Showing posts with label Bridges. Show all posts
Showing posts with label Bridges. Show all posts
Monday, July 18, 2011
Friday, July 15, 2011
World's Longest Bridge in Sea
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According to Xinhua, the bridge has the world's longest cross-sea span.The 41.58-km, eight-lane Qingdao Jiaozhou Bay Bridge, connecting the urban district of the city to its Huangdao district, cost 14.8 billion yuan ($2.3 billion). Construction started in May 2007. It will cut the length of the drive from Shanghai to Ningbo from 400 km to 80km.Somewhat inevitably, the bridge takes the world record from another Chinese sea crossing, the 22.5-mile Hangzhou Bay bridge, which opened in 2008, connecting the cities of Jiaxing and Ningbo, south of Shanghai.It makes the one-hour economy ring for downtown Qingdao and its surrounding districts a reality and strengthens the prospect of a four-hour economy ring among cities on the Shandong Peninsula.
The bridge supported by more than 5,000 pillars was built in four years.
China is constructing an even more ambitious bridge. Work began in December 2009 on a Y-shaped structure linking Guangdong province in southern China to Hong Kong and Macau. Building is expected to be finished in 2015, and the bridge is expected to cover about 31 miles, although only about 22 miles will Span the sea.
Danyang–Kunshan Grand Bridge, China (rail): 102.4 miles
China, which seems to complete greatest infrastructure projects on a routine basis, has claimed another world-beater with the construction of the longest sea bridge.
The bridge supported by more than 5,000 pillars was built in four years.
The 26-mileJiaozhou Bay crossing connects the bustling port city of Qingdao, south-east of Beijing, to the industrial district of Huangdao.
China is constructing an even more ambitious bridge. Work began in December 2009 on a Y-shaped structure linking Guangdong province in southern China to Hong Kong and Macau. Building is expected to be finished in 2015, and the bridge is expected to cover about 31 miles, although only about 22 miles will Span the sea.
The eight-lane, 35-metre-wide bridge opened to traffic Built over a four-year period the project cost about £1.4bn and uses 5,000 pillars. It shortens the driving route between the two locations by about 20 miles.
The Lake Pontchartrain Causeway in Louisiana, at almost 24 miles, is slightly longer but crosses an inland waterway rather than open sea.
Danyang–Kunshan Grand Bridge, China (rail): 102.4 miles
Top 10 World’s Longest Bridges
- Tianjin Grand Bridge, China (rail): 70.6 miles
- Weinan Weihe Grand Bridge, China (rail): 49.5 miles
- Bang Na Expressway, Thailand (road): 33.5 miles
- Beijing Grand Bridge, China (rail): 29.9 miles
- Qingdao Haiwan Bridge, China (road) – longest bridge over water: 26.4 miles
- Lake Pontchartrain Causeway, USA (road): 23.8 miles
- Manchac Swamp bridge, USA (road): 22.8 miles
- Yangcun Bridge, China (rail): 22.2 miles
- Hangzhou Bay Bridge, China (road): 22.1 miles
Saturday, July 9, 2011
Truss bridges
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Fig. some of the trusses that are used in steel bridges
Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
For short and medium spans it is economical to use parallel chord trusses
such as Warren truss, Pratt truss, Howe truss, etc. to minimise fabrication and
erection costs. Especially for shorter spans the warren truss is more economical
as it requires less material than either the Pratt or Howe trusses. However, for
longer spans, a greater depth is required at the centre and variable depth trusses
are adopted for economy. In case of truss bridges that are continuous over many
supports, the depth of the truss is usually larger at the supports and smaller at
midspan.
As far as configuration of trusses is concerned, an even number of bays
should be chosen in Pratt and modified Warren trusses to avoid a central bay
with crossed diagonals. The diagonals should be at an angle between 50o and
60o to the horizontal. Secondary stresses can be avoided by ensuring that the
centroidal axes of all intersecting members meet at a single point, in both vertical
and horizontal planes. However, this is not always possible, for example when
cross girders are deeper than the bottom chord then bracing members can be
attached to only one flange of the chords.
General design principles
Optimum depth of truss girder
The optimum value for span to depth ratio depends on the magnitude of
the live load that has to be carried. The span to depth ratio of a truss girder
bridge producing the greatest economy of material is that which makes the
weight of chord members nearly equal to the weight of web members of truss. It
will be in the region of 10, being greater for road traffic than for rail traffic. IS:
1915-1961, also prescribes same value for highway and railway bridges. As per
bridge rules published by Railway board, the depth should not be greater than
three times width between centres of main girders. The spacing between main
truss depends upon the railway or road way clearances required.
Design of compression chord members
Generally, the effective length for the buckling of compression chord
member in the plane of truss is not same as that for buckling out-of-plane of the
truss i.e. the member is weak in one plane compared to the other. The ideal
compression chord will be one that has a section with radii of gyration such that
the slenderness value is same in both planes. In other words, the member is just
likely to buckle in plane or out of plane. These members should be kept as short
as possible and consideration is given to additional bracing, if economical.
The effective length factors for truss members in compression may be
determined by stability analysis. In the absence of detailed analysis one can
follow the recommendations given in respective codes. The depth of the member
needs to be chosen so that the plate dimensions are reasonable. If they are too
thick, the radius of gyration will be smaller than it would be if the same area of
steel is used to form a larger member using thinner plates. The plates should be
as thin as possible without losing too much area when the effective section is
derived and without becoming vulnerable to local buckling.
Fig. some of the trusses that are used in steel bridges
Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
For short and medium spans it is economical to use parallel chord trusses
such as Warren truss, Pratt truss, Howe truss, etc. to minimise fabrication and
erection costs. Especially for shorter spans the warren truss is more economical
as it requires less material than either the Pratt or Howe trusses. However, for
longer spans, a greater depth is required at the centre and variable depth trusses
are adopted for economy. In case of truss bridges that are continuous over many
supports, the depth of the truss is usually larger at the supports and smaller at
midspan.
As far as configuration of trusses is concerned, an even number of bays
should be chosen in Pratt and modified Warren trusses to avoid a central bay
with crossed diagonals. The diagonals should be at an angle between 50o and
60o to the horizontal. Secondary stresses can be avoided by ensuring that the
centroidal axes of all intersecting members meet at a single point, in both vertical
and horizontal planes. However, this is not always possible, for example when
cross girders are deeper than the bottom chord then bracing members can be
attached to only one flange of the chords.
General design principles
Optimum depth of truss girder
The optimum value for span to depth ratio depends on the magnitude of
the live load that has to be carried. The span to depth ratio of a truss girder
bridge producing the greatest economy of material is that which makes the
weight of chord members nearly equal to the weight of web members of truss. It
will be in the region of 10, being greater for road traffic than for rail traffic. IS:
1915-1961, also prescribes same value for highway and railway bridges. As per
bridge rules published by Railway board, the depth should not be greater than
three times width between centres of main girders. The spacing between main
truss depends upon the railway or road way clearances required.
Design of compression chord members
Generally, the effective length for the buckling of compression chord
member in the plane of truss is not same as that for buckling out-of-plane of the
truss i.e. the member is weak in one plane compared to the other. The ideal
compression chord will be one that has a section with radii of gyration such that
the slenderness value is same in both planes. In other words, the member is just
likely to buckle in plane or out of plane. These members should be kept as short
as possible and consideration is given to additional bracing, if economical.
The effective length factors for truss members in compression may be
determined by stability analysis. In the absence of detailed analysis one can
follow the recommendations given in respective codes. The depth of the member
needs to be chosen so that the plate dimensions are reasonable. If they are too
thick, the radius of gyration will be smaller than it would be if the same area of
steel is used to form a larger member using thinner plates. The plates should be
as thin as possible without losing too much area when the effective section is
derived and without becoming vulnerable to local buckling.
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Introduction
The main types of bridges are beam bridges,arches, cable-stayed bridges, cantilever bridges and suspension bridges,and combinations.The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The towers hold up the cables. They have to be rigid enough to act as struts between the downward forces from the cables and the upward forces from the foundations.The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.
At various places in this website there are sections which explain that the boundaries between the various types of bridges are not completely impervious, and that in principle at least, bridges can be built that are not obviously in a simple category. The reason that the types of most bridges are obvious is that these types have become popular because they are successful, and success is greatest in the broad central regions of the available variable-space. For example, if you make an extremely flat suspension bridge, you could put the wires in a concrete matrix, and you would have a pre-stressed beam requiring no anchorages. arches - an extremely flat arch would generate enormous thrust, and a beam would be a better solution.
The same difficulty applies to many other other human activities, and indeed of many natural groups of species: although there are many genera and species which tax the powers of biologists to classify them, the vast majority fall more easily into groups. On the other hand, where there are very many closely related species, there may be sporadic disputes between "lumpers" and "splitters".
This diagram shows the length of a bridge and two definitions of span.
This chart shows the relative lengths of the longest bridges of different types, in 2004. The completion of new bridges may mean that the diagram needs updating. The spans are measured on the vertical axis, while the horizontal axis merely counts the spans in order of length. The types of materials used are greatly dependent on the span. The designer of a small footbridge may have greater freedom of choice than the designer of a large cable-stayed bridge, for example, though economic principles always play a part.
Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
If we look at the distribution of the longitudinal forces within a span, they can be summarized as follows.
Arch - the average line of the forces should be as near the centre line as possible, and certainly within the kern.
Cable - the forces will automatically be distributed across the cable.
Beam - the forces should be as far from the neutral axis as possible.
Cantilever - the forces should be as far from the neutral axis as possible.
This requirement leads to the use of constructions such as I-beams, truss girders and trusses.
BRIDGES
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Introduction
As discussed in earlier chapters the main advantages of structural steel
over other construction materials are its strength and ductility. It has a higher
strength to cost ratio in tension and a slightly lower strength to cost ratio in
compression when compared with concrete. The stiffness to weight ratio of steel
is much higher than that of concrete. Thus, structural steel is an efficient and
economic material in bridges. Structural steel has been the natural solution for
long span bridges since 1890, when the Firth of Forth cantilever bridge, the
world's major steel bridge at that time was completed. Steel is indeed suitable for
most span ranges, but particularly for longer spans. Howrah Bridge, also known
as Rabindra Setu, is to be looked at as an early classical steel bridge in India.
This cantilever bridge was built in 1943. It is 97 m high and 705 m long. This
engineering marvel is still serving the nation, deriding all the myths that people
have about steel. [See Fig.]
contributed to their popularity in Europe and in many other developed countries.
· They could carry heavier loads over longer spans with minimum dead weight,
leading to smaller foundations.
· Steel has the advantage where speed of construction is vital, as many
elements can be prefabricated and erected at site.
· In urban environment with traffic congestion and limited working space, steel
bridges can be constructed with minimum disruption to the community.
· Greater efficiency than concrete structures is invariably achieved in resisting
seismic forces and blast loading.
· The life of steel bridges is longer than that of concrete bridges.
· Due to shallow construction depth, steel bridges offer slender appearance,
which make them aesthetically attractive. The reduced depth also contributes to
the reduced cost of embankments.
· All these frequently leads to low life cycle costs in steel bridges
Introduction
As discussed in earlier chapters the main advantages of structural steel
over other construction materials are its strength and ductility. It has a higher
strength to cost ratio in tension and a slightly lower strength to cost ratio in
compression when compared with concrete. The stiffness to weight ratio of steel
is much higher than that of concrete. Thus, structural steel is an efficient and
economic material in bridges. Structural steel has been the natural solution for
long span bridges since 1890, when the Firth of Forth cantilever bridge, the
world's major steel bridge at that time was completed. Steel is indeed suitable for
most span ranges, but particularly for longer spans. Howrah Bridge, also known
as Rabindra Setu, is to be looked at as an early classical steel bridge in India.
This cantilever bridge was built in 1943. It is 97 m high and 705 m long. This
engineering marvel is still serving the nation, deriding all the myths that people
have about steel. [See Fig.]
Fig. Howrah bridge
The following are some of the advantages of steel bridges that havecontributed to their popularity in Europe and in many other developed countries.
· They could carry heavier loads over longer spans with minimum dead weight,
leading to smaller foundations.
· Steel has the advantage where speed of construction is vital, as many
elements can be prefabricated and erected at site.
· In urban environment with traffic congestion and limited working space, steel
bridges can be constructed with minimum disruption to the community.
· Greater efficiency than concrete structures is invariably achieved in resisting
seismic forces and blast loading.
· The life of steel bridges is longer than that of concrete bridges.
· Due to shallow construction depth, steel bridges offer slender appearance,
which make them aesthetically attractive. The reduced depth also contributes to
the reduced cost of embankments.
· All these frequently leads to low life cycle costs in steel bridges
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