Basic concepts of Earthquake- Resistant Design and Construction

Transcription

1 Post-earthquake School Reconstruction Project Seminar on Earthquake Resilient Construction for School Buildings Day-1 Session 1 Basic concepts of Earthquake- Resistant Design and Construction Naveed Anwar, PhD

2 Earthquakes cause disasters! Why do they cause disasters? Understanding the Risk Can such disasters be minimized? How can we reduce the consequences of such disasters Understanding the Response and Performance How structures, specially schools can be made safer 2

3 Why do Earthquake Cause Disasters?

4 4

5 What is a disaster

6 Environmental Sustainability Climate Change Natural Phenomena Population Growth Urbanization and Unplanned development Lack of Resources for Communities Low Quality of Built Environment Disaster Hazard Exposure Vulnerability Increased Consequences Lack of post-event management and recovery, and re-bound capacity 6

7 Seismic Risk Seismic Risk = Seismic Hazard x Seismic Vulnerability 7

8 What is Seismic Hazard

9 For Hazard Earthquake Source: Murty (2004) 9

10 Arrival of Seismic Waves at a Site Source: Murty (2004) 10

11 Reducing illumination with distance from an electric bulb Effected by Medium in between Clear, Fog Reflection, Abortion Source: Murty (2004) 11

12 Seismic hazard Maps Seismic hazard map of Asia, from the Global Seismic Hazard Assessment Program (GSHAP) 12

13 What is exposure

14 Exposure to Seismic Hazard The people, property, assets, infrastructure that could be effected by the damage caused by earthquake No exposure > No disaster Earthquake in a desert causes no disaster Same earthquake in a crowded city is disastrous Unoccupied building has no exposure to life, but exposure to assets still present Nepal Earthquake of 2015 less disastorus due to reduced exposure Schools closed Day time, on a holiday 14

15 What is vulnerability and its causes

16 Seismic Vulnerability The weaknesses in the location and structure that will be exploited by the earthquakes Site Vulnerability Soil type and profile that may amplify the hazard Structural Vulnerability The factors that will increase the seismic demands and or reduce capacity 16

17 Causes of Vulnerability Lack of awareness Inappropriate site location Poor soil condition Poor design and construction practice Inappropriate use of building materials Innovate l Integrate l Collaborate 17

18 Causes of Vulnerability Type of building construction (Brick/stone masonry, mud mortar, RCC frame, timber frame etc.) Non-engineered construction Low quality of construction & building materials Negligence of existing building design codes Untrained masons Innovate l Integrate l Collaborate 18

19 The Special Case of Nepal

20 Nepal lies in an active seismic belt. 20

21 The Historical Formation of the Earth 21

22 Location of Nepal Nepal sits astride the boundary between the Indian and the Tibetan plates along which a relative shear strain of about 2 cm per year has been estimated. 22

23 Location of Nepal Innovate l Integrate l Collaborate 23

24 Himalayan Range Existence of the Himalayan Range with the world s highest peaks is evidence of the continued tectonic activities beneath the country. As a result, Nepal is very active seismically. 24

25 Continuous Movement Indian subcontinent pushes against Eurasian, pressure is released in the form of earthquakes. The constant crashing of the two plates forms the Himalayan mountain range. 25

26 History of Earthquakes Nepal has a long history of destructive earthquakes. The earliest recorded earthquake event Significant earthquakes in 1833, 1934, 1960,1988 and

27 7 June August June 1505 January 1681 July August July Aug Jan Jun Jul Aug Sep Apr May-15 7 June August June 1505 January 1681 July August July Aug Jan Jun Jul Aug Sep Apr May ,091 2,200 2,500 3,500 4,500 4, , , ,519 8,922 History of Earthquake in Nepal Magnitude Fatalities 10 10, , , , , , , , , ,

28 History of Earthquake in Nepal History of Earthquake in Nepal 28

29 Nepal: 2015 Earthquakes On April 25, 2015 at NST 7.8 on Richter Scale Shallow: only 11 km below the ground >40 sec trembling Moved Kathmandu about 1.5m At least 240 aftershocks May 12, 2015: 7.3 on Richter Scale Innovate l Integrate l Collaborate 29

30 Nepal: 2015 Earthquakes AFTERMATH 30

31 Impacts of Nepal Earthquake Kathmandu Durbar Square Patan Durbar Square 31

32 Impacts of Nepal Earthquake Bhaktapur Durbar Square Dharahara Swoyambhu Boudha 32

33 Impacts of Nepal Earthquake Barpak Village Bhaktapur Kathmandu Gorkha Sindhupalchowk Dhading 33

34 Impacts of Nepal Earthquake Nuwakot Kavrepalanchowk Rasuwa Dolkha Makwanpur Sindhuli 34

35 Nepal is exposed to High Seismic risk Risk = Vulnerability X Hazard High Risk High Vulnerability High Hazard Consequences - Disaster (Death-Dollars-Downtime) 35

36 How to Reduce Damage due to Earthquakes And save human lives and property

37 What needs to be Done Minimize Disaster Consequences Difficult to reduce Exposure Reduce Risk to Disaster (and manage consequences due to disaster) Define Acceptable Risk Difficult to reduce Hazard Determine the Hazard Reduce Vulnerability to match Acceptable Risk (R = V x H) 37

38 Hazard Hazard-Vulnerability-Risk-Consequences Resta urant Resta urant School Loading Severity Consequences Resta urant Vulnerability Structural Displacement 38

39 School Restaurant School Restaurant Restaurant Operational (O) Immediate Occupancy (IO) Life Safety (LS) Collapse Prevention (CP) 0 % Damage or Loss 99 % Ref: FEMA 451 B Lowest Lowest Lowest Casualties Downtime for Rehab Rehab Cost to Restore after event Highest Highest Highest 39

40 Basic Concepts of Earthquake-Resistant Construction Basic of Seismic Design on the application of construction techniques, methods and criteria used for the design and construction of building structures exposed to earthquakes. A. Proper Site Selection B. Appropriate Planning C. Good foundation resting on a Firm Base D. Building has to act as a single unit for a good earthquake resistance E. Better bonding within masonry F. Controlled size and location of openings G. Light construction Innovate l Integrate l Collaborate 40

41 Considerations for Site Selection Proper Site Selection: Very important for stable & disaster safe construction i. Steep & Unstable Slopes ii. Areas susceptible to landslides & rock fall iii. Filled area iv. River banks v. Water logged area vi. Geological fault & Ruptured areas vii. Trees 41

42 Earthquake effects are Different Earthquake is different from all other loads It is not an applied external force Earthquake effects are generated by the structure itself in response to ground shaking Basically depends on stiffness and mass distribution Can be controlled by damping, ductility and energy dissipation mechanisms 42

43 43 Most loads Concept of 100% g (1g) Earthquake Mu Cu Ku F NL F 43

44 Earthquake Inertial Forces Effect of Inertia in a building when shaken at its base Flow of seismic inertia forces through all structural components Inertia force and relative motion within a building Source: Murty, (2004) 44

45 Building Behavior during Earthquakes 45

46 Appropriate Planning Shape, size and proportion of the building Sudden deviation in load transfer path along the height lead to poor performance of buildings. (a) Setbacks (c) Sloppy Ground (d) Hanging or Floating Columns (b) Weak or Flexible Story (e) Discontinuing Structural Members 46

47 Appropriate Planning Regular Configuration: Seismically ideal. Low heights to base ratio Symmetrical plane Uniform section & elevation Balanced resistance These configurations would have maximum torsional resistance due to location of shear walls and bracings. Uniform floor heights, short spans and direct load path play a significant role in seismic resistance of the building. 47

48 Appropriate Planning Irregular Configuration: Buildings with irregular configuration Buildings with abrupt changes in lateral stiffness Buildings with abrupt changes in lateral resistance 48

49 Appropriate Planning Fig. Buildings with one of their overall dimensions much larger or much smaller than the other two, do not perform well during earthquake. Fig. Simple plan shape buildings do well during earthquake 49

50 Appropriate Planning Adjacency of building: 2 buildings too close to each other, may pound on each other during strong shaking. With increase in building height, this collision can be a greater problem. When the two building heights do not match, the roof of the shorter building may pound at the mid-height of the column of the taller one. This is very dangerous. Fig. Pounding can occur between adjoining buildings due to horizontal vibrations of the two buildings. 50

51 Appropriate Planning Architectural features that are detrimental to earthquake response of buildings should be avoided or minimized. When irregular building features are included, a considerably higher level of engineering effort is required in the structural design. Even after doing so the building may not be as good as one with simple architectural features. Decisions made at the planning stage on building configuration are more important. 51

52 Earthquake-Resistant Construction The building has to act as a single unit for a good earthquake resistance: Can be achieved by incorporating; Vertical Reinforcement Horizontal bands well connected to the vertical reinforcements and embedded in masonry Diagonal bracing (horizontal and vertical) Lateral restraints 52

53 Proper Load Path The structural frame must have enough strength to securely bear the gravity loads throughout the entire life span of the building.. Fig. Load path from structure slab to the ground 53

54 Earthquake-Resistant Construction An adequate load bearing system is based on a continuous load path throughout the structure: Slabs carry the floor loads of each story. Beams carry the loads transferred to them by the slabs as well as the weight of the walls seated on them. Columns carry the beam loads and they transmit them to the foundation. Footings (foundation) carry the column loads and transfer them to the ground. 54

55 Earthquake-Resistant Construction Structural frame must be able to withstand not only the gravity loads but also the loads imposed in a few but vital cases during its life span such as during an earthquake. Fig. Frame deformation due to seismic action 55

56 Earthquake-Resistant Construction A seismic band is the most critical earthquake-resistant provision usually in a masonry building. Usually provided at lintel, floor, and/or roof level in a building, the band acts like a ring or belt. Seismic bands hold the walls together and ensure integral box action of an entire building. A seismic band acts like a belt (adapted from: GOM 1994) Pulling and bending of a lintel band in a stone masonry building (adapted from: Murty 2005) 56

57 Earthquake-Resistant Construction Seismic bands are constructed using either reinforced concrete (RC) or timber. Proper placement and continuity of bands and proper use of materials and workmanship are essential for their effectiveness. Locations of seismic bands in a stone masonry building (roof omitted for clarity) (adapted from: UNCRD 2003) 57

58 Earthquake-Resistant Construction Seismic bands should always be continuous; an offset in elevation is not acceptable (adapted from: GOM 1998) Merging RC floor and lintel bands RC seismic bands should always remain level without any dips or changes in height (adapted from: GOM 1998) Combining floor/roof and lintel band: a) timber band, and b) RC band 58

59 Importance of Foundations

60 Good foundation resting on a Firm Base foundation : Quality of foundation and the base on which the foundation rests Fig. Structural Foundation 60

61 Good foundation Vertical Reinforcement Tie Beam Shear Reinforcement Horizontal Reinforcement Stone Masonry Reinforcement Brick Soling PCC Fig. Strip Footing on Brick and Stone Masonry Innovate l Integrate l Collaborate 61

62 Good foundation resting on a Firm Base : Quality of foundation and the base on which the foundation rests Fig. Foundation consisting of flexible & rigid spread footings Innovate l Integrate l Collaborate 62

63 Good foundation Fig. Foundation consisting of flexible spread footings and connecting beams 63

64 Good foundation Innovate Fig. l Integrate Strip Foundation l Collaborate with connecting beams 64

65 Good foundation Fig. Foundation Innovate l consisting Integrate of l Collaborate spread footings eccentrically constructed 65

66 Good foundation Fig. Raft Foundation consisting Innovate connecting l Integrate beams l Collaborate Fig. Two level foundation 66

67 Proper Foundation Different foundation depths are required for building sites with variable soil properties (source: GOM 1998) 67

68 Special Considerations for Massonry

69 Better bonding within masonry Better bonding within masonry: Type & quality of bond within the walling units Based on the type of individual units used for masonry walls and their functions, types of masonry walls: Load Bearing Masonry Walls Reinforced Masonry Walls Hollow Masonry Walls Composite Masonry Walls Post-tensioned Masonry Walls 69

70 Better bonding within masonry Fig. Load Bearing Masonry Walls Fig. Reinforced Masonry Wall 70

71 Better bonding within masonry Fig. Hollow Masonry Wall Fig. Composite Masonry Wall 71

72 Better bonding within masonry Fig. Post-tensioned Masonry Wall 72

73 Better bonding within masonry Fig. Good bonding with stone masonry 73

74 Better bonding within masonry Proper placement of through-stones in stone masonry walls (adapted from: GSDMA 2001) 74

75 Better bonding within masonry Wooden battens can be alternatively used instead of long stones at wall (adapted from: Bothara et al. 2002) Through-stones in stone masonry walls: a) through stones act like interlaced fingers; b) a wall with through-stones, and c) a wall without through-stones (source: GOM 1998) 75

76 Better bonding within masonry Construction of stitches made from wire mesh embedded in mortar at the wall intersection Stitches made from wood dowels at wall corners and intersections Other Alternative Ways instead of long stones. (adapted from: Bothara et al. 2002) Wall stitches made from reinforced concrete with steel reinforcement 76

77 Better bonding within masonry Vertical Reinforcement Vertical Reinforcement U-Hook Horizontal Reinforcement Hor. Reinforcement U-Hook Fig. Good bonding with brick masonry Innovate l Integrate l Collaborate 77

78 Masonry failure mechanisms Fig. (a) Joist Displacement; (b) Joint Slipping; (c) Unit direct tensile cracking; (d) Masonry crushing; (e) Unit diagonal tensile cracking. 78

79 Masonry failure mechanisms Fig. Masonry building during earthquake shaking: (a) loosely connected walls without slab at the roof level; (b) a building with well-connected walls and a roof slab 79

80 Controlled size and location of openings Location and size of openings in walls has significance in deciding the performance of masonry buildings in earthquakes Recommendations regarding the length and story height of stone masonry walls 80

81 Controlled size and location of openings Large un-stiffened openings create soft story effect leading to a deformation of building during an earthquake. To prevent such effects the opening size and location has to be controlled. Recommended location and size of openings for stone masonry walls (source: IAEE 2004) 81

82 Controlled size and location of openings Figure: Regions of force transfer from weak walls to strong walls in a masonry building. Wall B1 pulls walls A1 & A2, while wall B2 pushes walls A1 & A2. 82

83 Light construction Lighter structures absorbs less seismic force, hence less effect. Shera Board Gypsum Board Plywood/OSB CGI Sheet GFRG Panel Fiber Cement Board Different types of light weight materials available in Nepal 83

84 Features for Rural Masonry Houses Horizontal Bands in different levels Corner strengthening with stitches Vertical Reinforcement Tying floor/roof rigidly with lateral load resisting elements (walls/columns) Diagonal Bracing Innovate l Integrate l Collaborate 84

85 Conclusion Building has to act as a single unit for a good earthquake resistance. Should be designed with the application of proper seimic design and construction principles Proper Site Selection Appropriate Planning Good foundation resting on a firm Base Better bonding within masonry Controlled size and location of openings Innovate l Integrate l Collaborate 85

86 Even if the buildings are designed properly using the best practices They can not perform well in earthquakes if the construction is not done properly! 86

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88 Reference Amod M. Dixit, 2004, 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, Promoting Safer Building Construction in Nepal Ministry of Physical Planning and Works, Earthquake Risk Reduction and Recovery Preparedness Programme for Nepal gharpedia.com. How Configuration of the Building Affect During Earthquakes? Building How: Earthquake Resistant Buildings Satish Kambaliya, Earthquake and Earthquake Resistant Design Innovate l Integrate l Collaborate 88