7/23/2019 2.Reddy Jaiswal and Earthquake Analysis of Tall Rc Chi0.Mneys

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511 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

Along-wind analysisThe along-wind analysis of chimney is performed using

Random Response Method of IS: 4998 (Part-1)1992.

Maximum shear force and bending moment for both thechimneys have been calculated with various options using

random response method and the results are given in Table

3.

Across-wind analysis

In across-wind analysis, those modes for which critical windspeed less than mean design wind speed are to be

considered. It is fond that the critical wind speed of only

first mode is less than the mean design wind speed, henceonly the first mode has been considered in the across-wind

analysis. Maximum shear force and bending moment

computed by random response method of IS: 4998 and theresults are presented in Table 3.

Table 3: Results of wind analysis

ChimneyHeight

(m)

SFkN

Along Across Combined217 1512 2239 2701

220 3410 5654 6654

Chimney

Height

(m)

BM

kNm

Along Across Combined

217 196822 348462 400205

220 454434 832453 948413

Earthquake analysis

To obtain the earthquake loads, the time period is calculated

using the Equation 10. Knowing the time period of chimneysthe horizontal seismic coefficients are calculated as per the

Equation 11. Then the design shear force and bendingmoments are evaluated by Equations 12 and 13 respectively.The results are obtained for different response reduction

factors with R equal to 3.0 and 1.5 and also various values of

zone factors. The design shear force and the bending

moment of 217 m chimney are presented in Table 4 and thatof 220 m chimney is given in Table 5.

Table 4: Earthquake analysis results of 217 m chimney

Sr.

no.

Zone

 No.

Zone

factor

R=3.0 R=1.5

SF

kN

BM

kNm

SF

kN

BM

kNm

1 II 0.10 454 74222 909 148443

2 III 0.16 727 118754 1456 237509

3 IV 0.24 1091 178131 2184 3562634 V 0.36 1637 267198 3276 356264

Table 5: Earthquake analysis results220 m chimney

Sr.no.

Zone No.

Zonefactor

R=3.0 R=1.5

SF

kN

BM

kNm

SF

kN

BM

kNm

1 II 0.10 522 87238 1045 174476

2 III 0.16 836 139581 1672 279162

3 IV 0.24 1254 209371 2508 418743

4 V 0.36 1881 314057 3762 628115

On studying the results presented in the above tables, it

shows that as the zone factor increases the values of

shear force and bending moment increases. It ismentioned that the response reduction factor of 3.0 is

sufficient for estimating the earthquake loads but the

results becoming double if the R equal to 1.5. The

results obtained with R equal to 1.5 and with zonefactor of 0.36 are almost matching with that of the

wind loads.

CONCLUSIONS

On comparison of the wind loads with that of the

earthquake loads, the following conclusions are drown,

The wind loads are always governing the design of

chimney shell.

In the most critical earthquake zone with zone

factor of 0.36 and response reduction factor of 1.5,

the earthquake response is almost matching with

that of wind response but never been crossing the

wind response.

For the design of the chimney shell, the combined

design wind loads are used.

REFERENCES

[1]  ACI 307:1998,’Standard practice for design and

construction of RC chimneys(ACI 307-98) and

commentary(ACI 307R-98)’,American Concrete

Institute, Detroit

[2]  IS 4998(part-1):1992, ‘Criteria for Design of

Reinforced Concrete Chimneys’, 2nd

  revision,

Bureau of Indian Standards, New Delhi.

[3]  IS 1893 (Part 4):2005, ‘Criteria for earthquake

resistant design of structures’, Bureau of Indian

Standards, New Delhi.

[4]  Jain, S.K., Singh, B.P., Guptha, B.K., 1990, ‘I.S.

code provisions for seismic design of tall

chimneys’, International of structures, Vol. 10,

 No. 2, pp 103-111

[5]  Jaiswal, O.R., Srinivas, V., 2005, ‘Effect of tuned

mass damper on across-wind response of tall RC

Chimneys’, Journal of Wind & Engineering, India,

vol.2, No.1, pp.9-21

[6]  Shivaji, M., Raju, V.S.N., ‘Dynamic analysis of

R.C.C. chimneys’