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IS 11089:1984 is the Indian Standard (BIS) for design and construction of ring foundation. This standard provides guidelines for the analysis, design, and construction of reinforced concrete ring foundations. It is specifically intended for tall, hollow circular structures like chimneys, silos, and towers, detailing methods to calculate stresses, moments, and shear forces resulting from vertical loads and overturning moments.
Code of practice for design and construction of ring foundation
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Ring foundations are most efficient for structures with large overturning moments where a central opening is necessary or desirable.
! The analysis method is based on the assumption that the ring beam is rigid compared to the soil. This assumption's validity should be checked, especially for large diameter rings on soft soil.
! Accurate assessment of soil parameters and soil-structure interaction is critical for an economical and safe design, as the code provides an analytical framework, not soil properties.
API 650, Appendix BAmerican Petroleum Institute (API), USA
HighCurrent
Welded Tanks for Oil Storage, Appendix B: Recommendations for Design of Foundations for Flat-Bottom Tanks
Provides specific design and detailing rules for ringwall foundations supporting oil storage tanks.
EN 1997-1:2004European Committee for Standardization (CEN), Europe
MediumCurrent
Eurocode 7: Geotechnical design - Part 1: General rules
Provides the overall geotechnical design framework (bearing resistance, settlement) for all foundation types, including ring foundations.
ACI 336.2R-17American Concrete Institute (ACI), USA
MediumCurrent
Guide to the Design and Construction of Combined and Mat Foundations
Covers analysis of foundations on soil using similar soil-structure interaction principles (e.g., beam on elastic foundation).
BS 8004:2015British Standards Institution (BSI), UK
LowCurrent
Code of practice for foundations
General code for all foundation types, providing principles that would inform a ring foundation design but without specific ring formulae.
Key Differences
≠IS 11089 is based on the Working Stress Method (WSM) of design, whereas modern international standards like Eurocode 7 are based on the Limit State Design (LSD) or Load and Resistance Factor Design (LRFD) philosophy, using partial safety factors.
≠IS 11089 explicitly provides formulae and coefficients based on the Winkler model (beam on elastic foundation). While international standards permit this, they also encourage or require more sophisticated soil-structure interaction (SSI) analyses, such as Finite Element Method (FEM), especially for complex or critical structures.
≠The load combinations specified in IS 11089 are specific to the 1984 code. International standards like those referencing ASCE 7 (for ACI codes) or Eurocode 0 (for EN codes) use more updated and comprehensive probabilistic load combinations.
≠API 650 provides highly specific requirements tied to tank design, such as anchorage details to resist shell uplift and specific considerations for hydrostatic test loads, which are not detailed in the general-purpose IS 11089.
Key Similarities
≈Both IS 11089 and international design guides are fundamentally based on modeling the ring as a structural beam supported by the underlying soil (elastic foundation).
≈All standards recognize the same primary internal forces to be designed for: circumferential bending moment, torsional moment, and radial shear force.
≈A core design requirement in all standards is that the pressure exerted by the foundation on the soil must not exceed the safe or allowable bearing capacity of the soil.
≈The principles of reinforcing the concrete ring are consistent: providing longitudinal steel to resist bending and torsion, and stirrups (links) to resist shear and torsion.
Parameter Comparison
Parameter
IS Value
International
Source
Primary Analysis Model
Prescribes analysis based on the theory of a circular beam on an elastic foundation (Winkler model).
Winkler model is permitted, but advanced methods like Finite Element Method (FEM) for soil-structure interaction are encouraged.
ACI 336.2R-17 / Eurocode 7
Minimum Foundation Width (Guideline)
Suggested to be between D/10 and D/15, where D is the mean diameter of the ring. (Clause 4.2.1)
Typically determined by analysis or prescribed minimums, e.g., minimum 300 mm (12 in.) plus bolt projections. Not directly a ratio of diameter.
API 650, Appendix B
Minimum Longitudinal Reinforcement
~0.20% of cross-section area (for Fe415 steel, as per reference to IS 456).
0.25% of the gross concrete cross-sectional area for circumferential and radial reinforcement.
API 650, Appendix B
Minimum Concrete Cover (to earth)
50 mm for 'moderate' exposure (as per reference to IS 456).
75 mm (3 inches) for surfaces cast against and permanently exposed to earth.
API 650, Appendix B
Design Philosophy
Working Stress Method (WSM).
Limit State Design (LSD) / Load and Resistance Factor Design (LRFD).
Eurocode 7 / ACI 318
⚠ Verify details from original standards before use
Key Values3
Quick Reference Values
Applicability criteriaGenerally used when ratio of inner to outer radius (a/b) is greater than 0.7
Soil pressure assumptionLinear (trapezoidal) distribution under the ring foundation
Ring beam assumptionThe ring beam is considered rigid
Key Formulas
p(θ) = (P/A) + (M/Z)cos(θ) — Soil pressure at any angle θ
Mθ = M₀ * C1 + M' * C'1 — Bending moment at any angle θ
Tθ = M₀ * C2 + M' * C'2 — Twisting moment at any angle θ
Qθ = (P/R) * C3 + (M'/R) * C'3 — Shear force at any angle θ
Tables & Referenced Sections
Key Tables
Table 1 - Coefficients C1, C2 and C3 for Bending Moment, Twisting Moment and Shear Force (Uniform Vertical Load)
Table 2 - Coefficients C'1, C'2 and C'3 for Bending Moment, Twisting Moment and Shear Force (Moment M')
It is suitable for tall, hollow, circular structures like chimneys and silos, especially when the ratio of inner to outer radius is greater than 0.7 (as a guideline).
What is the basic assumption for soil pressure distribution?+
The code assumes a linear (trapezoidal) distribution of soil pressure under the foundation, resulting from the combined effect of the total vertical load and the overturning moment (Clause 5.2).
Which codes are used for load calculation?+
Loads are calculated as per IS 875 (for dead, live, wind loads) and IS 1893 (for seismic loads).
How is the structural design of the ring beam done?+
The structural design for bending, shear, and torsion is carried out in accordance with IS 456, using the forces calculated from the analysis prescribed in this code (Clause 6).