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VIBRATIONAL ANALYSIS OF CHIMNEY

EQUIPPED WITH STRAKES AND TUNE MASS

DAMPER.

Pratik Wadgave1, Chittaranjan Nayak

2

1,2Civil Department, VPCOE Baramati, SavitribaiPhule Pune University, (India)

ABSTRACT

Chimneys are the structures which are built to greater heights as tall slender structures. Any damage to

chimneys may result in shut down of plants and industries.Construction of tall chimneys needs the better

understanding of loads acting on them and of the structural behaviour, so that with the help of modern

construction equipment and technique such as slip form, reinforced concrete, the most favoured 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. Chimneys are more vulnerable to wind and

earthquake loads which may cause severe problem in power plants and major industries. However, if they are

located in higher seismic zone with lower wind speeds, then, seismic forces may become analogous, if not more,

than the wind loads may become analogous. It is designed for both along wind and across wind loads.

Chimney with strakes is a modern technique which is used in most of the chimney now-a-days. Another

technique which is used commonly is the use of Tune Mass Damper (TMD). Both these techniques are used to

minimize the vibration of chimney but in totally different phenomenon. In this paper the use of both the

techniques are done and corresponding vibrational analysis are done using Finite Element Method (SAP 2000).

To expand more on this research work vents are provided andanalysis is then carried out with strakes and TMD

for base shear, base moment and time period.

Keywords: Tune mass damper, strakes, stresses, moments and time period.

I. INTRODUCTION

Chimneys or stacks are used as a medium to transfer highly contaminated polluted gases to atmosphere at

greater heights. The main function of chimney is to take highly poisonous gases which are not acceptable at

ground level were taken to greater heights with sufficient velocities however, with the increment in height, the

wind actions on it become important as these produce very high stresses. Although chimneys do not present as a

great hazard to life and limb as buildings with high human capacity, damage to chimneys may result in shut

down of plants and industries. The chimney may be self-supporting or cable supported. Chimneys are more

vulnerable to wind and earthquake loads which may cause severe problem in power plants and major industries.

However, if they are located in higher seismic zone with lower wind speeds, then, seismic forces may become

analogous, if not more, than the wind loads may become analogous.

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1.1 Methodology: The primary objective of this paper is to analyse the 90m chimney equipped with Strakes and

tune mass damper. This analysis varies as the height of strakes increases as 30m and 45m from top. The analysis

also varies with increase in damping percent as nominal, 5%, 10% and 15%. The analysis is then carried

forward with chimney equipped with both strakes and tune mass damper.

This further analysis is carried out in varies combinations of height of strakes and damping percentage of tune

mass damper with openings as air vent and door vent. Thus the comparison is made among them. The main

objectives of undertaking the present study are as follows.

To develop a model of chimney of 90m height, 6.4m diameter and 0.400m thickness in SAP 2000 as per

specification of chimney.

To analyse modelled chimney for time period of various cases and comparing it with corresponding models.

To develop relation between strakes height and time period of chimney with and without opening subjected

to seismic and wind loadings. To develop this relation height of strakes is increased

To develop relation between damping ratio, time period, base shear and base moment of chimney subjected

to seismic load and wind load. Also relation between tune mass damper and time period of chimney

subjected to seismic and wind load is to develop.

1.2 Structural description: Chimney is situated in zone IV and the top wind speed recorded is about 46 km/hr.

The height of the chimney to be constructed is about 90m and the internal diameter is about 6m. Shell thickness

of chimney is about 0.4m and external diameter is 6.4m. An air vent is also provided to facilitate gas thrust out

of the chimney. This chimney is to be analysed for both seismic effects and for wind effects. Also there is a

provision for the application of Tune Mass Damper (TMD) and strakes to minimise the vibrations caused due to

seismic forces and wind forces. Structural damping is to be considered as 5% of total weight of chimney.

Figure 1 chimney modelled in SAP2000

II. RESULTS:

Case i - chimney without opening for different strakes height and damping ratio.

The chimney without opening for without strakes, 30m and 45m strakes height and damping ratios as without

damping, 5%, 10% and 15% is analysed as follows.

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Case I (a): Chimney without opening and without strakes and for damping ratio as without damping, 5%, 10%

and 15%.

Figure 2 shows three mode shapes versus their time period in seconds for without strakes and for damping ratios

as without damping, 5%, 10% and 15%.

Figure 2 Comparison of time period for without strakes

It is observed from figure 2 it is clear that the time period decreases as the damping ratio increases

where strakes height is same for this case as without strakes. The time period drops dramatically between 0%

and 15% of damping ratio by 76% for first mode and is almost same for second mode. For third mode this

difference reduces to 45.38%.

Figure 3 shows damping ratio values versus their base shear in kN of for without strakes for

earthquake, response spectrum and wind forces.

Figure 3 Comparison of base shear for without strakes

It is observed from figure 3 it is clear that base shear increases as damping ratio increases for linear

earthquake and for response spectrum. Were as, it remains constant for wind pressure. The difference between

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0% and 15% damping ratio for linear earth quake is about 78.78% and that for response spectrum is about

13.33%.

Figure 4 shows damping ratio values versus their base moment in kN-m of for without strakes for earthquake,

response spectrum and wind forces.

Figure 4 Comparison of base moment for without strakes

It is observed from figure 4 it is clear that base moment increases as damping ratio increases for linear



10.65%.

Case I (b): Chimney without opening and with 30 metres of strakes and for damping ratio as without damping,

5%, 10% and 15%.

Figure 5 shows three mode shapes versus their time period in seconds for 30m strakes and for damping

ratios as without damping, 5%, 10% and 15%.

Figure 5 Comparison of time period for 30m strakes

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It is observed from figure 5 it is clear that the time period decreases as the damping ratio increases where strakes

height is same for this case as 30m. The time period drops dramatically between 0% and 15% of damping ratio

by 75.87% for first mode and is almost same for second mode. For third mode this difference reduces to 487%.

Figure 6 shows damping ratio values versus their base shear in kN of 3for without strakes for earthquake,


Figure 6 Comparison of base shear for 30m strakes




11.58%.

Figure 7 shows damping ratio values versus their base moment in kN-m of 3for without strakes for earthquake,


Figure 7 Comparison of base moment for 30m strakes

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112%.

Case I (c): Chimney without opening and with 45 metres of strakes and for damping ratio as without damping,

5%, 10% and 15%.

Figure 8 shows three mode shapes versus their time period in seconds for 45m strakes and for damping ratios as

without damping, 5%, 10% and 15%.


It is observed from figure 8 it is clear that the time period decreases as the damping ratio increases where strakes

height is same for this case as 45m. The time period drops dramatically between 0% and 15% of damping ratio

by 78.88% for first mode and is almost same for second mode. For third mode this difference reduces to

24.22%.

Figure 9 shows damping ratio values versus their base shear in kN of 45m height of strakes for earthquake,



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10.56%.

Figure 10 shows damping ratio values versus their base moment in kN-m of 45m height of strakes

for earthquake, response spectrum and wind forces.





10.33%.

Case II - chimney with opening as air vent and different strakes height for varying damping ratio.

The chimney with opening as air vent for without strakes, 30m and 45m strakes height and damping ratios as

without damping, 5%, 10% and 15% is analysed as follows.

Case II (a): Chimney with opening as air vent for without strakes and for damping ratio as without damping,

5%, 10% and 15%.

Figure 11 shows three mode shapes versus their time period in seconds for without strakes and for damping


Figure 11 Comparison of time period for without strakes

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It is observed from figure 11 it is clear that the time period decreases as the damping ratio increases where

strakes height is same for this case as without strakes. The time period drops dramatically between 0% and 15%

of damping ratio by 77.36% for first mode and is almost same for second mode. For third mode this difference

reduces to 43.71%.

Figure 12 shows damping ratio values versus their base shear in kN of for without strakes for earthquake,

response spectrum and wind forces. .

Figure 12 Comparison of base shear for without strakes

It is observed from figure 12 it is clear that base shear increases as damping ratio increases for linear earthquake

and for response spectrum. Were as, it remains constant for wind pressure. The difference between 0% and 15%

damping ratio for linear earth quake is about 70.58% and that for response spectrum is about 19.36%.

Figure 13 shows damping ratio values versus their base moment in kN-m of for without strakes for earthquake,


Figure 13 Comparison of base moment for without strakes



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18.77%.

Case II (b): Chimney with opening as air vent for 30 metres of strakes and for damping ratio as without

damping, 5%, 10% and 15%.

Figure 14 shows three mode shapes versus their time period in seconds for 30m strakes and for damping ratios

as without damping, 5%, 10% and 15%.



strakes height is same for this case as 45m. The time period drops dramatically between 0% and 15% of

damping ratio by 78.94% for first mode and is almost same for second mode. For third mode this difference

reduces to 45.71%.

Figure 15 shows damping ratio values versus their base shear in kN of 30m of strakes for earthquake, response

spectrum and wind forces.


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10.56%.

Figure 16 shows damping ratio values versus their base moment in kN-m of 30m of strakes for





0% and 15% damping ratio for linear earth quake is about 71% and that for response spectrum is about 116%.

Case II (c): Chimney with opening as air vent for 45 metres of strakes and for damping ratio as without

damping, 5%, 10% and 15%.

Figure 17 shows three mode shapes versus their time period in seconds for 45m strakes and for damping


Figure 17 Comparison of time period for 45 strakes

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strakes height is same for this case as 45m. The time period drops dramatically between 0% and 15% of

damping ratio by 78.94% for first mode and is almost same for second mode. For third mode this difference

reduces to 20.01%.

Figure 18 shows damping ratio values versus their base shear in kN of 45m height of strakes for earthquake,



It is observed from figure 18 it is clear that base shear increases as damping ratio increases for linear earthquake

and for response spectrum. Were as, it remains constant for wind pressure. The difference between 0% and 15%

damping ratio for linear earth quake is about 71.65% and that for response spectrum is about 15.38%.

Figure 19 shows damping ratio values versus their base moment in kN-m of 45m height of strakes for



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110%.

III. CONCLUSIONS

As the height of strakes of chimney increases time period increases for loading of seismic and wind loading.

It is clear from all the results that time period of chimney decreases with increase in damping ratio.

As time period of chimney after three modes decreases it can be concluded that the amplitude of vibration

decreases with increase in damping ratio and is true for all heights of strakes.

From all the discussed results it can be concluded base shear of the chimney increases as damping ratio

increases irrespective of the height of strakes.

From all the discussed results it can be concluded base moment increases as damping ratio increases

irrespective of the height of strakes.

Base shear and base moment due to wind forces remains constant irrespective of the increase in damping

ratio. However the value of stresses increases as height of strakes increases. It can be concluded that wind

forces are more dominant as strakes height is increases.

Base shear and base moment for response spectrum increases as damping ratio increases irrespective of the

increase in height of strakes.

When air vent as an opening is provided to chimney it does not alter the response of chimney considerably

but it surely increases stress concentration at particular area.

When door vent is provided at its base it causes serious increase in base shear and base moment at the area

where it is provided.

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