1. Earthquake Engineering
Dr.G.S.Venkatasubramani
Professor of Civil Engineering
VLB Janakiammal College of Engg. And Tech.,
Coimbatore
Oct 05, 2010

2. Objective of Presentation
Principles of Earthquake Resistant Design (ERD)
Basic Principles of Modern Codes
Design Philosophy
Structural Safety Issues
Features of Indian Seismic Code
ERD Features of High Rise Buildings and Industrial Structures
Principal Issues in ERD

3. Seismic Codes of India
IS: 1893 (Part1-5): 2002 Criteria for Earthquake Resistant Design of Structures, BIS, New Delhi
IS: 13920:1993 Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces, BIS, New Delhi
IS: 4326:1993 Earthquake Resistant Design and Construction of Buildings, BIS, New Delhi

4. Basic Intent of Seismic Code
The structures are able to respond without structural damage to shocks of moderate intensities and without total collapse to shocks of high intensities
Codes are made for normal structures
Site-specific study for special structures
It is not the intent of code to provide regulations that the structure shall suffer no damage during earthquake of all magnitudes

5. Types of Structures Non-Engineered buildings High-Rise Buildings
Industrial Structures
Civil Infrastructures

6. Structural Safety Issues
Design of New Structures
Retrofit of Existing Structures

7. Intervention of Engineer & Architect & Town Planner (Seismic Consideration)
Planning : Site selection, Conceiving the structure, Structural configuration, Type of foundation, Material of construction, Soil conditions
Analysis : Modeling and Assumptions, Seismic force estimation
Design : Detailing for ductility
Construction: Implementation of seismic provisions, quality assurance

8. Basic Principles of Modern Codes (Seismic Design Philosophy)
Serviceability limit state: Structures must resist low-intensity earthquake without structural damage. Elastic behaviour is required during small and frequent earthquakes
Ultimate limit state: Structures should withstand an earthquake of moderate intensity (design earthquake having PGA with 90% probability of not being exceeded in 50 years) with repairable damage in structural as well as nonstructural elements Collapse limit state: Structures should withstand high intensity earthquake with a return period much longer than design life without collapse

9. Design Philosophy IS: 1893 (Part1):2002
The structure should possess at least a minimum strength to withstand minor earthquakes without damage
The structures should resist moderate earthquake without significant structural damage, some non- structural damage may occur
The structures withstand a major earthquake without collapse

10. Design Philosophy for Special Structures
Low structural redundancy in structures like, Chimneys, Cooling towers, Water towers, Cantilever bridge piers, Core suspended structures
Enormous risk that a possible failure can cause to nuclear power plants, dams

11. Advances in ERD Seismic Hazard Assessment: DSHA, PSHA
PGA, Earthquake Response Spectrum, Response Reduction factors, Importance factors
Soil Structure Interaction
Experience of Performance in Earthquakes
Mathematical Modeling: 2D, 3D, FEM
Shake Table Testing, Quasi-static Testing, Ambient Vibration Testing, Forced Vibration Testing

12. Advances in ERD Continued…
Structural Analysis: Static linear, Static non- linear (Push over)
Dynamic Analysis: RSM, Time history, Complex response method
Material Behavior: Nonlinear models
Design Methods: Working stress, Ultimate load design, Limit-state design, Capacity design, Performance based design

13. Establishing Reference Hazard
Understanding seismic hazard is fundamental to seismic design
Most structures are affected by earthquake ground shaking
Other hazards: Surface fault rupture, liquefaction, differential settlement, lateral spreading, tsunami
Site hazard procedures
-Generalized procedure
-Site specific procedure

14. Seismic Zoning Map

15. Generalized Procedure
Codal procedure: Seismic Zoning
Assign a seismic zone factor to each zone
The zone factor is used to define total seismic force or base shear
Zone factors provide only a general representation of expected seismic intensity at a site. Specific site consideration such as proximity to active faults and type of soils are not included
Hazard defined by code is regarded as minimum design requirement to ensure life safety
For performing dynamic analysis, response spectrum is provided for three types of soils

16. Site Specific Procedure
Used for Industrial structures and important Civil Infrastructures
Site specific procedures are used where the need for more accurate definition of hazard can be justified
Improved accuracy is obtained due to consideration of regional and local geology, seismology and soil characteristics

17. Site Specific Procedure
Continued…
Expected ground shaking is presented in the form of site specific response spectrum or time histories
This requires a specialized knowledge in engineering seismology, geotechnical engineering and structural dynamics

18. Broad Features of Indian Seismic Code
Seismic Zoning
General Principles
Design Philosophy
Design Earthquake
Response Spectrum for 3 Types of Soils
Response Reduction Factors
Minimum Design Force on Empirical Fundamental Period
Structural Irregularities in Plan and Elevation
Load Combination for Limit State Design
Dynamic Analysis: RSM, Time History Method

19. Phases in Earthquake Resistant Design
Conceptual Design
Modeling and Analysis
Detailed Design and Proportioning
Detailing and Drafting for Seismic Safety

20. Uncertainties in Seismic Design
Earthquake force is most uncertain compared to other actions
Variability of action is much larger than that of resistance

21. Broad Features of Indian Seismic Code Continued….
Dynamic/static analysis of structure in a elastic range for seismic actions reduced to 1/2 to 1/5 of their elastic value depending upon ductility level in the structure
Lower seismic loading is combined with the requirement of ductile behaviour with the appropriate design of structural system: Strong-column weak beam, detailing of structural elements so that energy of earthquake is dissipated through large plastic deformation without collapse
Effect of local soil condition: By defining design spectrum (microzonation), modification in time period due to flexibility of soil, damping coefficient representing hysteretic damping of soil

22. Structural Systems Frame system Shear wall system Dual system
Braced frame system
Core system
Inverted pendulum system

23. Seismic Design Principles
Simple, symmetrical and regular configuration
Adequate lateral force resisting system
Adequate lateral strength
Stiffness
Ductility
Sound foundation, no liquefaction
Adequate separation between buildings
Non structural element to have suitable detailing
Floor diaphragms to distribute forces to vertical elements

24. Classical Design Process
Preliminary design
Selection of configuration, structural and foundation types, initial dimensions
Determination of potential loads
Environmental, man induced
Mathematical modeling and structural analysis
Detailed structural design
Detailing and specifications
Construction planning and scheduling

25. Ductility
It is the capacity of the structure to deform in-elastically without significant loss of strength

26. Ductility Consideration
Plain concrete is brittle
Steel is ductile
R.C. is not ductile
Ductile detailing introduces ductile behavior

27. Advantages of Ductility
Structure has capacity to withstand unexpected overloading, load reversals, impact, foundation settlement
Enables economical design using low seismic coefficient
Sudden failure does not occur
Enhances energy dissipation capacity

28. Design for Ductility Control of Minimum and Maximum Reinforcement
Bar cut offs
Splicing
Stirrups spacing, hoop/spiral spacing
Joint Detailing
Simple design detail can ensure ductility
Detailing considerations are specified for: Flexural Members, Columns, Beam-column joints

29. Factors Influencing Ductility
Increases with decrease in tension steel-As
Increases with increase in compression steel-Ac
Increases with increase in fcu and decrease in fy
Confinement of concrete by lateral reinforcement increases ductility

30. Principles of Ductile Design
Design lateral force is equal to elastic force divided by Response reduction factor (R)
Prevent failure due to shear and bond
Confinement of concrete by lateral reinforcement
Detailing of potential plastic hinge regions
Strong-column and weak-beam design
Detailing of members and joints
Under-reinforced design of beams
Addition of compression reinforcement in beams
Detailing of ductility: bar cut off, splices, anchorage, hooks, joint detailing, transverse reinforcement

31. Ductility consideration for flexural members
Avoid shear failure: Strength in shear should exceed the strength in flexure
Under reinforced design favored for ductile failure, compression steel further enhances ductility
Avoid bond and anchorage failure
Use closed stirrups
Splicing desired at section of minimum tension

32. Flexural Members Factored axial stress should not exceed 0.1fck
Width to depth ratio to be more than 0.3
Width of member to be not less than 200mm
Depth of member be not more than ¼ of the clear span
Longitudinal reinforcement
Top and bottom steel to consist of minimum two bars
Minimum tension steel on any face : pmin=0.24 fck/fy, fck and fy are in MPa
Maximum steel ratio on any face, pmax= 0.025
Anchorage of Beam bars in External joint
Lap splice in Beam: Not more than half the reinforcement shall be spliced at a section where yielding may take place

33. Anchorage of Beam Bars in an External Joint

34. Lap, Splice in Beam

35. Web reinforcement
The shear force to be resisted by the vertical hoops shall be maximum of (i) Calculated factored shear force, (ii) shear force due to formation of plastic hinges at both ends of beam plus factored gravity load on the span
The spacing of hoops over a length of 2d at either end of a beam shall not exceed (a) d/4, and (b) 8 times the diameter of smallest longitudinal bar; however it shall not be less than 100mm

36. Beam Web Reinforcement

37. Design Shear Force for Beam

38. Beam Reinforcement

39. Ductility consideration for columns Restriction on dimensions
Minimum dimension=200mm. However in frames which have beams with c/c span exceeding 5m or columns of unsupported length exceeding 4m, the shortest dimension of the column shall not be less than 300mm
The ratio of shortest cross sectional dimension to the perpendicular dimension shall not be less than 0.4

40. Special Reinforcement in Columns
Spiral reinforcement
Rectangular closed stirrups

41. Transverse Reinforcement in Columns

42. Ductility of columns
Spiral RC columns have greater ductility than tied RC columns
Avoid long columns
Design basis: Strong column weak beam
Confinement by special shear reinforcement
Design shear force for columns shall be maximum of (i) calculated factored shear force, (ii) a factored shear force based on plastic moments at column ends
Splicing desired near mid height of column. Hoops should be provided over entire splice length at a spacing not more than 150mm

43. Beam column joints Adequate lateral reinforcement for confinement
Shear reinforcement to resist shear force
Adequate anchorage for flexural reinforcement
Transverse steel in joint should not be less than special transverse reinforcement at column ends
Reduction in special confining reinforcement due to framing beams 50%

44. Provision of Special Confinement Reinforcement near Joints and Footing

45. Seismic Consideration for Foundations
The construction site should be free from risks of soil rupture, slope instability and permanent settlement caused by liquefaction or densification
The footing should be interconnected either by a mat foundation or by a grid of foundation beams, or at least with connecting beams
All footings should rest on the same horizontal level
Only one foundation type should in general be used for the same structure

46. Plan Irregularities

47. Plan Irregularities Continued…

48. Vertical Irregularities

49. Vertical Irregularities Continued…

50. Unfavorable and Favorable Consideration in Elevation

51. Earthquake Resistance Features of High Rise Buildings
Adequate lateral load resisting system in both the directions. Well suited arrangement of structural elements for high rise buildings
Simple regular geometry and uniformly distributed mass and stiffness in plan as well as in elevation
Avoid stiffness and mass irregularities in elevation, like soft storey, floating column, swimming pool on roof
Two adjacent buildings or two adjacent units of the same building should be separated according to code provisions to avoid pounding

52. Earthquake Resistance Features of High Rise Buildings
Continued…
Infill walls should be appropriately considered in building design
RC members should be designed to avoid premature failure due to shear and bond
Ductile detailing provisions of IS: should be followed for beams, columns, joints, plastic hinge region and shear walls
Strong-column weak beam design
Adequate treatment of nonstructural elements, floor equipments, appendages
Include shear walls to control drift, torsion
Build capacity of structure to deform in ductile manner

53. Key Factors for ERD of High Rise Buildings
Structural configuration
State-of-the-art analysis methods, design and detailing for ductility
Avoid abrupt or drastic changes in stiffness and strength: soft storey, floating column
Avoid irregular and asymmetric arrangements of structural elements in plan
Avoid mass irregularity

54. Inelastic action concentration in (a) beams and base of columns; (b) columns of soft storey; (c) beams and wall base in dual structures

55. Earthquake Resistant Features for Industrial Structures
Design philosophy: Higher degree of performance, Improved seismic protection through stringent design criteria, that is, immediate occupancy and operational level
Site specific response spectrum: Account for local geology and local soil characteristics
Mathematical modeling: 2D/3D modeling of structure, non-structural elements, foundation
Soil foundation interaction: Soil flexibility, damping
Choice of damping : Equivalent modal damping when structure is composed of different materials

56. Earthquake Resistant Features for Industrial Structures
Continued…
Dynamic Analysis: RSM, time history analysis
Proper choice of response reduction factors : variable reduction factors for different portions
Floor response spectrum : Equipment supported on floors to be designed for FRS
Components of ground motion: Horizontal and vertical ground motion
Multi-support excitation: Various supports of piping may be subjected to different ground motions

57. Principal Issues in Earthquake Resistant Design
Estimation of seismic forces
Adequate lateral load resisting system
Soil structure interaction
Influence of foundation type
Buildings with equipment and other systems
Quality of construction, quality assurance
Nonstructural elements
Structural irregularities
Addition to existing structures
Retrofitting of structures
Assessment of seismic capacity of existing buildings

58. Conclusions
Earthquake vulnerability can be greatly reduced when seismic code provisions are rigorously followed
Codal safety is based on design for low seismic coefficient combined with the requirement of ductile behaviour
Structural configuration is a key factor for the safety of seismically controlled structures
Structural irregularities should be avoided for seismic design

59. Conclusions Continued…
Special and industrial structures should be designed for achieving higher degree of performance on the basis of site-specific hazard assessment
Codal procedures should be continuously updated based on the results of research and performance of code designed structures in earthquakes

60. REFERENCES
Prof. S. K. Thakkar’s lectures notes (Former Professor of Earthquake Engineering, Indian Institute of Technology Roorkee)
IS: 1893 (Part1-5): 2002 Criteria for Earthquake Resistant Design of Structures, BIS, New Delhi
IS: 13920:1993 Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces, BIS, New Delhi
IS: 4326:1993 Earthquake Resistant Design and Construction of Buildings, BIS, New Delhi

61. THANK YOU