Design Requirements of Buildings and Good Construction Practices in

Seismic Zone

CII Safety Symposium & Exposition 2015: 11th September 2015: Kolkata

Stages of Structural Design

Concept

Finalisation of Architectural Drawings

Preparation of DBR

Structural Modeling as per Architectural Drawings

Finalise Member sizes

Provision for Services

Structural Analysis

Structural Design

Issue of GFC Drawings

Topographical SurveyContours

Soil InvestigationPile Load Tests

Equipment and Services Loading

Analysis and design

Loads on Structures

Gravity Loads Lateral Loads

Dead Loads Live Loads Snow Loads

Seismic LoadsWind Loads

Impact Loads Crane Gantry LoadsMachinery Loads (e.g. TG)

Deflected Behaviour of structure under different load combinations

(Wind)

Deflected ProfileDeflected ProfileGravity Loads

(Dead Load + Live Load) (Dead Load + Live Load + Lateral Load)

(Seismic)

Gravity Load

Typical Values of Dead and Live Load – (IS 875: 1987 Part I & II)

Material Unit Weight (kN/m3)

Reinforced cement concrete

25 kN/m3

Brick Masonry 18 to 20 kN/m3

Stone Masonry 22.55 kN/m3

Occupancy Classification Uniformly Distributed Load kN/m2

Concentrated Load (kN)

Residential buildings 2 to 3 1.8 to 4.5

Hotel buildings

• Dining rooms, cafeterias 4 2.7

Industrial Buildings

• Work areas w/o machinery 2.5 4.5

• Work areas with light duty machinery

5 4.5

Business/ Office Buildings 3 to 4 2.7 –4.5

Storage Buildings/ Warehouses 2.4 kN/m2 per m height of storage

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Lateral Loads - Wind Load

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Design Wind Speed in m/s

Vz = Vbk1k2k3

Vb = Basic wind speed in m/sk1 = Risk Coefficient depends on probable structure life & basic wind speedk2 = Terrain, height and structure size factor k3 = Topography factor

Design Wind Pressure in N/M2

Pz = 0.6 Vz2

F = (Cpe – Cpi) A Pz

Cpe = External pressure coefficient Cpi = Internal pressure coefficientA = Surface area of structural element

Lateral Loads – Seismic Load

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1. What is an earthquake?

2. Mechanism of Earthquake Damage

3. Factors governing the extent of damage

4. How to combat an Earthquake?

5. Principles of Earthquake Resistant Design

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1. What is an Earthquake?

An earthquake is a tremor of the earth's surface usually triggered by the release of underground stress along fault lines

This release causes extensive movement in underground mass and the shock progressively expands away in all directions, at high speed3 types of waves are generated (1) P Waves (2) S Waves (3) Surface Wave

•P waves travel fast and is less powerful

• S waves follow the P waves and is powerful

• Surface waves travel along the Ground surface which causes major damages

Seismic Waves

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Seismic Waves

Magnitude and Intensity

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Seismic Waves

Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake

Earthquake Occurrences

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Global occurrences of Earthquake

Group Magnitude

Annual Avg. No.

Great 8 & higher

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Major 7-7.9 18

Strong 6-6.9 120

Moderate 5-5.9 800

Light 4-4.9 6200

Minor 3-3.9 49000

Very Minor

< 3.0 8000 per day

Earthquake Occurrences

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Date Location Magnitude Death Toll

May 22, 1960 Valdivia, Chile 9.5 6,000

May 27, 1964 Alaska, USA 9.3 150

December 26, 2004 Sumatra, Indonesia 9.1 2,00,000

March 11, 2011 Tohoku, Japan 9.0 15,000

January 23, 1556 Shaanxi, China 8.0 8,30,000

October 11, 1138 Aleppo, Syria 8.5 2,30,000

January 12, 2010 Haiti 7.0 3,16,000

April 25, 2015 Nepal 7.8 9,000

Worst Earthquake in History

Effects of Earthquake

Ground Motion

Ground displacement

Landslides

Liquefaction

Tsunamis

After Shocks

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2. Mechanism of Earthquake Damage

Earthquake causes complex, irregular and time dependentoscillation of ground ( predominantly horizontal)

A building attracts earthquake forces because it has mass

During earthquake, structures intensely vibrate to and fro

Large inelastic deformations, over-stressing and fatigue of structural members take place

Complete /partial –structural & non structural damage takes place

Diagonal Cracks in infill walls

Heavy non-structural and significant structural damage

Shear failure of Bridge Deck

Collapse of Load Bearing wall

Overturning of Bridge Deck

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Disaster caused by Earthquake

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Tsunami caused by Earthquake

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Liquefaction caused by Earthquake

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Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced due to shaking caused by earthquake.

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3. Factors governing the extent of damage

2. Type of Structure

3. Material and Quality of Construction

1. Intensity and Duration of Ground Motion

(a) Richter Scale

(b) Comprehensive Intensity Scale (MSK 64)

4. Soil Foundation system

Earthquake measuring scales

(a) Richter Scale - It is defined as logarithm to the base 10 of themaximum trace amplitude. It measures energy released in anearthquake.

(b) Modified Mercelli Scale or MSK Scale – A scale used for measuringthe intensity of earthquake, based on effects of an earthquake on theEarth's surface, humans, objects of nature, and man-made structureson a scale of 1 through 12, with 1 denoting a weak earthquake and 12one that causes complete destruction. It measures strength of shaking.

Richter Scale MSK Scale EQ Zone

0.0-4.3 I – III

> 4.3 –4.8 IV – VI II

> 4.8-6.2 VII III

> 6.2 –7.3 VIII IV

> 7.3 –8.9 IX - XII V

ITC Establishments

Zone Intensity as perMSK Scale

II VI or less

III VII

IV VIII

V IX and above

4. How to Combat an Earthquake?

Design of Structures

Earthquake ResistantDesign

Make the building lighterMake the structural members tough and ductile to sustain large inelastic deformation

Vibration Isolation

Use energy absorbing cushions called dampers to absorb seismic energyDo not allow the earthquake to enter the building by replacing rigid connections between ground and building by flexible links

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5. Principles of Earthquake Resistant Design

Able to withstand minor earthquakes (<DBE) withoutdamage

Able to withstand moderate earthquakes (DBE) withoutsignificant structural damage though some non-structuraldamage may occur

Able to withstand major earthquake (MCE) without collapse

Actual seismic force may be much greater than design seismic force. However, structures are able to withstand the additional force due to

1. Higher Ductility (Ductile detailing)

2. Additional reserve strength over and above design strength

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Some Important Concepts

Indian subcontinent is divided into 4 seismic zones (II to V) in theincreasing order of severity and extent of damage

Importance Factor - A factor depending upon functional use &hazardous consequences of failure

Response Reduction Factor - A factor depending on perceived seismicdamage performance of the structure, characterized by ductile orbrittle deformation

Critical damping – The damping beyond which free vibration motionwill not be oscillatory

Damping – The effect of internal friction, imperfect elasticity ofmaterial, in reducing the amplitude of vibration and is expressed aspercentage of critical damping

Ductility - Capacity to undergo large inelastic deformation withoutsignificant loss of stiffness

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Seismic Analysis – Design Spectrum Method

Z = Zone factor (0.10 for II, 0.16 for III, 0.24 for IV, 0.36 for V)

I = Importance factor

R = Response Reduction Factor It is the factor by which the actual base shear force, that would be generated if the structure were to remain elastic during its response to the Design Basis Earthquake (DBE ) shaking, shall be reduced to obtain the design lateral force.

Sa/g = Average response acceleration Coefficient depends on natural period of vibration and damping

Ah = Design Horizontal Seismic Coefficient

gRSIZA a

h 2

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Seismic Analysis

Ah = Design horizontal acceleration spectrum

W = Seismic weight of the building

Fundamental Natural Period of vibration

Ta = 0.075 h0.75 for RCC frame building

= 0.085 h0.75 for steel frame building

Vertical distribution of base shear

Qi = Design lateral force at floor i

Wihi2Wihi2

Bi VQVb = AhW

Good Design and Construction Practices

Commonly Found Design Issues

1. Incorrect Loading

2. Modeling Errors

3. Under / Over Design of Structure

4. Incorrect Reinforcement Detailing

5. Absence of Ductile Details

6. Soft Storey

7. Story Drift

8. Errors and Omissions

9. Inadequate Concrete Cover

Good Structural Configuration

Size shape and structural system for ensuring direct transfer of forces to ground

Lateral Strength

To resist maximum lateral force so that the damage induced does not result in collapse

Adequate Stiffness

Lateral load resisting system to ensure that earthquake induced deformations do not damage under low to moderate shaking

Good Ductility

Capacity to undergo large deformations under severe earthquake is improved by design & detailing strategies

Good Construction Practices

Good Design Practices

Structural Configuration

Discontinuity in load carrying members should be avoided

• Soft storey configuration should beavoided

If unavoidable, Columns and beams shall be designed for 2.5 times the storey shear and moments under seismic load.d for 1.5 times the storey shear

Ground Floor being soft storey floor completely

destroyed

Vertical Geometric Irregularity should be avoided

Failure due to Vertical

Irregularity

Lateral Load Resisting System: (Braced Frame)

• Diagonal bracings create stabletriangular configurations within thesteel building frame

• Braced frames are the mosteconomical method of resisting windloads in multi-story steel buildings

• Types of Braced Frames are:

X Type

Nee Type

V Type

K Type

Lateral Load Resisting System: (Braced Frame) (X Type)

Diagonal members of X-Type Bracing go into tension and compression, similar to those of a truss

Nee-Type V-Type K-Type

Lateral Load Resisting System: (Braced Frame) (Nee, V & K Type)

Members are designed for both tension and compression forces

Nee-bracing allows for doorways or corridors through the bracing lines in astructure

Lateral Load Resisting System: (Rigid Frame)

Rigid frames, utilizing moment connections,are well suited for specific types of buildingswhere diagonal bracing is not feasible or doesnot fit the architectural design

Rigid frames generallycost more than the Bracedframes

Lateral Load Resisting System: (Combination Frame)

A combination of Braced and Rigid Frames

Moment (Rigid) Frame Combination Frame

Braced frame

Moment frame

Braced Frame

Lateral Load Resisting System: (A Comparison)

A Braced Frame deflects like a cantilever beam

A Moment (Rigid) Frame deflects more or less consistently from top to bottom

In a Combination Frame, reduced deflections are realized

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• Minimum member dimension

1. Beam – Minimum Width = 200 mm

2. Column- Minimum dim. =200 mm . However not lessthan 300 mm when beam span exceeds 5 mand/or unsupported height of column exceeds 4 m

Ductile Detailing

Twist during Earthquake

How to make Building Ductile

Shear Failure due to Short Column Effect

Base Isolation

Structure is rested on flexible pads

Induces flexibility to the structures

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Lead Rubber isolator

Made from rubber layers sandwitched between steels plates

Very strong in vertical direction but flexible horizontally

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Base Isolator and Dampers

• Lead Rubber isolator• Made from rubber layers

sandwitched between steels plates

• Very strong in vertical direction but flexible horizontally

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Ensure that:

Ductile detailing has been followed in construction as per thedrawings provided.

Proper development length are provided in case ductile detailingare not mandatory.

Laps are avoided in the places where negative moments aregoverning

Good quality of concrete

Construction joints are rough

Preferably vertical construction joints are provided

Expansion joints are more than the storey drift

Proper reinforcement have been provided in Brickwork in highseismic zone

Good Construction Practices

Anchorage in Beam Lap splice in Beam

Incorrect Reinforcement detailing at Beam-Column Joint

Shear Crack at Beam End -Result of insufficient stirrup spacing

Inadequate Anchorage of Hoop Bar in Column

Open ends of Hoop Bar

Inadequate Stirrup Spacing & Poor Quality Concrete

Inadequate stirrup spacing Inadequate Development Length

• Ductile detailing is mandatory for structures in Zone III, IV, V

Other Important Considerations

• Torsional Eccentricity to be considered

• Storey Drift Limitation : Shall not exceed 0.004 x Storey Height

• Soft Storey should be avoided