Natural disaster is a hot issue in international society and academy. The term "tropical cyclone" is a generic name used to refer to all organized, circulating weather systems of low pressure that develop over tropical waters. Tropical cyclone is perhaps the most devastative natural disasters both because of the loss of human life they cause and the large economic losses they induce. On an average, 80-100 tropical cyclones happen over the world and they cause 20,000 deaths and the economic losses worth 7 billion dollars per year. The vulnerability to tropical cyclones is becoming more pronounced because of the fastest population growth in tropical and subtropical coastal regions.
The vulnerability of a human settlement to a cyclone is determined by its siting, the probability that a cyclone will occur, and the degree to which its structures can be damaged by it. Buildings are considered vulnerable if they cannot withstand the forces of high winds. Generally those most vulnerable to cyclones are lightweight structures with wood frames, especially older buildings where wood has deteriorated and weakened the walls. Houses made of unreinforced or poorly-constructed concrete block are also vulnerable. Urban and rural communities on low islands or in unprotected, low-lying coastal areas or river floodplains are considered vulnerable to cyclones. Furthermore, the degree of exposure of land and buildings will affect the velocity of the cyclone wind at ground level, with open country, seashore areas and rolling plains being the most vulnerable. The destructive force of typhoon is strong and great harmful, and the typhoon often brings heavy rains, storm surges, river floods and flash floods in small basins and mud-rock flows, landslides and other disasters. More than 95% of the flood disasters caused due to the impact of typhoon, which threat seriously to the people's personal safety and property.
Disaster is merciless. And it is important to guard against disaster. Therefore, in the process of formulating the overall construction planning of villages and towns, disaster prevention and mitigation planning should be strengthened, based on local species vulnerable to the disaster and their characteristics.
Cyclones are vortices in the atmosphere having a core called the eye of extreme low pressure and light winds, surrounded by strong winds having nearly circular contours of equal pressure called isobars. They produce very strong winds rotating clockwise around a calm centre. Very strong winds, heavy rainfall with flooding and storm tide are all elements of a tropical cyclone. Strong winds generated during severe tropical cyclones can cause extensive property damage and turn wind-borne debris into missiles. Tropical cyclones can also produce heavy rainfall over extensive areas that can cause further damage to property and infrastructure and potential injury and loss of life. As well, the low central pressure and strong winds over the ocean can lift the sea water surface to produce a storm tide
The radial distance from the centre of the eye to the region where the maximum tangential wind velocity occurs is called the radius of maximum winds (RMW). The wind speed falls off gradually beyond this region and the approximate wind velocity distribution is given by:
velocity of wind at a radial distance, r;
radius of maximum wind (RMW);
velocity of maximum wind; and
a power law exponent varying between 0.4 to 0.6.
NOTE—r and ro are to be measured from the centre of the eye of the storm.
In the interior region to the radius of maximum wind the velocity distribution may be assumed as linear varying from zero at centre of eye.
2.1 CATEGORIES OF CYCLONES
This five-category system is based on the wind speeds generated by the cyclone. The Bureau of Meteorology quotes wind speeds measured under standard conditions – at a height of 10m above the ground and measured in flat, open terrain. The following table presents maximum expected sustained winds and estimated wind gusts near the centre of a tropical cyclone measured under standard conditions. The Bureau of Meteorology uses a 10 minute averaging time for reporting the sustained winds.
Table 2.1 categories of cyclones
Tropical cyclone causing negligible house damage. Damage to some crops, trees and caravans. Watercraft may drag moorings.
Severe Cyclonic Storm (SCS)
Cyclone causing minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small watercraft may break moorings.
Cyclone causing some damage to roofs and structural damage on older houses. Some caravans destroyed. Power failure likely.
Cyclone causing significant roofing loss and structural damage on older houses. Many caravans destroyed and blown away. Dangerous Air borne debris.Widespread power failures.
Super Cyclonic Storm
Extremely dangerous with potential for wide spread destruction.
2.2 BASIC WIND SPEED ZONES
The macro-level wind speed zones of India have been formulated and published in IS: 875 (parts) - 1987 titled " Indian Standard Code of Practice for Design Loads (other than earthquakes) for Building and Structures, Part 3, Wind Loads". There are six basic wind speeds ‘V0’ considered for zoning, namely 55, 50, 47, 44, 39 and 33 m/s. From wind damage viewpoint, these could be described as follows:
55 m/s (198 km/h) - Very High Damage Risk Zone - A
50 m/s (180 km/h) - Very High Damage Risk Zone - B
47 m/s (169.2 km/h) - High Damage Risk Zone
44 m/s (158.4 km/h) - Moderate Damage Risk Zone - A
39 m/s (140.4 km/h) - Moderate Damage Risk Zone - B
33 m/s (118.8 km/h) - Low Damage Risk Zone
The basic wind speeds are applicable to 10 m height above mean ground level in an open terrain with a return period of 50 years. At higher elevations and longer return periods, the values will be higher.
The earlier wind pressure maps (one giving winds of shorter duration and other excluding winds of shorter duration) were replaced by a single wind map giving basic maximum wind speed in m/s (peak gust speed averaged over a short time interval of about 3 seconds duration). The wind speeds were worked out for 50 years return period based on the up-to-date wind data of 43 dines pressure tube (DPT) anemograph stations and study of other related works available on the subject since 1964.
Fig.2.2 Basic wind speed in m/s (based on 50 year return period)
2.3 Kerala-an over view
Kerala is relatively safe from coastal cyclones, going by the database on cyclones. The State has been included in Lower Vulnerability Zone along with Tamil Nadu, Maharashtra, Goa, Karnataka, Daman and Diu, Pondicherry, Lakshadweep, and Andaman and Nicobar Islands. The Kerala region, in general, has lesser probability of getting affected by the occurrence of cyclonic storm or severe cyclonic storm, according to the report. Though relatively safe from coastal cyclones, there have been instances of localised coastal cyclones and speedy winds following water spouts.
3. THE QUALITY AND SAFETY PROBLEMS OF RURAL HOUSING PROJECTS
"The typhoon disaster" is essentially due to the insufficient strength of house. The 'self-collapse' result from the unreliable connection of the various components of house, poor quality of anchors, and no attention paid to resisting wind. It should be noted that the determinants of anti-typhoon capacity of house are the stiffness and integrity of whole house and not entirely dependent on wall thickness or all of the use of reinforced concrete. The better the integrity, the greater the stiffness is. The greater the stiffness, the stronger the wind- resistance is. The quality and safety of rural building problems are as follow:
3.1 BAD CONSTRUCTION QUALITY: Most of the rural houses are of brick buildings and most of them are constructed by masons. The poor construction quality has affected the housing integrity and weakened the housing stiffness, wind-resistance and lateral displacement resistance, which result in insufficient capacity of residential housing .
3.2 IRRATIONAL STRUCTURE: Most of the houses collapsed in typhoon are of simple structure, such as wood structure, brick-wood structure and so on. Housing Constructions do not set constructional column, which is deadly to the affected houses. To set constructional column is important for improving house anti-collapse ability House with large open windows and doors, are also contributed to the collapse of buildings in typhoon. After wind damages large doors and windows, the air flow directly acts on the lower part of the roof, which cause the roof damage, water penetrated the wall, so that the wall capacity declined.
3.3 SUB-STANDARD BUILDING MATERIALS: Money spent on the construction of houses is farmer's hard-earned money. villagers spend money as little as possible on building a new house, which leave them potential hazard. Some villagers have not enough money to whitewash the external walls, and the walls suffer from prolonged exposure to rain erosion, which reduce the bearing capacity of the walls. Although some houses adopt cement mortar or cement lime mortar, in which there are low cement content, impurities, leading to a low strength of cement sand mortar.
4. TYPES OF DAMAGE DURING CYCLONES
As a consequence of the wind pressures/suctions acting on elements obstructing the passage of wind the following types of damage are commonly seen to occur during high wind speeds:
· Uprooting of trees, which disrupt rail and road transportation;
· Failures of many cantilever structures such as sign posts, electric poles, and transmission line towers;
· Damage to improperly attached windows or window frames;
· Damage to roof projections and sunshades;
· Failure of improperly attached or constructed parapets;
· Overturning failures of compound walls of various types;
· Failure of weakly built walls;
· Failure of roofs and roof covering-This is perhaps the commonest area of failure in cyclones. The causes are usually inadequate fastening devices, inadequate sheet thickness and insufficient frequencies of fasteners in the known areas of greater wind suction.
Fig 4.1 Loss of corrugated metal, roof sheets
· Failure of large industrial buildings with light weight roof coverings and long/ tall walls due to combination of internal and external pressures;
· Brittle failure of asbestos - cement (AC) sheeting of the roofs of Industrial sheds; failure of AC sheets is generally along eaves, ridges, and gable ends;
· Punching and blowing off of corrugated iron roofing sheets attached to steel trusses;
· Foundations-The uplift forces from cyclone winds can sometimes pull buildings completely out of the ground.
Fig 4.2 Foundation pulled completely out of ground
· Steel frames: Usually the weakness in steel frames is in the connections. Economizing on minor items (bolts) has led to the overall failure of the major items (columns, beams and rafters).
· Timber House: The inherent vulnerability of light-weight timber houses coupled with poor connections is a dangerous combination which has often led to disaster.
· Rafters-Of particular interest in recent cyclones was the longitudinal splitting of rafters with the top halves disappearing and leaving the bottom halves in place. The splitting would propagate from holes drilled horizontally through the rafters to receive holding-down straps.
Table 4.1 shows the aerofoil effects of some cyclonic wind speeds.
Table 4.1: Aerofoil Effect of Wind
Wind Speed, m/sec.
Typical Possible Movement
Roof sheets fixed to battens fly
Small aircrafts take off speed
Roof tiles nailed to battens fly
Garden walls blow over
Unreinforced brick walls fail
Major damage from flying debris
75 mm thick concrete slabs fly
Given below are some typical effects of openings in the walls from the attack of winds as well as the pressure on each of the building components:-
• Wind generating opening on the windward side during a cyclone will increase the pressure on the internal surfaces. This pressure, in combination with the external suction, may be sufficient to cause the roof to blow off and the walls to explode.
Fig 4.3 Blow off of roofs due to wind generating opening
• Another mode of failure occurs when the windward side of the house collapses under the pressure of the wind.
Fig 4.4 Windward face of the building collapses under pressure of wind force
• During a cyclone an opening may suddenly occur on the windward side of the house. The internal pressure which builds up as a result may be relieved by providing a corresponding opening on the leeward side.
Fig 4.5 Opening on the leeward side due to internal pressure
• If the building is not securely tired to its foundations, and the walls cannot resist push/pull forces the house tends to collapse starting the roof with the building leaning in the direction of the wind.
Fig 4.6 Collapse start at the roof building leaning in the wind direction
• Failure of the Wall: Wind forces on the walls of the house may produce failure. Wind striking a building produces pressure which pushes against the building, on the windward side, and suction which pulls the building on the leeward side and the roof. If no air enters the building, then there is pressure inside which is pushing against the walls and the roof.
Fig 4.7 wall failure
• Overturning is another problem for light structures. This occurs when the weight of the house is insufficient to resist the tendency the house to be blown over.
Fig 4.8 Overturning of light structures
5. DESIGN WIND SPEED AND PRESSURES OF BUILDINGS
5.1 WIND SPEED
The basic wind speed is reduced or enhanced for design of buildings and structures due to following factors:
(i) The risk level of the structure measured in terms of adopted return period and life of structures (5,25,50 or 100 years),
(ii) Terrain roughness determined by the surrounding buildings or trees and, height and size of the structure.
(iii) Local topography like hills, valleys, cliffs, or ridges, etc.
(iv) Importance factor for the cyclonic region.
Thus general basic wind speed being the same in a given zone, structures in different site conditions could have appreciable modification and must be considered in determining design wind velocity as per IS: 875 (Part 3) - 1987.
It is known that higher wind speed occurs during cyclones compared to non-cyclonic storms. Further, there is a greater degree of turbulence in such storms and the probability of occurrence during the life time of a structure is also large. Therefore, structures are subjected to greater risk under cyclonic storms. To account for the enhanced risk, an importance factor ‘k4’, whose value is equal to unity for dwellings, 1.15 for industrial buildings, and 1.30 for structures of post-cyclone, importance shall be considered while determining the design wind speed. The design wind speed Vz at any height z in m/s shall be taken as:
Vz = Vb k1 k2 k3 k4,
Vz = design wind speed at any height z in m/s,
Vb = basic wind speed
k1 = probability factor (risk coefficient)
k2 = terrain roughness and height factor
k3 = topography factor and k4 = importance factor for the cyclonic region;
The values of k1, k2, k3, and Vb shall be as specified in IS 875 (Part 3).
5.2 WIND PRESSURE
The wind pressure at any height above mean ground level shall be obtained by the following relationship between wind pressure and wind speed:
Pz = 0.0006 Vz2
Vz = design wind velocity, m/s
Pz = design wind pressure, kN/m2
The design wind pressure pd can be obtained as,
pd = Kd. Ka. Kc. pz
Kd = Wind directionality factor
Ka = Area averaging factor
Kc = Combination factor
The value of wind pressure actually to be considered on various elements depends on
(i) Aerodynamics of flow around buildings.
(ii) The windward vertical faces being subjected to pressure.
(iii) The leeward and lateral faces getting suction effects and
(iv)The sloping roofs getting pressures or suction effects depending on the slope. The projecting window shades, roof projections at eave levels are subjected to uplift pressures. These factors play an important role in determining the vulnerability of given building types in given wind speed zones.Figure5.2.1 shows the various cladding areas of a building, which will have different pressure coefficients.
Fig.5.2.1 External wind pressure areas on building faces
Figures 5.2.2(a) and 5.2.3(b) show typical effects of openings in the walls for a given angle of attack of wind as indicated:
(a) Only one large opening in a wall will cause very large internal pressure say 0.7Pz.which combined with external pressures/suctions will modify the wind effects on cladding and their connections immensely.
(b) A building with all windows and doors locked will have zero or very small internal suction or pressure, 0.2 Pz. If a room has openings distributed in all walls or at least in opposite walls and the overall porosity is less than 5%, the passage of air will cause only low internal pressure say only 0.2 Pz. Effects of wind uplift on roof projections can also be seen in Fig.5.2.2(a), (b). For a design speed of 50 m/ s, the basic pressure will be 1.5 kN/m2 and the design pressure could be obtained by multiplying with the coefficients given in Fig. 3 (a) and (b) for the specimen cases shown. For other dimensions of length, width and height and direction of wind, reference may be made to IS: 875 Part 3-1987.
Fig.5.2.2 Structural Load Coefficients, Internal and External Pressures
5.3 COASTAL AREAS
The coastal areas are subjected to severe windstorms and cyclonic storms. It is known that in certain events, the wind gusts could appreciably exceed the specified basic wind speeds (by as much as 40 to 55%). But for design of structures (except those considered very important) the above macro-level zoning stated in 2.2 is considered as sufficient. The frequency of occurrence of cyclones on the different portions of the coast has been different.
5.4 STORM SURGE
Besides the very high velocity winds, the coastal areas suffer from the onslaught of seawater over the coast due to storm surge generated by cyclones. A storm surge is the sudden abnormal rise in sea level caused by the cyclone. The surge is generated due to interaction of air, sea and land. The seawater flows across the coast as well as inland and then recedes back to the sea. Huge loss of life and property takes place in the process. The height of the storm surge is even higher during the period of high tides.
6. PLANNING ASPECTS
6.1 BASIC REQUIRMENT OF CYCLONE RESISTANT BUILDINGS
Based on the above facts the housing plan, design and executions should be carried out
considering the following.
· Avoid a low pitched roof
· Use of Hip roof (or) High Pitched gable roof
· Avoid over hanging roofs.
· Cyclone shutters to be introduced in the opening places.
· Minimize the projection of eaves.
· Recommended steel reinforcement should be provided in the wall as seismic band in vertical and horizontal directions.
· The foundation of the building should be designed by taking bearing capacity under the soil strata conditions and also checked the uplift pressure.
· The wall should resist the horizontal forces developed by cyclone.
· The joints are the vulnerable points should be properly designed and inter connection is essential.
· The self weight of roof should be increased to counteract the uplift pressure developed by the cyclone, the structural components inter connected and anchored with the foundations so that the entire building should be act as one unit.
· The overhanging roofs eves projections are restricted.
· The sloped roof is preferable rather than flat roof and low pitched roof are to be avoided
· In the coastal area the steel section should be protected from corrosion and use the anti corrosion reinforcement for the buildings. The cover for the reinforcement and quality of concrete are ensured for durability.
6.2 SITE SELECTION
i. Though cyclonic storms always approach from the direction of the sea towards the coast, the wind velocity and direction relative to a building remain random due to the rotating motion of the high velocity winds. In non-cyclonic region where the predominant strong wind direction is well established, the area behind a mound or a hillock should be preferred to provide for natural shielding (Fig.6.2). Similarly a row of trees planted upwind will act as a shield (Fig.6.2.2). The influence of such a shield will be over a limited distance, only from 8 to 10 times the height of the trees. A broken tree close to the house may damage the house also hence distance of tree from the house may be kept about 1.5 times the height of the tree.
Fig.6.2.1 Shielding of house by hillock
ii. In hilly regions, construction along ridges should be avoided since they experience an accentuation of wind velocity whereas valleys experience lower speeds in general.
(a) No shielding from high wind due to absence of barriers
(b) Shielding from high winds due to permissible barriers such as strong tree,
Fig.6.2.2 Wind shielding of buildings
(b) Shielding from high winds due to permissible barriers such as strong tree,
Fig.6.2.2 Wind shielding of buildings
iii. In cyclonic regions, close to the coast, a site above the likely inundation level should be chosen. In case of non availability of high level natural ground, construction should be done on stilts with no masonry or cross-bracings up to maximum surge level, or on raised earthen mounds as shown in Fig. 6 to avoid flooding/inundation but knee bracing may be used.
Fig 6.2.3.Construction on raised ground / stilts to prevent inundation
Buildings usually have shallow foundation on stiff sandy soil and deep foundations in liquefiable or expansive clayey soils Following parameters need to be properly accounted for in the design of foundation.
· i. Effect of Surge or Flooding - Invariably a cyclonic storm is accompanied by torrential rain and tidal surge (in coastal areas) resulting into flooding of the low-lying areas. The tidal surge effect diminishes as it travels on shore, which can extend even upto 10 to 15 km. Flooding causes saturation of soil and thus significantly affects the safe bearing capacity of the soil. In flood prone areas, the safe bearing capacity should be taken as half of that for the dry ground. Also the likelihood of any scour due to receding tidal surge needs to be taken into account while deciding on the depth of foundation and the protection works around a raised ground used for locating cyclone shelters or other buildings
· ii. Building on Stilts- Where a building is constructed on stilts it is necessary that stilts are properly braced in both the principal directions. This will provide stability to the complete building under lateral loads. Knee braces will be preferable to full diagonal bracing so as not to obstruct the passage of floating debris during storm surge.
Fig.6.3.1. Building on stilts
6.4 MASONRY WALLS
6.4.1 EXTERNAL WALLS
All external walls or wall panels must be designed to resist the out of plane wind pressures adequately. The lateral load due to wind is finally resisted either by all walls lying parallel to the lateral force direction (by shear wall action) or by RC frames to which the panel walls must be fixed using appropriate reinforcement such as 'seismic' bands at window sill and lintel level.
6.4.2 STRENGTHENING OF WALLS AGAINST HIGH WINDS/CYCLONES.
For high winds in cyclone prone areas it is found necessary to reinforce the walls by means of reinforced concrete bands and vertical reinforcing bars as for earthquake resistance.
6.5 GLASS PANELLING
· One of the most damaging effects of strong winds or cyclones is the extensive breakage of glass panes caused by high local wind pressure or impact of flying objects in air. The large size glass panes may shatter because they are too thin to resist the local wind pressures [Fig.6.5.1.(a)]. A broken glass pane on windward side opening increases internal pressures abnormally [Fig.6.5.1(b)], and may lead to a chain of events including a roof failure.
Fig 6.5.1 Protection of Glass Panes
· The way to reduce this problem is to provide well designed thicker glass panes.
· Further, recourse may be taken to reduce the panel size to smaller dimensions. Also glass panes can be strengthened by pasting thin plastic film or paper strips [Fig.6.5.1(b)]. This will help in holding the debris of glass panes from flying in case of breakage. It will also introduce some damping in the glass panels and reduce their vibrations.
· Further, to prevent damage to the glass panels from flying wind borne missiles, a metallic fabric/mesh be provided outside the large panels [Fig.6.5.1 (c)].
· For the purpose of reducing wind forces on the roof, a hipped or pyramidal roof is preferable to the gable type roof (Fig.6.6.1)
· In areas of high wind or those located in regions of high cyclonic activity, light weight (Gl or AC sheet) low pitch roofs should either be avoided or strongly held down to purlins. Pitchgd roofs with slopes in the range 22 -30 , that is, pitch of 1/5 to 1/3.5 of span, will not only reduce suction on roofs but would also facilitate quick drainage of rain water.
Fig.6.6.1 Effects of roof architecture on uplift forces
6.7. SECURING THE RIDGE
If the rafters are not secure, the ridge can fall apart when strong wind passes over the roof.The ridge can be secured by using:-
Fig.6.7.1 Collar ties
(ii) GUSSETS - Usually made of steel/plywood. This is used at the ridge.
Fig 6.7.3 Metal straps
7. GUIDELINES FOR PLANNING
Though the cyclonic storms always approach from the direction of the sea towards the coast, the wind velocity and direction relative to a building remain random. Hence, reduction coefficients for directionality and orientation of buildings in a preferential direction are not feasible. The general guidelines on planning include:
- As far as possible, the building shall be founded on good ground. Part of the building on good ground and partly on made up ground shall be avoided [Fig. 7.1 (a)].
- Regular plan shapes are preferred. Reentrant corners are to be avoided [ Fig. 7.1 (b)].
- For individual buildings, a circular or polygonal plan is preferred over rectangular or square plans but from the view point of functional efficiency, often a rectangular plan is commonly used. Where most prevalent wind direction is known, a building should be so oriented, where feasible, that its smallest facade faces the wind.
- A symmetrical building with a compact plan-form is more stable than an asymmetrical building with a zig-zag plan, having empty pockets as the latter is more prone to wind/cyclone related damage [ Fig. 7.1 (c)].
- In case of construction of group of buildings with a row type or cluster arrangement, cluster arrangement can be followed in preference to row type. However, in certain cases, both may give rise to adverse wind pressure due to tunnel action and studies need to be conducted to look into this aspect [Fig. 7.1(d)].
- Long walls having length in excess of 3.5 m shall be provided with cross walls or integrated pilasters [ Fig. 7.1 (e)].
- Buildings are not to be located in low-lying areas as cyclones are invariably associated with floods.
- In hilly regions, construction along ridges should be avoided since they experience an accentuation of wind velocity whereas valleys experience lower speeds in general [ Fig. 7.1 (f)].
- Except in case of buildings with large span with sloped roofs, roof pitches having a slope less than 1 in 3 shall be avoided [ Fig. 7.2 (a)].
- Hipped roofs are preferred to gabled roofs for non-engineered and semi-engineered buildings as the peak suction pressures for all angles of attack are lower in the former case, and may be taken as 80 percent of those on pitched gabled roof in the absence of more detailed information [ Fig. 7.2 (b)].
- The percent of the total opening in the cross-section of the frontal wall shall be less than 50 percent of the width of the wall. Opening in load bearing walls should not be within a distance of h/6 from the inner corner for the purpose of providing lateral support to cross walls, where h is the storey height up to eave level [ Fig.7.2(c)].
- While planning a lay-out for group housing, if the inter-building spacing is less than twice the width of the building considerable shielding is available for the interior buildings though the first two columns/rows attract larger forces compared to a stand alone building.
- In regions where storm surges lead to coastal inundation, buildings should be located at higher ground levels. If high ground is not available buildings may be constructed at raised earthen mounds suitably surrounded by retaining walls. Alternatively, buildings may be constructed with stilts with no masonry up to maximum surge level. Suitable bracings may, however, be provided in case of multiple hazard zones, particularly due to earthquake, to avoid falures arising out of large variations in stiffness between stilt and higher floor levels.
7.1 GUIDELINES FOR NON-ENGINEERED CONSTRUCTION
All construction though using the conventional building materials but made intuitively without carrying out a proper structural design and or constructed without adequate control at site, with respect to both materials used and construction practices employed, may generally be termed as non-engineered construction. All construction in low strength masonry or clay mud and similar other forms of biomass with fall under the category of non-engineered construction.
Fig7.1 Improvements For Building Layouts To Reduce Damages Due to Cyclones
Fig.7.2 Improvements For Roofs and Walls of Buildings to Reduce Damages Due to Cyclones
Fig.7.2 Improvements For Roofs and Walls of Buildings to Reduce Damages Due to Cyclones
Fig.7.3 Improvements for Thatched Roofs and Mud Walls to Reduce Damages Due to Cyclones
a) The aerodynamics of flow around buildings, leads to large suction pressures on the roof. To reduce problems due to flying-off of thatched roof, it may be held down to the frame work of the roof or the building envelope using organic ropes. As organic ropes have short life, the holding down ropes alone may be changed every year prior to the most probable month of occurrence of cyclones. Diagonal pattern of rope is preferred [Fig. 7.3 (a)].
b) The overhang of the roof beyond the wall shall be limited to 450 mm. In case it exceeds this value, the projected portion of the roof may be properly tied back to the wall framework.
c) All the posts buried below ground level shall be painted with a coat of coal tar up to the level of maximum flood discharge.
d) The main posts shall be firmly anchored to the ground using suitable anchor poles. The minimum depth of anchorage for the main posts shall be 900 mm and the minimum length of anchorage bars shall be 450 mm with a minimum bearing area of 22 500 mm2. Each post shall have four anchor poles, as shown in Fig.7. 3 (b) at two levels at least at 500 mm interval in different directions.
e) As mud wall is erodable, protection barrier or revetment built with stone or brick shall be built up to the maximum flood level, and plastering with special water proof clay or cement/lime mortar on outer surface is essential [ Fig.7. 3 (c)].
f) In case of sloped roof, triangular frames as shown in Fig. 4 may be located with a maximum spacing of 2.0 m. The members of this triangular frame shall be sufficiently strong to hold back the cross runners. Suitable connections shall be ensured between various elements of this frame using metal straps, bolt and nuts, and steel flats to enable better integrity for the structure as a whole ( Fig.7.5 and Fig. 7..6).
g) The main triangular frames are to be firmly connected to anchorage elements/bond beams at the level of the eaves. The anchorage elements in turn are to be connected to the main posts of the wall using U bolts.
h) Brick work in weak mortars and random rubble masonry can be used for the walls. In these cases, the bond beam/anchorage beam provided on top shall be anchored to the foundation using mild steel rod properly encased in cement mortar. Alternatively if continuous lintel is provided with reinforced concrete or wood with sufficient height of brickwork/rubble masonry, the roof can be anchored to the continuous lintel. The total downward load due to weight of masonry and roof shall have a factor of 1.50 over the total uplift force on roof. The total area of anchorage reinforcement provided shall be twice that required for transmitting the uplift force.
i) Discrete anchorage of roof into brick/rubble masonry can be accomplished through anchorage reinforcement. An angle of dispersion of two verticals to one horizontal may be assumed. The shear strength of masonry shall be neglected in any computation, and the effective weight of masonry above shall be 1.5 times the uplift force at the given anchorage based on simplified load-flow pattern.
Fig. 7.6 Connections Using Wires and Straps in Thatched Buildings
7.2 GUIDELINES FOR SEMI-ENGINEERED CONSTRUCTION
Semi-engineered buildings are buildings which have certain elements structurally designed, such as, roof slabs and foundations but certain elements not properly designed such as walls of masonry buildings and in which the supervision may be through Engineering staff or otherwise. The following guidelines are useful in detailing semi-engineered buildings;
- To achieve a certain measure of restraint for tiled roofs provide concrete or masonry restraining bands at a spacing of approximately 1.2 m to 1.5 m. These bands may preferably be located over wooden rafters forming integral part of the truss system. In case the bands are connected to the purlins U bolts may be used and suitably anchored over the reinforcing rod. The dimension of the band may be about 100 mm × 50 mm. The restraining bands shall have at least one 8 10 mm diameter bar placed inside the band. Typical details of improvements to tiled roof are given in Fig.7.7 Hip, valley and ridge tiles shall be firmly embedded in continuous band of cement mortar. If nailing holes are available in these tiles, nails can be inserted through these into the mortar bed and these can effectively serve as shear connectors.
- The tiled roof system shall be securely fixed to a bond beam. The bond beam in turn is to be connected to the foundation by holding down bolts. The holding down bolt shall be designed with a factor of safety of 2.0.
- Wherever asbestos sheets are used for roof cladding, U bolts are preferred when compared to J bolts. The numbers of U bolts at various locations are indicated in Fig.
- In case hollow concrete block masonry is used for walls the designed reinforcements can be taken through the hollow concrete block forming a pilaster with reinforcement as shown in Fig.7.8 The spacing of such pilasters shall not be greater than 3.0 m. The reinforcements are to be anchored well into the foundation and integrated with lintel band and bond beam (Fig.7.9).
- Good connections are required among the various wooden elements in the roof and wall. Typical details shown in Fig.7.10 and Fig.7.11 may be adopted with modifications to suit the structural scheme. The important requirement is that the uplift force on the roof is to be safely transmitted to the foundation. The connections must have adequate strength to transfer the uplift force.
- If strong wall made of good quality brick work is provided, the roof can be anchored to the continuous lintel band through cyclone bolts.
7.3 GUIDELINES FOR ENGINEERED CONSTRUCTION
Engineered buildings are buildings designed by Architects and/or Engineers and properly supervised by Engineering staff during construction, such as, reinforced concrete and steel framed buildings. Public buildings, such as, schools and hospitals, cyclone shelters, etc, have to be carefully engineered.
In a cluster of buildings having similar heights and where the inter building spacing is less than 2 times the width of an individual building the following enhancement/shielding factors are to be considered:
- For corner buildings located on the periphery of the building clusters, the pressure loadings shall be enhanced by a factor of 1.50.
- For all interior buildings a shielding factor of 0.80 can be considered.
- The roof pressures on corner buildings of the outer rows shall be enhanced by a factor of 1.50 in industrial sheds.
- For evaluating the roof pressures on interior buildings, a shielding factor of 0.80 can be considered for gabled roofs.
- In all buildings where wind loading is the dominant loading no increase in allowable stresses in steel over and above that specified in IS 800 is permitted.
- In all buildings where load bearing masonry is used a parapet of minimum height 600 mm may be provided. Also the roof slab may be anchored to the continuous lintel through adequate ties.
Fig. 7.7 Improvements to Tiled/ Ac Sheets Roof to Reduce Damages Due to Cyclone
Fig. 7.8 Details of Bolts
Fig. 7.9 Construction of Concrete Block Masonry
Fig 7.10 Fixing of Walls to the Foundation Using Tie-Down Bolt.
- In multi-hazard prone areas with earthquake zones ITJ and above, even if the design forces are governed by wind loading, ductile detailing provisions as given in IS 13920 shall be followed. The design forces would however be computed based on wind loading in such cases.
- In flood prone areas all public buildings including cyclone shelters shall be constructed on raised ground with appropriate peripheral retaining walls.
- If buildings are constructed with openings at the ground level/stilted buildings, adequate symmetric shear walls shall be provided in both the principal directions of buildings. This is absolutely essential in multi-hazard prone areas for earthquake regions with zone-Ill and above.
- Wherever feasible, without compromising functionality, the corners of the buildings shall be rounded off with suitable radius of curvature so as to reduce the drag forces.
Fig.7.11 Connection of Roof Frame to Wall Frame
Fig.7.12 Connection Details Between Purlin and Rafter
- In industrial buildings with gable roof plan bracing shall invariably be provided at the bottom chord level of trusses to avoid bottom chord buckling due to uplift force as well as to distribute the horizontal loading from gable ends (see Fig.6.6.5). Upper chord bracing is also desirable at least near gable end walls.
Fig.7.13 Wind Bracing for Roof Trusses
From the recent cyclonic incidents it is evident that, cyclone shelter as a disaster management measure is the most effective tool. But the existing cyclone shelters in the coastal regions are not sufficient in number; they are not properly located, designed and maintained. On the basis of the new concept of disaster management, it is essential to consider a cyclone shelter not only as an evacuation space for cyclone affected people during emergency period but also as a community development centre throughout the whole year.
Disaster preparedness and planning in the past have been too dependent on massive financial, technical and infra-structural input by Government & NGO’s. While such interventions are necessary, these steps must be accompanied by local people’s participation as well as incorporating the age old wisdom of the people. This will have the double advantage of empowering the people, drawing them into plans which will no longer merely be injected from the outside, and will result in a more thought out, user and environment friendly response to extreme natural calamity.
However, it is important to note that traditional houses can only be cyclone resistant with a comprehensive approach for the implementation of all the recommendations in the guidelines for cyclone resistant houses. The critical aspects of the recommendations are a ) solving the social and environmental problems of housing, b) technology input for improving and enhancing the durability of building material such as bamboo, and c) careful consideration of the recommendations outlined in construction techniques, structural components and details. In short the cyclone resistant house is feasible with the simultaneous implementation of a community approved plan of tree plantation, preservative treatment of all components of building materials and following recommendations for technology input in construction techniques and structural components and details.
1. Mahendran.k, A.Zahir Hussain (2010) Disaster Resistant Rural House Design For Low Income People ,INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, Volume 1, No1,77-82
2. The Practice and Reflection on Disaster Prevention Construction of Rural Dwelling Houses in Coastal Typhoon Disaster Area;Pan Anping;College of Architectural and Civil Engineering(Wenzhou University,Wenzhou, P.R.China,) 978-1-4244-6932-1110/$26.00 ©2010 IEEE
3. The Practice and Management on the Construction of Typhoon Emergency Shelter in Coastal Rural Areas:Pan Anping:College of Architectural and Civil Engineering(Wenzhou University,Wenzhou, P.R.China,) 978-1-4244-5326-9/10/$26.00 ©2010 IEEE
4. Ankush Agarwal ( 2007) CYCLONE RESISTANT BUILDING Disaster Risk management Programme
5. Gujarat State Disaster Management Authority (2001) Guidelines for Cyclone Resistant Construction of Buildings in Gujarat
6. Indian Standard CODE OF PRACTICE FOR DESIGN LOADS (OTHER THAN EARTHQUAKE) FOR BUILDINGS AND STRUCTURES,PART 3 WIND LOADS(Second Revision),Fifth Reprint JULY 1997,UDC 624·042·41,BUREAU OF INDIAN STANDARDS,MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG,NEW DELHI 110002
LIST OF REFERRED INDIAN STANDARDS
800 : 1984
Code of practice for general construction in steel (second revision)
875 (Part 3) : 1987
Code of practice for design loads (other than earthquake) for buildings and structures: Part 3 Wind loads (second revision)
13920 : 1993
Ductile detailing of reinforced concrete structures subjected to seismic forces—Code of practice
DESIGN PROCEDURE FOR WIND RESISTANT BUILDINGS
The following procedure may be followed to design a building that will be resistant to damages during high winds/cyclones.
A.l Fix the Design Data
a. Identity the national wind zone in which the building is situated. This can be seen from wind code (IS: 875 Part 3-1987) or the Vulnerability Atlas of India (1997).
b. Corresponding to the zone, fix the basic design wind speed, Vb which can be treated as constant upto the height of 10m.
c. Choose the risk co-efficient or the importance factor k-|, for the building, as for example given below:
Building type Coefficient k1
i. Ordinary residential building 1.0
ii. Important building (e.g. hospital; 1.08 police station; telecommunication, school, community and religious buildings, cyclone shelters, etc.
d. Choose appropriate value of k2 corresponding to building height, type of terrain and size of building structure, as per IS: 875 (part 3), 1987. For buildings upto 10m height and category - A, which will cover the majority of housing, the values are:
Terrain Coefficient kg
i. Flat sea-coastal area 1.05
ii. Level open ground 1.00
iii. Built-up suburban area 0.91
iv. Built-up city area 0.80
e. The factor k3, depends upon the topography of the area and its location above sea level. It accounts for the acceleration of wind near crest of cliffs or along ridge lines and deceleration in valleys etc.
A.2 Determine the wind forces
a. Determine the design wind velocity Vz and normal design pressure Pz
Vz =Vbk1 k2 k3
Pz = 0.0006 Vz2 , Pz will be in kN/m2 for Vz in m/s
b. Corresponding to the building dimensions (length, height, width), the shape in plan and elevation, the roof type and its slopes as well as projections beyond the walls, determine the coefficients for loads on all walls, roofs and projections, taking into consideration the internal pressures based on size and location of openings. Hence calculate the wind loads on the various elements nornnal to their surface.
c. Decide on the lines of resistance which will indicate the bracing requirements in the planes of roof slopes, at eave level in horizontal plane, and in the plane of walls. Then, determine the loads generated on the following connections:
• Roof cladding to Purlins
• Purlins to rafters/trusses
• Rafters/trusses to wall elements
• Between long and cross walls
• Walls to footings.
A.3 Design the elements and their connections
a. Load effects shall be determined considering all critical combinations of dead load, live load and wind load. In the design of elements, stress reversal under wind suctions should be given due consideration. Members or flanges which are usually in tension under dead and live loads may be subjected to compression under dead load and wind, requiring consideration of buckling resistance in their design.
b. Even thin reinforced concrete slabs, say 75mm thick, may be subjected to uplift under wind speeds of 55 m/s and larger, requiring holding down by anchors at the edges, and reinforcement on top face! As a guide, there should be extra dead load (like insulation, weathering course, etc) on such roofs to increase the effective weight to about 375 kg/m .
d. Resistance to corrosion is a definite requirement in cyclone prone sea coastal areas. Painting of steel structures by corrosion-resistant paints must be adopted. In reinforced concrete construction, a mix of M20 grade with increased cover to the reinforcement has to be adopted. Low water cement ratio with densification by means of vibratos will minimise corrosion.
e. All dynamically sensitive structures such as chimney stacks, specially shaped water tanks, transmission line towers, etc. should be designed following the dynamic design procedures given in various IS codes.
f. The minimum dimensions of electrical poles and their foundations can be chosen to achieve their fundamental frequency above 1.25 Hz so as to avoid large amplitude vibrations, and consequent structural failure.
It may be emphasised that good quality of design and construction is the single factor ensuring safety as well as durability in the cyclone hazard prone areas. Hence ail building materials and building techniques must follow the applicable Indian Standard Specifications.