It is well known among engineers that absolute safety is unattainable, and inevitably there are p risks of collapse associated with any bridge. However, the bridge engineer should take every possible precaution to avoid failures, as serious major causes of bridge failure will often result in loss of Tevolinso lives, interruption to vital traffic, and costly repairs.
Every bridge engineer would do well to study the circumstances leading to any bridge failure that he may come across, so as to learn lessons from such failures. As bridge spans grow longer, and complex designs aim to result in lighter structures, the bridges tend to become more vulnerable to failures.
Complex club designs necessitate sophisticated checks to ensure the careful layout and detailing of the various on most members by the designers and correct compliance by the construction team. Lack of communication among the various key personnel involved in the design and construction, and lapse in respect for natural forces have often proved disastrous. The failure may be total or partial. Total failure refers to the collapse of the bridge.
Partial failure, on the other hand, deficiencies in meeting the intended requirements, necessitating reduced load limit, decreased speed, and implementation of substantial repair and rehabilitation. Total failures generally attract attention. But partial failures also merit careful study to avoid recurrence of the defects. The lessons learned from every major bridge failure would normally result in
Based on a study of 143 bridge failures that occurred throughout the world between 1847 and Istneineoni 1975, Smith’ has categorized the causes of failures as in Table 5.1. Though outdated, the qualitative inferences from the above report are still valid. About sixty of the bridge lo failures listed were due to natural phenomena, i.e. due to flood, earthquake, and wind.
Other major causes of failure include Defective design; Erections errors; Accidents (barge impact); Fatigue; and Corrosion. The failure of a bridge is normally due to a combination of several effects and errors. In this discussion, failures are grouped according to the major cause triggering collapse.
Discuss about falsework failures in bridges.
Failures of falsework can result in loss, injury, death, and interruption of traffic as much as 2ovil 2E to bridge collapse. Falsework failure can cause excessive settlement and deflection, besides catastrophic collapse of the superstructure. While flood, storm winds, and earthquakes to may contribute to failure, most falsework failures are attributable to human error. The problem of avoiding falsework failures is not easy to solve because of many economic and erected to last long enough to support the final structure during construction.
Traditionally, this has been left to the contractor and as an economic necessity, the formwork construction needs to use secondhand materials to the extent possible, thus lacking the finesse of a finely designed structure. With increased spans of our bridges, falsework design has become more sped complicated.
The bridge falsework design should be prepared by a competent engineer, should be checked by the government engineers and its erection should be under proper supervision. Immediately prior to and during the placing of concrete, the constructed falsework should be carefully checked for joint fits, bracing, stiffness, overturning possibilities, foundation settlement, and general adequacy. By improved methods of construction and constant vigilance, we can avoid falsework failures.
Causes of Failure of Bridge Structures.
What are the activities involved during the inspection of bridge construction? Enlist the responsibilities of inspection for quality assurance inspection.
Inspection of activities during bridge construction aids better quality assurance and promotes safety. Construction inspection includes checking of materials, operations to produce various components, and the temporary structures such as shorting systems, reinforcement, and structural steel for conformance to specifications.
Operation inspection relates to ensuring that the structure is being built in the correct locations, alignments, and elevations according to the project plans. The inspector should ensure that the ready-mixed concrete meets the specifications and it is placed in the forms with proper vibration and consolidation. Reinforcements should be placed as required in the plans in terms of grade, size, and location. Precast mortar blocks of proper quality and dimensions should be used to ensure correct cover.
The inspector is responsible for the quality assurance inspection of all welding: the welding equipment, procedures, and techniques should be in accordance with the relevant specifications. The component inspection involves checking of the various components during construction for dimensions and finish.
Temporary structures need special attend be prevented distortions to final structures. During the placement of concrete, formwork should be inspected to prevent excessive settlement and distortion of bracings. The forms should be mortar-tight and should be strong enough to prevent excessive deflection.
A safe working environment and practices should be maintained at the construction site. In addition to construction inspection, the inspector should also maintain an accurate record of work performed by contractors.
Explain the construction of short-span bridges.
For bridges involving spans up to about 40 m, the superstructure may be built on staging supported on the ground. Alternatively, the girders may be precast for the full span length and erected using launching girders or cranes, if the bridge has many équal spans. In the Tatter procedure, the additional cost on erection equipment should be Tess than the saving in the cost of formwork and the labour cost resulting from faster construction. Precast concrete bridge construction facilitates speedy erection. Hence it is one of the most favored construction techniques for bridge decks of small and medium spans.
An example of efficient site organization using precast prestressed girders and special erection procedures is the Sone bridge at Dehri comprising 93 spans of 32.9 m each”. Here the beams for the superstructure were precast in a casting yard at one end of the bridge. After prestressing, each beam in proper sequences was loaded on a tractor-trailer by a traveling gantry, moved to the span by the tractor-trailer, and picked up and placed in position by a launching gantry. The repetitive nature of work and the extensive use of precast components carefully incorporated the use of special machinery helped to cut down construction time considerably.
What are the factors considered while planning and designing formwork?
The following factors should be given careful consideration while planning and designing formwork.
1. Strength. The formwork should be capable of carrying the pressure of concrete and the weight of labour and plant engaged in its placement and compaction. The pressure due to concrete will vary depending on whether the formwork is horizontal or vertical. The factors affecting the pressure are the density of concrete, workability of the mix, rate of placing, method of concrete discharge into the forms, the temperature of the concrete, extent of vibration, the height of lift, dimensions of the section cast, reinforcement details, and stiffness of the formwork structure. In the can be calculated as that due to a liquid weighting 26 kN/m for horizontal surfaces and for vertical surfaces up to a depth of 1.8 m. For vertical surfaces deeper than 1.8 m, the stress may be increased up to a depth of 1.8 m. For vertical surfaces deeper than 1.8 m, the stress may be increased at 4 kN/m for every additional meter. Construction loads may be taken approximately at 3.6 kN/m actings vertically.
2. Stiffness. The forms should be rigid enough to ensure that the deflection of the completed work should not exceed 0.003 of the span and that the deflection of the form itself in any one span should not be more than 3 mm.
3. Repetition. The forms should be designed in such a manner that components are easy to handle and will be reused a number of times in the same work. Since formwork cost is a considerable part of the total cost of concrete work, this aspect requires careful analysis.
4. Durability. In order to ensure maximum economy, it is essential to provide for repetitive uses of the formwork. Hence careful consideration should be given to the use of durable materials so that the formwork. Hence careful considerations should be given to the uses of durable materials so that the formwork can be used and handled without undue wear. With proper handling and care, timber form panels should give about ten repetitive
uses without major repair. Many more reuses should be possible with steel forms.
5. Strippability. The ease of stripping without damage to the concrete or the forms is a requirement deserving special attention. Wedges and special insertions of smaller closing pieces are arranged to facilitate removal of forms from enclosed spaces, Unless easy stripping is ensured, the gains due to repetitive use of forms may be lost in costly
6. Cost. The final cost of forming an area of concrete is the sum of the cost of materials for the forms, the cost of labour in erecting, stripping, cleaning, and carrying forward to the next use, and the cost of expendable material such as form ties, The aim in design is to keep the total cost to a minimum. It is usually difficult to estimate the cost of
formwork reliably. With the universal rise in prices and wages, it would be well for a contractor to study the labour content of the formwork cost.
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