IRC 45-1972 offers comprehensive guidance for assessing soil resistance beneath the maximum scour depth in the design of well foundations for bridges supported on non-cohesive soils such as sand. It assists engineers in determining base and lateral soil pressures, moments, and safety factors by applying elastic theory alongside ultimate resistance principles to guarantee foundation stability under various loading conditions including live loads, water currents, seismic, and wind forces.
Overview
IRC 45-1972 offers comprehensive guidance for assessing soil resistance beneath the maximum scour depth in the design of well foundations for bridges supported on non-cohesive soils such as sand. It assists engineers in determining base and lateral soil pressures, moments, and safety factors by applying elastic theory alongside ultimate resistance principles to guarantee foundation stability under various loading conditions including live loads, water currents, seismic, and wind forces.
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Contents
Structure
This section introduces IRC 45, highlighting the general design aspects for road bridges and referencing the Standard Specifications and Code of Practice for Road Bridges, Section 1. It presents essential load combinations such as 1.1D + 1.4(Wc + Ep + Wor S) and 1.1D + 1.6L, defining variables like dead load, wheel load, impact factors, and others. Additionally, it includes the Ultimate Resistance Method table relating depth-to-breadth ratios with factor Q, crucial for foundational calculations.
This part defines the application range of IRC 45 in relation to the Standard Specifications for Road Bridges, focusing on design features. It details load combination formulas with factors for dead load, braking force, centrifugal force, seismic, and wind forces. Shape factors for wells of various geometries are explained, along with the Ultimate Resistance Method’s Q values based on depth-to-width ratios.
This section outlines the primary assumptions regarding soil behavior and foundation geometry. It details the formula for base resisting moment incorporating load, base dimensions, soil friction angle, and a Q factor dependent on shape and embedment ratio. Interpolation methods for intermediate Q values are also discussed.
Describes the computational procedures for determining soil resistance using elastic theory, involving subgrade moduli and soil properties. It emphasizes the use of elastic theory to estimate base pressures and soil resistance under various loads, referring readers to the full clauses for detailed formulas.
Explains the method for calculating base pressures on well foundations, presenting the formula for base resisting moment dependent on load, base dimensions, soil friction angle, and Q factor. It includes adjustments for circular bases and tabulated Q values for rectangular wells, alongside earth pressure coefficients for side resistance.
Details the evaluation of side resisting moments acting about a rotation point above the base, incorporating soil unit weight, embedment depth, passive and active earth pressure coefficients, and well dimensions. It also discusses additional moments and the stability condition requiring the total resisting moment to exceed applied moments.
Describes how frictional forces on the well’s sides contribute to resisting moments. Includes formulas for base resisting moment, side resisting moment, frictional moments, and the combined total resisting moment, with safety factors and load considerations as per the code.
Summarizes the total resisting moment of soil as the sum of base, side, and frictional moments. It reiterates the significance of the Q factor, soil parameters, and safety checks ensuring that reduced resisting moments meet or exceed applied loads.
Presents the recommended safety factors, including a reduction coefficient applied to soil resistance moments, and specifies load factors for dead load, live load, water current, buoyancy, wind, seismic, and earth pressure. It also enumerates the principal load combinations for use in design.
Covers the use of elastic theory to compute soil pressures at the base and sides of well foundations, considering soil stiffness through subgrade reaction coefficients. It explains the necessity of verifying elastic theory results with the ultimate resistance method to ensure design safety.
Details the ultimate resistance method for calculating the base resisting moment, incorporating soil friction and foundation geometry. It provides formulae, Q values for various embedment ratios, and adjustments for circular foundations, highlighting its role in assessing soil failure modes.
Outlines conditions needed for foundation stability, including load combination expressions, factor of safety criteria, and limits on soil pressures to avoid tension and overloading. It also emphasizes inclusion of moments from foundation tilt and horizontal soil reactions in the stability analysis.
Describes the stepwise design methodology involving initial checks for soil resistance and structural adequacy, iterative redesign if criteria are unmet, and separate consideration of wind and seismic load cases to ensure comprehensive foundation safety.
Provides detailed elastic theory calculations assuming soil behaves as an elastic medium. It includes parameters such as subgrade reaction coefficients, base area, depth, moments, and resultant soil pressures, along with stability conditions ensuring allowable pressure limits and no soil tension.
Focuses on ultimate soil resistance considering failure mechanisms around the well foundation. It presents Q values based on the depth-to-width ratio, load factor formulas, and stability conditions to confirm soil side and base pressures remain within permissible limits.
Frequently Asked
IRC 45 is intended for designing well foundations on non-cohesive soils like sand where the soil beneath and surrounding the foundation is homogeneous and behaves elastically. The provisions apply when the foundation is embedded at least half its width in the direction of lateral forces below the maximum scour level. The soil reaction is assumed linear with deflection, following elastic behavior, and both elastic theory and ultimate resistance methods are used to verify design safety.
Base soil pressure is computed by distributing the total vertical load over the foundation base, factoring in soil friction and shape via the Q constant. For rectangular bases, the base resisting moment is calculated using Mb = Q × W × B × tan φ, adjusting for circular bases with a factor of 0.6. Side pressures are evaluated through elastic theory using subgrade reaction coefficients, integrating soil reactions along depth, and considering earth pressure coefficients. Stability requires side pressures to not exceed passive earth pressure and base pressures to remain within allowable limits without tension.
The standard specifies load factors such as 1.1 for dead load, 1.6 for live load combined with dead load only, 1.4 for water current and earth pressure forces, and 1.25 or 1.4 for wind and seismic forces depending on combinations. Braking force is treated similar to live load. The main load combinations include variations like 1.1D alone, 1.1D plus braking and environmental loads, 1.1D plus live load, and combinations incorporating wind or seismic forces to ensure safety under all loading scenarios.
IRC 45 addresses scour by focusing on estimating soil resistance beneath the maximum predicted scour level, ensuring that the foundation design considers reduced soil support due to scour and maintains stability by evaluating soil-structure interaction at or below this depth.
The code advocates two primary approaches: calculating ultimate soil resistance with appropriate safety factors and using elastic theory to determine soil pressures under design loads. The total resisting moment combines contributions from base, side, and frictional soil forces, reduced by a factor (commonly 0.7) to accommodate strength variability. This ensures conservative design accounting for uncertainties in soil properties and loading.
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