What is meant by routine inspection; in-depth inspection and special inspection bridge?
The inspections could be classified as (i) routine inspeclion ; (ii) in-depth inspection : and (i) special inspection. The routine inspection is particularly applicable to short span bridges. It usually involves a general examination of the structure, conducted ona regular basis, to look for obvious outward physical evidence of distress that might require repair or maintenance attention. An in-depth inspection requires a detailed visual examination of all superstructure and substructure elements. This is particularly necessary in the case of old bridges and structures of major proportions where structural failure could result in catastrophic consequence. The in-depth inspection may be scheduled once in three to five years. The special inspection is undertaken after special events such as earthquake,
cyclone or passage of unusually heavy loads. For bridges subject to the constant action of the forces of nature, such as the daily ravages of a river, frequent expert inspection is essential. An inspection should be thorough and it must be conducted by knowledgeable investigators for the evaluation to be reliable.
What is the importance of bridge maintenance work? Explain the different types of bridge maintenance.
Maintenance is the work necessary to keep a bridge in operating condition and to prevent potential deterioration in the future. Systematic maintenance is essential to ensure long term conservation of bridge structures. To be effective, maintenance should be based on adequate inspection, besides monitoring and assessment procedures. Maintenance
solutions are normally evolved based on experience. There are four types of bridge maintenance
(A) Routine (b) Preventive (c) Repairs ; and (d) Strengthening/Replacement. All the above options are important. The appropriate measure to be adopted at a given time for a particular bridge would depend on the circumstances. Thus preventive maintenance to protect against corrosion would only apply to a bridge where corrosion has not yet started. A combination of the methods would be needed in a case where the bridge shows corroding reinforcement ; the actions to include repair the damage, arrest the corrosion process and to use a preventive measure to guard against further corrosion. The most effective way to reduce bridge maintenance costs is to include life cycle maintenance planning form the initial conceptual design through all phases of design, construction and operation, and to devote special attention to quality construction.
What do you mean by repairs? What are the main defects in bridges requiring repairs ?
Repair refers to action taken to correct damages or deterioration of structural elements to restore the structures to an acceptable condition with respect to strength and serviceability.
The main defects in bridges requiring repairs are normally of the following types
1. Defects in substructuro and foundation, Including scour and cracks in masonry Scoured portions may be filled with boulders and, if necessary, aprons may be provided. Cracks in masonry may be repaired by grouting under pressure with cement grout or with epoxy grout. When the substructure shows spelling of concrete, the substructure may be strongthened by guniting with cement sand mixture under air pressure or by jacketing with additional thickness of concrete and provision of dowel bars of 20 at about 450 mm spacing both horizontally and vertically to bond with the existing structure.
2. Defects in concrete decks
An understanding of the cause of deterioration is essential for effective repair of concrete decks. The most frequent cause is the corrosion of reinforcement, which is influenced by the cover, the quality of the concrete and the environment. The repair interventions may comprise mortar repair, crack sealing and structural crack repair. The aim in repairing cracks is to prevent water or chemicals intruding into concrete and to restore the appearance of the structure, Cracks in R.C. decks can be repaired by injecting epoxy grout under pressure for cracks below 0.25 mm wide, and by filling with cement grout for cracks wider than 0.25 mm.
3. Excessive vibrations and defiections in prestressed concrete decks This condition generally indicates loss of prestress, and can be repaired by introducing external prestressing.
4. Cracks and corrosion in steel work in superstructure
If the cracks are located in isolated places, cover plates may be added by riveting or welding. If similar cracks are noted in identical locations in all the spans in a multi-span bridge, the design should be checked. Patch repair may be attempted when corrosion is local al in a girder. The defective member may be replaced by a new member when
5. Deterioration of Kerbs and Railings
Kerbs and railings suffer damage due to vehicle collision, cracking and spalling, corrosion and poor original design details. Collision related damage should be attended to immediately after the event. For deterioration due to cracking and corrosion, repairs as for similar defects in the superstructure are to be adopted.
Write short note on the following Retrofitting & Reconstruction
Retrofitting of a highway bridge involves procedures to improve the function of an existing bridge for a better performance or to increase the load carrying capacity. For example, many existing suspension bridges were strengthened with additional stays to reduce vibrations after the Tacoma collapse. Fairings were attached to the stiffening plate girders in the Deer Isle suspension bridge in USA and these were found to be very effective in enhancing the aerodynamic behavior of the structure. Some cable stayed bridges were provided with dampers on the stays to reduce vibrations in the cables. Addition of external prestressing to existing prestressed concrete bridges to reduce deflections and vibrations is another example of retrofit, as accomplished in the strengthening and rehabilitation of the Zurai river bridge on NH-17 in Goa27
Based on observations of seismic vulnerability of bridges in a number of earthquakes such as the 1971 San Fernando quake and the 1995 Kobe quake, many existing bridges have been retrofitted to improve their performance. The seismic retrofit projects included joint restrainers, confinement jackets to columns to ensure ductile behaviour, and
modifications to column footings and pile caps. Bearing retrofits often involve replacementof steel rocker-type bearings with LRB elastomeric bearings along with provision ofguides and seat extenders to prevent unseating of the spans.
Reconstruction of an existing bridge may become necessary when it fails to satisfy itsfunctional requirements. The main causes of reconstruction include : (a) inadequatecarriageway for the volume of traffic : (b) structural inadequacy due to deterioration orincrease in design loadings ; (c) insufficient waterway for river bridges and (d) inadequateclearances for road under bridges. Of these causes, the predominant reason forreplacement of an existing bridge is the inadequate width to carry the planned volume oftraffic.
How the maintenance of submersible bridges ?
Submersible bridges should be devoted greater maintenance attention than comparablehigh level bridges. The duration of submergence and number of traqffic obstruction duringevery flood should be noted. The scour around foundations during the immediately aftersubmergence should be recorded. The bearings should be inspected before and aftersubmergence. When the deck is of box section, water holes are provided in the deck toof water during the submergence. These holes should be kept clean before floods. After the submergence, the entire superstructure should be cleaned of thesilt deposit and the water holes should be cleaned free of debris. The superstructureshould be carefully examined for any signs of distress or cracks and any remedialmeasures necessary should be implemented early. The expansion joints should beinspected and maintained. During flood, collapsible railing, if provided, should be loweredin time.