IS 4998 establishes the structural design criteria for reinforced concrete chimneys. It outlines procedures for evaluating dead loads, along-wind and across-wind forces, seismic actions, and thermal stresses caused by internal flue gas temperature gradients, alongside providing detailing guidelines for the chimney shell.
Provides constructional details for buildings to enhance their resistance to cyclone forces.
Quick Reference Values
minimum shell thickness150 mm
minimum vertical reinforcement0.25% of gross concrete area
minimum horizontal reinforcement0.20% of gross concrete area
maximum reinforcement spacing250 mm
minimum clear cover50 mm
Key Formulas
T_x = T_i - (T_i - T_o) * (R_x / R_t) — Temperature distribution across shell thickness
f_c = (N / A) + (M / Z) ± f_temp — Combined stresses due to axial load, moment, and temperature
Practical Notes
Across-wind loads due to vortex shedding often govern the structural design of tall chimneys and require dynamic analysis.
Thermal stresses induced by temperature gradients across the concrete shell are significant and must be combined with wind or seismic load combinations.
Special attention must be given to concrete mix design and cover to protect reinforcement from aggressive chemical exposure due to flue gases.