Criteria for design of reinforced concrete chimneys, Part 1: Assessment of loads
1992 Edition

According to IS 4998 Part 1 (1992), wind loads on reinforced concrete chimneys are derived by calculating the wind force per unit height at any elevation z using the formula w_z = p_z × C_D × d_z, where p_z is the design wind pressure at height z, C_D is the drag coefficient (typically 0.8 for circular chimneys), and d_z is the chimney diameter at that height. For chimney groups spaced less than three times the largest diameter, aerodynamic interference effects must be accounted for. Oscillatory wind effects are characterized by peak and RMS lift coefficients of 0.16 and 0.12 respectively. For non-circular or closely spaced chimneys, wind tunnel testing or expert consultation is recommended to capture dynamic effects such as buffeting and vortex shedding.

IS 4998 Part 1 addresses dynamic wind-induced effects including vortex shedding and aerodynamic interference by relating vortex shedding frequency to wind speed through the Strouhal number (0.2). Resonance may occur when the vortex shedding frequency aligns with the chimney’s natural frequency, potentially causing significant oscillations known as vortex locking within ±10% of the critical wind speed. The standard provides formulas to compute oscillation amplitudes using a peak lift coefficient of 0.16 and prescribes amplitude adjustments if deflections exceed 4% of the diameter. Aerodynamic interference effects are especially considered for chimney clusters with spacing less than 20 diameters at two-thirds height, with magnification factors derived from model studies applied to account for increased oscillation amplitudes.

Earthquake loads on chimneys per IS 4998 Part 1 are assessed using the seismic design criteria outlined in IS 1893:1984, incorporating the chimney’s natural frequencies computed as per Annex A of IS 4998. The procedure involves calculating spectral accelerations corresponding to these frequencies to determine seismic base shear forces. The chimney is treated as a dynamic structure, requiring modal analysis to distribute earthquake forces along its height appropriately. These seismic forces are combined with other loads such as dead, wind, and temperature effects to ensure structural safety.

To mitigate vortex-induced oscillations, IS 4998 Part 1 recommends installing discrete helical strakes arranged along three helices around the chimney, each offset by 30° in azimuth. The vertical spacing between strakes is specified as 5d/12, where d is the chimney diameter. Coverage of the strakes depends on oscillation magnification: the top one-third of the chimney if magnification is under 6, or the top half if greater. The minimum strake cross-sectional area and the resulting increase in drag coefficient are prescribed based on magnification levels. This method effectively suppresses vortex shedding amplitude and improves structural stability. For non-circular or clustered chimneys, additional aerodynamic considerations or wind tunnel tests are advised.

The standard requires that temperature effects, including vertical and circumferential thermal stresses, be incorporated into load assessments as they induce expansion, contraction, and associated bending and hoop stresses in chimneys. Temperature loads are combined with other load types such as dead, wind, and seismic loads in prescribed combinations. Circumferential effects due to temperature are explicitly considered, and secondary effects from deflection under thermal loads should be accounted for, at least for one load cycle. The design must consider temperature gradients along the height and circumference to ensure chimney safety and serviceability under combined loading conditions.