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Full Paper Proc. of Int. Conf. on Recent Trends in Transportation, Environmental and Civil Engineering 2011

Wind Analysis Of Tall Stack Like Structure Jigar K. Sevalia1 , Dr. Atul K. Desai2 1

Ph. D. Scholar, Applied Mechanics Department, S. V. National Institute of Technology, Surat, Gujarat, India. email:jigar_sevalia@yahoo.co.in 2 Head and Associate Professor, Applied Mechanics Department, S. V. National Institute of Technology, Surat, Gujarat, India. e-mail : akd@amd.svnit.ac.in Case –1 The Chimney having conical portion up to 24 m from bottom and then cylindrical portion for remaining 6 m.

Abstract - It has been undergone a considerable development of industrial chimney in past few years in terms of structural system as well as method of analysis. Also the height of chimney has been increased for the better control of environment pollution in populated areas. With the increase in height, the wind forces have become predominant forces while analyzing and designing such structures. Here in this paper, an attempt has been made to analyse the industrial steel chimney for the prevailing wind forces considering chimney with and without guy ropes. Index Terms: Chimney, Wind Force, Moment, Shell Stresses, Dead Load

I. INTRODUCTION

Case – 2 The same chimney as mentioned in Case – 1 but supported by 3 Numbers 25 mm Ø Guy Cables.

The chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. Scientific discoveries have lead to the establishment of various types of industries. These industries supply smoke and harmful gases in to the atmosphere. Due to heavy industrialization and installation of high capacity power plants together with the growing consciousness about pollution has led to the construction of tall chimneys. 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 guyed chimney. In this paper, an attempt has been made to analyse the industrial steel chimney with and without guy ropes for prevailing wind conditions and forces.

Case – 3 The same chimney as mentioned in Case – 1 but supported by 6 Numbers 25 mm Ø Guy Cables at equal angles.

II. ANALYSIS METHOD AND GEOMETRIC CONFIGURATION The wind force analysis of steel chimney is carried out using SAP 2000 v 14.0.0. Five different models have been developed for the steel chimney using shell elements to study effects of seismic forces acting on it. The height of the chimney is assumed to be 30 m. The diameter at the base is considered 1.3 m and at top is 0.36 m. The five different models of chimney are as under;

Full Paper Proc. of Int. Conf. on Recent Trends in Transportation, Environmental and Civil Engineering 2011 IV. SIGN CONVENTION

Case – 4 The Chimney having Conical Bottom up to bottom 10 m height and then cylindrical portion for remaining 20 height.

Case – 5 The Chimney having Conical Bottom up to bottom 10 m height and cylindrical portion for remaining 20 height with stiffener plates at junction of conical bottom & cylindrical top.

Figure 1 Sign Convention for Shell Stresses (S11) & (S22)

Considering above mentioned cases of chimney and seismic forces, the dynamic analysis of chimney was carried out using software SAP 2000 v 14.0.0 III. LOADS CONSIDERED Dead Load : Self Weight of Chimney considering density of steel material – 78.5 kN/m3 Wind Load Basic Wind Speed = 44 m/sec

Figure 2 Sign Convention for Shell Forces (M11) & (M22)

Risk Coefficient k1 = 1.07

V. RESULTS AND DISCUSSION

Topography Factor k3 = 1.0

Considering abovementioned cases of chimney and wind forces, the structural analysis of chimney was carried out using softwareSAP2000 v 14.0.0 and results aretabulated as under where S11 and S22 are thedirect stresses acting along axis 1 and 2 respectively and M11 and M22 are the direct moments acting about axis 2 and 1 respectively.

Terrain Category = 2 Class of the Structure = B

Full Paper Proc. of Int. Conf. on Recent Trends in Transportation, Environmental and Civil Engineering 2011 TABLE 1 SHELL ELEMENT STRESSES

TABLE 2 SHELL ELEMENT MOMENTS

Graph 1 Direct Stress (S11) along Axis - 1

From above mentioned table 1 and graph 1, it can be seen that out of the five cases of chimney, Case – 2, Case -4 & Case – 5 is developing minimum shell stresses S11. Graph 3 Direct Moment (M11) along Axis - 2

Graph 4 Direct Moment (M22) along Axis - 1 Graph 2 Direct Stress (S11) along Axis - 2

From above mentioned table 2, Graph 3 and Graph 4, it can be seen that out of the five cases of chimney, Case – 2 & Case – 3 is developing minimum shell moments i.e. M11 & M22 under action of Dead Load combined with wind load.

From above mentioned table 1 and Graph 2, it can be seen that out of the five cases of chimney, Case – 2, Case -4 & Case – 5 is developing minimum shell stresses S22. Chimney Case – 3 is developing maximum stresses due to more number of guy ropes as wind forces acting on guy ropes are adding wind forces on chimney.

Full Paper Proc. of Int. Conf. on Recent Trends in Transportation, Environmental and Civil Engineering 2011 The Chimney with 6 Guy Cables as compared to Chimney without Guy Cables reduces the deflection at the Top considerably about by 78 %. Though Chimney with 3 guy cables shows more deflection than the chimney with 6 guy cables, the stresses in the walls of chimney are found very less compared to the chimney with 6 guy cables as the more number of guy cables are producing more wind forces on the structure of chimney and hence more stresses. Also it is found that Case – 4 & Case – 5 are more vulnerable to lateral deflection due to wind load and exceeding the permissible limits prescribed by IS 6533 (Part-2): 1989. Due to increase flexibility in Case 4 & Case 5 of Chimney, the cross sectional shape of the chimney in Case -4 & Case 5 is deforming largely at the top.

TABLE 3 DEFLECTION AT TOP OF CHIMNEY (MM)

REFERENCES 1. B. C. Punmia, Ashok Kumar Jain and Arun Kumar Jain, “Design of Steel Structures”, Laxmi Publications (P) Ltd., 2008. 2. Dayaratnam P., “Design of Steel Structures”, A. H. Wheeler & Co. Ltd., 199. 3. IS: IS: 800-2007 “Code for practice for general construction in steel” Second revision, Bureau of Indian standards, New Delhi. 4. IS: 875(Part-1) -1987, “Code of Practice for design loads for buildings & structures – Dead Load”, Bureau of Indian standards, New Delhi. 5. IS: 875(Part-2) -1987, “Code of Practice for design loads for buildings & structures – Live Load”, Bureau of Indian standards, New Delhi. 6. IS: 875(Part-3) -1987, “Code of Practice for design loads for buildings & structures – Wind Load”, Bureau of Indian standards, New Delhi. 7. IS: 875(Part-5) -1987, “Code of Practice for design loads for buildings & structures – Special Loads & Load Combinations”, Bureau of Indian standards, New Delhi. 8. IS: 6533(Part-1) -1987, “Code of Practice for Design and Construction of Steel Chimney –Mechanical Aspect”, Bureau of Indian standards, New Delhi. 9. IS: 6533(Part-2) -1987, “Code of Practice for Design and Construction of Steel Chimney –Structural Aspect”, Bureau of Indian standards, New Delhi. 10. Subramanian N., “Design of Steel Structures”, Adage Printers (P) Ltd., 2008.

Graph 5 Deflections at Top of Chimney

Considering the deflection at the top of the chimney, it can be seen from the table 3 and Graph 5 that Chimney Case – 2 & Chimney Case – 3 deflect less under the Chimney combined action of dead load and wind load. Although, the deflection at the top of the Chimney in first three cases is within permissible limit i.e. (h/200) = 150 mm. In chimney case – 4 and chimney case – 5, the deflection at the top of the chimney is beyond permissible limit i.e. 150 mm. CONCLUSION From the above mentioned results one can conclude that chimney with three guy ropes is performing well during the action of wind compared to chimney with six numbers of guy ropes as well as self supporting chimney. The Chimney with 3 Guy Cables as compared to Chimney without Guy Cables reduces the deflection at the Top considerably about by 71 %.