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ASeismic Structural
Configurations
Much of the material contained in this lecture handout is used with the permission of Prof.Durgesh Rai from his 2009 Myanmar lectures. This marerial is the property of Professors
Sudhir K. Jain, C.V.R.Murty and Durgesh C. Rai of IIT, and is for the sole and exclusive useof the participants of the December 2012 Ahmedabad Course at IITGN
Gregory MacRae
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Why Do We Care
About SeismicConfiguration?
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We want to have the building resist
earthquake as simply as possible
This results in best seismic
performance
Consider also travel from one point to another.
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ANALOGIES
Travel Between Building Seismic Different Points Performance
Traveller health Proper design/detailingDirect route Direct load path
Steady gradient No concentration of
effort – e.g. Soft storey
No overhanging trees No dangerous non-structural elementsNo pirates/crocodiles Gaps between adjacent structures
Solid/nonslip path Good foundation
No extra burdens No attachments that may interfere
with performance
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Configuration
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• Important Factors in Design
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Seismic Configuration
• Seismic Configuration
– Geometry, Size and Shape– Location and size of Structural Elements
– Location and size of Non-Structural Elements
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• Geometry, Size and Shape
Seismic Configuration…
???
Convex ConcaveRai, Murty and Jain
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Seismic Configuration…
Simple Complex
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Seismic Configuration…
• Overall size
Buildings with one overall size much larger/smaller than other two,do not perform well during EQs
Too long
Too large in planToo tall
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OFFSET BUILDINGS
• Alphabetic Shapes
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• Re-entrant
corners
OFFSET BUILDINGS…
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Seismic Configuration…
• Shape
Simple plan shapebuildings do well
during EQs
Simple Plan::good
Separation joints make complex plans
into simple plans
Corners and Curves:: poor
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Seismic Configuration…
• Indirect load path
Sudden deviations in load transfer path
along height lead to poor performance
Setbacks
Weak or Flexible Storey
Slopy Ground Hanging or Floating Columns
UnusuallyTall Storey
RC WallDiscontinued in
Ground Storey
Discontinuing Structural Members
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• Simple, direct load transfer path gives
better performance
• Regular configuration preferred
–
Even distribution of mass and stiffness in building plan and with building height
• In buildings with irregular configuration
– Ductility demand gets concentrated in a few storeysor elements
– Simple code expression not valid;
quantitative guidelines required
Seismic Configuration…
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Symmetric Buildings
• Swing Analogy
Rope swings and buildings, both swing back-and-forth when shaken horizontally.The former are hung from the top, while the latter are raised from the ground.
1-storeybuilding
3-storeybuilding
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• Symmetry–
Mass– Stiffness
Identical vertical members placed uniformly in plan of building causeall points on the floor to move by same amount.
EQ GroundMovement
UniformMovement of
Floor
Identical Vertical Members
Symmetric Buildings…
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UNSymmetric Buildings
• Swing Analogy:: Mass Unsymmetry
Even if vertical members are placed uniformly in plan of building, more mass on one sidecauses the floors to twist.
Twist
EQ Ground Shaking
Light Sideof Building
Heavy Sideof Building
Swingwith equal ropes
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• Swing Analogy:: Stiffness Unsymmetry
Buildings have unequal vertical members;they cause the building to twist about a vertical axis.
Vertical Axis about whichbuilding twists
Earthquake
Ground Movement
Building on slope ground
Swingwith unequal ropes
UNSymmetric Buildings…
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• Location and size of Structural Elements
Seismic Configuration…
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• Other types of stiffness unsymmetry
Buildings with walls on two/one sides
(in plan)
Wall
ColumnsColumns
Wall
Wall
UNSymmetric Buildings…
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• Combined Unsymmetry
One-side open ground storey building twistsduring EQ shaking
EarthquakeGround
Shaking
UNSymmetric Buildings…
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• Consequences
These columns aremore vulnerable
Vertical members of buildings that move more horizontallysustain more damage
EarthquakeGround
Movement
UNSymmetric Buildings…
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• Pounding
Seismic Design…
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• Connectors between buildings to be
designed carefully
Seismic Design…
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Diaphragms
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Floor Diaphragms
• Out-of-plane versus In-plane flexibility
– Out of plane is important for gravity loads
–
In-plane is important for seismic loads
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• Floor rigid in-plane
– Moves like a rigid block in its own plane•Translation
Floor Diaphragms…
Earthquake Ground Movement
Uniform Movement of Floor
EarthquakeGround
Movement
Uniform Movement ofFloor
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•Rotation
•Combined Translation and Rotation
Floor Diaphragms…
EarthquakeGround
Movement
Earthquake
Ground Movement Rai, Murty and Jain
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– Floors apply horizontal force on vertical elements
•
Vertical elements act as horizontal springs
Floor Diaphragms…
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• Rigid diaphragm
– Forces are proportional to stiffness of walls
0.4k 0.4k0.2k
Floor Diaphragms…
0.4F 0.2F 0.4F
F
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• Flexible diaphragm
– Forces are proportional to tributary area
– Wall stiffnesses do not come into calculations
0.4k 0.4k0.2k
Floor Diaphragms…
F/4 F/2 F/4
F
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• Act like beams in horizontal plane
Floor Diaphragms…
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• Many of them…
Floor Diaphragms…
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• Cut-outs ??
Floor Diaphragms…
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• Openings only under compulsion
Floor Diaphragms…
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Open Ground Story
O G d St
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Open Ground Storey
• Known for decades
– Poor seismic configuration
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• Garage doors
Open Ground Storey…
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• Three Situations…
Open Ground Storey…
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• Understood… well!!
Open Ground Storey…
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• Lessons learnt !!
Open Ground Storey…
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Structural Walls
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Structural Walls
• Steel or RC
– Do not interrupt wall in lower levels
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Short Column Effect
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Short Column Effect
• Mistaken Identity of Infills
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Irregularities provisions
in Codes
Building Configuration
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Building Configuration
• Irregularities in Buildings
– Mass– Stiffness
– Geometry
Indian Standards
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• Building Codes
Configuration emphasised in buildings– Comprehensive section on identifying irregularities
– Quantitative definitions of irregular buildings
• Two types of Irregularities addressed
– Plan Irregularities
–
Vertical Irregularities
Building Configuration…
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• Plan Irregularities
–
Torsion Irregularity
22.1
21
2
Heavy Mass
Irregular Orientation of LateralForce Resisting System
Floor 2
Building Configuration…
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– Re-entrant Corners
L
L
A A
A
A
A
20.015.0 L
A
Building Configuration…
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– Out of Plane Offsets
Shear Wall Shear Wall
Shear Wall
Building Configuration…
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– Non-Parallel System
x
y
Building Configuration…
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C f
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• Vertical Irregularities
–
Stiffness Irregularity (Soft Storey)
kiki-1
ki+1
17.0
ii k k
38.0
321 iii
i
k k k k
Building Configuration…
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B ildi C fi ti
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– Mass Irregularity
W iW i-1
W i+1
12
ii W W
12
ii W W
Building Configuration…
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– Vertical Geometric Irregularities
L
A A A
L L
A A
20.015.0 L
A
Building Configuration…
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L2
L1
L2
L1
12 5.1 L L
Building Configuration…
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– In-plane Discontinuity in Lateral Load Resisting Elements
Upper FloorPlan
Lower FloorPlan
Building Configuration…
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– Strength Irregularity (Weak Storey)
SiSi-1
Si+1
18.0
ii S S
Building Configuration…
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SEISMIC CODES
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SEISMIC CODES
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Seismic Codes
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Seismic Codes…
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Non Structural
Components
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Non-structural Elements
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Non structural Elements
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Facades
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Facades
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Non structural Elements
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Non-structural Elements…
Bare
Finished
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Non structural Elements
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Non-structural Elements…
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Non-structural Elements
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• Infringements understood
Non-structural Elements…
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Non-structural Elements
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• Problems
Non-structural Elements…
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Non-structural Elements
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• Electrical power failures
Non structural Elements…
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Non-structural Elements
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• Secondary disasters
Non structural Elements…
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Non-structural Elements
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• Chemical Spill
Non structural Elements…
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Non-structural Elements
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• Chemical Disasters
Non structural Elements…
Non-structural Elements…
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• Overhead safety
Non structural Elements…
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Non-structural Elements…
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• Indirect losses
Non structural Elements…
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• Disruption of services
Non structural Elements…
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• May be acceptable
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Non-structural Elements…
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• Not acceptable
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Solutions
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Solutions…
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Solutions…
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• Securing masonry chimneys
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Solutions…
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• Securing masonry parapets
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Solutions…
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Solutions…
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Solutions…
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Non-structural Elements…
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• Half-hearted… not good!
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Importance of Configuration
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• Henry Degenkolb,
a noted Earthquake Engineer of USA– aptly summarised the intense importance of seismic
structural configuration in his words:
“If we have a poor configuration to start with,all the engineer can do is to provide a band-aid- improve a basically poor solution as best as he can.
Conversely, if we start-off with a good configuration andreasonable framing system, even a poor engineer cant harmits ultimate performance too much.”
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ANALOGIES
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Travel Between Building Seismic Different Points Performance
Traveller health Proper design/detailingDirect route Direct load path
Steady gradient No concentration of
effort – e.g. Soft storey
No overhanging trees No dangerous non-structural elements
No pirates/crocodiles Gaps between adjacent structuresSolid/nonslip path Good foundation
No extra burdens No attachments that may interfere
with performance
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Signature Tower:Jakarta
638m
#5 in the world in 2020The tallest in a seismic
region
111 Stories
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