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IS 11504:1985 is the Indian Standard (BIS) for criteria for the structural design of reinforced concrete natural draught cooling towers. This standard provides criteria for the structural design of reinforced concrete natural draught cooling towers. It covers the determination of loads (dead, wind, seismic, thermal), methods of analysis for the hyperbolic shell, and design requirements for the shell, top ring, bottom ring beam, and supporting columns. The code places significant emphasis on wind load analysis due to the unique geometry and height of these structures.
Criteria for the structural design of reinforced concrete natural draught cooling towers
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! This code is from 1985 and may not cover all complexities of modern analysis. Designers often supplement it with international standards like ACI-ASCE 334.2R or detailed finite element analysis (FEA).
! Wind load calculation is the most critical aspect of the design. For major projects, wind tunnel testing is highly recommended to validate or replace the static coefficients provided in the code.
! Construction tolerances are extremely important. Deviations from the specified hyperbolic profile can significantly reduce the shell's buckling capacity.
Directly addresses the structural design and analysis of natural draught cooling towers.
ACI 318-19 & ASCE/SEI 7-22American Concrete Institute / American Society of Civil Engineers (USA)
MediumCurrent
Building Code Requirements for Structural Concrete & Minimum Design Loads and Associated Criteria for Buildings and Other Structures
Used in combination; ACI 318 for concrete design and ASCE 7 for wind/seismic loads.
CICIND Model Code for Concrete Cooling Towers, 2018International Committee for Industrial Chimneys (International)
HighCurrent
Model Code for Concrete Cooling Towers
Provides comprehensive international guidelines for the design of concrete cooling towers.
BS 4485-4:1998British Standards Institution (UK)
MediumWithdrawn
Water cooling towers. Part 4: Code of practice for structural design and construction
Former UK code of practice covering the structural design and construction of cooling towers.
Key Differences
≠Wind Load Analysis: IS 11504 uses a simplified quasi-static approach based on IS 875 (Part 3). Modern codes like VGB-S-610 and ASCE 7 mandate more sophisticated analyses, including detailed dynamic response calculations for along-wind and across-wind effects, and often require wind tunnel studies for complex sites or tower groups.
≠Durability and Reinforcement: The Indian standard specifies minimum reinforcement percentages (0.3-0.35%) and concrete cover (25mm) that are significantly lower than modern international standards. For example, VGB-S-610 often leads to total reinforcement of over 0.6% and cover of >40mm to ensure long-term durability and crack control in the aggressive, moist environment.
≠Seismic Design Philosophy: Seismic provisions in IS 11504 are based on the principles of IS 1893 from the 1980s. Current international practice (e.g., per ASCE 7) involves more advanced methods like multimodal response spectrum analysis or nonlinear time-history analysis, providing a more accurate representation of the tower's dynamic behavior during an earthquake.
≠Thermal and Operational Loads: Modern standards like the CICIND Model Code provide highly detailed guidance on analyzing thermal gradients (between inner and outer faces, and vertically), shrinkage, and creep. The treatment in IS 11504 is less comprehensive, which can lead to an underestimation of stresses due to operational and environmental cycles.
Key Similarities
≈Structural Form and Primary Analysis: All standards are based on the analysis of a thin-walled reinforced concrete hyperbolic shell of revolution.
≈Membrane Theory Application: The fundamental approach of using membrane theory to determine the primary meridional (vertical) and circumferential (hoop) forces in the shell is a common basis across all standards.
≈Load Combinations Principle: The core principle of combining various loads (dead, live, wind, seismic, thermal) using specified load factors to check for ultimate and serviceability limit states is a shared methodology, although the specific factors and combinations differ.
≈Stiffening Elements: All codes recognize the necessity of stiffening elements, requiring a stiff upper edge beam (top lintel) and a robust lower ring beam to distribute shell forces into the supporting columns.
Parameter Comparison
Parameter
IS Value
International
Source
Minimum Shell Thickness
125 mm
≥ 160 mm (often 180 mm in practice)
VGB-S-610-00-2016-12-EN
Minimum Concrete Cover (Shell)
25 mm
≥ 40 mm (for durability in aggressive environments)
VGB-S-610 / Modern Practice
Minimum Meridional Reinforcement (Total %)
0.30% of gross area
≈ 0.60% (e.g., 0.3% per face for crack control)
VGB-S-610-00-2016-12-EN
Minimum Circumferential Reinforcement (Total %)
0.35% of gross area
≈ 0.60% (e.g., 0.3% per face for crack control)
VGB-S-610-00-2016-12-EN
Basis for Design Wind Speed
3-second gust speed (per IS 875-3)
10-minute mean wind speed
EN 1991-1-4 (referenced by VGB)
Ultimate Load Factor for Wind Load (in typical combo)
1.5 (in 1.5(DL+WL))
1.6 (in 1.2D+1.6W+...)
ASCE/SEI 7-22
Analysis for Shell Openings
Prescriptive rules for additional reinforcement.
Requires detailed Finite Element Analysis (FEA) to determine stress concentrations and design reinforcement accordingly.
CICIND Model Code, 2018
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Minimum grade of concreteM20
Minimum clear cover to shell reinforcement40 mm
Minimum thickness of shell160 mm
Minimum vertical reinforcement in shell0.3%
Minimum horizontal (hoop) reinforcement in shell0.3%