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HCTL Open International Journal of Technology Innovations and Research (IJTIR) http://ijtir.hctl.org Volume 14, April 2015 e-ISSN: 2321-1814, ISBN (Print): 978-1-62951-946-3

Megha Bhatt, Rakesh Shah, Aanal Shah, Comparison between Limit State Method and Working Stress Method for RCC Chimney Design.

Comparison between Limit State Method and Working Stress Method for RCC Chimney Design Megha Bhatt1, Rakesh Shah2, Aanal Shah3 [email protected] Abstract Nowadays because of strict rules and regulations of environmental board each industry is forced to construct the structures like chimney for effective means of controlling the environmental pollution.

IS: 4998 (Part I) Criteria for Design of Reinforced Concrete Chimneys, is using working stress method for chimney designing. There are some limitations of working stress method. Also the designing is difficult involving lengthy and iterative computational effort. So we should recognize this problem and we should use some time saving techniques like interaction envelopes to optimize the structural design.

Chimneys with various heights i.e. 65m, 70m, 85m and 220m are analyzed and designed by working stress method and limit state method for collapse and comparison of results are discussed in this study.

Keywords RCC Chimney, Working Stress Method, Limit State Method, IS 4998, Draft Code Introduction Chimneys, as we all know today, are hollow, tall and slender vertical structures that carry smoke or steam away from a fire or engine at a high enough elevation so that after dilution due to atmospheric turbulence, their concentration and that of their entrained solid particulates is within acceptable limits on reaching the ground. Need for Study IS: 4998 (Part I) uses the working stress method of design for RCC chimney. The Hook’s law is not applicable to concrete structures. Also the effect of creep and shrinkage of concrete is totally ignored in working stress method, while in the design based on Limit



State concept i.e. described in the Draft Code, the structure is designed to with stand safely all loads liable to act on it throughout its life; it shall also satisfy the serviceability requirements, such as deflection and cracking.

With control regulations becoming stringent, chimneys of heights over 400m are being erected. Design of R.C.C. chimneys by trial and error methods is difficult involving lengthy and iterative computational effort.

So, we should recognize this problem and use some time saving techniques like interaction envelopes to optimize the structural design and to reduce the computational time.

Formulation of Problem

To get comparison of analysis and designing results at different levels in various heights of chimneys i.e. 65m, 70m, 85m and 220m have been considered in present study.

- Lining thickness for chimneys: throughout the height of chimney 0.115 m - Spacing between shell and lining: throughout height of chimney 0.17 m - Hourly mean wind speed factor: 0.67 Up to height 10m from ground terrain category

- Mean probable design life of structure: 100years

- Location for all chimneys: Agartala - Basic wind speed: 55m/s - Probability factor: 1.08 - Topography factor: 1 - Upwind slope is less than 300 - Terrain category: 2 - Class of the structure: A - Number of flues: single - Number of boiler openings: 00 - Temperature of flue gas: 1500C - Ambient temperature: 50C - Proximity of other chimney: No

Results

Chart 1. Temperature Gradient for 65m Chimney

Chart 2. Temperature Gradient for 70m, Chimney





Chart 5. Governing Moment at Section for 65m Chimney




Table 1. Designing Results for 65m Chimney



Range of Section, m

fck, N/mm2

IS: 4998 (Part I) - 1975 Draft Code

From To Pt pt 65 60 25 0.25 0.25

60 55 25 0.25 0.25

55 50 25 0.26 0.25

50 45 25 0.55 0.5

45 40 35 0.851 0.7

40 35 35 0.91 0.7

35 30 35 0.92 1.05

30 25 40 0.92 1.2

25 20 40 0.92 1.2

20 15 45 0.95 0.9

15 10 45 1 0.9

10 5 45 1.03 0.9

5 0 45 1.05 0.9

0 -2 45 1.05 0.9


Range of Section, m

fck, N/mm2


From To pt pt 70 65 25 0.25 0.25 65 60 25 0.25 0.25 60 55 25 0.25 0.25 55 50 25 0.3 0.5 50 45 35 0.5 0.7 45 40 35 0.55 1.05 40 35 35 0.55 1.05 35 30 40 0.63 1.2 30 25 40 0.8 1.2 25 20 45 0.92 1.35 20 15 45 1.02 1.35 15 10 45 1.1 1.35 10 5 45 1.15 1.35 5 0 45 1.15 1.35 0 -2 45 1.15 1.35




Range of Section, m

fck, N/mm2


From To pt Pt

85 80 25 0.25 0.25

80 75 25 0.25 0.25

75 70 25 0.25 0.25

70 65 25 0.25 0.5

65 60 35 0.3 0.7

60 55 35 0.48 0.7

55 50 35 0.5 1.05

50 45 40 0.49 1.2

45 40 40 0.47 1.2

40 35 40 0.45 1.2

35 30 40 0.43 1.2

30 25 40 0.43 1.2

25 20 40 0.42 1.2

20 15 40 0.425 1.2

15 10 40 0.425 1.2

10 5 40 0.42 1.2

5 0 40 0.41 1.2

0 -2 40 0.41 1.2


Range of Section, m

fck, N/mm2


From To pt pt 220 210 25 0.25 0.25 210 200 25 0.25 0.25 200 190 25 0.25 0.25 190 180 25 0.25 0.75 180 170 50 0.33 1.5 170 160 50 0.7 1.5 160 150 50 1.17 2.5 150 140 50 0.935 3 140 130 50 0.745 2.5 130 120 50 0.58 2.5 120 110 50 0.44 2.5 110 100 50 0.31 2 100 90 40 0.25 1.6 90 80 40 0.25 1.6 80 70 35 0.25 1.75



70 60 35 0.25 1.75 60 50 35 0.25 1.75 50 40 35 0.25 1.75 40 30 35 0.25 1.75 30 20 35 0.25 1.75 20 10 35 0.25 1.75 10 0 35 0.25 1.75 0 -2 35 0.25 1.75

Conclusion

From the study the following conclusions are drawn; Limit state design of RC circular hollow sections can be followed for the purpose to achieve a suitable combination of load factors and Availability of interaction envelopes and computer algorithm immensely help the designer in expeditiously solving the design problem.

According to Draft Code for all chimneys due to higher value of Vcri at higher mode of vibration according to IS 4998 (part 1), 1992, for rest of the chimneys along wind load for the 1st mode is governing case. For considered parameters IS 4998 (part 1), 1992 gives 1.05 times higher values in along load calculation and 1.5 to 1.76 times higher values in across wind load calculations. Draft Code gives 0 to 1.6 times higher values for percentage of steel for most of the sections.

Acknowledgements

I deeply obliged to my guide Mr. Rakesh Shah and Ms. Aanal Shah.

References

[1] Prem Chand, “Design of circular RC chimney section subjected to axial load and bending moment”, The Indian Concrete Journal, Vol. 68, No. 7, July 1994, pp. 357-364.

[2] Gupta S. R. Davalath and Murthy K. S. Madugula, “Analyses/design of reinforced concrete circular cross section”, ACI Structural Journal, Vol. 85, No. 6, Tital No. 85-s55, Nov-Dec 1988.

[3] R. Ranganathan and A Muftha, “Evaluation of reliability RCC Chimneys”, International Journal of Structures, Vol. 17, No. 1, paper No-154, Jan-Jun 1997, pp. 19-35.

[4]Wadi S. Rumman and Ru-Tung Sun, “Ultimate strength design of reinforced concrete chimneys”, ACI Journal, Vol. 74, No. 4, Titalno. 74-18, July-August, pp. 179-184.

[5] ACI Committee 307, “Standard practice for the design and construction of cast-in-place reinforced concrete chimneys”, ACI Structural Journal, Vol. 88, No. 1, Tital no. 88-512, Jan-Feb 1998, pp. 99-101.



[6] K. R. C. Reddy, O. R. Jaiswal and P. N. Godbole “Wind and Earthquake Analysis of Tall RC Chimneys”*International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp. 508-511

[7] K.S. Babu Narayan and Subhash C. Yaragal, “Load-moment interaction envelopes for design of tall stacks -A limit state approach”, The Indian Concrete Journal, Vol. 68, No. 7, September 2007, pp. 21-25.

[8] K. S. Babu Narayan, Subhash C. Yaragal, and Yukio Tamura “Interaction Envelopes For Limit State Design Of Chimneys” The fourth International symposium on computational wind engineering (CWE2006), Yokahama 2006 pp. 439

[9] Charles E. Rynolds and James C Steedman, “Reinforced concrete designers hand book”, Ninth edition.

[10] S. N. Manohar, “Tall chimneys design and construction”, Tata McGraw Hill Publishing Company Limited New Delhi and Tor Steel Research Foundation in India, Bangalore 1985.

[11] G. M. Pinfold, “Reinforced concrete chimneys and towers”, A Viewpoint Publication 1975.

[12] Mark Fintal, “Hand book of reinforced concrete engineering” Second Edition, pp. 565-573.

[13] Shah and Karve “Limit State Theory and Design of Reinforced Concrete” Structures

[14] IS: 4998 (Part I)-1975, “Criteria for design of reinforced concrete chimneys”, Bureau of Indian Standards, New Delhi.

[15] IS: 4998 (Part I)-1992, “Criteria for design of reinforced concrete chimneys”, Bureau of Indian Standards, New Delhi.

[16] IS: 456-2000, “Code of practice for plain and reinforced concrete”, Bureau of Indian Standards, New Delhi.

[17] IS: 1893-1984, “Criteria for earth quake resistant design of structures”, (Fourth Revision), BIS, New Delhi.

[18] IS: 875 (Part 3)-1988, “Code of practice for design loads (Other than earthquake) for buildings and structures”, BIS, New Delhi.

[19] Draft code, BIS, CED 38 (7892) WC April 2013, Draft Indian Standard Code of Practice for Design of Reinforced Concrete Chimneys Revision of IS 4998(Part 1) ICS No.: 91.060.040; 91.100.30

[20] ACI 307-08 Code Requirements for Reinforced Concrete Chimneys and Commentry Reported by ACI committee 307, American Concrete Institute



[21] CICIND Model Code for Concrete Chimney, Part A: The shell, Second Edition, Revision1

Authors

Megha Bhatt

M.Tech (Structural Designing)

Working as Assistant Professor in Department of Civil Engineering, LDRP – ITR, Gandhinagar.

This article is an open access article distributed under the terms and conditions of the

Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).

© 2015 by the Authors. Licensed by HCTL Open, India.

comparison between limit state method and working · pdf filecomparison between limit state...

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