What are the various causes of erection Failures? Explain with example.
The erection of a major bridge invariably involves special risks, which could lead to injuries and loss of lives. A major cause of erection failures in the past has been the underestimation Go of the construction loads and their effects on the unfinished structure. The failure during eve le erection of the first Quebec cantilever bridge in 1907 highlighted the need for reviewing the bool erection stresses in the bridge members for strength and stability at each critical stage of ow erection.
The bottom chords of the side span failed in compression at an advanced cantilever erection stage. This failure also evidenced the essentiality of the design engineer to 01 ol mi constantly interact with the construction engineers in order to initiate timely remedial action should any defects be noticed during erection.
The failures during the erection of steel box girder bridges during 1970-71 were basically due to instability of thin steel plates in compression and secondary stresses arising from minor Ounlenc geometric imperfections. At the West Gate Bridge in Australia, the failure of the 112 m long fed y steel box girder span resulting in the loss of 35 lives was triggered due to premature removal of 37 top flange splice bolts in an effort to facilitate connection of the two longitudinal halves Dbi Dof the top flange.
At Koblenz, geometric imperfections caused critical secondary stresses to stop resulting in buckling of the bottom flange. The lesson to be learned is that the design derived from sophisticated analysis should be tempered with realistic allowances for construction methods and erection tolerances.
A recent example of errors in erection procedures is the failure in 1998 of the Injaka bridge in 16 T8t n South Africa”, which resulted in 14 deaths including that of the designer of the bridge. The prestressed concrete bridge of box section collapsed during erection by incremental End a launching method, mainly due to wrong placement of temporary bearings, which punched o through the inadequately designed soffit slab. The disaster highlighted the importance of a proper erection procedure.
State the precautions you would take in design and construction to protect a bridge against impact from ships passing below.
Damages to bridges across navigable rivers caused by barges or ships are on the increase. It has been reported that, during the period 1960 to 1998, there were 30 major bridge collapses worldwide due to ship or barge collision with a total loss of 321 lives”. A familiar example is the 1980 collapse of the Sunshine Skyway bridge in Florida, the USA causing the loss of 35 lives as a result of a collision by a bulk carrier.
Another example is the damage to the Tasman bridge in Australia in 1975. Barge tows often hit the piers across waterways, e.g. the 2002 collapse of the 1-40 bridge over the Arkansas river at Webber Falls, Oklahoma, USA. The vessels may be adrift or may hit the piers under power. The damage to the bridge can be minimized by providing properly designed protective fendering.
When potential damage due to barge impact exists, it is prudent not to use a pile foundation with exposed piling above the river bed. In such cases, a sturdy well and heavy caisson foundation with protective fendering will be desirable, as adopted for the new Sunshine Skyway bridge in Florida, USA, and álso the Zurai bridge in Goa.
Bridge design for barge collision is not based on the worst-case scenario due to economic and structural constraints. A certain amount of risk is considered acceptable. The risk acceptance criteria are specified in codes with consideration of the probability of occurrence of a vessel collision and the consequences of the collision.
It is advisable to incorporate protection against vessel collision in the initial design. The horizontal and vertical clearances of the navigation span have to be determined based on a study of the anticipated vessel movements. For Storebelt East suspension bridge with a main span of 1624 m and 1800 vessel passages per year, vessel impact has been the governing criterion for the design of piers, and probability-based criteria were derived from comprehensive vessel simulations and collision analysis. There is scope for a research study to improve our understanding of vessel collision mechanics and the development of cost-effective protection measures.
What type of failures have been noticed during earthquakes? Also, suggest some preventive measures against Earthquake.
Several types of bridge failures have been noticed during earthquakes”. A common failure was by span shortening. Decks slid off their supports due to violent shaking as in the Showa bridge in the 1964 Niigata earthquake (m = 7.5) and the Nishinomiya bridge in the 1995 Kobe earthquake (m = 6.9) in Japan. As abutments and piers moved together, some decks bucked, some were crushed and some collapsed.
This problem is critical for bridges with simply supported spans located in soft soil. Another type was the horizontal displacement of piers due to the movement of piles in liquefied soils subjected to lateral loading. A third type involved the differential settlement of piers and abutments due to differences in soil characteristics due to liquefaction. Liquefaction of approach fills have resulted in settlement of fills in relation to abutments, causing accidents to motor vehicles by impact against the abutment back wall.
Columns in substructures have been found to suffer extensive damage during earthquakes. The damage is mainly attributable to inadequate detailing, which limits the ability of the column to deform inelastically. In concrete columns, inadequate ductility results from insufficient reinforcement to achieve confinement of concrete within the core.
Termination of longitudinal bars at mid-height led to splitting failure or shear failure near the end cutoff point
in a few bridges, as noticed in the 1995 Kobe earthquake. Column failures by crushing of concrete due to extreme torsion have also been noticed. In steel columns, local buckling may cause inadequate ductility.
Several preventive measures have been suggested. Heavier and closer spaced spiral reinforcement should be provided for columns. Such reinforcement would retain the concrete in the core and prevent collapse. Restraint should be provided at expansion joints and articulations such that ordinary expansion due to temperature is permitted but larger enable) 1omovements under earthquake are restrained.
No splices are to be allowed in columns of less than 6 m height, as lapped splices of column bars have been found to be ineffective under 2otaip earthquakes. Approach slab with one end resting on abutment should be provided to permit a smooth transition in case of settlement of approaches due to liquefaction of the fill.
earthquake resistant building design || In hindi
Discuss the bridge failure a occurred due to wind.
Bridge failures have occurred due to wind. Major examples include the collapse of the Tary bridge in 1879, and the Chester bridge over Mississippi in 1944. Tay bridge consisted of eighty-five though-lattice-truss spans of malleable iron supported 26.8 m above the water level. The Sisnnerl bridge failed due to aerostatic instability, as the design of cross bracing and its fastenings ni envoy was inadequate to sustain wind forces, though the design was otherwise in conformity with contemporary practice.
Sir Thomas Bough, the designer, was unjustly blamed and made the scapegoat by the Chairman of the Court of Enquiry, who summarized his report in concise and quotable form. The majority report which was supportive of Bough was voluminous and was not read by many. This example highlights the need for the bridge engineer to cultivate the art of advocacy and to learn the benefits of brevity in the oral and written presentation of the technical reports.
The main channel stretch of Chester bridge was a 408.8 m through truss, continuous over two equal spans. This stretch was blown off into the river during a tornado. While very little Can be done to save a structure from the direct attack of a severe tornado, the damage can be minimized by providing proper anchorage of the deck with the substructure. Tacoma Narrows’ first bridge (1940) failed due to aerodynamic instability. The o recurrence of these
types of failures is avoided in recent designs through streamlining the deck and adequate stiffening.
Why corrosion prevention is important in the maintenance of Prestressed Concrete bridges?
Corrosion of reinforcement in a reinforced concrete bridge may lead to cracking and spalling of concrete, rendering the bridge unsafe for modern traffic. Potential damage due to corrosion in a backwater area can be prevented only by careful attention to concrete cover to reinforcement, by proper placement and compaction of concrete to avoid honeycombing, and by proper curing with potable water.
The risk of corrosion for concrete bridges may be reduced by the adoption of a combination of preventive measures such as the provision of adequate concrete cover, use of high-performance concrete, application of admixtures to inhibit corrosion, use of corrosion-resistant reinforcement, such as epoxy-coated bars, and exercise of good quality control.
in pretension structures, corrosion prevention is mainly accomplished through the use of high-performance concrete and the addition of corrosion-inhibiting admixtures. In post-tensioned concrete bridges, special care should be devoted to ensuring the integrity of the duct and grouting of prestressing cables soon after stressing.
Delayed grouting and inadequate grouting of tendons especially near the anchorage may cause corrosion of the tendons, contributing to the failure of a prestressed concrete bridge. Corrosion of tendons may remain undetected until the loss of strength to a significant level result in sin major evidence of Imine be deterioration or even a sudden failure. The failure of the Mandovi first bridge at Panjim, Goa, in 1986 is attributed to the corrosion of prestressing steel.
Corrosion of prestressing tendons also caused the failure of the single-span bridge in Wales, UK in 1985. This bridge built in 1953 on a minor road was a segmental post-tensioned structure of 18.3 m span with thick mortar joints and defective grouting for tendons. The corrosion took place at the transverse joints between precast segments at which chloride-containing water could penetrate. Grouting deficiencies have contributed to the distress of several post-tensioned bridges in many countries “.
While stressing operations have been devoted extensive care by construction engineers, adequate attention to subsequent grouting has been lacking. Such failures have shown the importance of durability design, besides the load and resistance-based structural design. Modern segmental construction, however, uses matchcast joints sealed with epoxy.
List the modified design criteria adopted for recent flyovers in Mumbai and Chennai.
Some of the modified criteria, as followed successfully for the construction of flyovers in Mumbai and Chennai are indicated below
1. The flyovers need not be designed for IRC Class 70R loading, as such heavy vehicles can take the ground-level road if at all they are encountered.
2. Continuous superstructures are encouraged as foundations are generally rested on a rock. Continuity results in a lower depth of deck and also enhances riding comfort due to a reduction in the number of expansion joints. Besides unsatisfactory performance, most bridge expansion joints leak and contribute to the deck and substructure corrosion damage, and hence their reduction promotes durability.
3. When precast prestressed girders are used, continuity may be achieved by the cast in situ concrete transverse beams at or near the piers. The continuity connection may consist of wide in situ integrated crossheads over the pier providing 1 m embedment of the beams. The reinforcement in the composite deck slab provides longitudinal continuity. One set of bearings uu is adequate. Alternatively, a narrow in sited integral crosshead may be adopted; but this will need two sets of bearings on the pier. Another type of continuity connection relies on the lo s deck slab flexing to accommodate the rotations of the simply supported deck beams. Here the slab is separated from the support beams for about 1.5 m by a layer of compressible material. Two sets of bearings are required.
4. High-strength concretes of grade M 40 and above only are used for the deck. The permissible compressive and tensile stresses are adopted as 0.33 fk and 0.033 concrete is adopted, the permissible stresses are allowed with the characteristic strength scaled down by 10 MPa, as a conservative measure.
5. For prestressed concrete superstructure, partial prestressing has been permitted with the proviso that there will be no tension under ‘Dead load + 60%. Live load’ condition. Under full design load, tensile stress in concrete is limited to 1.0 MPa for severe exposure and 2.0 MPa for moderate exposure.
6. As a precaution against corrosion of steel, corrosion resisting steel reinforcement or steel rods coated with anti-corrosive treatment are used. The minimum clear cover is kept at 40 side mm, 50 mm, and 75 mm for reinforcement in components above ground level, components below ground level, and for prestressing cables, respectively.
7. Since the durability of the structure is of paramount importance, the minimum concrete grades Suoue adopted are M30, M35 and M40 for plain cement concrete, reinforced concrete, and prestressed concrete, respectively, with water-cement ratio limited to 0.45, 0.40, and 0.40, respectively
8. Approach embankment height is restricted to be less than 3.0 m. The sides of the embankment are retained with reinforced earth retaining structures avoiding massive wing walls.
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9. Pile foundations with hydraulically operated rotary drilling equipment are encouraged. Pile caps are so arranged as to have the top of the pile cap at 500 mm below the ground level.
10. crash barriers are essential to achieve vehicle containment, for the Mumbai flyovers the crash barriers were designed for the impact of 300 kN vehicle at 64 km/h at an angle of 20° at the eid6o o Ine top of the crash barrier, and R.C. crash barriers laid with kerb laying machinery are provided. Steel barriers are adopted in Chennai and Bangalore.
What are the measures adopted for innovative construction techniques?
Emphasis on the high quality and fast construction of recent flyovers necessitated the adoption of innovative construction techniques. Typical measures are mentioned below :
1.Pile foundations using rotary pile driving machines were made mandatory. Besides facilitating speedy construction, the method was economical. The liner plates were of 4 mm thickness instead of the usual 6 mm thickness. Also, the seating of the pile on the rock was assured.
2. Since space was not available around the flyover sites for storing and handling of materials for concrete production, the use of ready mixed concrete was made compulsory. This led to many spin-off benefits in quality assurance and reduction in construction joints due to the fast and continuous placement of concrete.
Precast prestressed concrete girders were used extensively in flyovers in Delhi, Mumbai, and Chennai. For flyover at Bangalore, precast segments of box shape were used for the entire bridge. The adoption of precast elements for the superstructure results in considerable saving in time, as the foundations and precasting can be startede simultaneously, and the superstructure beams/ segments are ready for placing in position by the time the substructure is completed.
3. To improve riding comfort, the number of expansion joints was reduced. In the case of the girder and slab system, this was achieved by providing continuity of the deck slab over the supports. For some of the flyovers, the flexibility of the deck slab in the longitudinal direction was increased by introducing neoprene pads of 12 mm thickness between the slab and the girders for a length of 1.0 to 1.5 m near the bearings. This continuity in the deck slab is not
likely to reduce the total positive moment in the girders to any appreciable extent.
4. Use of an automatic casting machine was made compulsory for casting kerbs and crash barriers for the Mumbai flyovers. For Delhi flyovers, precast crash barriers with shear keys for interlocking were used.
What are the special features of Extradosed bridges?
A recent innovation combining the principles of cable-stayed bridges and prestressed E concrete box girders in the span range of 100 to 275 m is the evolution of extradosed bridges, a concept attributed to Mathivat in France. These bridges are similar to cable-stayed bridges as both use stay cables for strengthening. They differ in that the extradosed bridges generally use lower tower heights, usually about half that normally used for cable stayed bn odeon bridges. In the case of cable stayed bridges, the prestressed concrete box deck structure is A0 be suspended from stay cables. Most of the load, such as the dead load and the live load, is carried through the stay cables to the top of the towers and then down to the foundation.
On the other hand, in an extradosed bridge both the box deck and the stay cables share the load, with the box deck carrying the major part of the dead load and the stay cables supporting the live load and a portion of the dead load. Its structural behavior is close to that open of a girder bridge. For this reason, the relation between the center span and the end span for the Javal bran extra dose bridge should be similar to that of a normal prestressed concrete girder bridge. Also for a three-span bridge, the stay weight will be of the order of 10 to 20 kg/m for an extra dosed bridge compared with 40 to 50 kg/m for a corresponding cable stayed bridge
The stay cables may be in two planes or in a single plane. The vertical component of cable-stay force is low, especially for cables in two planes. This would facilitate easy construction, reducing the need for structural diaphragms at the stay anchorages. The cable stays in extradosed bridges in Japan are stressed to 0.60 fy, in view of the low fatigue stress range in the cables of such bridges. In the case of a single plane scheme, inclined web members in the lo middle of the cross-section may be used for the transfer of stay forces to the main girder.
Discuss the advantages of using High-Performance Concrete for Bridges.
High-Performance Concrete (HPC) is increasingly adopted in bridge construction as a high-quality concrete choice for high strength, durability, and optimum life-cycle costs. HPC can be designed to achieve specific requirements such as : enhanced durability; high strength greater than M60 high early gain in strength for precast bridge components high impermeability to withstand marine exposure; air entrainment to improve resistance to
freezing and thawing; and special applications like spraying, pumping, and placement underwater.
Limited quantities (up to about seven percent of cement) of micro silica (which is much finer than cement) can be added to produce HPC. Microsilica (also known as silica fume) and cement together constitute the binding material. The water-binder ratio of HPC is in the range of 0.3 to 0.4. The production and use of HPC demand stringent quality control to ensure a high degree of uniformity between batches and efficient curing.
The constituents of HPC are the same as in normal concrete, but proportioned and mixed (with appropriate materials such as super-plasticizer, fly ash, blast furnace slag, and silica fume) so as to yield a stronger and more durable product. The cement content of concrete inclusive of any mineral admixtures should not less than 380 kg/m”, and the cement content excluding any mineral admixtures should not exceed 450 kg/m”.
HPC structures are likely to last longer and suffer less damage from traffic and climatic conditions, resulting in reduced costs on repairs in the long run. Thus the present goals are to achieve durability and economical long-term maintenance along with economical construction. When HPC with silica fume is used for bridge piers, curing should be done with extra care to avoid surface shrinkage cracks.
What are the precautions to be taken in bridge design and construction to enhance durability?
The durability of concrete may be defined as its ability to resist deterioration from weathering duration, chemical attack, abrasion, and other degradation processes. Concrete should be durable in order to provide the desired performance in the conditions of exposure during its service life. It should maintain its integrity and should afford protection from corrosion to the embedded reinforcement.
Concrete durability can be enhanced by careful selection of materials to control and optimize their properties; reducing variability in the mixing, transport, placement, and curing of concrete specifications to evaluate in-place concrete. A predominant characteristic influencing the durability of concrete is its impermeability to the ingress of water, carbon dioxide, oxygen, chlorides, sulfates, and other potentially deleterious substances. Low permeability is achieved by using proper cement content, low water-cement ratio, and dense concrete obtained by thorough compaction and efficient curing.
The importance durability of concrete bridge structures has gained better recognition in recent years, based on the experience of deterioration of the existing bridges mainly due to corrosion of prestressing steel and untensioned reinforcement. Presently it is realized that high-quality concrete with the required strength and good durability can be produced with a little extra care in mix design and construction. This awareness has resulted in many precautionary measures in design and construction, including the following
1. Dimensions of structural elements are increased to avoid congestion of reinforcement ;
2.Concrete of low permeability is being adopted, with the use of pozzolanic materials ;
3. High-performance concrete is increasingly specified, with a wáter/cement ratio less than 0,4.
4. The minimum concrete cover for all structural elements is increased, typically to 50 mm in the superstructure and up to 100 mm for the foundation in the marine environment.
5. Enhanced quality control is exercised in the various construction processes such as batching. mixing, transporting, placing, consolidation, finishing, and curing. al ben While the current codes and standards attempt to achieve durability through a set of prescriptive specifications, the future trend is to include additional considerations such as
design life, life cycle costs, and specifications for planned maintenance and repair.
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