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DRAFT IN WIDE CIRCULATION DOCUMENT DESPATCH ADVICE
Reference
Date
CED 39/T- 16
15 10 2009
TECHNICAL COMMITTEE: EARTHQUAKE ENGINEERING SECTIONAL COMMITTEE , CED 39 ADDRESSED TO : 1 Interested Members of Civil Engineering Division Council, CEDC 2. All members of CED 39 and CED 39/AP/Tsunami 3. All others interested Dear Sir, Please find enclosed the following document:
Doc No. CED 39(7545)
Title
Draft Indian Standard Tsunami Resistant Design of Buildings and Structures — Recommendations ICS No. 91.120.25
Kindly examine the draft standard and forward your views stating any difficulties which you are likely to experience in your business or profession, if this is finally adopted as Indian Standard. Last date for comments : 31 12 2009 Comments if any, may please be made in the format as given overleaf and mailed to the undersigned at the above address. In case no comments are received or comments received are of editorial nature, you will kindly permit us to presume your approval for the above document as finalized. However, in case of comments of technical in nature are received then it may be finalized either in consultation with the Chairman, Sectional Committee or referred to the Sectional Committee for further necessary action if so desired by the Chairman, Sectional Committee. The document is also being hosted on BIS website www.bis.org.in . Thanking you, Yours faithfully,
Encl: as above
(A.K. Saini) Sc `F’ & Head (Civil Engg.) email :
[email protected]
FORMAT FOR SENDING COMMENTS ON BIS DOCUMENTS (Please use A4 size sheet sheet of paper only and type within fields indicated. Comments on each clause/subclause/table/fig etc. etc. be started started on a fresh box. Information in column 3 should include reasons for the comments and suggestions for modified working of the clauses when the existing text is found not acceptable. Adherence to this format facilitates Secretariat’s work)
Please e-mail your comments to
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[email protected] or 011 23235529
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Doc : CED 39(7545)
DRAFT FOR COMMENTS ONLY (Not to be reproduced without the permission of BIS or used as an Indian Standard)
Draft Indian Standard TSUNAMI RESISTANT DESIGN OF BUILDINGS AND STRUCTURES — RECOMMENDATIONS FOREWORD The Great Indian Ocean Tsunami on 26 December 2004 caused massive damage and a great deal of it was structural. Furthermore a large proportion of the loss of life could be ascribed to this structural failure, since it had not planned for vertical evacuation, and the resulting debris became an added hazard. Although, structural failure was widespread, many structures did survive inundation by the tsunami, particularly if it was only partial inundation, suggesting that if communities are going to be built in tsunami prone areas there may be structural solutions that could be expected to provide a safe refuge in all but the most extreme events. There is also a need for critical infrastructure in tsunami prone areas to be resilient to tsunami inundation. inundation. To date there has been very little research undertaken on the design of structures to resist tsunamis, primarily because major tsunamis were perceived as being so rare as not to warrant attention from the point of view of public safety. However public perceptions in this regard are changing. If suitable structural solutions are to be found they will need to be based on a fundamental understanding of the forces imposed on structures by tsunami inundation, and the response of structures to them. This will require considerable knowledge about the physical characteristics of the tsunamis as they penetrate over land. Three important variables variables are penetration, depth and velocity. Some information is available on penetration and depth, but very little is known about velocities during inundation, and the effect of the entrained debris, which are critical to estimating forces. In the context of the above a need was felt to formulate a code, which can provide a guideline to professional engineers and government bodies for designing tsunami resistant structures near coasts Tsunami forces are so high that structures cannot be designed to resist the full impact of tsunami forces either elastically or in-elastically. in-elastically. Buildings subjected to tsunami are likely to experience extensive damage even if designed to conform to the provisions of this code. Tsunami force could be 8-10 times the earthquake force. It will be difficult to design the normal residential structures to sustain tsunami forces. Hence normal structures must be protected from design tsunami waves and need not be designed for tsunami forces. However the coastal protection structures such as walls, dykes, embankment and the structures inside the sea (e.g. bridges, jetty etc) must be able to sustain the tsunami forces. Though earthquake occurrence time is of the order of few seconds, the tsunami arrival time on the coast may range from few minutes to few hours. The simultaneous impact of both the phenomenon on a structure is not possible, hence, while combining the loads the effect of forces for earthquake and tsunami need not be taken simultaneously.
1
The presence of suitable coastal protective measures have been found to mitigate the extent of damage to structures. Coastal protection measures include hard solutions like groins, seawalls, break waters, bulk heads, water gates, etc. and soft solutions like artificial beach nourishment, bioshields, mangroves etc. The presence of coastal protection measures is an added advantage to safety and it should not be considered for reducing the design forces stipulated in this code. With the availability of time gap between initiation of a tsunami event and of its striking a region, it is recognized that installation of appropriate tsunami warning system plays a very important role to enable evacuation and prevent loss of life. In the aftermath of the December 2004 tsunami, India has developed its own tsunami warning system, which includes a seismic network and ocean bottom pressure recorders. This system enables a warning to be issued within 20-30 minutes of an earthquake The effects of sea level variation due to climate change are beyond the scope of this standard and hence is not addressed. This draft Indian Standard has been prepared based on studies carried out by various research groups and the papers published in national and international journals. In the preparation of this standard, assistance has been taken from the following documents : Tsunami Glossary:International Tsunami Information Center (ITIC) Intergovernmental Oceanograhic Commission (of UNESCO): International Co-ordination Group for the Tsunami Warning System in the Pacific (ICG/ITSU) , 2006. Guidelines for Reconstruction of House affected by Tsunami in Tamil Nadu: Revenue Administration, Administration, Disaster Management Management & Mitigation Department, Government of Tamil Nadu, 2005. Development of Design Guidelines for Structures that Serve as Tsunami Vertical Evacuation Sites (52-AB-NR-20051) By Harry Yeh (Oregon State University), Ian Robertson)University Robertson)University of Hawaii), Janes Preuss, Planwest Partners, 2005 Preventive / Protection and Mitigation from Risk of Tsunami, A Strategy Paper by Anand.S. Arya, Ministry of Home Affairs, Government of India, 2005. ‘Reducing Tsunami Risk-Strategies for Urban Planning and Guidelines for Construction Design’ by Asian Disaster Preparedness Centre. ‘Designing for Tsunami’ by National Tsunami Hazard Mitigation Program Steering Committee, USA, March 2001 For the purpose of deciding whether a particular requirement requirement of this standard is complied with, the final value observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS 2 : 1960 ‘Rules for rounding of numerical values (revised)’. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.
2
Doc : CED 39(7545)
Draft Indian Standard TSUNAMI RESISTANT DESIGN OF BUILDINGS AND STRUCTURES — RECOMMENDATIONS
1 SCOPE 1.1 This standard deals with the strategies for protection against tsunami and the design of structures located on coastal sites to resist the forces induced due to tsunami. 1.2 This standard also includes special criteria for multistory buildings that may be used for vertical evacuation. 2 REFERENCES 2.1 The standards listed in Annex A contain provisions which through reference in this text, constitute provisions of this standard. At the time of publication, the editions listed were valid. All standards are subject to revision and the parties to agreements based on this standard are encouraged to apply the most recent editions of the standards indicated in Annex A. 3 TERMINOLOGY 3.1 Arrival Time Time of arrival of the first wave of a tsunami at a particular location. location. 3.2 Crest Length The length of a wave along its crest. Sometimes it is also called as crest width. 3.3 Datum Reference level for measurement of elevation on land (mean sea level extended landward is considered as datum in the context of tsunami). 3.4 Dynamic Wave Pressure The pressure that a moving mass of water associated with tsunami would induce while encountering encountering an obstruction. 3.5 Estimated Time of Arrival Computed arrival time of the first wave of a tsunami at coastal zone after a specific major disturbance in the ocean like, earthquakes, earthquakes, landslides or volcanoes that has occurred.
3
3.6 Elapsed Time Time interval between observed time of arrival of the first wave of a tsunami at a specific location on the coast and the time of reaching the normal water level conditions. 3.7 Evacuation Map A drawing or representation representation that outlines danger zones and designates limits beyond which people must be evacuated to avoid any harm from tsunami waves. 3.8 Far-Field Tsunami A tsunami capable capable of widespread widespread destruction, destruction, not only in in the immediate immediate region of its its generation, but across the entire ocean basin. 3.9 Force Pressure distribution integrated integrated over a given area of the structure. 3.10 Hazard A Hazard is a situation which poses a level of threat to life, health, property or environment. environment. Most hazards are dormant dormant or potential, with only a theoretical risk of harm, however, once a hazard becomes active, it can create an emergency situation. 3.11 Impact Standing Wave Pressure The pressure that acts on the structure experiences due to formation of a standing wave. 3.12 Inundation depth Depth of water measured at a given location inland inland at the time of occurrence of tsunami. 3.13 Inundation Distance The distance that a tsunami wave penetrates inland, measured horizontally from the intersection point of mean sea level and the beach face (also known as shoreline). 3.14 Intensity Intensity is the degree of damage to buildings. 3.15 Local Tsunami A tsunami of which which destructive effects effects are confined confined to coasts within within a hundred hundred km of the source. 3.16 Mean Sea Level The average height of sea surface, based upon hourly observations of the height of tide on the open coast or in adjacent waters which have free access to the sea.
4
3.17 Mean Tsunami Height Average height height of a tsunami measured measured from the trough to the crest. crest. 3.18 Near–Field Tsunami A tsunami from a nearby source, generally generally less than 200 km or associated with a short travel time of less than 30 minutes. 3.19 Reference Sea Level It is level of water at the time of tsunami occurrence. occurrence. 3.20 Regional Tsunami A tsunami capable of causing destruction in a particular geographic region, generally within about 1000 km of its source. Regional tsunami also occasionally have very limited and localized effects outside the region. 3.21 Run up Maximum vertical height of the water level inland, measured above mean sea level. 3.22 Sustained Wave Pressure The pressure that a structure continues to experience for a short period to time. 3.23 Terrain Slope The tangent of angle made by the ground surface with respect to the mean sea level (symbolically (symbolically indicated as tan β in the fig.1) 3.24 Travel Time Time required for the first tsunami wave to propagate from its source to a given point on a coastline. 3.25 Tsunami A Japanese term term derived from the characters characters "tsu" meaning meaning harbor and and "nami" meaning meaning wave. A tsunami is a series of waves with a long wavelength and period (time between crests) usually generated by disturbances associated with earthquakes/landslide or volcanoes occurring below or near the ocean floor. Time between crests of the wave can vary from a few minutes to over an hour. Tsunamis are often incorrectly called tidal waves; they have no relation to the daily ocean tides. Tsunamis can occur at any time of day or night. 3.26 Tsunami Amplification Tsunami amplification is the increase in the height of tsunami as it travels from deep ocean to near shore region.
5
3.27 Tsunami Dispersion Redistribution of tsunami energy, particularly as a function of its period, as it travels across a body of water. 3.28 Tsunami Height It is the vertical distance between the crest (highest point over the water surface) and trough (lowest point over the water surface) of a tsunami. 3.29 Tsunami Magnitude, Mt A number characterizing the strength of a tsunami based on the tsunami wave wave height. Mt = log 2H, where H = maximum run-up height or amplitude on a coast line near the generating area Or, Mt = logH + a logR + D, where R is the distance in km from the earthquake epicenter to the tide station along the shortest oceanic Path, and ‘a’ and ‘D’ are constants 3.30 Tsunami Period Time that a tsunami wave takes to complete a cycle. Tsunami period typically ranges from 5 minutes to two hours. 3.31 Tsunami Wavelength Wavelength is the horizontal distance between successive crests of a tsunami wave. 3.32 Wave Celerity The speed with which a wave crest moves horizontally across the ocean surface is defined as wave celerity (c) or phase speed, and is usually measured in meters per second. Note: Refer to Fig.1 ffor or some of the important glossary of terms.
6
4. TSUNAMI CHARACTERISTICS 4.1 Generation of Tsunamis Tsunamis are generated by any large, impulsive displacement of the sea bed level (Fig.2). Earthquakes generate tsunamis by vertical movement of the sea floor. If the sea floor movement is horizontal, a tsunami is not generated. Earthquakes of M > 6.5 are critical for tsunami generation. Tsunamis are also triggered by landslides into or under the water surface, and can be generated by volcanic activity and meteorite impacts.
4 3 2
As waves approach approach shore they slow down, the waves lengths shorten and amplitudes become higher
Possible bore formation on shore
Tsunami wave train formation
Submarine fault movement, landslide, or volcanic activity
Fig. 2 Wave train of Tsunami*
4.2 Characteristics of Tsunamis Tsunami velocity is dependent on the depth of water through which it travels (Velocity equals the square root of water depth h times the gravitational acceleration g, that is V = g h ) (see fig.3). Tsunamis Tsunamis travel approximately approximately at a velocity of 700 kmph kmph in 4000 m depth of sea water. Thus, the tsunami from Sumatra coastal earthquake traveled to Tamil Nadu coast in about two hours. In 10 m of water depth, the velocity drops to about 36 kmph. Even on shore tsunamis speed is 35 – 40 km/h, hence much faster than a person can run.
*Source:- International Tsunami Information Information Centre – Gerologic Hazard **Source: http://www.prh.noaa.gov/pr/itic/library/pubs/great_waves/tsunami_great_waves_4.html
7
Possible Bore
Fig.3 Tsunami Velocities**
Tsunamis range in size from few centimeters to over 30 m height. Most tsunamis however are less than 3 m in height. In deep water (greater than 200 m), tsunamis are rarely over 1m high and will not be noticed by ships due to their long time period. The scientific and technical studies carried out after Indian Ocean Tsunami have provided some lessons and guidelines for the construction of tsunami safe buildings and structures and these have been covered in Annex B. 5 GUIDELINES FOR PLANNING OF BUILDINGS AND EVACUATION OF HUMANS The tsunami waves always approach from the direction of sea towards the coast. The general guidelines for planning of buildings shall be as follows: 5.1 Minimizing Tsunami Pressures The buildings constructed on reinforced stilt columns with sufficient clearance under the building superstructure, the tsunami wave will be able to pass though exerting only the minimum pressures on the columns (see Fig.4). For further reduction in such hydrodynamic pressures, the columns may be made circular, octagonal or square with chamfered/rounded corners. The risers in stairs should be left open for water to cross through.
Tsunami Max
Fig. 4 - House constructed on stilts
8
5.2 Providing Soft Breaking Obstructions Buildings may be built as per 5.1 but with infill/cladding infill/cla dding wall panels which would break easily and give way to the tsunami tsunami wave to pass through under the upper structure structure of the building. Such a lower level space may be used to perform functions like seating for primary education schooling or community gathering purposes in the normal course. 5.3 Protecting the Building by Strong Walls On the coastal side of the building, strong walls may be constructed by which the wave water will be deflected back towards the sea (see Fig.5 a). The walls may be curved concavely towards the sea in vertical or the horizontal plane. Needless to say that the walls will have to be designed for the resulting very large reactive forces. Blocking Wall
Fig. 5a Construction of Blocking walls for deflection of tsunami waves.
NOTE – Recommended heights of bund above high tide line (on the basis of Dec 2004 Indian Ocean Tsunami) given in Annex C.
5.4 Use of Break Waters On the coastal side of the building, appropriate energy dissipation blocks of concrete or stone may be arranged as under the canal falls or the spill way dams which will dissipate the energy of the fast moving waters of the tsunami so that the impact on the building elements will be minimized to safe level (see Fig. 5 b).
Fig. 5b Construction of wave breakers breakers for slowing speed of waves.
5.5 Designing the Building Resistance Resistance It is known that the tsunami forces can even be ten times larger than the maximum earthquake or cyclonic wind pressures. It will therefore require a very heavy wall structure in the lower storyes of the building to make it safe against tsunami impacts. The kind of actions created on the building are shown in Fig. 6.
9
OVERTURNING
SLIDING WAVE BREAK
SCOURING
Fig. 6 Actions on Structures created by Tsunamis
5.6 Evacuation of the Population Evacuation of the people could be affected by vertical evacuation through raised platforms with proper staircase approach, or into multistoreyed upper floors, or to platforms constructed at high enough elevation as part of elevated water towers, or by creating safe areas at higher elevations provided with easy and direct approach to the nearby communities as shown in Fig. 7. The design approach for structures to be used for evacuation purposed should be chosen suitably for the sites under consideration. consideration.
VERTICAL EVACUATION HORIZONTAL EVACUATION
HIGH MOUND
Fi . 7 Vertic Vertical al & Horizo Horizonta ntall Evacu Evacuatio ation n
6 TSUNAMI HAZARD MAP The Tsunami hazard map at present may be empirically defined using a deterministic approach based upon potential maximum Tsunami wave heights. The definition of the tsunami hazard zones, as preliminary estimates, is given in Table 1. For the terrestrial environment the hazard has been presented as inundation depths. Tsunami hazard map indicating the elevation data of coasts (contour maps) is yet to be developed . However, Maximum Probable Storm Surge Height and Seismic Zone in Coastal Districts of India are given in ANNEX C. For the marine environment (“In water”) Harbor, Bay and Reefs – hazard has been specified in terms of potential tsunami amplifications. 10
Table 1 Tsunami Hazard Hazard Zones Zones Definition Characteristics
On land structures: Inundation depth above GL (m) In water Structures: Tsunami amplification above MSL (m)
Tsunami Hazard Zone Very high
High
Medium
Low
Very Low
>9
6-9
3-6
1-3
< 1.0
-
>2
1-2
0.5-1.0
< 0.5
7 GENERAL DESIGN CRITERIA Due to the effect of tsunami, structures are subjected to the following additional pressures such as dynamic and sustained wave pressures in addition to the impact wave pressure. The effects of long term erosion, storm-induced erosion and local scour should be considered in the design of foundations of buildings and structures. 7.1 Category of Structures For tsunami resistant design of structures, the structures are classified into the following categories:
Category
Description
I
Load bearing or Non-engineered buildings.
II
RCC & steel buildings which are not intended for vertical evacuation.
III
Structures for vertical evacuation. evacuation.
IV
All near shore and on shore structures not covered in I, II & III categories.
7.2 Materials and Methodology for Category I Structures No special analysis and design provisions are envisaged for category I structures. However, applicable clauses of IS 4326 / IS 13828 may be used as recommended in 8 of this standard. 7.3 Materials for Category II, III and IV Structures The requirements in this clause are applicable to structures of category II, III & IV only. 11
7.3.1 Concrete Exposure condition for coastal environment shall be taken as defined in IS 456. Minimum grade of concrete, cement content, maximum w/c ratio, maximum aggregate size and cover to reinforcement shall be as per the provisions of IS 456. NOTE – In case, prestressed concrete is used, it shall conform to IS 1343. 7.3.2 Reinforcement Steel Reinforcement steel Fe 415 or TMT bars (Fe 500 or Fe 550) conforming to IS 1786 and having minimum elongation of 14.5 percent shall be used. 7.3.3 Structural Steel Structural steel conforming to IS 2062 shall be used with suitable corrosion protection measures. Provisions pertaining to durability and corrosion protection as given in IS 800 shall be complied. 7.4 Methodology for Designing Category II Structures The provisions of IS 1893 and IS 13920 shall be applied for analysis and design of category II structures as per seismic zone V requirements for the structures in the tsunami affected area of the district. district. However, for rest rest of the area, design design of structures shall be done as per seismic zones given in Annex C. 7.5 Forces due to Tsunami Impact on Category II Structures Structures 7.5.1 Estimation of Tsunami Amplification Near the Shore Tsunami amplification amplification near the shore can be estimated from the following equation:
H 2 × (g d ) = H 2 × (g d) 0 0 1 or
=
⎛ ⎜ ⎜ ⎝
(1a)
⎞ ⎟ ⎟ ⎠
(1b)
where Ho =Approximate height of tsunami in flat deep ocean floor (can be determined from the fig. 8. For rugged deep ocean floor adopt fig. 9) H1 = height of tsunami near the shore (m) do = water depth in the deep sea (m) d = water depth near the shore (m) g = acceleration due to gravity (9.81 m/s2)
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Figure 8 Height of tsunami in flat deep ocean floor (depth more than 4000 m )
Figure 9 Height of tsunami in rugged ocean floor
7.5.2 Estimation of Impact Forces for Structures Located Near the Sea Front 7.5.2.1 For Structures Located Within 50 m from the Sea Front The following procedure for estimating the design force for structures within 50 m from the sea front shall be adopted. For structures located beyond 50 m upland, procedure given in7.5.2.2 in7.5.2.2 shall be adopted
13
a) Determination Determination of dynamic dynamic wave pressure The following equation gives the dynamic wave pressure:
=
(2a)
or
= where
(2b)
Pdm = maximum dynamic wave pressure (N/m 2) c = wave celerity (m/s) h = initial water depth (m) H1 = Height of tsunami near shore (m) ρ = density of seawater (kg/m 3) g = acceleration due to gravity (m/s 2) K = kinetic wave coefficient (0.3 to 0.51)
b) Determination Determination of Sustained Wave Pressure Pressure
=
where
( +
)
(3)
θ1 : structure slope (Fig. 10)
Psm : maximum sustained wave pressure (N/m 2) Pdm : maximum dynamic wave pressure (N/m 2) c : wave celerity (m/s) g : acceleration due to gravity (m/s2) H1: Height of tsunami near shore (m)
Figure 10 Definition sketch for θ1
14
c) Determination of Impact Standing Wave pressure The relation between the sum of maximum dynamic wave pressure P dm, maximum sustained wave pressure P sm and maximum impact wave pressure P im is given by the following equations.
+
=
+ where
+
⎛ ⎜ ⎝
=
−
<
⎛ ⎜ ⎝
⎞ ⎟ ⎠
(4)
+
≥
⎞ ⎟ ⎠
(5)
Pim : maximum impact standing wave pressure. (N/m2) θ1 : structure slope (Fig. 10) Psm : maximum sustained wave pressure (N/m 2) Pdm : maximum dynamic wave pressure (N/m 2) c : wave celerity (m/s) g : acceleration due to gravity(m/s2) H1:height of tsunami near shore (m)
d) Estimation of wave force
Figure 11 Pressure distribution over the vertical face
Assuming a triangular triangular pressure pressure distribution distribution (Fig. 11), 11), wave force per unit length of the structure can be calculated based on equation
=
×
×
(6)
where F : Wave force / meter length of the structure (N/m) Pim : maximum impact standing wave pressure. (N/m 2) H1: Height of tsunami near shore (m) 15
7.5.2.2 For Structures Located Upland (More than 50 M from Sea Front ) For structures located upland maximum of the wave force obtained from the following two methods: Method 1 a) Determination of inundation distance and inundation depth (see fig. 12)
Figure 12 Definition sketch for inundation distance and inundation depth
X1= Location of the structure upland from the edge of the beach. X2= total inundation distance from the edge of the beach. H1 = Tsunami height at 6m water depth or the chosen depth. tan β = slope of the terrain ( above the beach berm ) h* = inundation depth at the structure = H 1 [ 1-(x1 / x2)] Evaluate distance X 2 from equation (7), where suitable value of the factor has to utilized.
⎛ =⎜ ⎝
⎞ ⎟ ⎠
(7)
where the factors* are given in Table 2. Table 2 Factors Recommended Recommended for the Given Slope of the Terrain Terrain Tan β < 0.003 0.003 – 0.006 0.006 – 0.011 0.011 – 0.025 0.025 – 0.03
Factor (rugged terrain) 0.75 0.85 1.12 1.6 1.8
16
Factor (plain surface) 1.2 1.5 2.24 3.2 3.6
b) Estimation of wave force The force can be directly evaluated as follows (see Fig. 12)
=
] ⎡⎢
⎛ − ⎜⎜ ⎣ ⎝
[
⎞⎤ ⎟⎟⎥ ⎠⎦
(8)
=
(9)
Where a : the projected area of the building (h* x 1) / meter length of the building . v* : tsunami flow velocity (m/s) F : Wave force / meter length of the structure (N/m) Method 2 Let Hb be the height of the building
≥
a) Estimation of wave force for
=
( see fig. 13 a for pressure distribution ) Pressure distribution is obtained based on equation 10 (z varies from zero to 3h*)
= =
− ×
(10)
×
(11)
b) Estimation of wave force for
<
=
( see fig. 13 b for pressure distribution distribution ) Pressure distribution is obtained based on equation 11 ( z varies from zero to Hb)
( )= ⎡⎧⎪⎛ = ⎢⎨⎜⎜ ⎣⎢⎪⎩⎝ Where
− −
(12)
⎞⎫⎪ ⎟⎟⎬ ⎠⎪⎭
⎤ ⎥ ⎦⎥
(13)
ρ : density of seawater kg/m3
g : acceleration due to gravity m/s2 Hb : height of the building pm : pressure (N/m2) F : wave force / meter length of the structure (N/m)
17
Figure 13 a Pressure distribution over a structure 3 * b H ≥ h
Figure 13 b Pressure distribution over a structure (Hb < 3 h*)
Maximum of the forces estimated based on equations (9) and (11 or 13 as applicable) shall be adopted as design wave load. 7.6 Forces Due to Tsunami Impact on Category IV Structures For structures which are classified as Category IV structures, the following forces due to Tsunami such as hydrostatic force, hydrodynamic force, buoyancy force and fluid flow drag, foundation scour and impact due to waterborne debris shall be considered. These loads may cause large structural deformation, yielding, fracture and collapse and / or dislodgment of coastal structures from their bases, hence should be properly considered in design. 7.6.1 Hydrostatic Force (Fh) Hydrostatic force occurs when standing or slow moving water encounters a building or building component. This load always acts perpendicular to the surface to which it is applied. It is caused by an imbalance of pressure due to differential water depth on 18
opposite sides of a structure or structural members. The lateral hydrostatic force is given by:
⎛ + ⎜ ⎜ ⎝
=
⎞ ⎟ = ⎟ ⎠
⎛ ⎜ + ⎜ ⎝
⎞ ⎟ ⎟ ⎠
where, Fh : Hydrostatic force ρ : Density of water g: Gravity h: water depth up : Water velocity normal to the wall (as obtained from simulation ) The resultant force will act horizontally at a distance of h R above the base of the wall where:
=
⎛ + ⎜ ⎜ ⎝
⎞ ⎟ ⎟ ⎠
This formula applies to steady state situation of a bridge column supported by a foundation underneath the sea. In the case of tsunami, the hydrodynamic loads are transient and the effects of water velocity are accounted for by the hydrodynamic and / or the surge force. The above formula may not be relevant to a building with finite breadth, for which the water can flow around and quickly fill up behind the building. Hydrostatic force is usually important for 2-D structures such as seawalls and dikes or for evaluation of an individual wall panel where the water level outsides differ substantially from the level inside. 7.6.2 Buoyant Force (Fb) The buoyant or vertical hydrostatic forces on a structure or structural member subjected to partial or total submergence will act vertically through the center of mass of the displaced volume. Buoyant forces are a concern for basement, empty above ground and below ground tanks, and for swimming pools. The buoyant force is given by:
= or
= where, V = volume of water displaced by the structure considered. CL= lift coefficient (normally=0.8) (normally=0.8) A= projected area Uv = vertical velocity ( ≈ √gd) 7.6.3 Hydrodynamic Drag Force (Fd) When the water flows around a building (or structural element or other object) hydrodynamic loads are applied to the building. These loads are a function of flow 19
velocity and structure geometry, and include frontal impact on the upstream face, drag along the sides and suction on the downstream side. These loads are induced by the flow of water moving at moderate to high velocity. They are usually called drag forces, which are combination of lateral loads caused by the impact of the moving mass of water and the friction forces as the water flows around the obstruction. The hydrodynamic drag force on a structure component component in the direction of a steady flow can be expressed as:
= Where A : Projected area area normal to the the direction of the flow CD : Drag coefficient, the value of which is taken as follows Circular piles Square piles Wall sections
1.0 2.0 1.5
For large obstructions this value is given in the following table : Width to Depth Ratio
Drag Coefficient CD
from 1-12
1.25
13-.20 21-32 33-40 41-80 81-120 > 120
1.3 1.4 1.5 1.75 1.8 2
7.6.4 Surge Impingement (Fs) Surge forces are caused by the leading edge of a surge of water impinging on a structure. The hydrodynamic force of the leading edge of the fluid flow acting per unit area of a structure due to tsunami surge is given by: Fs = 4.5 ρ g h2 Where, h is the height of surging flow. The resultant acts at a distance of approximately h above the base of the wall. This equation is applicable for walls within heights equal to or greater than 3h. Walls whose heights are less than 3h require surge forces to be calculated using appropriate combination of hydrostatic and hydrodynamic force equations for the given situation 7.6.5 Impact Force (Fi) During the tsunami or storm surge, water-borne objects (e.g. boats, oil rigs, vehicles, drift wood etc.) may hit a coastal structure with tremendous impact force. This scenario involves highly non-linear coupled fluid (tsunami or storm surge flow)- structure-(debris)structure interaction and the physics is often very complex. This load can be estimated by 20
parametric study using complex finite–element model and simulation. The generalized expression for impact force F i is given by following equation:
=
⎛ ⎞ ⎜ ⎟ ⎝ ⎠
where, uI = approach velocity that is assumed equal to the flow velocity m = mass of the body, ∆t = impact duration that is equal to the time between the initial contact of the body with the building and the maximum impact force. The value of ∆t may be taken from the table. Value of ∆ t Type of construction
Duration (t) of impact (sec)
Wood Steel Reinforced Concrete Concrete Masonry
Wall 0.7-1.1 Na 0.2-0.4 0.3-0.6
Pile 0.5-1.0 0.2-0.4 0.3-0.6 0.3-0.6
7.6.6 Wave Breaking Force (Fbrkw) Following expression for wave breaking force may be used:
= Where, Cdb is a shape coefficient (value = 2.25 for square or rectangular piles and 1.75 for round piles), D is the pile diameter, and H b is the wave breaking height (H b = 0.78 ds, where ds is the design still water depth). 7.6.7 Tsunami and Storm Surge Scour Scour of supporting material at the foundation base of a structure or a bridge pier due to tsunami or storm surge differs from the ordinary case of a bridge scour, which occurs gradually caused by periodic waves and steady current loads. In a tsunami or storm surge, the leading wave may scour away much of the supporting materials around the base of a structure and weaken the foundation so much that the foundation of structure or pier of bridge fails under the subsequent fluid drag load. The behavior of tsunami and storm surge scour is very complex and dependent on the geometric properties of the bridge columns as well and the material properties of the surrounding soil at the base. Currently, no simple formula exists for scour prediction. Much experimental work needs to be conducted to provide data for empirical prediction prediction and analysis. Note : As tsunami is a very low probability event and the forces due to tsunami are very huge, the economic implication of designing all the members of the structure for tsunami forces shall be deliberated. The infill walls may be allowed to collapse in the event of a tsunami. However, the frame members shall be designed to withstand the tsunami forces. Alternatively, Alternatively, measures such as wave arrestors can be provided at the coast to reduce the magnitude of forces so that the full impacts of the waves are not experienced by the structures to be designed.
21
7.7 Load Combination When tsunami forces are considered on a structure, these shall be combined as per 7.7.1 per 7.7.1 and 7.7.2 where the terms DL, IL and Ft stand for the response quantities due to dead load, imposed load and tsunami load for the members facing the tsunami wave. These load combinations are in addition to all other relevant load combinations that should be considered while designing the structures as per relevant Indian Standards. Even though tsunamis are generally caused due to earthquake, the earthquake and tsunami forces are not considered simultaneously simultaneously due to the difference in their arrival time. 7.7.1 Load Factors for Plastic Design of Steel Structures In the plastic design of steel structures following additional load combinations involving tsunami forces shall be accounted for: i) 1.7 (DL + Ft) ii) 1.3 (DL (DL + IL + Ft) 7.7.2 Partial Safety Factors for Limit State Design of Reinforced and Prestressed Concrete Structures In the limit state design of reinforced and pre-stressed concrete structure, following additional load combinations involving tsunami forces shall be accounted for: i) 1.2 (DL+IL+Ft) ii) 1.5 (DL + Ft) iii) 0.9 DL DL + 1.5 Ft 8 Construction Aspects of Category I Buildings For earthquake resistant design and construction of buildings of Category I, IS 4326 & IS 13828 shall be followed as appropriate for the material of construction and the Seismic Zone of the area where such buildings are to be constructed. Besides special considerations will be required if the buildings are situated in high cyclone prone and storm surge prone or tsunami prone coastal areas. Such special considerations are provided in the following clauses: 8.1 Siting of Buildings Coastal areas of low elevations within 500 m to 1.5 Km from the shore may suffer due to impact of tsunami flow and inundations. Also such areas may also suffer on account of high wind speeds in the tropical cyclones and storm surge of the sea water under the action of cyclones. Therefore it is recommended recommended that: i) The selection of site should preferably avoid areas likely to be submerged under tsunami or storm surge inundation. ii) Building should be founded on soil strata reliably stable against scour and erosion and should not be susceptible to liquefaction due to earthquake. iii) The site should preferably be selected at higher elevation as possible
22
8.2 Foundation The following safety considerations considerations may be applied for determining the depth and type of foundation: i) Shallow foundations have the risk of being scoured by the receding tsunami wave hence a minimum depth of foundation of 1.5 m below natural ground level is recommended. ii) Use of under ream piles or concrete pedestal piles or reinforced brick pedestal piers going to more than 2 m depth will be preferable. Such foundation shall have a ground level reinforced concrete beam at the top of piles/pedestals for supporting the super structure walls. iii) Where the storm surge or tsunami wave height is estimated to be more than 5 to 6m, the building may be constructed on stilts, the columns being founded on piles with a ground level interconnecting beam and knee braces provided near the top of the stilt columns (full diagonal brasses are to be avoided so as not to obstruct the passage of the floating debris during storm surge & tsunami) Note: Note: The ground floor in stilt buildings can be used for various temporary purposes like storage, running of classes for small children, play area of children or any community function.
8.3 Planning of the Building i) An integrated enclosure of a room by the four walls creates a stable structure against the onslaught of lateral water pressure. But if the rooms have very long walls, those may be destroyed under the hydrostatic and hydrodynamic pressures since such walls act like vertical cantilevers. A crate like plan will be much more stable. Hence wall lengths in the rooms may be restrained to 4 m. ii) Every building may be planned to work for vertical evacuation of the residents by providing a flat roof accessible through a stair case. The roof may also have a strong parapet to assure the safety of the people on the roof. iii) The staircase may be made without vertical risers having only treads so as to permit free flow of water without breaking the staircase. iv) The external wall corners may be made chamfered or curved in plan to permit smoother flow of water. 8.4 Building Super Structure i) The construction of walls, masonry piers etc. should follow the Guidelines given in IS 4326 or IS 13828 as the case may be. Notwithstanding the earthquake safety requirements provided in these codes for moderate seismic zones, in the storm surge or tsunami prone area the following safety measures should be adopted for the load bearing masonry buildings: ii) The safety measures provided in IS 4326 for the most sever Seismic Zone V should be adopted for strengthening the walls by the following measures:
23
a) Use of rich mortar as specified for zone V b) Control on the size and placing of door and window openings c) Provision of seismic bands at plinth and lintel levels in buildings with reinforced concrete slab floor and roofs d) Additional provision of seismic bands at eave level, around the gable masonry in the case of pitched roofs e) Provision of vertical reinforcement on all corners and junctions of walls from the foundation masonry through the floor and anchored into the roof f) Provision of vertical bars at the jambs of door and window openings, anchored into the plinth and the lintel bands Note: Note: All these provisions will incorporate such strengthening measures that the total disintegration disintegration may not not occur under the tsunami impact as itit may not be able to destroy the building totally but create damages in opposite walls to create openings for the water to follow through.
24
ANNEX A ( Clause 2.1) LIST OF REFERRED INDIAN STANDARDS
IS No.
Title
IS 456: 2000
Code of practice for plain and reinforced concrete ( fourth revision )
IS 800: 2007
Code of practice for general construction in steel (second revision )
875(Parts 1 to 5):1987
Code of practice for design loads (other than earthquake) for building structures: Part 1 Dead loads - Unit weights of building material and stored materials ( second revision ) Part 2 Imposed loads (second revision ) Part 3 Wind loads ( second revision ) Part 4 Snow loads ( second revision ) Part 5 Special loads and load combinations ( second revision )
IS 1786 : 2008
High strength deformed steel bars and wires for concrete reinforcement reinforcement – Specification Specification
1893 (Part 1): 2002
Criteria for earthquake design of structures: Part 1 General Provisions and buildings
IS 1904 : 1986
Code of practice for design and construction of foundations in soils : General requirements
IS 1905 : 1987
Code of practice for structural use of unreinforced unreinforced masonry
IS 2062 : 2006
Hot rolled low, medium and high tensile structural steel
IS 2911 : Part 1 : Sec 1 : 1979
Code of practice for design and construction of pile foundations: Part 1 Concrete piles, Section 1 Driven cast insitu concrete piles ( Revision under print )
IS 2911 : Part 1 : Sec 2 : 1979
Code of practice for design and construction of pile foundations: foundations: Part 1 Concrete piles, Section 2 Bored cast-in-situ piles ( Revision under print )
IS 2911 : Part 1 : Sec 3 : 1979
Code of practice for design and construction of pile foundations: Part 1 Concrete piles, Section 3 Driven precast concrete piles ( Revision under print ) 25
IS 2911 : Part 1 : Sec 4 : 1984
Code of practice for design and construction of pile foundations: Part 1 concrete piles, Section 4 Bored precast concrete piles ( Revision under print )
IS 2911 : Part 2 : 1980
Code of practice for desing foundations: foundations: Part 2 Timber piles
construction
of
pile
IS 2911 : Part 3 : 1980
Code of practice for design and construction foundations: Part 3 Under reamed piles
of
pile
IS 2911 : Part 4 : 1985
Code of practice for design and construction of pile foundations: foundations: Part 4 Load test on piles Code of practice for earthquake resistant design and construction of buildings ( third revision )
IS 4326: 1993
and
IS 13828 : 1993
Improving earthquake resistance of low strength masonry buildings - Guidelines
IS 13920: 1993
Ductile detailing of reinforced concrete structures subjected to seismic forces
26
ANNEX B SOME LESSONS FROM THE GREAT INDIAN OCEAN TSUNAMI OF DEC. 26, 2004 B-1 Dynamic Forces The force of some tsunamis is enormous. Large rocks weighing several tones along with boats and other debris can be moved inland hundreds of meters by tsunami wave activity. Trees are uprooted, and homes and other buildings are destroyed. All this material and water move with great force and can kill or injure people. B-2 Effect on Off-Shore/On-Shore Structures It was observed that a number of jetties and other harbor walls were severely damaged by the tsunami through its hydro-dynamic force as well as scouring of foundations acting simultaneously. This damage very adversely affected the relief work to be carried to the Andaman & Nicobar Nicobar Islands Islands through ships ships from the Indian Indian mainland. mainland. B-3 Lessons for Protection and Structural Safety The important lesson learnt from the Tsunami impact on off-shore structures and buildings near the coast and is that the brute force of tsunami waters if allowed to flow freely, may not cause any damage. But if it is resisted structurally, the resistance required will have to be very high against the breaking of the bore and the hydro-dynamic force of the flowing water. These forces will be further enhanced due to the debris created by the tsunami and flowing with the water creating impact on any obstructing element, wall or column. The lessons from behavior of various structures impacted by the Indian Ocean Tsunami are summarized in Tables 1 & 2, which would be helpful in planning the various new structures in coastal areas considered prone to impact of future tsunamis.
27
Table 1 Phenomenon of Inundation Effect
Design Solution
Flooded basement
Choose sites at higher elevations
Flooding of lower floors
Raise the buildings plinth above flood elevation
Flooding of mechanical electrical & communication system & equipment Damage to building materials & contents
Do not stack or install vital material or equipments on floors or basement lying below tsunami inundation level
Contamination of affected areas with water borne pollutants
Protect hazardous material storage facility located in tsunami prone area. •
Locate mechanical systems & equipments at higher location in the building
•
Use corrosion resistant concrete & steel for the portions of the building which are liable to inundation.
Hydrostatic forces (Pressure • on walls by variation in water depth on opposite sides •
Provide adequate openings such as louvers to allow water to reach equal heights inside & outside of buildings.
Buoyancy floatation or uplift forces caused by buoyancy
Design for static water pressure on walls.
•
Consider suction tensions on walls under receding waters.
•
Elevate building to avoid floatation due to flooding.
•
Anchor building to foundation to prevent floatation
Saturation of soil causing • slope instability and/or loss of bearing capacity
Evaluate bearing capacity & shear strength of soil that support building foundation & embankment slopes under condition of saturation.
•
Avoid slopes or setbacks from slope that may be destabilized destabilized when inundated.
Table 2 Phenomenon of Currents, (wave break & bore) Effect Hydrodynamic forces (pushing forces on the front face of the building and drag caused by flow around the building
Debris Impact
Scour
Design Solution •
Elevate building on stilts to avoid hydrodynamic pressures
•
Design infill wall panels on ground floor, in R.C. frame buildings, to fail under flowing water pressure without causing failure of columns. Anchor columns to foundations deep enough to escape soil erosion under receding waters.
•
Design for dynamic water forces on walls: off-shore and on-shore jetties, protection walls, break waters etc.
•
Elevate building to permit free flow of water and avoid debris impact.
•
Design for Impact loads.
•
Use deeper foundation (piles or piers).
•
Protect against scour and erosion around foundation.
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ANNEX C Maximum Probable Storm Surge Height and Seismic Zone in Coastal Districts of India Sl. No.
Coastal States and UTs
Coastal Districts
1
Andhra Pradesh
Srikakulam
Seismic Zone (IS 1893:2002)
Height of Tsunami Run-up, m
Recommended Height of bund above High Tide Line, m
4
II
4.5
4.5
Vizianagaram
4
II
4.5
4.5
Vishakha Pattanam
4
II
4.5
4.5
East Godawari
4.5
III
5
5.0
West Godawari
5
III
5.5
5.5
Krishna
5.5
III
6
6.0
Guntur
7.5
III
8
7.0
6
III
6.5
6.5
Nellore
4.5
III
5
5.0
North Goa
4.5
III
5
5.0
South Goa
4.5
III
5
5.0
Kachchh
3.5
V
4
4.0
Surendra Nagar
3.5
IV
4
4.0
Rajkot
3.5
IV
4
4.0
Jam Nagar
3.5
IV
4
4.0
Porbandar
3.5
III
4
4.0
Junagarh
3.5
III
4
4.0
4
III
4.5
4.5
Bhavnagar
4.5
III
5
5.0
Ahemdabad
4.5
III
5
5.0
Anand
4.5
III
5
5.0
Bharuch
4.5
III
5
5.0
Surat
4.5
III
5
5.0
Navsari
4.5
III
5
5.0
Valsad
5
III
5.5
5.5
Prakasam
2
3
Goa
Gujarat
Amreli
Strom Surge height above Concurrent Sea Level, m
Indian Ocean Tsunami run-up and Inundation, m (m)
29
4
5
Karnataka
Kerala
Uttara Kannada
4.5
III
5
5.0
Udupi
4.5
III
5
5.0
Dakshina Kannada
4.5
III
5
5.0
Kesaragod
4
III
4.5
4.5
Kannur
4
III
4.5
4.5
Kozhikod
4.5
III
5
5.0
Malappuram
4.5
III
5
5.0
Thrissur
4.5
III
5
5.0
Ernakulam
4
III
4.5
4.5
Kottayam
4
III
4.5
4.5
3.5
III
4
4.0
Thrivunanthapuram
3
III
3.5
3.5
Thane
5
III
5.5
5.5
Mumbai
5
III
5.5
5.5
Raigarh
5
IV
5.5
5.5
Ratnagiri
4
IV
4.5
4.5
Sindhudurg
4
III
4.5
4.5
Baleshwar
11
III, II
11.5
7.0*
Bhadrak
9.5
II
10
7.0*
Kendrapara
8.5
III
9
6.0*
Jagatsinghpur
6.5
III
7
6.0*
Puri
4
III
4.5
5.0
Ganjam
4
II
4.5
4.5
Kollam
6
7
8
Maharashtra
Orissa
Tamil Nadu
Thiruvallur
3.5
2.1-3.5 (500700)
III
4
4.0
Chennai
3.5
III
4
4.0
Kanchipuram
3.5
III, II
5.2**
4.0
Viluppuram
3.5
II
5.3**
4.0
Cuddalore
3.5
II
5.1**
4.0
Thiruvarur (East Coast)
3.5
2.2-2.3 (500700) 2.7-4.7 (100400) 3.3-4.8(300500) 1.8-4.6(1401500) 3.5-4.8 (7001000)
II
5.3**
4.0
Thiruvarur (South Coast) Nagapattinam
5.5
3.5-4.8 (7001000) 3.0-5.0 (3001500)
II
6
6.0
II
5.5**
5.0
Thanjavur
5.5
?
II
6
6.0
7
?
II
7.5
6.0*
Pudukkottai
4.5
30
9
10
11
West Bengal
Andaman & Nicobar Islands
Daman & Diu
Ramnathapuram (East Coast)
12
?
II
12.5
8.0*
Ramnathapuram (South Coast)
7
?
II
7.5
6.0*
Toothukudi
7
II
7.5
6.0*
Tirunelveli
7
II
7.5
6.0*
Kaniyakumari
3
3.5-5.1 (60600) 3.6-4.0(40750) 2.2-3.3(90200
III
3.8**
3.5
South 24 Parganas
12
IV
12.5
8.0*
Medinipur
13
III
13.5
8.0*
Chattam wharf
?
3.4
V
3.9**
3*
Jugli ghat
?
3.8
V
4.3**
3.5*
Bamboo flat jetty
?
4.0
V
4.5**
3*
Hut bay
?
6.1
V
6.6**
5*
Chidiatapu
?
3.9
V
4.4**
3*
Havelock island
?
3.0
V
3.5**
2*
Mayabandar
?
2.9
V
3.4**
2*
Diglipur
?
2.9
V
3.4**
2.5*
Nicobar Island
?
6-10
V
6-10.5**
4.6*
Daman
5
III
5.5
5.5
3.5
III
4
4.0
?
III
3
3.0
3.5
III
4
4.0
Mahe
4
III
4.5
4.5
Pondicherry
5
III
5.5
5.5
Yanam
?
III
3
3.0
Diu 12
Lakshdeep
13
Pondicherry
Karaikal
* Overtopping to be considered considered in design, design, ** Places where measured Tsunami run-up exceeded storm surge height NOTE -1) Indian Ocean Tsunami run-up and Inundation Inundation (m) have been provided by IIT Madras. 2) The recommended height of bund above high tide lines is, in most cases, 0.5m above the storm surge height which is more frequent than occurrence of the worst tsunami hence not considered even if higher than surge height. height. In few cases where the maximum maximum surge height was very high, say more than 6 m, the bund height is recommended from economy consideration with the guiding footnote that design may take the overlapping possibility in a rare event into account.
31
