International Journal of Advanced Engineering Technology E-ISSN

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1 Research Article PERFORMANCE OF ELEVATED WATER TANKS SUJECTED TO WIND FORCES Dr. Hirde Suchita K. 1, ajare Asmita A. 2, Hedaoo Manoj N. 3 Address for Correspondence 1 Professor in Applied Mechanics, Govt. College of Engineering, Karad, Satara, Maharashtra, India 2 Assistant Engineer grade I, PWD trainee officer, Amravati, Maharashtra, India 3 Associate Professor in Civil Engineering, Govt. College of Engineering, Karad, Satara, Maharashtra, India hirde.suchita@gmail.com ASTRACT In areas with high probability of natural disasters, ability of lifeline systems to resist disaster related damages is one of the most important civil engineering challenges. Elevated water tanks are one of the most important lifeline structures. In this paper an extensive computational study has been conducted to find out the performance of elevated water tank under wind force. Since these structures have large mass concentrated at the top of slender supporting structure, these structures are especially vulnerable to horizontal forces due to wind. Finite element models of 24 elevated water tanks have been analyzed. Elevated water tanks are analyzed with different parameters to study the effect of capacity, height of staging, terrain category and wind zone. Findings of the present study shall lead us to better understanding of the behavior of elevated water tank under wind load and safer design of such structure. KEYWORDS Elevated water tank, wind analysis. INTRODUCTION Indian sub-continent is highly vulnerable to natural disasters like earthquakes, draughts, floods, cyclones, landslides, avalanches etc. Majority of states or union territories are prone to one or multiple disasters. These natural calamities are causing many casualties and innumerable property loss every year. There has been an increasing trend in the occurrence of hazardous events over the last few decades. High wind storms occur in many parts of India, the coastal states of Gujarat, Tamilnadu, Andhra, Orissa and West engal get more seriously affected because of the occurrence of cyclonic storms. Each such storm causes widespread damage 1. Hence, the response of structures under wind effects is very important area where the researchers should concentrate and bring out effective disaster mitigating techniques so that life line facilities remain in function. Ferocity of the wind is the main reason for wind induced disaster. Hence we must be prepared to fight the impending disaster due to wind by understanding the performance of the structure under wind load. In this paper an extensive computational study has been presented to understand the behaviour of elevated water tank under wind force because these structures have large mass concentrated at the top of slender supporting structure and hence these structures are especially vulnerable to horizontal forces due to wind. Finite element models of 24 elevated water tanks have been analyzed to study the performance of this structure under wind force by changing various parameters like capacity, height of staging, terrain category and wind zone. The results presented in this paper will be useful to the designer to understand the effect of various factors as mentioned above on the magnitude of wind force acting on the elevated water tank. IMPORTANCE OF ELEVATED WATER TANK AND ITS STUDY Water supply is a life line facility that must remain functional following disaster. Most municipalities in India have water supply system which depends on elevated tanks for storage. Elevated water tank is a large elevated water storage container constructed for the purpose of holding a water supply at a height sufficient to pressurize a water distribution system. In major cities the main supply scheme is augmented by individual supply systems of institutions and industrial estates for which elevated tanks are an integral part. These structures have a configuration that is especially vulnerable to horizontal forces due to the large total mass concentrated at the top of slender supporting

2 structure. Also in Gujarat cyclone it was observed that the storage tanks were displaced by few meters and some were overturned due to wind. They were swept away by the wind. Flying debris caused dents on the surfaces when they hit the tanks. So it is important to check the severity of these forces for particular region. The study of damage histories revealed damage/failure of reinforced concrete elevated water tanks of low to high capacity. Damage of the important lifeline facility like elevated water tanks often results in significant hardships even after the occurrence of the disaster, claiming human casualties and economic loss to built environment. Investigating the effects of wind has been recognized as a necessary step to understand the natural hazards and its risk to the society in the long run. Most water supply systems in developing countries, such as India, depend on reinforced cement concrete elevated water tanks. The strength of these tanks against lateral forces, such as those caused by wind, needs special attention. COMPUTATIONAL MODELING It is very important to analyze reinforced cement concrete elevated water tank properly against horizontal forces. The present study has been planned to check the severity of wind forces with height and capacity of the elevated water tank in different zones of India. The analysis is carried out using SEPL Esr-Gsr software as per IS 875 (Part 3): The magnitude of wind force mainly depends on following factors: Classification of Structure The structures are classified into the following three different classes depending upon their sizes; Class A- Structures and/or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) less than 2m. Class - Structures and/or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) between 2 and 5 m. Class C- Structures and/or their components such as cladding, glazing, roofing, etc, having maximum dimension (greatest horizontal or vertical dimension) greater than 5m. Terrain Category There are four terrain categories. Terrain in which a specific structure stands shall be assessed as being one of the following terrain categories: Category 1- Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the structure is less than 1.5 m. Category 2- Open terrain with well scattered obstructions having heights generally between 1.5 to 1 m. Category 3- Terrain with numerous closely spaced obstructions having the size of structure up to 1 m in height with or without a few isolated tall structures. Category 4- Terrain with numerous large high closely spaced obstructions. Wind Speed ased on basic wind speed, there are six zones, zone I to zone VI. asic wind speed shall be modified to include following effects to get design wind velocity at height for the chosen structure; a) Risk level; b) Terrain roughness, height and size of structure; and c) Local topography. The design wind speed at any height can be mathematically expressed as follows: V z = V b x k 1 x k 2 x k 3 Where, V z = design wind speed at any height z in m/s; V b = basic wind speed in m/sec; k 1 = probability factor (risk coefficient); k 2 = terrain, height and structure size factor; and k 3 = topography factor. Elevated water tank is considered as an important structure so for analysis probability factor (risk coefficient) k 1 is taken as given in table 1 for different zones. There are four terrain categories as per the code depending on the obstruction to the wind. From

3 the wind zone map of India 2 shown in Figure 1 it is observed that based on basic wind speed, India is divided into six wind zones i.e. Zone I to zone VI. STUDY PARAMETERS The present study is all about the effect of height of water tank, zones and terrain category on magnitude of wind force. The staging heights considered for study are, 16 m, 2 m, 24 m and 28 m with 4 m height of each panel. Also, to see the effect of capacity 5, liter and 1,, litre capacity tanks are studied. The study is carried out on reinforced cement concrete circular elevated water tanks which are commonly used in practice in water distribution system. Grade of concrete and steel used are M25 and Fe415 respectively. Figure 2 shows the pictorial view of the sample of 24 models considered for study of 5, liter capacity reinforced cement concrete elevated water tank for 12 m staging height. For each staging height four terrain categories are considered and for each terrain category six wind zones are considered. Similar models have been prepared for 16 m, 2 m, 24 m, and 28 m height. Hence total one hundred and twenty models are studied for 5, liter capacity reinforced cement concrete elevated water tank and similar one hundred and twenty models are studied for 1,, liter capacity reinforced cement concrete elevated water tank. The other relevant data used in the modeling is tabulated in table 2. Figure 1: Wind Zone Map of India 2 Table 1: Risk coefficient k 1 for structure Zone asic wind speed (m/sec) k 1 factor I II III IV V VI

4 Table 2: Dimension of Elevated Water Tank Components Capacity 5, liter 1,, liter Diameter of container 4.65 m 5.89 m Depth of water in container 3. m 4. m Free board.3 m.3 m Roof slab 12 mm 14 mm ottom slab 2 mm 27 mm ottom beam 25 x 6 mm 3 x 7 mm Wall 2 mm 2 mm racing 3 x 45 mm 25 x 35 mm column 4 nos.- 45 mm dia. 4 nos.- 5 mm dia Depth of footing below ground level 2. m 3. m c/c distance between column 3.43 m 4.31 m CAPACITY 5, Liter 12 m 16 m 2 m 24 m 28 m TERRAIN CATEGORY 1 TERRAIN CATEGORY 2 TERRAIN CATEGORY 3 TERRAIN CATEGORY 4 I I II III IV V VI I II III IV V VI I II III IV V VI Figure 2: Models for Wind Analysis RESULTS AND DISCUSSIONS Calculation of wind force is a very important parameter for elevated water tank which is most susceptible to horizontal forces because of large mass concentrated at considerable height. The magnitude of wind force by changing various parameters has been evaluated and compared for 24 water tanks. The results obtained are shown in table 3 and table 4. Effect of wind zone on wind forces Graphs are plotted taking wind zone as abscissa and the wind forces as ordinate for reinforced cement concrete elevated tanks of 5, liter capacity with different staging heights to study the effect of wind zone on wind forces. Wind forces for different staging heights for terrain category 1, 2, 3 and 4 are shown in Figure 3 to Figure 6. Wind forces increases with increase in

5 basic wind speed i.e. zone. Wind forces for zone I is about 28-31% less than that of zone II, about 44-47% less than that of zone III, about 51-54% less than that of zone IV, about 58-6% less than that of zone V, about 65-66% less than that of zone VI. Wind forces for zone II is about 21-23% less than that of zone III, about 31-33% less than that of zone IV, about 4-42% less than that of zone V, about 5-52% less than that of zone VI. Wind forces for zone III is about 12-13% less than that of zone IV, about 22-24% less than that of zone V, about 35-38% less than that of zone VI. Wind forces for zone IV is about % less than that of zone V, about 26-29% less than that of zone VI. Wind forces for zone V is about % less than that of zone VI. Wind forces increases from wind zone I to VI because basic wind speed increases from lower to higher zone because of which risk coefficient k 1 increases. Effect of staging height on wind forces These observations are also analyzed and shown in graphical form to study the effect of staging height on wind forces. Graphs are plotted taking staging height as abscissa and the wind forces as ordinate for reinforced cement concrete elevated tanks of 5, liter and 1,, liter capacity. Wind forces for elevated water tanks of different staging heights for terrain category 1, 2, 3 and 4 are shown in Figure 7 to Figure 1. From above graphs it is observed that, wind forces increases with increase in capacity of reinforced cement concrete elevated water tanks. Since exposed area increases with increase in capacity. Wind forces also increases with increase in staging height. This is because exposed area, terrain height and size factor k 2 increases with increase in staging height. It is observed that wind force increases by about 1% for increase of 4 m in staging height. Table 3: Wind forces for 5, liter capacity reinforced cement concrete elevated water tanks having different staging height Staging Class Terrain Wind force (kg) height 2m A A category I II III IV V VI

6 Table 4: Wind forces for 1,, liter capacity reinforced cement concrete elevated water tank having different staging height Staging height 2m Class A A Terrain Wind force (kg) category I II III IV V VI m I II III IV V VI Zone Figure 3: Wind forces for terrain category Forces (kg ) 5 2m I II III IV V VI Zone Figure 4: Wind forces for terrain category 2

7 6 5 F o rc e s (k g ) 2m I II III IV V VI Zone Figure 5: Wind forces for terrain category 3 F o rc es (kg ) I II III IV V VI Zone 2m Figure 6: Wind forces for terrain category I 5 - II 5 - III 5 - IV 5 - V 5 - VI 1 - I 1 -II 1 - III 1 - IV 1 - V 1 - VI Staging height (m) Figure 7: Wind forces for terrain category 1

8 5 - I 5 - II 5 - III 5 - IV 5 - V 5 - VI 1 - I 1 -II 1 - III 1 - IV 1 - V 1 - VI Staging height (m) Figure 8: Wind forces for terrain category I 5 - II 5 - III 5 - IV 5 - V 5 - VI 1 - I 1 -II 1 - III 1 - IV 1 - V 1 - VI Staging height (m) Figure 9: Wind forces for terrain category 3

9 5 - I 5 - II 5 - III 5 - IV 5 - V 5 - VI 1 - I 1 -II 1 - III 1 - IV 1 - V 1 - VI Staging height (m) Figure 1: Wind forces for terrain category 4 Effect of obstructions on wind forces It is observed that wind forces for wind terrain category 1 are most critical among terrain categories 1, 2, 3 and 4. This is because in category 1, it is exposed open terrain with few or no obstructions and the average height of any object surrounding the structure is less than 1.5 m and hence terrain height and size factor k 2 is more for terrain category 1 as compared to terrain categories 2, 3 and 4. It is observed that for terrain category 4 where numerous large high closely spaced obstructions exists the wind force reduces by 4 to 5% than open terrain where average height of any object surrounding the structure is less than 1.5 m. Present study will be useful to civil engineers to understand the behaviour of elevated water tank for various staging height and also to get the feel of effect of zones and terrain category on wind forces. REFERENCES IS: 875 (Part3): 1987, Code of practice for design loads (other than earthquake) for buildings and structures, IS, New Delhi, Tansel, and N. Ahmed, Structural stability of elevated water reservoirs under hurricane force wind conditions, 8th ASCE Specialty Conference on Probabilistic Mechanics and Structural Reliability, 112(1-5),. 4. Jorge Gutierrez, Wind code evaluation, Manual for Wind Design,.