Criteria for earthquake-resistant design of the structure, Part 4: Industrial structures including stack-like structures )
2005 Edition

IS 1893 Part 4 categorizes industrial structures into four groups based on the consequences of their failure during seismic events: Category 1 involves structures whose failure could result in severe loss of life or property in nearby areas, such as hazardous plants; Category 2 includes those that may cause serious fire hazards or significant damage within the plant, including emergency facilities; Category 3 covers structures where failure causes costly damage but no major hazard; and Category 4 encompasses all other structures. This classification helps determine seismic detailing and ductility requirements.

Seismic force calculation for stack-like structures involves first estimating the fundamental natural period using Rayleigh’s method, which accounts for lumped weights and their lateral deflections. Then, the horizontal seismic coefficient is derived from the zone factor, structure importance, response reduction factor, and spectral acceleration coefficient per IS 1893 Part 1. The lateral earthquake force is obtained by multiplying this coefficient by the total seismic weight. Design can proceed using equivalent static lateral force methods or dynamic response spectrum analyses.

For RCC chimneys, vertical reinforcement must be at least 25% of the concrete cross-sectional area; if two reinforcement layers are provided, the outer layer should contain no less than 50% of the total vertical bars. Circumferential reinforcement near the top 20% of the chimney’s diameter is doubled compared to normal requirements. Around openings, additional reinforcement is required on both faces extending beyond the opening to develop full bond strength, with a minimum extra area equal to half the interrupted circumferential reinforcement. These provisions ensure structural stability and account for stress concentrations.

The standard acknowledges soil-structure interaction (SSI) as the effect of foundation soil on structural seismic response. SSI may be disregarded if the foundation rests on rock or very stiff materials. However, for soils susceptible to settlement, liquefaction, or strength degradation during earthquakes, SSI must be accounted for since it influences the seismic forces and displacement demands. While detailed SSI analyses are not explicitly mandated, they are implied for soft or problematic soils, often employing impedance functions or coupled dynamic soil-structure models.

The code recommends three main analysis approaches: the Equivalent Static Lateral Force Method for simpler industrial structures, using fundamental vibration period and seismic coefficients; the Dynamic Response Spectrum Modal Analysis for important or stack-like structures, capturing significant modal mass and combining modal responses per guidelines; and Linear Time-History Analysis for detailed site-specific studies, requiring compatible ground motion records and spectral data. The choice depends on structure complexity, importance, and data availability.