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.
| 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 |