DYNAMIC ANALYSIS OF RC CHIMNEYS · 2018. 3. 21. · 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

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INTRODUCTION

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.

1. Aim and Objectives of the Study

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,

To model 16 varieties of RC chimneys by varying H/D

and D/T ratios.

To carry out free vibration analysis to find out the

fundamental natural time period/frequency and

mode shapes of the chimneys.

To carry out seismic analysis using Response Spectrum

method as per 1893 (part 1): 2002.

To investigate the effect of H/D and D/T ratios on the

response of the RC chimneys in terms of top

displacement and base shear.

2. Literature Review

The parametric study of RC chimney in analysis of self

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DYNAMIC ANALYSIS OF RC CHIMNEYS

By

ABSTRACT

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.

Keywords: Modal Analysis, Fundamental Frequency, Base Shear, Top Displacement, Response Spectrum Analysis,

Geometric Parameters H/D, D/T Ratios of Chimney.

T. SHARVANI * B.D.V. CHANDRA MOHAN RAO **

* PG Student, Department of Civil Engineering, VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, India.** Professor, Department of Civil Engineering, VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, India.

li-manager’s Journal on Structural Engineering Vol. No. 4 l, 5 December 2016 - February 2017

Date Received: 05/12/2016 Date Revised: 18/01/2017 Date Accepted: 16/02/2017

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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

31li-manager’s Journal on Structural Engineering Vol. No. 4 - February 2017l, 5 December 2016

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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].

3. Methodology

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.

4. Numerical Study

4.1 Description of RCC Chimney

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.

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

4.2 Geometric Parameters

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.

5. Results and Discussions

5.1 Modal Analysis

Modal analysis is a procedure which evaluates free-

vibration 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.

This analysis is performed to get the dynamic

characteristics, such as natural time period and mode

shapes of the chimney.

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Figure 1. 3D View of RC Chimney

H/D D/d D/T D (m) D (m) T (m)

10 0.5 50 18.00 9.00 0.360

15 0.5 45 12.00 6.00 0.267

20 0.5 40 9.00 4.50 0.225

25 0.5 35 7.20 3.60 0.210

Table 1. Summary Chart of Geometric Parameters

Height of the chimney H; Bottom diameter D; Top diameter d; Thickness of the shell T.


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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.

5.2 Response Spectrum 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 %

5.2.1 Displacements

Case I:

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

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H/D D/d D/T D (m) D (m) T (m)

Modal Analysis

Time Period (Sec)

Frequency(Hz)

10 0.5 50 18 9 0.36 2.66 0.37

15 0.5 45 18 9 0.27 2.66 0.37

20 0.5 40 18 9 0.23 2.67 0.37

25 0.5 35 18 9 0.21 2.67 0.37

10 0.5 50 12 6 0.36 3.21 0.31

15 0.5 45 12 6 0.27 3.21 0.31

20 0.5 40 12 6 0.23 3.21 0.31

25 0.5 35 12 6 0.21 3.21 0.31

10 0.5 50 9 4.5 0.36 3.5 0.28

15 0.5 45 9 4.5 0.27 3.5 0.28

20 0.5 40 9 4.5 0.23 3.5 0.28

25 0.5 35 9 4.5 0.21 3.5 0.28

10 0.5 50 7.2 3.6 0.36 3.67 0.27

15 0.5 45 7.2 3.6 0.27 3.67 0.27

20 0.5 40 7.2 3.6 0.23 3.67 0.27

25 0.5 35 7.2 3.6 0.21 3.67 0.27

Table 2. Results of Modal Analysis

Figure 2. Modal Analysis

H/D D/d D/T D (m) d (m) T (m)

Response Spectrum Analysis

Displacement(mm)

Base Shear(kN)

10 0.5 50 18 9 0.36 7.60 462.09

15 0.5 45 18 9 0.27 8.00 342.70

20 0.5 40 18 9 0.23 8.60 295.09

25 0.5 35 18 9 0.21 9.60 269.51

10 0.5 50 12 6 0.36 12.80 254.77

15 0.5 45 12 6 0.27 15.90 191.07

20 0.5 40 12 6 0.23 24.60 162.34

25 0.5 35 12 6 0.21 27.60 148.61

10 0.5 50 9 4.5 0.36 24.60 174.75

15 0.5 45 9 4.5 0.27 26.70 162.20

20 0.5 40 9 4.5 0.23 26.70 97.05

25 0.5 35 9 4.5 0.21 28.70 97.00

10 0.5 50 7.2 3.6 0.36 12.60 133.29

15 0.5 45 7.2 3.6 0.27 12.70 96.26

20 0.5 40 7.2 3.6 0.23 12.90 83.30

25 0.5 35 7.2 3.6 0.21 14.90 74.04

Table 3. Results of Response Spectrum Analysis

li-manager’s Journal on Structural Engineering Vol. No. 4 - February 2017l, 5 December 2016

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from 0.36 m to 0.21 m, the displacement increased by

20.8 %.

Case II:


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


from 0.36 to 0.21 m, the displacement increased by

53.6%.

Case III:

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



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%.

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



shell thicknesses for a chimney with D=7.2 m, d=3.6 m. It is


from 0.36 to 0.21 m, the displacement increased by

15.4%.

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.

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Figure 3. Variation of Displacement with shell thicknesses

for a Chimney with D=18 m, d=9 m

Figure 4. Variation of Displacement with shell thicknesses for a Chimney with D=12 m, d=6 m

Figure 5. Variation of Displacement with shell thicknesses for a Chimney with D=9 m, d=4.5 m

Figure 6. Variation of Displacement with H/D Ratio for a

Chimney with D=7.2 m, d=3.6 m


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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.

5.2.2 Base Shear

Case I:

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



(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%.

Case II:


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


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%.

Case III:

Chimney with height 180 m, bottom diameter 9 m, and top


to 0.21 m was chosen. These results correspond to the



(Thickness of the shell vs. Displacement) base shear with D/T

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Figure 8. Variation of Base Shear with D/T ratio for a Chimney with D = 18 m, d = 9 m

Figure 9. Variation of Base Shear with D/T ratio for a Chimney with D=12 m d=6 m

Figure 10. Variation of Base Shear with D/T ratio for a

Chimney with D=9 m, d=4.5 m

Figure 11. Variation of Base Shear with H/D ratio for a Chimney with D=7.2 m, d=3.6 m

li-manager’s Journal on Structural Engineering Vol. No. 4 - February 2017l, 5 December 2016

Figure 7. Variation of Displacement with H/D Ratio for Chimneys of different Diameters

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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%.

Case IV:

Chimney with height 180 m, bottom diameter 7.2 m, and top


to 0.21 m was chosen. The results correspond to Response

spectrum analysis, performed in SAP2000 are presented in


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%.

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 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.

Conclusion

The following conclusions can be drawn for the dynamic

analysis of RC chimneys.

As H/D ratio changes from 10 to 25, fundamental

period of structure is increased by 27.59%.

There is no variation in the fundamental natural time

period of the structure, even though the thickness of

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the shell is varying from 0.36 to 0.21 m keeping the top

and bottom diameters constant.

Top displacement of the chimney is observed to be

within the permissible limits, for all the models

considered.

As the thickness of the shell decreases from 0.36 to

0.21 m, top displacement increased by 53.6%

As the thickness of the shell decreases from 0.36 to

0.21 m, base shear decreased by 44.2%

References

[1]. Amit Nagar, Shiva Shankar. M., and T. Soumya, (2015).

“Non Liner Dynamic Analysis of RCC Chimney”.

International Journal of Engineering Research &

Technology (IJERT), Vol.4, No.7, pp.530-535.

[2]. Aneet Khombe, Anand Bagali, Md Imran G, Irayya R, and

Sachin R Kulkarni, (2016). “Seismic Analysis and Design of RCC

Chimney”. International Journal of Advance Engineering and

Research Development, Vol.3, No.5, pp.903-913.

[3]. Indian Standards, (2002). Criteria for Earthquake

Resistant Design of Structures - Part 1 General Provisions

and Buildings (Fifth Revision) (IS 1893 -Part 1). Bureau of

Indian Standards, New Delhi, India.

[4]. K. Anil Pradeep, and C.V. Siva Rama Prasad, (2014).

“Governing Loads for Design of a 60 m Industrial RCC

Chimney”. International Journal of Innovative Research in

Science, Engineering and Technology. Vol.3, No.8,

pp.15151-15159.

[5]. Rajkumar, Vishwanath, and B. Patil, (2013). “Analysis of

Self-Supporting Chimney”. International Journal of

Innovative Technology and Exploring Engineering (IJITEE),

Vol.3, No.5, pp.85-91.

[6]. Sagar S, and Basavarj Gudadappanavar, (2015).

“Performance Based Seismic Evaluation of Industrial

Chimneys by Static and Dynamic Analysis”. International

Research Journal of Engineering and Technology (IRJET)

Vol.2, No.4, pp.1670-1674.

[7]. Yoganantham. C, and Helen Santhi. M., (2013).

“Modal Analysis of RCC Chimney”. International Journal of

Research in Civil Engineering, Architecture & Design, Vol.1,

No.2, pp. 20-23.

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Figure 12. Variation of Base Shear with H/D ratio for

Chimneys of different diameters


T. Sharvani is currently pursuing her M.Tech (Structural) in the Department of Civil Engineering at VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, India. She has 4 years of experience as Assistant Engineer in Era Groups, Hyderabad.

Dr. B.D.V Chandra Mohan Rao is currently working as a Professor in the Department of Civil Engineering at VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, India. He has received Sir Arthur Cotton Memorial Prize (Gold Medal) for the best paper published in Journal of the Institution of Engineers. He has 20 years of teaching experience and his research areas, include Earthquake Engineering, Structural Dynamics, Finite Element Analysis, and Structural Optimization.

ABOUT THE AUTHORS

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37li-manager’s Journal on Structural Engineering Vol. No. 4 - February 2017l, 5 December 2016

Documents

DYNAMIC ANALYSIS OF RC CHIMNEYS · 2018. 3. 21. · 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