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introduction to a bridge foundation
- 1. INTRODUCTION TO BRIDGE FOUNDATION
- 2. WHAT IS A BRIDGE ? Bridge is a structure which covers any gap or any obstacle to give a passage through it. Generally bridges carry a road or railway across a natural or artificial obstacle such as, a river, canal or another railway or another road. Approach slab Deck slab PARAPET ABUTMENT PIER BEARING PILE CAP LONG GIRDER CROSS GIRDER
- 3. WHAT IS FOUNDATION ? A foundation is the part constructed under the pier/abutment and over the underlying soil or rock. The loads transmitted by the foundation to the underling soil must not cause soil shear failure or damaging settlement of the superstructure. The foundations of a bridge are particularly critical because they must support the entire weight of the bridge and the traffic loads that it will carry.
- 4. PURPOSE OF FOUNDATION Purpose of foundation is to distribute the load of bridge over a large bearing area and provide stability of bridge against settlement and tilting. It is provided sufficiently deep so that it is not affected by the scouring. FORCES ACTING ON A BRIDGE
- 5. HOW THE LOAD TRANSFER FROM BRIDGE TO THE FOUNDATION? Beam Pier FIG3. WORKING OF FORCES
- 6. IN ORDER TO DESIGN THE FOUNDATION MUST DETERMINE THE FOLLOWING REASONABLY AND ACCURATELY : The maximum grip length required, The maximum likely scour depth, The soil pressures at the base, and The stresses in the structure constituting the foundation. The foundation should be taken to a depth which is safe from scour, and is adequate from considerations of bearing capacity, settlement stability of suitability of strata at the founding level. FIG4. FORCES ON BRIDGE Beam Pier
- 7. TYPES OF FOUNDATIONS The type of foundation is designed when subsoil data of bridge site is obtained. The different types of foundations for bridges are given following; Shallow Foundation (D<B) Spread Foundation and Raft foundation Deep Foundation (D>B) Pile and Well foundation B D FIG5. DEPTH AND WIDTH OF FOUNDATION
- 8. SHALLOW FOUNDATION A shallow foundation is sometimes defined as one whose depth is smaller than its width. Shallow foundations can be laid using open excavation by allowing natural slopes on all sides. This is normally convenient above the water table and is practicable upto a depth of about 5m. Shallow foundation transfer the load to the ground by bearing at the bottom of the foundation. FIG6. SHALLOW FOUNDATION
- 9. DEEP FOUNDATION A deep foundation is defined as one whose depth is larger than its width. Deep foundations can not be laid using open excavation. Deep foundation transfer the load to the ground partly by end bearing at the bottom of the foundation and partly by skin friction with the soil around the foundation along its embedment in the soil. Piles are preferred over wells as they are more economical than wells. FIG7. DEEP FOUNDATION
- 10. Bridge Bearings The devices which are provided over the support of bridge to accommodate the change in the main grinder due to deflection, temperature, vertical movement due to sinking of the support,shrinkage ,prestressing creep. The bearing are provided for the distribution of the load evenly over the substructure material which may not have sufficient bearing strength to bear or take up the load superstructure directly 10
- 11. TYPES OF BRIDGE BEARING ROCKER BEARING PIN BEARING ROLLER BEARING LAMINATED ELASTOMERIC BEARING POT BEARING KNUCKLE BEARING
- 12. ROCKER BEARING A rocker bearing is a type of expansion bearing that comes in a great variety. It typically consists of a pin at the top that facilitates rotations, and a curved surface at the bottom that accommodates the translational movements. Rocker and pin bearings are primarily used in steel bridges
- 13. PIN BEARING A pin bearing is a type of fixed bearings that accommodates rotations through the use of a steel. Translational movements are not allowed. The pin at the top is composed of upper and lower semicircularly recessed surfaces with a solid circular pin placed between. Usually, there are caps at both ends of the pin to keep the pin from sliding off the seats and to resist uplift loads if required.
- 14. POT BEARING A POT BEARING consists of a shallow steel cylinder, or pot, on a vertical axis with a neoprene disk which is slightly thinner than the cylinder and fitted tightly inside. A steel piston fits inside the cylinder and bears on the neoprene. Flat brass rings are used to seal the rubber between the piston and the pot. The rubber behaves like a viscous fluid flowing as rotation may occur
- 15. PIERS • The intermediate supports of a bridge susperstructure are known as piers. • FUNCTION:- • The only purpose of providing piers is to divide the total length of bridge into suitable spans with minimum obstruction to the stream or rivers.
- 16. TYPES OF PIERS 1. COLUMN BENTS 2. DUMB-BELL PIERS. 3. CYLINDER PIERS.
- 17. Choice of Pier Wherever possible slender piers should be used so that there is sufficient flexibility to allow temperature, shrinkage and creep effects to be transmitted to the abutments without the need for bearings at the piers, or intermediate joints in the deck. A slender bridge deck will usually look best when supported by slender piers without the need for a downstand crosshead beam.
- 18. Types of Bridge Decks Simply defined, a bridge deck is the portion of a bridge that acts as the roadway in the support of vehicular or pedestrian traffic. While deck parts like trusses, girders, rails, arches, posts and cantilevers assume a number of forms and types, there are relatively few bridge deck types given the utilitarian nature of the component. Deck types are defined by the materials from which they are made and the manner in which those materials are fit together. Bridge decks must be sturdy enough to withstand constant pressure and extreme weight.
- 19. Concrete Decks Concrete bridge decks are much what they sound like: flat roadway surfaces made of poured concrete that is laid in sections on large bridges or in one fell swoop on small bridges. Thin steel beams are laid into the concrete to provide both strength and flexibility. Concrete is a rigid material that will break if under overburdening pressure. Steel laid into concrete bridge decks permit the deck to bow and sway gently under the
- 20. Reinforced Decks Reinforced bridge decks are similar to concrete decks in that they are poured of reinforced concrete and interwoven with steel support bars for added strength and flexibility. Such decks differ from standard concrete decks in that they apply further support systems beneath the concrete for additional support. A typical reinforced bridge deck consists of a thick layer of concrete to form the roadway and solid steel sheets laid beneath the concrete to offer additional relief from the pressure of weight. Distribution bars and support beams are laid beneath the steel sheets. Distribution bars relieve pressure from the deck incrementally and transfer weight- bearing responsibility to support beams, much like flying buttresses do for buildings.
- 21. Beam and Girder Decks Beam and girder decks are those that employ side reinforcement. Girders are support beams made from steel that are divided into sections, or plates, and affixed to the sides of a concrete bridge deck for added support. Beams work much in the same manner but are generally one enormous piece of steel that provides structural support by helping a bridge maintain its shape in the face of elements that may cause the concrete to bow or flex. Girders help alleviate deck weight by segmenting the deck and relieving pressure from the structure as a whole.