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IS 4091:1979 is the Indian Standard (BIS) for design and construction of foundations for transmission line towers and poles. This code specifies the design and construction guidelines for foundations of overhead transmission line towers and poles. It emphasizes load determination, particularly the critical calculation of uplift resistance using the earth frustum method for various soil and rock classifications.
Code of practice for design and construction of foundations for transmission line towers and poles
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Uplift is often the governing criteria for transmission tower foundations, unlike standard building foundations where downward bearing pressure governs.
! Using assumed conservative soil parameters instead of actual geotechnical investigation can lead to massively oversized pad and chimney foundations, increasing project costs.
! Undercut foundations should only be proposed in stiff, cohesive soils that can remain stable without shoring during excavation.
ASCE/SEI 48-19American Society of Civil Engineers (ASCE), USA
HighCurrent
Design of Steel Transmission Pole Structures
Includes a comprehensive chapter on the design and analysis of foundations for transmission structures.
CIGRE Technical Brochure 783International Council on Large Electric Systems (CIGRE), International
HighCurrent
Foundations for Transmission Line Towers
A state-of-the-art global technical guide dedicated entirely to transmission tower foundations.
EN 50341-1:2012 + A1:2018European Committee for Electrotechnical Standardization (CENELEC), Europe
MediumCurrent
Overhead electrical lines exceeding AC 1 kV - Part 1: General requirements - Common specifications
Defines loads and design principles for OHL, referencing Eurocode 7 (EN 1997) for geotechnical design.
EN 1997-1:2004European Committee for Standardization (CEN), Europe
MediumCurrent
Eurocode 7: Geotechnical design - Part 1: General rules
Provides the fundamental geotechnical design rules (LSD) used for foundations under standards like EN 50341.
Key Differences
≠IS 4091 uses a Working Stress Design (WSD) methodology with a single global Factor of Safety, whereas modern standards like ASCE 48 and Eurocode 7 use Limit State Design (LSD/LRFD) with separate partial factors for loads and resistances.
≠The Indian standard provides a simplified empirical model for uplift resistance (frustum of a pyramid/cone). CIGRE TB 783 and ASCE 48 discuss and validate multiple, more sophisticated models like the cylindrical shear model and variable-angle cone models.
≠IS 4091 has limited guidance on cyclic loading effects from wind. CIGRE TB 783 and other modern research-based documents provide much more detailed analysis of potential soil strength degradation and accumulated displacement under cyclic loads.
≠Modern standards place a stronger emphasis on performance-based design, considering foundation movement and rotation and their effect on the overall structure, whereas IS 4091 uses prescribed allowable settlement limits.
Key Similarities
≈All standards are based on the fundamental equilibrium principle that the geotechnical and structural resistance of the foundation must exceed the applied tower loads (uplift, compression, shear) by an adequate safety margin.
≈The primary failure modes considered are consistent across standards, including uplift failure, bearing capacity failure, sliding, and structural failure of the foundation itself.
≈The basic types of foundations covered, such as open-cast pad & chimney (spread footings), pile foundations, and rock anchors, are common to IS 4091 and its international counterparts.
≈All codes recognize the same fundamental soil properties (unit weight, angle of internal friction 'φ', cohesion 'c') as the primary inputs for calculating geotechnical capacity.
Parameter Comparison
Parameter
IS Value
International
Source
Design Philosophy
Working Stress Design (WSD)
Limit State Design (LSD) / Load and Resistance Factor Design (LRFD)
ASCE/SEI 48-19, EN 1997-1
Factor of Safety on Uplift (Normal Conditions)
2.2 on soil resistance
No single factor; uses separate load factors (e.g., 1.2-1.6) and resistance factors (e.g., 0.3-0.5) in LRFD.
ASCE/SEI 48-19
Uplift Model (Spread Footing)
Weight of soil in an inverted frustum with side angle to vertical of φ/2 or 20°, whichever is less.
Multiple models are permitted, including cone models where the angle can be φ, and cylindrical shear models.
CIGRE TB 783
Concrete Curing Period
28 days before backfilling and loading.
Generally performance-based; backfilling and loading allowed when concrete reaches a specified percentage (e.g., 75%) of its 28-day design strength.
General practice in ASCE/ACI standards
Minimum Concrete Grade (RCC)
M20 (equivalent to M200 in old nomenclature)
Typically 3000 psi (~21 MPa / M20), but 4000 psi (~28 MPa / M25-M30) is common practice.
ASCE/SEI 48-19
Permissible Total Settlement (Tower)
25 mm
Not a fixed value; depends on tower analysis. Can range from 40 mm to 100 mm depending on structure type and criticality.
CIGRE TB 783
Permissible Differential Rotation
Not explicitly defined in detail.
Often limited to 0.002 to 0.005 radians (approx. 0.1° to 0.3°) depending on the structure's sensitivity to rotation.
CIGRE TB 783
⚠ Verify details from original standards before use
Key Values5
Quick Reference Values
Factor of safety against uplift (Normal)1.5
Factor of safety against uplift (Broken wire)1.5
Angle of earth frustum for ordinary dry soil30 degrees
Angle of earth frustum for hard rock0 degrees
Unit weight of dry soil (Standard assumption)1440 kg/m3
Key Formulas
Uplift capacity (Tu) = Wc + We (where Wc is weight of concrete and We is weight of earth frustum)
Tables & Referenced Sections
Key Tables
Table 1 - Angle of Earth Frustum and Unit Weight of Soil
What is the primary method for calculating uplift capacity in this code?+
The earth frustum method, which calculates the combined weight of the foundation and the volume of soil within a specific angle from the foundation base.
What is the recommended angle of earth frustum for normal dry soil?+
30 degrees to the vertical.
What factor of safety is required against uplift?+
A minimum of 1.5 is required under normal conditions.