Comparative Study on NDCT with Different Shell Supporting Structures

INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
59
Comparative Study on NDCT with Different Shell
Supporting Structures
K.Vignesh1
M.Ramesh Kannan2
Graduate Student, Division of Structural Engineering Assistant Professor, Division of Structural Engineering
School of Mechanical and Building Sciences School of Mechanical and Building Sciences
VIT Chennai, Chennai, India VIT Chennai, Chennai India
vignesh.k2013@vit.ac.in rameshkannan.m@vit.ac.in
Abstract— Natural draft cooling towers are very essential in modern days in thermal and nuclear power stations. These are the
hyperbolic shells of revolution in form and are supported on inclined columns. Several types of shell supporting structures such
as A,V,X,Y are being used for construction of NDCT’s. Wind loading on NDCT governs critical cases and requires attention.
In this paper a comparative study on reinforcement details has been done on NDCT’s with X and Y shell supporting structures.
For this purpose 166m cooling tower with X and Y supporting structures being analyzed and design for wind (BS & IS code
methods), seismic loads using SAP2000.
Index terms—Geometry, Raker Columns, SAP2000V17.0, Sandwich method, Soil Loads, Wind Pressure.
——————————  ——————————
1.INTRODUCTION
A cooling tower is a structure which is designed for the
evaporative cooling of water where hot water gets cooled by direct
contact with air. Cooling towers are subjected to its self-weight
and the dynamic load such as an earthquake motion and a wind
effects. In the absence of earthquake loading, wind constitutes the
main loading for the design of natural draught cooling towers. The
shell structure is supported on inclined raker columns. There are
several types of columns such as A,V,X,I,Y etc. The columns used
for the cooling tower should withstand the self-weight of the
structure and the loads acting on the structure. Therefore a
comparative study on analysis and design has been done on
cooling towers with X and Y supporting structures considering
wind loads.
N.Prabakar [1] gave a brief description about the working
principle and salient features of the cooling towers and the loads
considered in cooling towers such as dead load, wind forces, soil
pressure, earthquake loads and temperature loads. the shell
reinforcement gives the clear idea about the stress distribution in
circumferential and meridional surface. Dieter Buscha
, Reinhard
Harteb
, Wilfried B. Kra¨tzig c
, Ulrich Montagd
[2] has given a clear
idea in design of 200m cooling tower here the stress distribution
between the meridional and circumferential direction. In this paper
it shows the description of geometry and followed by elucidation
of the conceptual shape optimization. A.M. Nasira
, D.P.
Thambiratnama
, D. Butler b
, P. Austinb
[3] in this paper the
hyperbolic tower is analyzed and the behavior of the tower is
studied under earthquake loading. maximum meridional stress
resultants and lateral deflection of the shell is found out. Tejas G.
Gaikwad, N. G. Gore, V. G. Sayagavi, Kiran Madhavi, Sandeep
Pattiwar[4] this paper gives an idea about the behavior of cooling
tower in wind load conditions. Gust and peak winds methods are
adopted in finding the wind pressure along the height and
circumferential. Hoop stress is greatly affected by the changes in
shell curvature. G. Murali, C. M. Vivek Vardhan and B. V.
Prasanth Kumar Reddy [5] in this paper three different cooling
towers of varying height and parameters are being analyzed for
wind loads and the optimization of the towers is done by
comparing the meridional and circumferential stress resultants.
2.SALIENT FEATURES OF TOWER
The shell structure consist of two hyperbolic shell of
revolution both meeting at the throat of the cooling tower. The
overall height of the cooling tower is 166m, it consist of varying
thickness throughout the height of the structure. The top diameter
of the tower is 79m, diameter of throat level is 78.5m, base
diameter of the shell is 125.6m, height of throat from sill level is
about 124.5m, and height of raker columns with respect to sill
level is 9.81m. The geometry of hyperbola is derived from the
equation of hyperbola with respect to the height of tower.
The tower shell is supported on inclined raker columns. There
are several types of raker columns such as A,V,X,I and Y. In this
paper X and Y raker columns are used for analysis and design.
raker columns consists of inclined circular columns and pedestals.
The diameter of circular column is 1.4m and 1.2m and size of
pedestal is 3mx1.5m and 3.5mx1.8m for X and Y columns
respectively. The tower foundation consists of annular raft
foundation is about 3mx1.8m in depth.
2.1. Geometry of Hyperbolic Curve
The hyperbolic curve is derived from the hyperbolic equation.
NDCT are mostly designed as thin shell structures supported along

60
the circumference by a system of columns. Here the tower is
designed for variation of thickness over the height of the tower.
TABLE1
Variation of Radius and Thickness Along the Height.
Fig1. Cooling tower specifications. Tower specifications with top, throat and
bottom radius.
3. LOADS
Hyperbolic cooling towers may be subjected to various
loading such as dead load, wind loads, earthquake loads,
temperature loads and soil loads. For the proportioning of
elements in cooling towers the effect of various loading conditions
should be factored and combined as per codes and standards.
The loads considered for analysis and design of NDCT are
dead loads, wind loads, and soil loads. The effects of these loads
are factored and combined by using BS4485-4-1996 codes and
IS875-3-1987.
c.1. Wind Forces
Wind forces are the major externally applied force in design
of cooling towers. In this paper the gust and mean wind forces
applied on NDCT as per IS875-3-1987 and BS4485-4-1996.
3.1.1. IS 875-3-1987 Gust Wind Procedure
In this procedure actual design wind speed is calculated from
the product of k1, k2 and k3 with basic wind speed. The wind
pressure is calculated along the height of the tower and along the
circumference of the tower. The basic wind speed is chosen as per
the location of the tower, the risk coefficient (K1) is taken as per
IS875, and the terrain category (K2) is taken as per table 33 of
IS875 and topography factor (K3) is taken as 1 as per IS875.
Pressure coefficient, Cp is calculated as per IS11504-1985. Gust
effectiveness factor and interference factor should be multiplied
with design pressure to get actual design pressure.
actual design pressure, PZ = 0.6 x (VB.K1.K2.K3)2
.CP.GF.IF
3.1.2. IS 875-3-1987 Mean Wind Procedure
In this procedure actual design wind speed is calculated from
the product of k1, k2 and k3 with basic wind speed. The wind
pressure is calculated along the height of the tower and along the
circumference of the tower. The basic wind speed is chosen as per
the location of the tower, the risk coefficient (K1) is taken as per
IS875, and the terrain category (K2) is taken as per table 2 of
IS875 and topography factor (K3) is taken as 1 as per IS875.
Pressure coefficient, Cp is calculated as per IS11504-1985.
Interference factor should be multiplied with design pressure to get
actual design pressure.
actual design pressure, PZ = 0.6 x (VB.K1.K2.K3)2
.CP.IF
3.1.3. BS 4485-4-1996 Gust Wind Procedure
In this procedure site wind speed is calculated from the
product of k1, k2 and k3 with basic wind speed. The design mean
wind speed is calculated by product of site wind speed(Vs) and
gust wind factor (Sgz). The wind pressure is calculated along the
height of the tower and along the circumference of the tower.
Pressure coefficient, Cp is calculated as per BS4485-4-1996.
actual design pressure, PZ = 0.613 x (Vgz)2
.CP
3.1.4. BS 4485-4-1996 Mean Wind Procedure
In this procedure site wind speed is calculated from the
product of k1, k2 and k3 with basic wind speed. The design mean
wind speed is calculated by product of site wind speed(Vs) and
mean wind factor (Smz). The wind pressure is calculated along the
height of the tower and along the circumference of the tower.
Pressure coefficient, Cp is calculated as per BS4485-4-1996.
actual design pressure, PZ = 0.613 x (Vmz)2
.CP

61
Fig2. Circumferential net wind pressure distribution.
The wind pressure outside the shell is assumed to be
symmetrical about the centre line in the direction of wind. The
wind pressure is calculated as per IS11504-1985 and BS4485-2-
1996. The pressure distribution is calculated over the height of the
tower.
P1
= Σ Fncosnϴ
c.2. Soil Pressure
Raker columns are constructed usually below the ground level.
The ground level lies on the top level of the pedestal. The total
load from the shell transferred to foundation through raker
columns. The soil load can be calculated as vertical and inclined
portion that acts on the foundation and pedestals.
c.3. Load Combination
The various design loads should be combined in accordance
with the relevant design codes. Whichever combination provides
maximum effect to the building or structure should be concerned
for the design. In this paper the loads considered are dead load,
wind loads and soil loads. The load combination is in accordance
with IS456-2007and BS4485-4-1986.
Load combinations as per IS456-2007
a) 1.5 Dead load + 1.5 IS Gust wind + 1.5 soil vertical
b) 0.9 Dead load + 0.9 IS Gust wind + 0.9 soil vertical
Load combinations as per BS4485-4-1996
For design of tower shells:
c) Dead load + 1.5ΦG BS Mean wind + soil vertical
d) 1.4 Dead load + 1.5ΦG BS Mean wind + 1.4 soil vertical
For design of raker columns
e) Dead load + 1.5ΦF BS Mean wind + soil vertical
f) 1.4 Dead load + 1.5ΦF BS Mean wind + 1.4 soil vertical
ΦG, ΦF- wind amplification factors should be evaluated using
meridional stress resultants from the windward side of the tower.
ΦG is for the tower shell, here the tower shell is divided into three
parts as per BS4485-4-1996. ΦF is for the shell supporting
structures (i.e) raker columns which is evaluated at a height above
the base of the shell equal to the distance between adjacent shell
support nodes.
4. DESIGN ASPECTS
4.1. Analysis
The analysis of the towers are challenging task for a
structural designer because of their shape combined with non-
axisymmetric horizontal loads. In this paper the tower is modeled
and analyzed using SAP2000 V17.0. The tower is divided into 105
slices of horizontal plates. Hence each horizontal slice is divided
into 288 shell areas. The advantage of the finite element model of
tower is that the structural modeling can include raker columns
and tower foundations. Two sets of tower is being model one with
X supporting structure and the other with Y supporting structure.
The loads considered for the analysis of both the towers are dead
loads, wind loads, soil loads. The behavior of the two towers has
been studied by using the mode shapes and frequencies of the
respective towers.
Fig 3. Frequency vs time period
Frequency of first mode of tower with X shaped raker columns is
0.86815 cyc/sec, similarly frequency of the first mode of tower
with Y shaped raker column is 0.955 cyc/sec. as per IS875-3-
1987 buildings and closed structures with a height to minimum
lateral dimension ratio of more than about 5.0 and buildings and
closed structures whose natural frequency in the first mode -is
less than 1 Hz. Any building or structure which does not satisfy
either of the above two criteria shall be examined for dynamic
effects of wind. In this paper the tower with Y shaped raker
column is much stiffened when compare to tower with X shaped
raker columns.
4.2. Shell Design
The shell design is generally governed by the direct
tension and moment acting on the section due to dead load, wind
load and soil loads. In this paper the design of hyperbolic shells
are done using eurocode EN1992-2-2005. Sandwich method is
adopted to design the shells. In this method the shells are divided
into three layers upper layer, shear layer and lower layers and a
total of eight forces will act on the shell. In this method out of
plane forces are also should be taken into account for design of
shells. The outer layer carries the membrane action and the inner
layers carry the shear forces.
4.3. Shell Reinforcement
Shell reinforcement will be in meridional and
circumferential direction. The shell reinforcement is very sensitive
to wind loads. In this paper the shell reinforcement is compared
between the towers with X and Y raker columns. Usually the
reinforcements in the meridional govern more when compare to

62
circumferential. But circumferential reinforcements hold the
meridional reinforcements and prevent from displacements.
Fig4. Circumferential Vs Meridional top reinforcement
Fig5. Circumferential Vs Meridional bottom reinforcement
In the above figure it clearly shows that meridional reinforcements
govern the circumferential reinforcements. But the circumferential
reinforcements will be provided outside the meridional
reinforcements in order to hold the total compression and tension
forces in the meridional reinforcements.
Fig 6. Circumferential top reinforcement
From the above graph it clearly shows that tower with Y raker
columns governs more area of steel when compare to tower with
X raker columns but over a small scale. From the height of 80m to
120m both the towers have minimum area of steel.
Fig7. Circumferential bottom reinforcement
From the above graph it clearly shows that tower with X raker
Y raker columns.
Fig8. Meridional top reinforcement
Y raker columns.
Fig9. Meridional bottom reinforcement
Y raker columns.
4.4. Column Design
The column design is generally governed by axial loads
acting on the column due to load combinations. The loads that take
place on the columns are dead load, wind loads and soil loads acts
on the pedestals. There are two types of columns short and long
columns. Columns are designed as short columns when the ratio of
effective length to the breadth or depth is less than 12 similarly for
long columns when the ratio between the effective length and
breadth or depth of column is greater than 12. In this paper all the
columns are designed as short columns as per SP16-1980.
Fig10. Circular columns
here the Y type of columns governs more area of steel when
compare to X type of columns due to its axial force and the
percentage of reinforcement 0.8% for X type column and it 1.8%
for Y type column.

63
Fig11. Rectangular columns
Here the Y type of columns governs more area of steel when
compare to X type of columns. The percentage of reinforcement is
0.8% and its same for both the X type columns and Y type
columns. Due to geometric parameters there is increase in the area
of steel.
4.5. Foundation Design
The load of the structure is being transferred to foundation
through raker columns. The type of foundation adopted is anular
raft foundation. It’s a ring type foundation. The foundation is
being designed for the maximum moments occur in the face of the
column. And the area of steel is provided for the concerned
moments. Minimum reinforcements are provided for both tower
with X type columns and Y type columns.
5. CONCLUSION
Cooling towers are exceptional structures which require
special expertise in design. Since its huge structure the quantity of
the structure will also be high. Proper selection of parameters and
elements will reduce the quantity of the structure. This paper
shows the behavior of the towers with respect to wind forces, self-
weight and soil loads and the design of hyperbolic shells using
sandwich method. This paper emphasizes the idea of using the
proper columns for the towers. The hyperbolic shell is rests on the
raker columns which distributes the loads from the towers to the
foundation. So a proper care should be taken in selection of raker
columns. In this paper it shows that tower with X type of columns
governs more steel when compare to tower with Y type of
columns.
REFERENCES
[1] N.Prabakar (1990), Gammon india, “Structural Design Aspects of
Cooling Tower”.
[2] Dieter Buscha
, Reinhard Harteb
, Wilfried B. Kra¨tzig c
, Ulrich
Montagd
(2002), “New natural draft cooling tower of 200 m of
height”.
[3] A.M. Nasira
, D.P. Thambiratnama
, D. Butler b
, P. Austinb
(2001),
“Dynamics of axisymmetric hyperbolic shell structures”.
[4] Tejas G. Gaikwad, N. G. Gore, V. G. Sayagavi, Kiran Madhavi,
Sandeep Pattiwar (2014), “Effect of wind loading on analysis of
natural draft hyperbolic cooling tower”
[5] G. Murali, C. M. Vivek Vardhan and B. V. Prasanth Kumar Reddy
(2012) “Response of Cooling Towers to Wind Loads”
[6] BS4485-4-1996 “Water cooling towers code of practice for
structural design and construction”
[7] BS6399-2-1997 “ Loading for buildings code of practice for wind
loads”
[8] EN1992-2-2005 “Design of concrete structures, concrete bridges,
design and detailing rules.
[9] IS11504-1985 “Criteria for structural design of reinforced
concrete natural draft cooling towers”
[10] IS875-3-1987 “code of practice for design loads for buildings and
structures,part-3 wind loads”
[11] SP16-1980 “Design aids for reinforced concrete to IS456:2000”

Comparative Study on NDCT with Different Shell Supporting Structures

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Comparative Study on NDCT with Different Shell Supporting Structures