Figure 1. 3D View of RC Chimney
Reinforced Concrete chimneys are commonly used in major industries and power plants .Chimneys are slender structures, and loads acting on the chimney are self weight, wind loads, earthquake loads, and temperature loads. The designs of chimneys are normally governed by wind or earthquake loads. The dynamic characteristics of the RC chimney will vary in a wider range with respect to the aspect ratio (ratio of height to longitudinal section). In this paper, Dynamic analysis of an RC chimney in zone III is carried out by Response spectrum analysis as per IS 1893:2005 for different top and bottom diameters of the chimney and varying thickness of the shell using software SAP2000 version 18.0.1. Grade of concrete used is M25. The influence of various geometric parameters H/D and D/T ratios on the modal parameters and dynamic response of the structure, i.e., top displacement, fundamental frequencies, and Base shear are investigated.
A chimney is a system for venting flue gases or smoke from a boiler or furnace to the outside atmosphere. Chimneys are generally provided in the industries and power plants to discharge pollutants into certain height of atmosphere with certain velocities so that the pollutants do not harm the environment. To lessen the atmospheric pollution, the height of the chimney is increased. They are used to discharge waste/flue gases at higher elevation with sufficient exit velocity. They are typically tall and slender structures such that the gases and the suspended soils (ash) are dispersed into the atmosphere over a defined spread so that their concentration, on reaching the ground is within acceptable limits.
As the load exerted by the wind and earthquake on the chimney is dynamically sound, and effective, that will tend the chimney to undergo peak displacement and acceleration. Because of its slenderness, chimneys are the structures supposed to retain the critical loads by seismic and wind effects.
Chimneys with height exceeding 150 m are considered as tall chimneys.
The main purpose of the study is to analyze and compare the responses, such as base shear and top displacement of the RC chimney subjected to seismic lateral loads.
The objectives of the present study are as listed below,
The parametric study of RC chimney in analysis of self supporting chimney, which is made by obtaining the results from software for different heights, diameter, earthquake zones, wind zones, type of soils, and different load conditions. Due to changes in the dimensions of the chimney, structural analysis, such as response to earthquake and wind oscillations have become more critical to influence on the response and design of the chimney? Varying heights of the chimney from 150 meters to 250 meters at an interval of 5 meters, for Zone II, Hard soil & Critical Zone of Zone V, Soft soil with wind speed varying from 33 m/s to 55 m/s with an internal temperature of 100 Degrees is studied. The analysis is carried out using programming software Microsoft Visual Basic 6.0. The results obtained from the above cases are compared. The maximum values obtained in wind analysis and seismic analyses are then compared to deciding the design value [5].
Wind and seismic analysis of 60 m reinforced concrete chimney by Anil and Siva (2014) were studied and compared. Seismic analysis is done as per IS 1893 (part 4): 2005 and wind analysis as per Draft Code CED38 (7892): 2013. If a chimney is located in a higher seismic zone with lower wind speeds, then, seismic forces may become analogous, if not more, than the wind loads. It is designed for both, along wind and across wind loads. In this Governing Loads for Design of a 60 m Industrial RCC Chimney paper procedure given by the draft code as mentioned above was used to obtain the combined design loads. Designing of 60 m RCC chimney has seismic loads and wind loads; both loads are compared to decide the governing loads for the design of the RCC chimney shell by ensuring proper design and construction. Results of the analysis confirmed that the effect of wind force for 55 m/s wind speed is quite significant as compared with the earthquake forces in Zones in II and III. Moment due to seismic forces in Zone III is almost equal to the combined moment due to wind speed of 55 m/s [4].
The effects of changes in the dimensions of the chimney on the modal parameters such as fundamental frequency, Displacement etc. was studied. The modal analysis of a RCC chimney in a cement factory is carried out using the FEM software package ANSYS. The chimney height is taken as 60 m. The outer diameter of the chimney on top is 3.052 m and bottom is 5.73 m, respectively. The thickness at the top is 160 m and bottom is 330 mm, and the geometric parameter D/T ratio has a positive response to the fundamental frequency of the chimney. The displacement of the chimney is found to decrease with the increase in all geometric parameter ratios [7].
Aneet, et al. (2016), in their paper comprises a literature review of latest papers published in the field of industrial chimneys. This study offers a comprehensive review of the research papers published in the field of dynamic analysis carried out on the chimneys. The article gave the latest information and developments taken place in chimney analysis and design. This paper mainly focused on dynamic analysis, linear and non-linear analysis, soil structure interaction studies, Seismic and wind analysis, etc. This paper gave a complete collection of the studies carried out on dynamic analysis and would give an updated material for researchers [2].
The effect of base shear, maximum lateral displacement, fundamental time period, and frequency of all the zones from zone 2 to zone 5 and their comparison of the results of all the zones with the linear static and dynamic analysis of RC and Steel chimneys having a height of 65 m and chimneys were modelled with the help of the SAP2000 Version 12.00 Software [6].
The dynamic behavior of the RC chimney that varies in a wider range with respect to the height and longitudinal section of the chimney as the load exerted by the wind and earthquake on the chimney are dynamically sound and effective and tending the chimney to undergo peak displacement and acceleration. Because of its slenderness, chimneys are the structures supposed to retain the critical loads by seismic and wind effects. Amit, et al. (2015) presented the study of along wind load and earthquake load effects on RC chimneys in zone I (basic wind speed 33m/s). Seismic analysis is carried out by time history analysis as per IS 1893 (part 4): 2005 and wind analysis by along wind effects by gust factor method as per draft code CED 38 (7892): 2013 (third revision of IS 4998 (part 1:1992) for different heights varying from 150 to 300 m and for different longitudinal sections such as uniform, tapered and uniform-tapered by using the software SAP- 2000. They concluded that RC chimney with more height and uniform section will be critical compared to other types and the best suitable section will be uniform tapered for both seismic and wind load effects exhibiting minimum displacement [1] .
The steps included in the methodology of this paper are described below.
Step 1: Modeling of RC chimney by varying H/D and D/T ratios.
Step 2: Performing Modal analysis using SAP2000 for determining the fundamental natural time period (T) and frequency.
Step 3: Performing linear dynamic response spectrum analysis using SAP2000 to get the maximum displacements and base shear.
Step 4: Summarizing, tabulating, and comparing the results.
The chimney 180 m height with fixed support and elements of the chimney to ensure the cantilever action of the chimney were used.
For the present studies, 16 models of RC chimney are chosen with four different diameters and four different thicknesses of the shells. The diameter of the tapered chimney at the bottom is 18 m and is varying uniformly up to the top 9 m. The thickness of the RC shell is 0.36 m. The slope of tapering in 1 in 50. 3D view of chimney is shown in Figure 1.
Figure 1. 3D View of RC Chimney
The following are the sample data.
Outer diameter of Chimney at top = 18 m
Outer diameter of chimney at bottom = 9 m
Height of Chimney = 180 m
Taper of Chimney = 1in 50
Thickness of concrete shell = 0.360 m
Grade of concrete = M25
Seismic zone = Zone III
Type of Soil = Hard soil
Grade of the steel = Fe415
Geometric parameters that have an influence on dynamic response, such as H/D and D/T ratios are considered in the analysis. Table 1 shows the summary chart of varying geometric parameters such as the thickness of the shell varying from 0.36 m to 0.21m, Top diameter varying from 18 m to 7.2 m, bottom diameter varying from 9 m to 3.6 m. 16 models are analyzed by taking 4 different shell thicknesses for each H/D ratio.
Table 1. Summary Chart of Geometric Parameters
Modal analysis is a procedure which evaluates freevibration mode shapes to characterize displacement patterns. Mode shapes describe the configurations into which a structure will naturally displace. The primary concern is lateral displacement patterns. The fundamental mode shape of the chimney is shown in Figure 2.
Figure 2. Modal Analysis
This analysis is performed to get the dynamic characteristics, such as natural time period and mode shapes of the chimney.
Chimney with height 180 m, bottom diameter 18 m, top diameter 9 m with varying thickness of the shell from 0.36 m to 0.21m (Thickness of the shell vs. Time period). These results correspond to Modal analysis, performed in SAP2000 and are shown in Table 2. It is observed that with constant top and bottom diameters and varying thicknesses, Natural time period is almost same.
Table 2. Results of Modal Analysis
Response spectrum analysis is the structural analysis to get a response of the structure when subjected to earthquakes.
This method involves the calculation of only the maximum values of the displacements and member forces in each mode of vibration using smooth design spectra that are the average of several earthquake motions. The structure is analyzed by Response spectrum method using software SAP2000 18.0.1, in accordance with codal provisions given in IS 1893 (Part-1): 2002 [3] . Data considered for the analysis are given below.
Seismic zone : III
Importance factor : I: 1.5
Response reduction factor : R: 3.00
Soil type : Hard
Damping : 5 %
Chimney with height 180 m, bottom diameter 18 m, and top diameter 9 m with varying thickness of the shell from 0.36 to 0.21m was chosen. These results correspond to the Response spectrum analysis, performed in SAP2000 are presented in Table 3. Figure 3 shows the graph variation of (Thickness of the shell vs. Displacement) displacement with shell thicknesses for a chimney with D=18 m, d=9 m. It is observed that when the thickness of the shell decreases from 0.36 m to 0.21 m, the displacement increased by 20.8 %.
Table 3. Results of Response Spectrum Analysis
Figure 3. Variation of Displacement with shell thicknesses for a Chimney with D=18 m, d=9 m
Chimney with height 180 m, bottom diameter 12 m, top diameter 6 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. The results correspond to Response spectrum analysis, performed in SAP2000 are shown in Table 3. Figure 4 shows the graph variation of (Thickness of the shell vs. Displacement) displacement with shell thicknesses for a chimney with D=12 m, d=6 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the displacement increased by 53.6%.
Figure 4. Variation of Displacement with shell thicknesses for a Chimney with D=12 m, d=6 m
Chimney with height 180 m, bottom diameter 9 m, and top diameter 4.5 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. The results correspond to the Response spectrum analysis, performed in SAP2000 are presented in Table 3. Figure 5 shows the graph variation of (Thickness of the shell vs. Displacement) displacement with shell thicknesses for a chimney with D=9 m, d=4.5 m. It is observed that when thickness of the shell decreases from 0.36 to 0.21 m, the displacement increased by 6.9%.
Figure 5. Variation of Displacement with shell thicknesses for a Chimney with D=9 m, d=4.5 m
Case IV:
Chimney with height 180 m, bottom diameter 7.2 m, and top diameter 3.6 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. The results correspond to Response spectrum analysis , performed in SAP2000 are presented in Table 3. Figure 6 shows the graph variation of (Thickness of the shell vs. Displacement) displacement with shell thicknesses for a chimney with D=7.2 m, d=3.6 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the displacement increased by 15.4%.
Figure 6. Variation of Displacement with H/D Ratio for a Chimney with D=7.2 m, d=3.6 m
Case V:
Chimney with 180 m height, varying top and, bottom diameters and thickness of the shell from 0.36 to 0.21 m was chosen. The results corresponding to Response spectrum analysis are presented in Table 3. Figure 7 shows the graph variation of (Thickness of the shell vs. Displacement) displacement with H/D ratio for chimneys of different diameters. The displacement of the chimney is found to increase with the decrease in thickness of the shell.
Figure 7. Variation of Displacement with H/D Ratio for Chimneys of different Diameters
Chimney with height 180 m, bottom diameter 18 m, and top diameter 9 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. These results correspond to the Response spectrum analysis , performed in SAP2000 are presented in Table 3. Figure 8 shows the graph variation of (Thickness of the shell vs. Displacement) base shear with D/T ratio for a Chimney with D = 18 m, d = 9 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the base shear decreases by 41.7%.
Figure 8. Variation of Base Shear with D/T ratio for a Chimney with D = 18 m, d = 9 m
Chimney with height 180 m, bottom diameter 12 m, top diameter 6 m with varying thickness of the shell from 0.36 to 0.21m was chosen. The results correspond to the Response spectrum analysis, performed in SAP2000 are presented in Table 3. Figure 9 shows the graph variation of (Thickness of the shell vs. Displacement) base shear with D/T ratio for a chimney with D=12 m d=6 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the base shear decreases by 41.7%.
Figure 9. Variation of Base Shear with D/T ratio for a Chimney with D=12 m d=6 m
Chimney with height 180 m, bottom diameter 9 m, and top diameter 4.5 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. These results correspond to the Response spectrum analysis , performed in SAP2000 are presented in Table 3. Figure 10 shows the graph variation of (Thickness of the shell vs. Displacement) base shear with D/T ratio for a chimney with D=9 m, d=4.5 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the base shear decreases by 44.2%.
Figure 10. Variation of Base Shear with D/T ratio for a Chimney with D=9 m, d=4.5 m
Chimney with height 180 m, bottom diameter 7.2 m, and top diameter 3.6 m with varying thickness of the shell from 0.36 to 0.21 m was chosen. The results correspond to Response spectrum analysis, performed in SAP2000 are presented in Table 3. Figure 11 shows the graph variation of (Thickness of the shell vs. Displacement) base shear with D/T ratio for a Chimney with D=7.2 m, d=3.6 m. It is observed that when the thickness of the shell decreases from 0.36 to 0.21 m, the base shear decreases by 44.4%.
Figure 11. Variation of Base Shear with H/D ratio for a Chimney with D=7.2 m, d=3.6 m
Chimney with 180 m height, varying top and, bottom diameters and thickness of the shell from 0.36 to 0.21 m was chosen. The results corresponding to Response spectrum analysis are presented in Table 3. Figure 12 shows the graph variation of (Thickness of the shell vs. base shear) base shear with shell thickness for chimneys of different diameters. The base shear of the chimney is found to decrease with the decrease in thickness of the shell.
Figure 12. Variation of Base Shear with H/D ratio for Chimneys of different diameters
The following conclusions can be drawn for the dynamic analysis of RC chimneys.