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 Survey Contours Soil Investigation Pile Load Tests Equipment and Services Loading
Analysis and design Loads on Structures Gravity Loads Dead Loads Live Loads Snow Loads Lateral Loads Seismic Loads Wind Loads Impact Loads Crane Gantry Loads Machinery Loads (e.g. TG)
Deflected Behaviour of structure under different load combinations Gravity Loads Deflected Profile (Seismic) (Wind) Deflected Profile (Dead Load + Live Load) (Dead Load + Live Load + Lateral Load)
Gravity Load Typical Values of Dead and Live Load (IS 875: 1987 Part I & II) Reinforced cement concrete Material Unit Weight (kn/m 3 ) 25 kn/m 3 Brick Masonry 18 to 20 kn/m 3 Stone Masonry 22.55 kn/m 3 Occupancy Classification Uniformly Distributed Load kn/m 2 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 5 4.5 machinery Business/ Office Buildings 3 to 4 2.7 4.5 Storage Buildings/ Warehouses 2.4 kn/m 2 per m height of storage 7
Lateral Loads - Wind Load Design Wind Speed in m/s V z = V b k 1 k 2 k 3 V b = Basic wind speed in m/s k 1 = Risk Coefficient depends on probable structure life & basic wind speed k 2 = Terrain, height and structure size factor k 3 = Topography factor Design Wind Pressure in N/M 2 P z = 0.6 V z 2 F = (C pe C pi ) A P z C pe = External pressure coefficient C pi = Internal pressure coefficient A = Surface area of structural element 6
Lateral Loads Seismic Load 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 7
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 speed 3 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 8
Seismic Waves 9
Seismic Waves 10
Magnitude and Intensity 11
Seismic Waves Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake
Earthquake Occurrences Global occurrences of Earthquake Group Great Magnitud e 8 & higher Major 7-7.9 18 Strong 6-6.9 120 Moderate 5-5.9 800 Annual Avg. No. Light 4-4.9 6200 Minor 3-3.9 49000 1 Very Minor < 3.0 8000 13 per day
Earthquake Occurrences Worst Earthquake in History 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 14
Effects of Earthquake Ground Motion Ground displacement Landslides Liquefaction Tsunamis After Shocks 15
2. Mechanism of Earthquake Damage Earthquake causes complex, irregular and time dependent oscillation 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 16
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 23
Tsunami caused by Earthquake 24
Liquefaction caused by Earthquake Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced due to shaking caused by earthquake. 25
3. Factors governing the extent of damage 1. Intensity and Duration of Ground Motion (a) Richter Scale (b) Comprehensive Intensity Scale (MSK 64) 2. Type of Structure 3. Material and Quality of Construction 4. Soil Foundation system 26
Earthquake measuring scales (a) Richter Scale - It is defined as logarithm to the base 10 of the maximum trace amplitude. It measures energy released in an earthquake. (b) Modified Mercelli Scale or MSK Scale A scale used for measuring the intensity of earthquake, based on effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale of 1 through 12, with 1 denoting a weak earthquake and 12 one 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 per MSK Scale II VI or less III VII IV VIII V IX and above
4. How to Combat an Earthquake? Design of Structures Earthquake Resistant Design Make the building lighter Make the structural members tough and ductile to sustain large inelastic deformation Vibration Isolation Use energy absorbing cushions called dampers to absorb seismic energy Do not allow the earthquake to enter the building by replacing rigid connections between ground and building by flexible links 29
5. Principles of Earthquake Resistant Design Able to withstand minor earthquakes (<DBE) without damage Able to withstand moderate earthquakes (DBE) without significant structural damage though some non-structural damage 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) 30 2. Additional reserve strength over and above design strength
Some Important Concepts Indian subcontinent is divided into 4 seismic zones (II to V) in the increasing 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 seismic damage performance of the structure, characterized by ductile or brittle deformation Critical damping The damping beyond which free vibration motion will not be oscillatory Damping The effect of internal friction, imperfect elasticity of material, in reducing the amplitude of vibration and is expressed as percentage of critical damping Ductility - Capacity to undergo large inelastic deformation without significant loss of stiffness 31
Seismic Analysis Design Spectrum Method A h Z 2 I S R g a 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 A h = Design Horizontal Seismic Coefficient 32
Seismic Analysis A h = Design horizontal acceleration spectrum W = Seismic weight of the building Fundamental Natural Period of vibration Ta = 0.075 h 0.75 for RCC frame building = 0.085 h 0.75 for steel frame building Vertical distribution of base shear Vb = A h W Q i V B Wihi2 Wihi2 Q i = Design lateral force at floor i 33
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 Design Practices 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
Structural Configuration
Discontinuity in load carrying members should be avoided
Soft storey configuration should be avoided 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 stable triangular configurations within the steel building frame Braced frames are the most economical method of resisting wind loads in multi-story steel buildings Types of Braced Frames are: XType Nee Type VType KType
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 a structure
Lateral Load Resisting System: (Rigid Frame) Rigid frames, utilizing moment connections, are well suited for specific types of buildings where diagonal bracing is not feasible or does not fit the architectural design Rigid frames generally cost more than the Braced frames
Lateral Load Resisting System: (Combination Frame) A combination of Braced and Rigid Frames
Moment frame Braced frame Braced Frame Moment (Rigid) Frame Combination 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
Ductile Detailing Minimum member dimension 1. Beam Minimum Width = 200 mm 2. Column- Minimum dim. =200 mm. However not less than 300 mm when beam span exceeds 5 m and/or unsupported height of column exceeds 4 m 52
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 64
Lead Rubber isolator Made from rubber layers sandwitched between steels plates Very strong in vertical direction but flexible horizontally
Base Isolator and Dampers Lead Rubber isolator Made from rubber layers sandwitched between steels plates Very strong in vertical direction but flexible horizontally 66
Good Construction Practices Ensure that: Ductile detailing has been followed in construction as per the drawings provided. Proper development length are provided in case ductile detailing are not mandatory. Laps are avoided in the places where negative moments are governing 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 high seismic zone 67
Lap splice in Beam Anchorage in Beam
Incorrect Reinforcement detailing at Beam-Column Joint
Shear Crack at Beam End - Result of insufficient stirrup spacing
Open ends of Hoop Bar Inadequate Anchorage of Hoop Bar in Column
Inadequate Stirrup Spacing & Poor Quality Concrete
Inadequate stirrup spacing Inadequate Development Length
Other Important Considerations Ductile detailing is mandatory for structures in Zone III, IV, V Torsional Eccentricity to be considered Storey Drift Limitation : Shall not exceed 0.004 x Storey Height Soft Storey should be avoided