A Seismic Structural Configurations Sudhir k Jian c v r Murty and Durgesh C Rai 2012

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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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Simple Complex

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• 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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• 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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• 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


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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


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• Other types of stiffness unsymmetry

Buildings with walls on two/one sides

(in plan)

Wall

ColumnsColumns

Wall

Wall


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• Combined Unsymmetry

One-side open ground storey building twistsduring EQ shaking

EarthquakeGround

Shaking


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• Consequences

These columns aremore vulnerable

Vertical members of buildings that move more horizontallysustain more damage

EarthquakeGround

Movement


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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…


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• Understood… well!!


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• Lessons learnt !!


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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


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– Re-entrant Corners

L

L

A A

A

A

A

20.015.0 L

A


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– Out of Plane Offsets

Shear Wall Shear Wall

Shear Wall


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– Non-Parallel System

x

y


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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


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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


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B ildi C fi ti


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– Vertical Geometric Irregularities

L

A A A

L L

A A

20.015.0 L

A


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B ildi C fi ti


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L2

L1

L2

L1

12 5.1 L L


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B ildi C fi ti


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– In-plane Discontinuity in Lateral Load Resisting Elements

Upper FloorPlan

Lower FloorPlan


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B ildi C fi ti


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– Strength Irregularity (Weak Storey)

SiSi-1

Si+1

18.0

ii S S


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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…

Bare

Finished

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• Infringements understood


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• Problems


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• Electrical power failures

Non structural Elements…

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• Secondary disasters


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• Chemical Spill


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• Chemical Disasters




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• Overhead safety


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• Indirect losses


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• Disruption of services


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• May be acceptable

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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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• 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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Documents

A Seismic Structural Configurations Sudhir k Jian c v r Murty and Durgesh C Rai 2012