seismic behavior of rc elevated water tankunder different types of staging pattern

Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________

www.borjournals.com Blue Ocean Research Journals 23

Seismic Behavior of RC Elevated Water Tankunder Different Types of Staging Pattern

Pavan .S. Ekbote, P.G. Student, Civil Engineering Department, Government Engineering College, Haveri, Karnataka, India Dr. Jagadish .G. Kori, Prof. & Head of Civil Engineering Department, Government Engineering College, Haveri, Karnataka, India ABSTRACT As known from very upsetting experiences, elevated water tanks were heavily damages or collapsed during earthquake. This was might be due to the lack of knowledge regarding the proper behaviour of supporting system of the tank again dynamic effect and also due to improper geometrical selection of staging patterns. Due to the fluid-structure interactions, the seismic behaviour of elevated tanks has the characteristics of complex phenomena. The main aim of this study is to understand the behaviour of supporting system which is more effective under different response spectrum method with SAP 2000 software. In this Paper different supporting systems such as radial bracing and cross bracing. Introduction Water is human basic needs for daily life. Sufficient water distribution depends on design of a water tank in certain area. An elevated water tank is a large water storage container constructed for the purpose of holding water supply at certain height to pressurization the water distribution system. Many new ideas and innovation has been made for the storage of water and other liquid materials in different forms and fashions. There are many different ways for the storage of liquid such as underground, ground supported, elevated etc. Liquid storage tanks are used extensively by municipalities and industries for storing water, inflammable liquids and other chemicals. Thus Water tanks are very important for public utility and for industrial structure. Elevated water tanks consist of huge water mass at the top of a slender staging which are most critical consideration for the failure of the tank during earthquakes. Elevated water tanks are critical and strategic structures and damage of these structures during earthquakes may endanger drinking water supply, cause to fail in preventing large fires and substantial economical loss. Since, the elevated tanks are frequently used in seismic active regions also hence, seismic behaviour of them has to be investigated in detail. Due to the lack of Knowledge of supporting system some of the water tank were collapsed or heavily damages. So there is need to focus on seismic safety of lifeline structure using

with respect to alternate supporting system which are safe during earthquake and also take more design forces. The present study is an effort to identify the behaviour of elevated water tank under Response Spectrum Method with consideration and modelling of impulsive and convective water masses inside the container for different fluid conditions, types of bracings and bracing levels using structural software SAP2000. Model Provisions Two mass model for elevated tank was proposed by Housner (1963) which is more appropriate and is being commonly used in most of the international codes including Draft code for IS 1893 (Part-II). The pressure generated within the fluid due to the dynamic motion of the tank can be separated into impulsive and convective parts. When a tank containing liquid with a free surface is subjected to horizontal earthquake ground motion, tank wall and liquid are subjected to horizontal acceleration. The liquid in the lower region of tank behaves like a mass that is rigidly connected to tank wall. This mass is termed as impulsive liquid mass which accelerates along with the wall and induces impulsive hydrodynamic pressure on tank wall and similarly on base Liquid mass in the upper region of tank undergoes sloshing motion. This mass is termed as convective liquid mass and it exerts convective hydrodynamic pressure on tank wall and base. For representing these two masses and in order to include the effect of their hydrodynamic pressure in analysis, spring mass model is adopted for ground-supported tanks and two-mass model for elevated tanks.

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Fig 1.1: Two mass model for elevated tank

In spring mass model convective mass (mc) is attached to the tank wall by the spring having stiffness (Kc), where a impulsive mass (mi) is rigidly attached to tank wall. For elevated tanks two-mass model is considered, which consists of two degrees of freedom system. Spring mass model can also be applied on elevated tanks, but two-mass model idealization is closer to reality. The two- mass model is shown in Fig 1.1(a). where, mi, mc, Kc, hi, hc, hs, etc. are the parameters of spring mass model and charts as well as empirical formulae are given for finding their values. The parameters of this model depend on geometry of the tank and its flexibility. For elevated tanks, if the shape is other than circular or rectangular, then the values of spring mass parameters can be obtained by considering an equivalent circular tank having same capacity with diameter equal to that of diameter at top level of liquid in original tank. The two-mass model was first proposed by G. M. Housner (1963) and is being commonly used in most of the international codes. The response of the two degree of freedom system can be obtained by elementary structural dynamics. However, for most of elevated tanks it is observed that both the time periods are well separated. Hence, the two mass idealizations can be treated as two uncoupled single degree of freedom system as shown in Fig.1.1 (b). The stiffness (Ks) is lateral stiffness of staging. The mass (ms) is the structural mass and shall comprise of mass of tank container and one-third mass of staging as staging will acts like a lateral spring. Mass of container comprises of roof slab, container wall, gallery if any, floor slab, floor beams, ring beam, circular girder, and domes if provided. Fluid-Structure Interaction

The analysis of elevated tank under seismic load of Fluid- structure-interaction problems can be investigated by using different approaches such as added mass Westergaard or velocity potential, Lagrangian (Wilson and Khalvati), Eulerian (Zienkiewicz and Bettes), and Lagrangian Euclidian approach (Donea). These analyses can be carried out using FEM or by the analytical methods. The added mass approach as shown in Fig.1.2 can be investigated by using some of conventional FEM software such as SAP2000, STAAD Pro and LUSAS. Whilst in the other approaches, the analysis needs special programs that include fluid elements in the elements library, such as ANSYS, ABAQUS ADINA, ALGOR and etc. The general equation of motion for a system subjected to an earthquake excitation can be written as, M + C + Ku= -M g …….. 1.1 In which M, C and K are mass, damping and stiffness matrices with , and u are the acceleration, velocity and displacement respectively, and is the ground acceleration. In the case of added mass approach the form of equation 1.1 become as below.

M* + C + Ku= -M* g …………1.2 In which M* is the new mass matrix after adding hydrodynamic mass to the structural mass, while the damping and stiffness matrices are same as in equation 1.1

Fig 1.2: FEM Fluid-Structure-Interaction Model Westergaard Model’s method was originally developed for the dams but it can be applied to other hydraulic structure, under earthquake loads i.e. tank. In this paper the impulsive mass has been obtained according to GSDMA guideline equations and is added to the tanks walls according to Westergaard Approach as shown in Figure 1.3 using equation 1.3.



Where, ρ is the mass density, h is the depth of water and Ai is the area of curvilinear surface.

Fig 1.3: (a) Westergaard added mass concept (b) Normal and Cartesian directions. mai =[ ⍴ ]Ai …………1.3 In the case of Intze tank where the walls having sloped and curved contact surface, the equation 1.3 should be compatible with the tank shape by assuming the pressure is still expressed by Westergaard's original parabolic shape. But the fact that the orientation of the pressure is normal to the face of the structure and its magnitude is proportional to the total normal acceleration at the recognized point. In general, the orientation of pressures in a 3-D surface varies from point to point; and if it is expressed in Cartesian coordinate components, it would produce added-mass terms associated with all three orthogonal axes. Following this description the generalized Westergaard added mass at any point i on the face of a 3-D structure is expressed by the equation 1.4

1.4 Ai is the tributary area associated with node i, λi is the normal direction cosine(λ2y , λ2x , λ2z ) and ai is Westergaard pressure coefficient. Problem Description An Intze shape water container of 250 m3 capacity is supported on RC staging of 6 columns with

horizontal bracings of 300 x 600 mm at three levels. Details of staging configuration are shown in Figure 5. Staging conforms to ductile detailing as per IS 13920. Grade of concrete and steel are M20 and Fe415, respectively. Tank is located on hard soil in seismic zone IV. Density of concrete is 25 kN/m3. A FEM structural software SAP 2000 is used to model the elevated intze water tank as shown in Fig 5. Columns and beams in the frame type support system are modelled as frame elements (with six degrees of freedom per node). Conical part, bottom and top domes and container walls are modelled with thin shell elements (with four nodes and six degrees of freedom per node). Other dimensions of the elevated tanks are illustrated in Table 1.

TABLE 1: STRUCTURAL DATA FOR FRAME TYPE

Capacity of the tank 250 m3

Diameter of tank 8.6 m

Number of columns 6

Height of staging 16 m

Height of Cylindrical Wall 4.6 m

Rise of Top Dome 1.75 m

Rise of Bottom & Conical Dome 1.5 m

Number of Bracing Level 3, 4, 5

Top Dome 120 mm

Top Ring Beam 250 x 300 mm

Cylindrical Wall 200 mm

Bottom Ring Beam 500 x 300

mm

Circular Ring Beam 500 x 600

mm Bottom Dome 200 mm

Conical Dome 250 mm

Braces 300 x 600 mm

Columns 650 mm In the present study an alternate staging configurations are those which can be achieved by simple modifications to the Hexagonal Bracing Fig 2; for instance by adding cross bracing Fig 3 and radical bracing Fig 4 and levels of bracing as shown in Fig 6, Fig 7, Fig 8.



Fig 6: 3 Level Bracing Fig 7: 4 level Bracing Fig 8: 5 Level Bracing Results

TABLE 2- BASE SHEAR FOR 3 LEVEL BRACING

Fluid Level

Condition

BASE SHEAR (kN)

Bracing types

Hexagonal

Bracing

Hexagonal &

Cross Bracing

Hexagonal &

Radical Bracing

Hexagonal & Alt Cross & Radical

Bracing

Hexagonal & Alt Radical & Cross

Bracing Empty

227.29

255.34

260.81

257.41

258.49

Half Full

276.14

306.38

315.78

311.98

313.13

Full

316.14

352.44

360.43

355.06

356.94



TABLE 3- OVER-TURNING MOMENT FOR 3 LEVEL BRACING

Fluid Level

Condition

OVER-TURNING MOMENT (kN-m)

Bracing types

Hexagonal

Bracing

Hexagonal &

Cross Bracing

Hexagonal &

Radical Bracing


Bracing


Bracing Empty

3933.84

4320.02

4405.85

4351.24

4370.28

Half Full

4841.06

5231.85

5387.12

5325.28

5347.15

Full

5635.11

6136.79

6269.54

6179.97

6210.43

TABLE 4- MAXIMUM BENDING MOMENT AT THE BOTTOM OF COLUMN FOR 3 LEVEL BRACING

Fluid Level

Condition

MAX BENDING MOMENT (kN-m)

Bracing types

Hexagonal

Bracing

Hexagonal &

Cross Bracing

Hexagonal &

Radical Bracing


Bracing


Bracing Empty

115.69

122.84

119

123.65

118.23

Half Full

140.63

148.44

143.97

149.47

142.98

Full

160.91

169.80

164.22

170.74

163.26

TABLE 5- MAXIMUM STORY DISPLACEMENT FOR 3 LEVEL BRACING

Fluid Level

Condition

MAX STORY DISPLACEMENT (mm)

Bracing types

Hexagonal

Bracing

Hexagonal &

Cross Bracing

Hexagonal &

Radical Bracing


Bracing


Bracing Empty

15.693

15.419

15.251

15.336

15.318

Half Full

19.127

18.913

18.479

18.634

18.571

Full

21.942

21.366

21.175

21.287

21.238



Fig 9: Base Shear For 3 Level Bracing Fig 10: Over-Turning Moment For 3 Level Bracin

0

50

100

150

200

250

300

350

400

Baa

se S

hear

(kN

)

Fluid Level Condition

Hexagonal Bracing

Hexagonal & Cross Bracing

Hexagonal & Radical Bracing

Hexagonal & Alt Cross & Radical Bracing

Hexagonal & Alt Radical & Cross Bracing

0

1000

2000

3000

4000

5000

6000

7000

Ove

r-tu

tnin

g M

omen

t (kN

-m)

Fluid Level Condition

Hexagonal Bracing

Hexagonal & Cross Bracing

Hexagonal & Radical Bracing

Hexagonal & Alt Cross & Radical Bracing

Hexagonal & Alt Radical & Cross Bracing



Conclusion a) Base shear increases as bracing level increases

for different types of bracings. b) Base shear is more for Hexagonal & Radical

bracings of Full tank condition than Half Full and Empty condition.

c) Over-turning moment increases as bracing level increases for different types of bracings.

d) Over-turning moment is more Hexagonal & Radical bracings of Full tank condition than Half Full and Empty condition.

e) Bending Moment at bottom of column goes on decreasing as level of bracing increases for different bracing types.

f) Story displacement goes on decreasing as level of bracing increases and Hexagonal & Radical type bracing gives less story displacement as compared to other bracing types.

g) The performance of Hexagonal and Radical type bracing is better.

References [1] George W. Housner (1963) “The dynamic

behavior of water tanks” Bulletin of the Seismological Society of America. Vol.53, No. 2, pp. 381-387.

[2] IS: 11682-1985 “Criteria for design of RCC staging for over head water tanks”, Bureau of Indian Standards, New Delhi.

[3] IS:1893-2002(PartII) “Criteria for Earthquake Resistant Design of Structure (Liquid Retaining Tanks)”, Bureau of Indian Standards, New Delhi.

[4] Sudhir K. Jain, O R Jaiswal (2007) “IITK-GSDMA Guidelines for Seismic Design of Liquid Storage Tanks”.

[5] Stuctural Analysis Program SAP2000. “User’s manual, Computers and Structures, Inc., Berkley, Calif.

[6] H. Shakib, F.Omidinasab and M.T. Ahmadi (2010) “Seismic Demand Evaluation of Elevated Reinforced Concrete Water Tanks” International Journal of Civil Engineerng. Vol. 8, No. 3.

[7] Soheil Soroushnia, Sh. Tavousi Tafreshi, F. Omidinasab, N. Beheshtian, Sajad Soroushnia (2011) “Seismic Performance of RC Elevated Water Tanks with Frame Staging and Exhibition Damage Pattern” Procedia Engineering 14 ,pp.3076–3087.

[8] Dr. Suchita Hirde, Ms. Asmita Bajare, Dr. Manoj Hedaoo (2011) “Seismic Performance of Elevated Water Tanks” International Journal of Advanced Engineering Research and Studies, IJAERS/Vol. I/ Issue I/October-December, pp. 78-87.

[9] Pravin B.Waghmare, Atul M. Raghatate & Niraj D.Baraiya (2012) “Comparative Performance of Elevated Isolated Liquid Storage Tanks (With Shaft Staging)” International Journal of Advanced Technology In Civil Engineering, ISSN: 2231 –5721, Volume-1, Issue-2.

[10] Chirag N. Patel, Burhan k. kanjetawala, H. S. Patel (2013) “Influence of Frame Type Tapered Staging on Displacement of Elevated Water Tank” GIT-Journal of Engineering and Technology, Sixth volume, ISSN 2249 – 6157.