17-05-2014, 03:39 PM
Governing Loads for Design of A tall RCC Chimney
Governing Loads for Design.pdf (Size: 448.62 KB / Downloads: 115)
Abstract:
Design of tall chimneys requires dynamic analysis for loads due to self weight, earthquake and
wind. Because of changes in the dimensions of chimney, structural analysis such as response to earthquake and
wind oscillations have become more critical. The present paper discusses analysis of reinforced concrete tall
chimney. The main focus is to compare the wind analysis result with that due to seismic one. Wind analysis is
done for along wind by peak factor method as well as by gust factor method and for across wind by simplified
method as well as by random response method (shell completed case). The results obtained in above cases are
compared. The seismic analysis is performed using response spectrum method. Finally, the maximum values
obtained in wind analysis and seismic analysis are then compared for deciding the design values.
INTRODUCTION
As large scale industrial developments are taking place all around, a large number of tall chimneys would be
required to be constructed every year. The primary function of chimney is to discharge pollutants into
atmosphere at such heights and velocities that the concentration of pollutants deemed harmful to the
environment are kept within acceptable limits at ground level. Due to increasing demand for air pollution, height
of chimney has been increasing since the last few decades, and these are valid reasons to believe that this trend
towards construction of taller chimneys will continue. However, chimneys being tall slender structures, they
have different associated structural problems and must therefore be treated separately from other forms of tower
structure.
Construction of such tall chimneys needs the better understanding of loads acting on them and of the
structural behavior, so that with the help of modern construction equipment and technique such as slip form,
reinforced concrete, the most favored material for chimney construction, could be used efficiently. The proper
design and construction of such chimneys will create self standing structures to resist wind load and other forces
acting on them. It is a common practice to consider the effects of wind and earthquake separately in the design.
The present paper discusses analysis of reinforced concrete tall chimney. The main focus is to compare the wind
analysis result with that due to seismic one. Wind analysis is done for along wind and across wind (shell
completed case) and the results so obtained are compared with seismic analysis for deciding the design criteria.
HEADINGS
1. Description of Structure: A single flue reinforced concrete chimney is considered for the analysis
situated in seismic zone III. The flue gas emission point will be 220 m above the finished floor level. The liner is
essentially constructed from structural steel and shall be hung from the liner support platform near the chimney
top. The liners are provided with resin bonded wool type thermal insulation; there will be several internal
platforms of structural steel provided along the height of the chimney. Except for the roof platform, all the other
internal platforms will have steel grillage of beams covered with galvanized gratings; internal platforms are
provided for enabling access to various elevations of the chimney and provide restraint to the steel liners.
External concrete platforms are supported by the chimney shell. The chimney roof shall however comprise of a
reinforced concrete slab supported over a grid of structural beams. The roof slab shall be protected by layer of
acid resistant tiles. The grade level slab shall be of reinforced concrete. An internal structural steel staircase
supported from the floor at the bottom with a guide support from shell is considered up to the support platform.
There shall be rack and pinion elevator. Both the elevator and staircase will provide access to all internal and
external platforms.
CONCLUSION
The effect of wind forces is quite significant as compare to earthquake forces over 220 m height R.C.C chimney.
The geometry of chimney has to be so chosen that the deflection of chimney at the top is within permissible limits. The presence of gust wind over a considerable height of chimney plays important role as the forces
obtained by gust factor method are quite high along the sections considered except the top four sections where
the forces obtained in seismic analysis are higher. Elsewhere, the effect of earthquake forces seems
comparatively lesser along the height of the chimney. The cross wind analysis is taken care of by considering
first mode of oscillation as the critical wind speed is well within the design wind speed for the first two modes.
Having known this, a given tall reinforced concrete chimney can now be designed for respective wind and
seismic forces obviating the need for empirical formulae.