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IS 2950:2000 (Part 1) is the Indian Standard (BIS) for design and construction of raft foundations, part 1: design. This code establishes the criteria for the design of raft (mat) foundations. It provides detailed guidance on evaluating soil-structure interaction, selecting between rigid and flexible methods of analysis, calculating the modulus of subgrade reaction, and properly proportioning the foundation to limit differential settlements and ensure safe stress distribution.
Code of practice for design and construction of raft foundations, Part 1: Design
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Punching shear (two-way shear) under heavily loaded columns frequently governs the thickness of the raft and must be checked strictly as per IS 456.
! The modulus of subgrade reaction (k) given in tables or obtained from plate load tests (30 cm plate) must be scaled to the actual dimensions of the raft.
! The rigid method of analysis assumes the raft acts as a rigid body; this is only valid when the structural stiffness is high and column spacing is relatively close.
EN 1997-1:2004+A1:2013CEN (European Committee for Standardization), Europe
HighCurrent
Eurocode 7: Geotechnical design - Part 1: General rules
Covers geotechnical design principles for all foundation types, including rafts, using a limit state design philosophy.
ACI 336.2R-19ACI (American Concrete Institute), USA
HighCurrent
Guide to the Analysis and Design of Combined Footings and Mats
Provides specific and detailed guidance on the analysis and structural design of mat (raft) foundations.
BS 8004:2015+A1:2020BSI (British Standards Institution), United Kingdom
MediumCurrent
Code of practice for foundations
Offers broad recommendations for the design and construction of various foundation types, including rafts, often referencing Eurocode 7.
Key Differences
≠IS 2950 is primarily based on the Working Stress Method (WSM), using a single global factor of safety, whereas modern standards like Eurocode 7 mandate a Limit State Design (LSD) approach with partial safety factors applied to loads, material properties, and resistances separately.
≠IS 2950 describes conventional rigid and simplified flexible methods for analysis. In contrast, ACI 336.2R and Eurocode 7 strongly advocate for more advanced soil-structure interaction (SSI) analysis using methods like Finite Element Method (FEM) or a detailed coefficient of subgrade reaction (Winkler) approach.
≠The Indian standard provides basic guidelines, while documents like ACI 336.2R are comprehensive guides that include detailed commentary, example calculations, and extensive discussion on various analysis techniques and practical considerations.
≠Seismic design considerations in IS 2950 are less detailed compared to modern international standards, which incorporate more sophisticated analysis of dynamic soil-structure interaction and liquefaction potential as per codes like ASCE 7 and Eurocode 8.
Key Similarities
≈All standards universally recognize that raft design must prevent bearing capacity failure and ensure total and differential settlements are within tolerable limits for the superstructure.
≈A comprehensive geotechnical site investigation is mandated as a fundamental prerequisite for design across all codes, forming the basis for determining soil parameters and profiles.
≈The structural design of the raft slab (e.g., calculating bending moments, shear forces, and required reinforcement) is based on common principles of reinforced concrete theory in all standards, even though the specific detailing rules may vary.
≈All codes acknowledge the importance of considering groundwater levels, soil layering, eccentric loading, and the stiffness of the raft and superstructure in the foundation design.
Parameter Comparison
Parameter
IS Value
International
Source
Design Philosophy
Working Stress Method (WSM)
Limit State Design (LSD) / Load and Resistance Factor Design (LRFD)
EN 1997-1 / ACI 336.2R-19
Global Factor of Safety (Bearing Capacity)
2.5 to 3.0
Not used; partial factors on resistance (e.g., γR ≈ 1.4) and actions are applied instead.
EN 1997-1
Minimum Concrete Cover (against earth)
50 mm (for foundations, from IS 456)
75 mm (3 in.) for concrete cast against and permanently exposed to earth.
ACI 318-19 (used with ACI 336.2R-19)
Permissible Total Settlement (Rafts)
75 mm on sand/hard clay; 100 mm on plastic clay (from IS 1904)
No single prescribed value; depends on structure, but 50 mm is a common serviceability limit.
EN 1997-1 (Informative Annex)
Permissible Angular Distortion
1/500 to 1/750 for avoiding cracking in panel walls (from IS 1904)
1/500 is a common limit for frames with brittle finishes.
BS 8004:2015
Minimum Shrinkage & Temperature Reinforcement
0.12% of gross area for HYSD bars (from IS 456)
0.18% of gross area (for Grade 60 rebar).
ACI 318-19 (used with ACI 336.2R-19)
⚠ Verify details from original standards before use
Key Values4
Quick Reference Values
relative stiffness factor limit for rigid analysis> 0.5
relative stiffness factor limit for flexible analysis< 0.5
minimum factor of safety against uplift1.2
recommended minimum thickness at edge150 mm (subject to shear checks)
When should the flexible foundation approach be used?+
It should be used when the relative stiffness factor (K) is less than 0.5, meaning the raft will bend significantly under loads, causing non-planar soil pressure distribution.
How is the soil modulus of subgrade reaction determined?+
It is typically evaluated using a standard plate load test (IS 1888) and then corrected for the size of the full raft foundation using formulas provided in Appendix B.
What happens if the center of gravity of the loads does not coincide with the centroid of the raft?+
Eccentricity is introduced, causing an unequal pressure distribution on the soil. The raft must be proportioned to keep this eccentricity as small as possible to avoid excessive differential settlement.