ARCHITECTURAL AND PLANNING ASPECTS

Dr. G. P. ChandradharaProfessor of Civil EngineeringS. J. College of EngineeringMysore- 570 006

E mail : chandu_gpc@yahoo.comMobile: 094482 46425

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PERFORMANCE OF STRUCTURES DURING EARTHQUAKES

Rational studies along with a knowledge regarding theperformance of buildings in previous earthquakes haveshown that

Poor Building architectural design and planning woulddecrease the seismo resistant capacity of the buildingand also become the cause of collapse of the building

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PERFORMANCE OF STRUCTURES DURING EARTHQUAKES

The seismic analysis and design alone do notensure good performance of the structure duringearthquakes. The building planning should alsoconform to the principles of earthquake resistantconfiguration

A real compatibility between the planning and thestructural design avoids a stepping of the seismoresistant capacity of the building and also providesa positive, efficient and integral seismic resistantsystem

Montreal’s Expo World’ Fair were 354 individual pre-cast concrete units were inter-

connected with post-tensioning cables, grouped into 158 apartments.

What will Architects Do

Develop Innovative ideas in creating ArchitecturallyImpressivebuildings

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CCTV Headquarters, CHINA

What will Architects Do (Unsymmetrical in Plan and Elevation)

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WHAT WILL EARTHQUAKE ENGINEER DO?

Earthquake engineers are concernedwith creating earthquake resistantdesigns and construction techniquesto build all kinds of bridges, roadsand buildings.

Earthquake engineers are faced withmany uncertainties and must besmart in their decisions in developingsafe solutions to challengingproblems. They rely on state-of-the-art technology, materials science,laboratory testing and fieldmonitoring.

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Indian Standards For Earthquake Design

IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision)

IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces

IS 13935, 1993, Indian Standard Guidelines for Repair and

Seismic Strengthening of Buildings

IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision)

SP: 22 –Explanatory Handbook On Codes For Earthquake Engineering

IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings

IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings

Different Parts of IS:1893-2002

Part Description

Part 1 General Provisions and Buildings

Part 2 Liquid Retaining Tanks– Elevated and Ground Supported

Part 3 Bridges and Retaining Walls

Part 4 Industrial Structures Including Stack Like Structures

Part 5 Dams and Embankments

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BUILDING CONFIGURATION

IS 1893 (Part 1): 2002 has recommended buildingconfiguration system in Section 7 for the betterperformance of buildings during earthquakes.

An important feature in building configuration isits regularity and symmetry in horizontal andvertical plane .

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BUILDING CONFIGURATION

The Irregularities in building configuration system areclassified into two types.

1. Vertical Irregularities- sudden change in strength,stiffness, geometry and mass results in irregulardistribution of forces over the height.

2. Horizontal Irregularities – Refers to asymmetric planshape or discontinuous in the horizontal resistingelements such as openings and re-entrant corner andother changes resulting in torsion.

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Example of Plan Irregularity

Example of Vertical Irregularity

BUILDING CONFIGURATION PROBLEMS

BUILDING CONFIGURATION: PROBLEMS

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CHOICE OF METHOD FOR MULTISTORIED BUILDING

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FOUR VIRTUES OF EARTHQUAKE RESISTANT BUILDINGS

Strength

Stiffness

Ductility

Configuration

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IMPORTANCE

Good Structural Configuration: Its size, shape and structural system carrying loads are such that they ensure a direct and smooth flow of inertia forces to the ground.

Lateral Strength: The maximum lateral (horizontal) force that it can resist is such that the damage induced in it does not result in collapse.

Adequate Stiffness: Its lateral load resisting system is such that the earthquake-induced deformations in it do not damage its contents under low-to moderate shaking.Good Ductility: Its capacity to undergo large deformations under severe earthquake shaking even after yielding, is improved by favourable design and detailing strategies.

VERTICAL IRREGULARITIES

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VERTICAL IRREGULARITIES

Stiffness Irregularity -Soft Story

Stiffness Irregularity - Extreme Soft Story

Weight (Mass) Irregularity

Vertical Geometry Irregularity

In-plane Discontinuity in Vertical

Lateral Force Resisting Elements

Discontinuity in Capacity - Weak Story

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Type 1: Stiffness Irregularities

Soft story

◦ Architectural considerations Taller first story

Unreinforced infill in upper stories

Completely open ground story

◦ Soft Story Stiffness related

Lateral stiffness of adjacent stories

RC beamsRC columns

Infill

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Example

◦ Stilts (open ground storey) Used as parking lots, garages

Apartments & commercial buildings

Prevalent in many countries like India, Iran Turkey, etc.

◦ Intermediate open storey Machinery, recreations, etc.

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Soft Story RC Frames …

Seismic performance

◦ Under lateral loads Excessive story deformation

Shear failure of columns

Excessive deformation

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Type 1: Stiffness Irregularities

1a - Soft Story

◦ the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.

1b - Extreme Soft Story

◦ the lateral stiffness is less than 60% of that in the story above or less than 70% of the average stiffness of the three stories above.

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Soft Storey Collapse

Soft Storey

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Where is ground storey? (BHUJ, 2001)

Ground Storey Collapse of a 4-Storey Building with Open Ground Storey at Bhuj

Olive View Hospital, which nearly Collapsed due to Excessive Deformation in the First Two

Storeys during the 1972 San Fernando Earthquake

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AVOID SOFT STOREY GROUND FLOORS

Often the columns are damaged by the cyclic displacements between the moving soil and the upper part of the building

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AVOID SOFT STOREY UPPER FLOORS

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Soft mid storey failure

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What happened to the upper floors (Turkey 1999))

Did they not know column of building shouldbe stronger than beam?

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Type 2: Weight (Mass) Irregularity

Mass irregularity shall beconsidered to existwhere the effectivemass of any story ismore than TWO timesthe mass of an adjacentstory.

Heavy load at Top

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Short column failure due to insufficient transverse reinforcementKrishna Aparments, Airport Road, Bhuj

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Type 3: Vertical Geometry Irregularity

Vertical geometry irregularity shall be considered to exist when the Maximum horizontal projection exceeds

15 to 20% of the Base width.

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Stress concentration & change in the stiffness

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Crushing of corner columnBuilding – Insufficient reinforcement : Tera Nam Mandir, Main Road, Bhuj

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Type 4: In-Plane Discontinuity in Vertical Lateral Force Resisting Elements

An in-plane offset of the lateral force-resisting elements greater than the length of those elements or a reduction in stiffness in the resisting element in the story below.

Design forces for lateral force connections to be increased

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Type 5: Discontinuity in Capacity - Weak Story

A weak story is one in which the story lateral strength is less than 80% of that in the story above. The story strength is the total strength of all seismic-resisting elements sharing the story shear for the direction under consideration.

Do not confuse STIFFNESS with STRENGTH.

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Weak & soft story

◦Weak story Strength related

Lateral strengths of adjacent stories

◦ Soft Story Stiffness related

Lateral stiffness of adjacent stories

Plan Structural Irregularities

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Building Configuration

The Irregularities in building configuration system areclassified into two types.

1. Vertical Irregularities- sudden change in strength,stiffness, geometry and mass results in irregulardistribution of forces over the height.

2. Horizontal Irregularities – Refers to asymmetric planshape or discontinuous in the horizontal resistingelements such as openings and re-entrant corner andother changes resulting in torsion.

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L shaped Building

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L – Shape in Plan / Elevation ?

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Seismic behaviour of irregular shaped plansdiffers from regular shapes because the first canbe subjected to their asymmetry and/or canpresent local deformations due to the presenceof re-entrant corners or excessive openings. Botheffects give origin to undesired stressconcentrations in some resisting members of thebuilding

Plan Structural Irregularities

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Plan Structural Irregularities

On the contrary, the ideal rectangular or square plane, structurally symmetric, with enough in-plane stiffness in its diaphragm, presents an ideal behaviour. Building shaped like a box, such as rectangular, both in plan and elevation, is inherently stronger than one that is L-shaped or U-shaped, that is a building with wings

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Plan Structural Irregularities

1-Torsional Irregularity

2 - Re-entrant Corners

3 - Diaphragm Discontinuity

4 - Out-of-plane Offsets

5 - Nonparallel Systems

PLAN IRREGULARITIES

Torsion Irregularities with Stiff Diaphragm

If max drift is more than 1.2 times the average

Building with Re-entrant Corners

Non-parallel SystemsDiaphragm Discontinuity

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Type 1: Torsional Irregularities

1a - Torsional Irregularity

◦ larger story drift more than 1.2 times average story drift

1b - Extreme Torsional Irregularity

◦ larger story drift more than 1.4 times average story drift

Design forces for lateral force connections to be increased

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Torsion of unsymmetrical Plans

Direction of Ground Motion

CG. Of Building mass

Applied force

Center of lateral resistant /stiffness center

Resisting force

Proper

Improper

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Torsion of Buildings on Sloping Ground

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Torsion Analysis

The analysis must take into account any torsional effects resulting from the location of the masses relative to the centers of resistance.

In addition to the predicted torsion, accidental torsion must be applied for structures with rigid diaphragms by assuming the center of mass at each level is moved from its actual location a distance equal to 5% the building dimension perpendicular to the direction of motion.

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Type 2: Re-entrant Corners

Both projections beyond the corner are more than 15% of the plan dimension of the structure in the same direction

Un-symmetrical Plans

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Guidelines for Planning

Desirable geometry of buildings

Separation

L< 3B

B

B

< 0.2B

PLANS

A should be smallIf L =100 m , A can be max of 15 m

Definition of Irregularity as per IS 1893

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ROLE OF FLOOR When beams bend in vert

direction during earthquake the thin slabs move along with them (fig) & when beams move with columns in horz direction the slab forces the beams to move together with it

In most buildings the geometric distortion of slab is negligible in the horizontal plane; this behavior is known as rigid diaphragm action .

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Type 3: Diaphragm Discontinuities

Diaphragms with abrupt discontinuities or variations in stiffness, including those having cutout or open areas greater than 50% of the gross enclosed diaphragm area, or changes in effective diaphragm stiffness of more than 50% from one story to the next.

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Type 4: Out-of-Plane Offsets

Discontinuities in a lateral force resistance path, such as out-of-plane offsets of the vertical elements.

Design forces for lateral force connections to be increased

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Type 5: Nonparallel Systems

The vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral force resisting system.

Analyze for forces applied in the direction that causes the most critical load effect

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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

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Regular and Irregular Configurations

To perform well in an earthquake, a building should possess four main attributes:◦ Simple and regular configuration◦ Adequate lateral strength◦ Stiffness◦ Ductility

Buildings having simple regular geometry and uniformly distributed mass and stiffness in plan as well as in elevation, suffer much less damage than buildings with irregular configurations.

Buildings with plan irregularity suffer from torsional modes of vibration whose effects may not be adequately represented in the equivalent static seismic coefficient method.

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