16-09-2016, 02:24 PM
1454930628-EQUATIONFORESTIMATIONOFFUNDAMENTALTIMEPERIODFORELEVATEDWATERTANK2.pdf (Size: 217.04 KB / Downloads: 61)
ABSTRACT
Earthquakes are one of the most devastating natural hazards that cause great loss of life and
livelihood. The determination of the natural period of vibration of a reinforced concrete structure is
an essential procedure in earthquake design and assessment. The fundamental period of vibration of
a reinforced concrete structure appears in the equation specified in building codes to calculate the
design base shear and lateral forces. To estimate the period, building codes provide empirical
formulas that depend on the building material, building type and overall dimensions of the structure.
The aim of the present investigation is to propose a simplified period – height equation for use in the
seismic force estimation of elevated water tank. The period of vibration which has been derived
herein represents the period of first mode of vibration. The study includes the seismic response of
elevated water tank with different size, shape, capacities and varying height for high seismic zone in
India. Various analytical models were prepared using SAP 2000 V-14.2 software. The new
formulation for the estimation of time period of Elevated water tanks are developed using regression
analysis from the statistical data generated. The combined general equation thus arrived at, can be
used in general for Intz and funnel type of elevated water tanks irrespective of the capacities, height,
type of staging, and type of bracing as an alternative to the already available codal provisions of IS:
1893-2002.
1. INTRODUCTION
Water is essential to humans and other life forms. Sufficient water distribution depends on
design of a water tank in certain area. Many new ideas and innovation has been made for the storage
of water and other liquid materials in different forms and fashions. There are different ways for the
storage of liquid such as underground, ground supported, elevated. 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.
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. These 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 water tanks are
frequently constructed and used in seismic active regions also, seismic behaviour of them has to be
investigated in detail.
The present study is an effort to identify the behaviour of elevated water tank under different
parameters such as various size, shape, capacity, height, type of supporting structure etc. with
consideration and modeling of elevated water tank using structural software SAP2000 and hence
generating empirical equation for estimation of fundamental time period.
2. 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).[1] For representing two masses and in order to include the effect of their
hydrodynamic pressure in analysis, spring mass model is applied on elevated tanks. 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 [1].
The behaviour of supporting system, which is more effective under different earthquake time
history records with SAP 2000 software was carried out by Ayazhussain M. Jabar, H. S. Patel [2].
Here, two different supporting systems such as radial bracing and cross bracing were compared with
basic supporting system for various fluid level conditions. Tank responses including base shear,
overturning moment and roof displacement have been observed, and then the results have been
compared and contrasted. Conclusions were drawn that cross bracings are more stable and suitable
for construction from earthquake point of view.
Sloshing response of elevated water tank over alternate column proportionality was studied
by Chirag N. Patel, Shashi N. Vaghela and H. S. Patel [3]. Amongst all type of column
proportionality, circular and rectangular wide were proven highly competitive to withstand against
sloshing displacement under different earthquake characteristics.
H. Shakib, F. Omidinasab and M.T. Ahmadi studied the Seismic Demand Evaluation of
Elevated Reinforced Concrete Water Tanks [4]. The elevated tanks period showed that simultaneous
effects of mass increase and stiffness decrease of tank staging lead to increase in the natural period.
Durgesh C. Rai and Bhumika Singh studied the seismic design of concrete pedestal supported
tanks and concluded that when a tank is empty, flexure strength governs the failure mode for all
aspect ratios (ratio of height to diameter) of the support shaft and time periods of the tanks. And
when tank is full, shear mode is found to be governing failure of stiffer shafts having short time
period and low aspect ratios
3. PROBLEM DEFINITION
The typical plan in 2D and 3D models of elevated water tank are prepared. The models are
kept symmetric in both orthogonal directions. Container vessel for storage of water is considered in
circular shape and funnel shapes with varying diameter and height. The height of supporting
structure is varied respectively in proportions.
3.1 Analysis of Water Tank
The loads are applied to the prepared model as the guidelines provided in IS 875-1987, IS
1893-2002. Then the time period values are calculated and the behaviour is studied using structural
analysis software SAP-2000.
3.2 Study Parameters
The present study is all about the behaviour of elevated water tank under free vibration and
generation of empirical equation for computation of fundamental time period. Various parameters
are as listed below,
• Shape- Intz and funnel shapes.
• Capacity - 10, 15, 20 lakh liters.
• Height of supporting structure- 16m, 20m and 24m
• Type of supporting structure - Framed support, shaft support.
• Tank fill condition - Full, half full, empty
CONCLUSIONS
The important findings of this study are summarized below.
1. The time period for the tank with full filled condition is more than the time period for empty
condition.
2. The fundamental natural period of a Elevated water tank increases with increase in the height
of supporting structure for the tank.
3. The fundamental natural period of a Elevated water tank increases with increase in the
storage capacity of the tank.
4. Intz tanks with shaft type of supporting structure are to be preferred.