVE COLUMNS
opening opening
masonry masonrymasonry
column
Beam Captive column
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masonry masonrymasonry
column
column
Beam
CAPTIVE COLUMNS: SOLUTIONS.
OPENINGOPENING OPENING
L
LL LL
1 1a
2
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Severe Shearing Effects on Columns
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Short Column behaviour (Kachchh, 2001)
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Short column failure due to insufficient transverse reinforcement or ?Krishna Aparments, Airport Road, Bhuj
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Failure of Express highway in Kobe
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Strong Column & Weak Beam Concept
The seismic inertia forces generated at its floor levels are transferred through the various beams and columns to the ground. The correct building components need to be made ductile. The failure of a column can affect the stability of the whole building, but the failure of a beam causes localized effect. Therefore, it is better to make beams to be the ductile weak links than columns.
Strong column and weak beams
Good – Failure in beams Poor– Failure in Columns
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Weak column and strong beams
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PLASTIC HINGE
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Large Span Cantilevers
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No failure – Earthquake Resistant DesignBhuj 2001
Weak Link
Stair case orLift well
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Weak Link, Bhuj 2001
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Weak Link, Bhuj 2001
No lateral load force transfer mechanism to the core Punjal Apartments, Ahmedabad
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Himagiri Apt-weak link
Weak Link, Bhuj 2001
Simple Supports
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LOSS OF SUPPORT
Column Beam connection
Movement of supports due to horizontal force
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Bad construction practice of casting the ground floor columns up to the bottom of the beam and leaving a gap
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Weak joint,Bhuj 2001
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Discontinuity of the longitudinal reinforcement Bombay Dying Market, Station Road, Bhuj
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Poor quality materials and workmanship
Neelima Park Apartments, Ahmedabad
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L,T, + SHAPE COLUMNS CAN BE USED TO INCREASE THE STIFFFNESS OF THE COLUMNS
Behavior of Symmetrical and Asymmetrical Buildings
Location of Centre of Mass
Location of Centre of Mass
The center of mass wrt A(9.76, 4.10)
AA
Location of Centre of Stiffness
Plan of one Storey Building with size of columns being same
Stiffness of all the columns are same
Location of Centre of Stiffness (COS)
In Y direction the Three frames are symmetrical, COS lies along symmetrical X axis
In X direction there are 4 frames.Let the lateral Stiffness of each frame be ‘k’
X X
The center of mass w.r.t .A (8.75, 5.0)
A
Y
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