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1457695904-IS116821985watertankstaggingdesigncode.pdf (Size: 1.85 MB / Downloads: 12)
FOkEWORD
0.1 This Indian Standard was adopted by the Indian Standards Institution
on 29 November 1985, after the draft finalized by the Criteria for Design
of Structures Sectional Committee had been approved by the Civil
Engineering Division Council.
0.2 Water tanks are important public utility and industrial structure. The
design and construction methods in reinforced concrete are influenced by
the prevailing construction practices, the physical property of the material
and the climatic conditions At present there is no national standard for
the design of overhead tanks. This code is being published in order to fulfil
the need and to lay down uniform requirements of water towers.
0.3 While the common methods of design have been covered in this code,
design of structures of special forms or in unusual circumstances should be
left to the judgement of the Engineer-in-charge and in such cases special
systems of design and construction may be permitted on production of
satisfactory evidence regarding their adequacy and safety by analysis or
test or by both. ‘Care may be taken for dust load.
0.4 In this standard it is assumed that the design of water tower is entrusted
to the qualified engineer and that the execution of the work is carried
out under the direction of an experienced supervisor.
0.5 The construction aspects of liquid retaining structures and design aspects
of the side walls and floor of the same design aspects of the side
walls and floor of the same which mainly rest on the ground level have
been covered by IS : 3370 ( Pdrts 1 to 4 )* and this standard lays down the
principles of design of water towers for arriving at sizes of various components
of water tank.
1. SCOPE
1.1 This standard lays down guidelines for layout for overhead water
tanks and criteria for analysis for RCC staging both for steel and concrete
tanks.
1.2 While some of the provisions of this standard in the case of RCC staging
for elevated tanks, though refer to the storage of water, the recommendations
are equally applicable to other materials stored.
1.3 The requirements given in this code applicable for column type staging
and circular and polygonal shaft staging for overhead water tanks.
2. TERMINOLOGY
2.0 For the purpose of this standard, the following definitions shall apply.
2.1 Capacity - Capacity of the tank shall be the volume of water it can
store between the designed full supply level and lowest supply level ( that
is, the level of the lip of the outlet pipe ). Due allowance shall be made
for plastering the tank from inside if any when calculating the capacity of
tank.
2.2 Height of Staging - Height of staging is the difference between the
lowest supply level of tank and the average ground level at the tank site.
2.3 Water Depth - Water depth in tank shall be difference of level between
lowest supply level and full supply level of the tank.
LOADS
3.1 Dead Loads - Dead loads shall be calculated on the basis of unit
weights taken in accordance with IS : 1911-1967*. Unless more accurate
calculations arc warranted, the unit weight of reinforced concrete made
with sand and gravel or crushed natural stone aggregate may be taken as
2 500 kg/ms. Loads due to pipings and stair cases should also be considered.
3.2 Imposed Loads - Imposed loads like live loads, snow loads and
wind loads shall be in accordance with IS : 875-1964t. Weight of the
water may be taken as live load for members directly containing the same.
The weight of water shall be considerd as dead load in the design of
staging.
3.3 Wind Load - Wind load shall be applied in accordance with IS :
875-19641 while analying the stresses the combination shall be as
follows:
a) wind load with tank empty; and
b) wind load with tank full.
The worst combination of the stress on account of the above shall be
considered while working out the permissible stresses.
3.4 Seismic Forces - When seismic loading is considered, following two
cases may be considered :
a) tank empty; and
b) tank full.
In additiqn, wherever required the effect of surge due to wave formation
of the water may be considered. The seismic force acting on the
support for the tank and its analysis shall be in accordance with IS :
1893-1975$.
3.5 Vibration Forces - Vibration forces such as due to blast forces
( see IS : 6922-1973s ) as experienced in mines, collaries and in the close
proximity of railway tracks shall be considered in the design ( see
also 7.1.1 ).
4. ANALYSIS
4.1 General - Provision shall be made for conditions of stresses that
may occur in accordance with principles of mechanics, recognized methods
of design and sound engineering practice. In particular, adequate consideration
shall be given to the effects of monolithic construction in the
assessment of bending moment and shear.
4.1.1 Before taking up the design, the designer should first decide the
most suitable type of staging of tanks and correct estimation of loads including
statical equilibrium of structure particularly in regard to overturning
of overhanging members shall be made. The design should be based
on the worst possible combination of loads, moments and shears arising
from vertical loads and horizontal loads acting in any direction when the
tank is full as well as empty.
4.2 Loading Combination - Loading combination may be considered
as given in IS : 456-1978* and IS : 875-1964t. Both conditions, tank full
and tank empty shall be considered.
NOTE - Wherever required the effect of wave action of water on the sides o f
the tank may be considered.
5. BASIS OF DESIGN FOR REINFOIiCED CONCRETE MEMBERS
5.1 Staging and other reinforced concrete members including foundation
shall be designed in accordance with the requirements of IS : 456-1978*.
5.2 Permissible Stresses - The permissible stresses in the concrete
and steel reinforcement shall be in accordance with IS : 456-1978* for
c&nnn staging.
5.3 Seismic Effect - Earthquake resistant design of water tanks shall be
carried out in accordance with IS : 1893-1975$.
3.4 Increase in Permissible Stresses - Increase in permissible stresses
for column staging shall be as per IS : 456-1978*.
5.4.1 The increase in permissible stresses as per 5.4 need not be allowed
in the design of braces for forces as wind or earthquake which are primary
forces in them.
LAYOUT OF OVERHEAD TANKS
6.0 Generally the shape and size of elevated concrete tanks for economical
design depends upon the functional requirements such as:
a) Maximum depth for water;
b) Height of staging;
c) Allowable bearing capacity of foundation strata and type of foundation
suitable;
d) Capacity of tank; and
e) Other site conditions.
6.1 Classification and Layout of Elevated Tanks - Based on the
capacities of the tank, the possible classification for types of elevated tanks
may be as followed as given in 6.1.1 to 6.1.4 for general guidance.
6.1.1 For tank up to 50 ms capacity may be square or circular in shape
and supported on staging three or four columns.
6.1.2 Tanks of capacity above 50 ms and up to 200 ms may be square
or circular in plan and supported on minimum four columns.
6.1.3 For capacity above 200 ms and up to 800 ms the tank may be
square, rectangular, circular or intze type tank. The number of columns
to be adopted shall be decided based on the column spacing which normally
lies between 3.6 and 4.5 m. For circular, intze or conical tanks, a
shaft supporting structures may be provided.
6.1.4 Different shapes of water towers with certain arrangements of bottom
construction are shown in Fig. 1 to 4.
6.2 Besides the general shapes given in 6.1.1 to 6.1.4, tanks of unusual
shapes, such as spherical, conical or multi’cell may also be adopted depending
upon the discretion of the designer.
7. ANALYSIS OF STAGING
7.1 Staging Components
7.1.1 Columns
7.1.1.1 Forces and moments on columns - The entire load of the tanks
shall be considered to be transferred to the columns in the manner in which
the floor of the tank contributes to each column. The effects of continuity
of the beam at the top of columns, if any, shall be accounted for in calculating
the reactions on columns. In addition to tank load, axial forces, forces