Week 1 Challenge

Aim:

To briefly explain the concepts of foundation of the bridge with appropriate drawings and codal references.

Introduction:

The bridge foundation is a critical element of any bridge since they maintain the stability of structure against all the impending design loads such as Dead load, Live load which is the traffic loads moving across the span of the bridge, Earth pressure, Water forces, Wave forces, Seismic forces, Snow loads, Temperature loads, Impact loads, Centrifugal loads etc.,

The bridge foundation is the component constructed under the pier or abutments and above the founding strata which might be soil or rock. The bridge foundation transfers the loads evenly from the superstructure to the soil below without drastic settlements, or shear failures of the soil.

Solution:

Question No. I: What are the types of bridge foundations and how are selection type is made?

The type of foundation depends on the depth / location of the stratum in which the bearing capacity of the soil is adequate against the loads impending on it. The basic selection type is made as follows.,

Shallow foundation:

If the bearing capacity of the stratum at a depth nearer to the nominal ground level is adequate to the design considerations, then shallow foundations would be adopted. The width of the foundation would be greater or equal to the depth of the foundation.

Shallow foundation = d≤b

Types of shallow foundations are as follows.,

• Combined foundation - The combination of two column loads acting on a single foundation is termed combined footing, generally combined footing would be adopted if there is less spacing between two isolated footings or if the actual settlement is greater than the allowable settlement.
• Mat foundation - Assuming the bridge of span length of 24m and width of 10m with 4 series of piers along the transverse direction of the bridge and 4 series along the direction of traffic. If the entire set of piers is combined under the nominal ground level as the same continuous foundation in the shape of a rectangular slab covering the whole area of the longitudinal span, this type would be termed as a mat foundation. This foundation type is used when there is a differential settlement at the site due to the presence of highly cohesive soil such as clay and black soil. Also to avoid punching shear due to heavier load transfer at a localised point.
• Spread or Isolated foundation - The isolated foundation supports a load of the super-structure and transfers the load to the subsoil from a single pier. This type of foundation is economical and used when the soil strata are of high bearing capacity and has a lesser settlement rate.
• Strip foundation - Strip foundations are the combined foundation but only in a linear manner supporting the piers along the transverse direction of the bridge. These types of footing are adopted when the centre-to-centre distance between the foundation is lesser and when the load distribution of the sub-soil profile intersects which might cause the shear failure of soil.
• Strap or Cantilever foundation - When two newly constructed or new and existing foundations are connected by means of a tie beam is called a strap or cantilever beam. When improvisation is required at the site of construction where there is existing footing, strap or cantilever foundation is advised.

Deep foundation:

If the bearing capacity of the stratum at a depth nearer to the nominal ground level is not adequate to the design considerations, then the further stratums will be checked against the bearing capacity, if the bearing capacity is found at a greater depth or in the hard rock, deep foundations would be adopted. Deep foundations are adopted when the soil strata are susceptible to liquefaction and higher capillary action causing uplift at the foundation level. The width of the foundation would be lesser than the depth of the foundation.

The slenderness ratio of piles is generally higher because it is of greater length. Unlike columns surrounded by atmosphere piles are surrounded by soil strata which by itself provides greater stiffness and stability though its depth is greater.

Types of deep foundation are as follows.,

• End-bearing piles - These end-bearing piles are up to the depth of hard strata upon which they are rested. The load is transferred from the superstructure to the pile and to the hard strata. These piles are preferred when hard strata are found at reasonable depths and in cohesive soil.
• Skin friction piles - These piles are supported only by skin friction and are adopted in non-cohesive soils and when the hard strata are not available at economical / considerable depths.
• Under reamed pile - These piles have two or more bulbs nearer to the end of the pile generally their radius being twice to thrice that of pile foundation. It is adopted when the soil is highly cohesive such as clay and black cotton soil.
• Well foundation or Caisson foundation - Well foundation is of circular in shape either precast or cast in-situ and is adopted when lateral forces are large. It is usually in waterbodies and in soils with active earth pressures.

Deep foundation = d>>b

The types of bridge and its foundation are influenced by several other factors and will be decided upon by conducting various preliminary studies. The bridge foundation is influenced by many factors and is as follows.,

• Topographical survey - Topography is the study of the forms and features of land surfaces. The recorded topography of an area contains the basic three-dimensional datum of latitude, longitude and altitude. Apart from these data, there are features such as roads, land boundaries, water bodies, conservatories and buildings. This data is essential for the selection of a bridge site
• Catchment area - It is the area of a boundary which is susceptible to collecting rainwater. This affects the highest flood level when the bridge is built across a river, lake or ocean.
• Hydrology - The hydrological data of a site has valuable datum such as water cycle, water movement and distribution, underwater drainages and groundwater across a terrain.
• Geotechnical investigation - This investigation is important as it provides information about the types and physical properties of soil stratum after laboratory tests such as density, moisture content, specific gravity and atterberg limits such as plastic limit, liquid limit, shrinkage limit and bearing capacity of soil
• Navigation - In case of bridges spanning across a river, lake or ocean, the navigation of ships under the bridge influences the clear height from the water body.
• Seismology - Seismic zone of the construction site of the bridge influences it at the start during the design stages as the bridge needs to be designed to withstand the colossal effects caused during an event of an earthquake.
• Construction resources - The availability of resources in the vicinity of the construction site influences the type of bridge and its foundation to be constructed.
• Traffic data - The volume of moving traffic across a region influences the span, type and width of the bridge which affects the type of bridge and its foundation to facilitate adequate movement of traffic.

Question No. II: Name all the components of a RCC girder bridge resting on open foundation?

Solution:

The components of a RCC girder bridge are as follows.,

Open Foundation - Open foundation is also called as spread foundation and is constructed where there is less or zero susceptibility of scouring and the hard strata is found near the nominal ground level. The bridge foundation is the component constructed under the pier or abutments and above the founding strata which might be soil or rock. The bridge foundation transfers the loads evenly from the superstructure to the soil below without drastic settlements, or shear failures of the soil.

Pier - Piers are the vertical structural element of the bridge which transfers the load from the superstructure to the subsoil via the foundation.

Pier cap - Pier caps are located above the piers which transfer the load from the girders to the piers via bearings evenly. The pier and pier cap acts as a monolithic rigid frame unlike the deck slab and pier cap.

Bearings - The transfer of loads and forces from the girders to the pier cap causes abrasion, wear and tear due to the shear and torsional effects. These effects cause a reduction in the size of section of contact between pier cap and girder leading to failure. To reduce such effects, bearings are provided which allow movement to a certain degree without compromising the lattices of the bridge as constructed.

Girders - The girders are the main horizontal structural member which transfers the load from the deck slab to the pier cap via bearings. If the single span of the bridge is greater, then cross girders are provided at equal intervals to provide adequate stiffness.

Deck slab - Deck slab is the component constructed above the girders and is upon which asphalt or concrete road is laid for the passing of traffic.

Crash barriers - They are life-saving mechanisms capitalising on the impact force during an accident reducing loss of life and reducing the damage to traffic below and the bridge structure.

Case a) Seismic case 5 & Case b) 3 span continuous bridge:

The seismic case 5, the bridge is assumed to be a 3-span continuous bridge and built in a seismic zone V which is a highly active seismic zone having major fault systems. A magnitude of 7 or higher has occurred and an intensity of 9 is on record. Seismic waves are the vibration of the earthquake and are recorded in the seismograph as amplitudes. The logarithmic amplitude of these waves recorded by seismograph is called magnitude. The damage to life and infrastructure due to the earthquake is called Intensity. Considering the magnitude of 7 which causes an intensity of 9, will definitely create fractures in the nominal ground level.

Assuming the fault line is transverse to the bridge span, a Normal fault or reverse fault will cause a side of the span to lift and the connecting another end across the fault to dip. Structural stability will fail in such conditions.

Assuming the fault line is transverse to the bridge span, a shear fault or strike-slip fault will cause the bridge to shear from its original lattices. Structural stability will fail in such conditions.

Assuming the fault lines are far away from the bridge site, being the bridge designed to withstand seismic forces, if rubber bearings or lead rubber bearings are provided as bearings between pier caps and girders the horizontal inertial forces which cause the maximum destruction will be absorbed as the bridge substructure moves with the inertial motion and the superstructure is semi-isolated and failure can be minimised. These types of bridges are termed as load dispersion bridges and seismic isolation bridges at the event of an earthquake.

In Japan, 3-span continuous bridges are designed as seismic isolation bridges where the bearings are lead rubber bearings which elongate and damps the vibration which reduces the effects of seismic forces leading to minor cracks which are possible to be repaired and rehabilitated.

Clauses of IRC 6 used for assumptions and designs are as follows.,

• Dead load
• Live load
• Impact load
• Water currents
• Longitudinal forces
• Centrifugal forces
• Buoyancy
• Earth pressure
• Temperature
• Seismic load
• Barge Impact load
• Snow load
• Vehicle collision loads

Q. What are the various forces acting on the shallow foundations?

The forces acting on the shallow foundation are as follows.,

Earth pressure: When the soil is prevented from moving by a structure in-between and the potential energy the soil has is called earth pressure at rest. When the structure moves due to settlement or scouring or any other factor, the soil at rest moves along and creates active earth pressure. These forces act on the foundation and can cause shear failure.

Seismic load: Seismic waves are caused due to movement of tectonic plates over each other causing primary waves and secondary waves. The latter is the damage causing earthquake wave. These horizontal and vertical wave forces move the original lattices of the bridge foundation and which will cause the failure of the bridge.

Dead load and Live load: The self-weight of the bridge components and the live loads such as traffic loads will be transferred from the super-structure to the subsoil via the foundation.

All other loads such as Impact load, water current, Centrifugal forces, Uplift or Buoyancy, Temperature, Barge Impact load, Snow load, and Vehicle collision load acts at the foundation level.

Fixed and Free Piers:

Based on the type of bearing provided in the bridge it is classified as fixed or free bearing. Piers supporting a fixed bearing are called as fixed piers. Piers supporting a free bearing are called as free piers. Fixed piers restrict movement and allows only rotation and hence it is subjected to longitudinal and transverse forces. Free piers are only subjected to axial loads.

Abutments:

Abutments are subjected to dead load, live load, earth pressure, seismic load, Impact load, water current, Centrifugal forces, Uplift or Buoyancy, Temperature, Barge Impact load, Snow load, and Vehicle collision loads etc.,