This document provides guidance on designing reinforced concrete chimneys and windshields according to Indian codes IS 4998 and IS 15498. Key points covered include: 1. Chimneys must be designed to resist along-wind and across-wind loads from wind, considering factors like earthquake loads and wind speed. 2. Foundations must safely transfer vertical and lateral loads to the sub-grade while preventing excessive deflection. For raft foundations, uplift is not permitted under critical load combinations. 3. Dynamic wind loads are estimated using mean drag coefficients and gust factors to account for turbulent wind fluctuations and vortex shedding contributing to along-wind and across-wind loads.

1. RCC Chimney windshield: A learning document
Lesson learned during the drawing & design calculation inspection of chimney foundation & RCC wind-shield.
Reference codes:
 IS 4998: 2015 (latest version): Design of Reinforced concrete chimneys
 IS 15498: 2004: Guidelines for improving the cyclonic resistance of low rise houses & other
buildings/structures
Learning:
1. As chimney is a high-rise structure & wind sensitive in nature. Due to this along-wind & across wind-load
effect is taken into consideration. Considering the earthquake load & wind load, the prominent load taken in
this calculation is wind load.
2. The hollow-circular c/s is designed to resist circumferential pressure distribution.
3. In the design provisions, the philosophy of limit state design is adopted in IS 4998, in line with international
practice.
4. Uplift is not permitted for a raft foundation under the critical load combination of (0.9DL+1.0WL).
5. During the design calculation stage query were raised regarding the following:
a. Load due to slip-form activities (in shell only case) was not considered.
Ans.: The chimney shell and foundation are design two cases viz shell only condition and shell completed condition. Slip
form loads are very negligible loads compared to the liner and platform loads. Chimney shell and foundation is designed
for the liner and platform loads which are higher than slip-form loads at shell completed case during which the slip form
loads will not be present. Under shell only case where stability is a concern, it is only necessary to check for 0.9DL case
without considering any additional load from slip form etc to check the stability. Hence slip-form loads are not required to
be considered.
b. As the area i.e. TSK is wind prone, no wind strakes are provided to brake strong winds to avoid vacuum
formation and avoid formation of differential pressure.
Ans: The wind strakes were provided to alleviate across wind loads as mentioned in earlier versions of Chimney codes. But
IS 4998 (2015) does not specify use of wind strakes for any amount of wind speeds to alleviate across wind loads. Hence
Chimney will be designed for the wind loads of Along wind and Across wind forces as required as per the latest code.
Hence wind strakes are not envisaged.
6. A chimney in isolated condition is subjected to atmospheric buffeting effect due to turbulent velocity fluctuations in the
approach flow contributing to along wind loads and vortex shedding & lateral component of the turbulence in the
approach flow contributing to across-wind loads.

2. Design
5.1: Dead loads: As per IS 875 (Part-1)
5.2: Imposed loads: As per IS 875 (Part-2)
5.3: Earthquake loads: This load will be computed in accordance to IS 1893 (Part 4) with a response reduction factor
R= 2.0 & with a importance factor I=1.75.
5.4: Temperature effects: loads due to temperature effects depend on individual requirements of chimneys & they
should be considered accordingly.
5.5 Wind loads:
5.5.1: General
Circular chimneys are wind resistant structures & are designed to resist:
1. Across-wind load- for the estimation of these loads, the widely used method developed by Vickery is
recommended.
2. Along-wind loads- for these loads, the effects of dynamic fluctuations are considered as static equivalent loads
through the concept of gust response factor, following Davenports Method.
3. Loads caused by circumferential pressure distribution
5.5.2: Basic Wind speed (Vb): The value of basic wind speed is recommended by IS 875 (Part-3). This corresponds to
3s averaged wind speed at 10 m height above ground level, in an open terrain country, having an annual exceedance
probability of 0.02.
5.5.3: Design Hourly mean wind speed (𝑽z): For computing along-wind loads at various heights of a chimney, the
HMWS shall be taken at any height z.
Vz (m/sec) = Vb*k1*k2*k3*k4
k1 & k3= IS 875 part-3
k4= 1.15 (for industrial structure as per IS 875 part-3)
k2- is the modification factor for z>10 m
k2= 0.1423[ln(z/z0)](z0^0.0706)
z0= aerodynamic roughness height which shall be taken as 0.02 m for all terrain categories.
5.5.4: Design wind pressure due to HMWS, p(z)
p(z)= 0.5*𝜌a[Vz]^2
ρa= mass density of air= 1.2 kg/m3.
5.5.5: Along-wind loads: The along wind responses are computed using Gust factor approach.
In general, the chimney shall be discretized into a number of segments along its height with each segment not
exceeding 10m. The load at any section shall be calculated by suitably averaging the loads above & below it. The
moments are calculated from the sectional forces treating the chimney as a free-standing structure.
The along wind load, F(z) per unit height at any level=z, on a chimney is equal to the sum of the mean along-wind
load, F~z and the fluctuating component of along-wind load, F’z. Therefore:
F(z)=F~z+F’(z)
F~z (N/m)=Cd*d(z)*p(z); where, Cd is mean drag co-efficient taken as 0.8
d(z)=outer dia of chimney at ht. z
The fluctuating component of along wind
F’z= 3
( )
^
∫ 𝐹(𝑧)𝑧𝑑𝑧

3. Where G is Gust Response Factor
H= Height of the Chimney above the ground (m)
5.5.6: Gust Response Factor:
gf= peak factor, defined as the ratio of expected peak value to root mean square value of the fluctuating load
rt= 0.622-0.178log10H
B= {1+(H/265)^0.63}^-0.88 ; background factor indicating the slowly varying component wind load fluctuations.
E is a measure of available energy in the wind at the natural frequency;
S= size of reduction factor
5.5.7: Across wind loads: Across wind loads due to vortex shedding in the 1st
& 2nd
modes shall be considered shall
be considered in the design of the chimney shells;
Conditions: critical wind speed Vcr
0.5V mean(z ref)<Vcr<1.3V mean(z ref)
Across wind loads beyond these limits should not be considered.
The across wind base bending moment depends on the shape factor of the two modes , effective diameter which is
taken as average dia over top 1/3rd
height of chimney, critical wind speed & natural frequencies of unlined chimney,
average mass in top 1/3rd
height of chimney.
The maximum value of Mac, which is the bending moment, determined in the region of 0.8 Vcr & 1.2 Vcr shall be
taken as the design across-wind base bending moment.
The across wind load per unit length at any height i.e. Fac(z) in N/m is calculated with the bending moment Mac & the
corresponding mode shape of chimney at any level z & ith
mode.
V(10)=mean hourly wind speed at 10m ht
AGL (m/s).
b= Str. Damping as a fraction of critical
damping to be taken as 0.016 for along wind
loads
f1=natural frequency of unlined chimney in
the first mode of vibration Hz, as per 5.5.8

4. Using Fac(z), the across wind bending moment at any height Mac(z), can be obtained.
Natural frequencies: For preliminary design f1 & f2, i.e. the natural frequencies are calculated by the following
formulas:
5.5.10: Combination of Across-wind & Along wind loads:
The combined bending moment Mcomb(z) at any section shall be taken as the resultant of across wind bending moment
& the co-existing mean along -wind bending moment.
Mcomb(z)={(Mac(z))^2+(Mal(z))^2}^0.5
5.5.11: Circumferential ring moments due to wind
Mo(z)(N-m/m)=0.33 *p(z)*(rm(z))^2 where; p(z)- wind pressure due to 3 second gust wind speed at height z in
N/sqm. The pressure shall be increased by 50% for a distance of 1.5d(H) not
exceeding 15m from the top
rm(z): mean radius of shell at the section.
The hoop forces & shear force due to ovalling need not be considered.
5.6: Load combinations:
Wind & earthquake loads shall not be acting simultaneously.
The various load combination shall include:
1) Dead load
2) Dead Loads + wind loads+ loads due to temperature effects
3) Dead loads+ earthquake loads+ loads due to temperature effects
4) Circumferential ring moments due to wind+ due to temperature effects
Foundations
6.4.3.1: Based on Geo-technical considerations, shallow raft or deep pile foundations may be provided.
Foundations must be designed to transfer the vertical (gravity) & lateral (wind/earthquake) loads safely to the sub-
grade. The foundations must also be sufficiently rigid to prevent excessive deflection of the chimney.
But, for final design, natural frequency shall be computed
y dynamic analysis & by discretizing the chimney into
segments of not more than 10m.
If the lining is supported by the shell in any manner, te
effect of lining on the natural frequency will be
investigated.

5. Tall chimneys are more susceptible to differential settlement than ordinary structures firstly because the width of the
foundation is small in relation to the height of the structure. & secondly because of the lack of redundancy in the
structure.
Uplift shall not be permitted for a raft foundation under critical load combination of (0.9 DL+1.0WL).
For piled foundations, the tension capacity of the piles may be utilized to permit a small amount of uplift.
Simple elastic analysis based on assumption of rigid foundation & uniform thickness may be used strictly within
following limitations:
A) The foundation is relatively rigid. The foundation may be assumed to be rigid, if:
 Dia to depth ratio does not exceed 12
 Overhang of the foundation beyond the shell does not exceed four times the thickness
B) The foundation consists of a solid raft or pile cap
C) The raft/ pile cap is uniform in depth or tapered only very slightly ( taper not exceeding 1 in 8).
The structural design of the foundation shall comply with limit state designs requirements for strength & serviceability
(cracking) as per IS 456.
Annex C: Dynamic wind loads on chimney
A chimney in isolated condition is subjected to
a) Atmospheric buffeting effect due to turbulent velocity fluctuations in the approach flow contributing to along
wind loads
b) Vortex shedding & lateral component of the turbulence in the approach flow contributing to across-wind
loads.
Since wind speed varies randomly , the analysis of wind loads is based on the principles of statistics & theory of
random vibrations.
The mean along wind loads per unit height, at any given height, is estimated by multiplying the design wind pressure
due to HMWS at that height with mean drag co-efficient & outer dia.
The estimation of across wind loads, due to vortex shedding is relatively a more complex issue. The challenges which
retard a better understanding of the behavior of chimney are due to:
a) The effect of Reynolds number & the turbulence intensity on the aerodynamic parameters such as mean &
fluctuating drag & lift co-efficient, Strouhal number, correlation length, wake pressure characteristics
b) Proper extrapolation of boundary layer wind tunnel experimental results to full-scale chimney conditions
c) Limited information from full-scale measured data on various aerodynamic parameters with considerable
scatter.