Ventajas y Desventajas de Viga Cj

8/4/2019 Ventajas y Desventajas de Viga Cj

1/6

The continuing expansion of highway network throughout the world is largely

the result of great increase in traffic, population and extensive growth of

metropolitan urban areas. This expansion has lead to many changes in the use

and development of various kinds of bridges. The bridge type is related to

providing maximum efficiency of use of material and construction technique,

for particular span, and applications. As Span increases, dead load is an

important increasing factor. To reduce the dead load, unnecessary material,

which is not utilized to its full capacity, is removed out of section, this results in

the shape of box girder or cellular structures, depending upon whether the

shear deformations can be neglected or not. Span range is more for box bridge

girder as compare to T-beam Girder Bridge resulting in comparatively lesser

number of piers for the same valley width and hence results in economy.

A box girder is formed when two web plates are joined by a common flange at

both the top and the bottom. The closed cell which is formed has a much

greater torsional stiffness and strength than an open section and it is this

feature which is the usual reason for choosing a box girder configuration.

Box girders are rarely used in buildings (box columns are sometimes used but

these are axially loaded rather than in loaded in bending). They may be used inspecial circumstances, such as when loads are carried eccentrically to the

beam axis

When tension flanges of longitudinal girders are connected together, the

resulting structure is called a box girder bridge.

Box girders can be universally applied from the point of view of load carrying,

to their indifference as to whether the bending moments are positive or

negative and to their torsional stiffness; from the point of view of economy.


2/6

Historical development and description of box girder:

The first box girder cross section possessed deck slabs that cantilevered out

only slightly from the box portion shown in figs a to e. With the prestressed

concrete the length of cantilever could be increased. The high form work costs

caused a reduction in the number of cells fig (f, g, h). In order to reduce the

construction loads to minimum possible extent or to require only one

longitudinal girder in working states even with multiple traffic lanes.

It was only with the development of high strength prestressing steel that it

became possible to span longer distances. The first prestressed concrete

bridges, most of I-cross sections were built towards the end of the 1920s.The

great breakthrough was achieved only after 1945. THE SCLAYN bridge over

the river Maas, which was built by Magnel in 1948, was the first continuous

prestressed concrete box-girder bridge with 2 spans of 62.70m. In following

years the ratio of wages to material costs climbed sharply. This thereby shifted


3/6

the emphasis of development of construction method. The box girder cross-

section evolved structurally from the hollow cell-deck bridge or T-beam Bridge.

The widening of the compression zone that began as a structural requirement

at the central piers was in the extended throughout the entire length of bridge

because of advantages transverse load-carrying characteristics.

Evolution of Box Girder:

The spanning of bridges started with simple slabs. As the spans increased, the

design depth of slab is also increased. It is known that material near centre of

gravity contributes very little for flexure and hence can be removed. This leads

to beam and slab systems. The reinforcement in bottom bulb of beam providedcapacity for tensile forces and top slab concrete, the capacity to resist the

compression. They formed a couple to resist flexure.

As the width of slab is increased more number of longitudinal girders are

required resulting in reduction of stiffness of beams in transverse direction and

relatively high transverse curvature. The webs of beams get opened out

spreading radially from top slab. Under high transverse bending these will no

longer be in their original position. To keep it in their original position the bulbs

at bottom should be tied together which in-turn leads to evolution of box

girder. Long spans with wider decks and eccentric loading on cross-section will

suffer in curvature in longitudinal and transverse direction causing heavy

distortion of cross-section. Hence the bridges should have high torsional rigidity

in order to resist the distortion of cross-section deck to a minimum.


4/6

Accordingly box girders are more suitable for larger spans and wider decks,

box girders are to be suitable cross-section. They are elegant and slender.

Economy and aesthetics further lead to evolution of cantilevers in top flanges

and inclined webs in external cells of box girder. The dimension of cell could be

controlled by prestressing.

As the span and width increases the beams and bottom slabs are to be tied to

keep the geometry which in turn leads to evolution box girder.

Any eccentric load will cause high torsional stresses which will be counter acted

by the box section. The analysis of such sections are more complicated due

combination of flexure, shear, torsion, distortion. But it is more efficient cross-

section. It is used for larger spans with wide cross-section. It can be used for

spans up to 150m depending upon the construction methods. Cantilever

method of construction is preferred most.

Advantages Associated with Box Girders:

o In recent years, single or multi-cell reinforced concrete box Girder Bridge

have been proposed and widely used as economic aesthetic solution for the

over crossings, under crossings, grade separation structures and viaducts

found in modern highway system.

o The very large Torsional rigidity of the box girders closed cellular

section provides structures beneath is more aesthetically pleasing than open-

web type system.

o In case of long span bridges, large width of deck is available to

accommodate prestressing cables at bottom flange level.

o Interiors of box girder bridges can be used to accommodate service such

as gas pipes, water mains etc.

o For large spans, bottom flange could be used as another deck

accommodates traffic also.

o The maintenance of box girder is easier in interior space is directly

accessible without use of scaffolding.

o Alternatively space is hermetically sealed and enclosed air may be dried

to provide a non-corrosive atmosphere.


5/6

o It has high structural efficiency which minimizes the prestessing force

required to resist a given bending moment, and its great Torsional strength

with the capacity this gives to re-centre eccentric live loads, minimizing the

prestress required to carry them.

Disadvantages:

One of the main disadvantages of box decks is that they are difficult to cast in-

situ due to the inaccessibility of the bottom slab and the need to extract the

internal shutter. Either the box has to be designed so that the entire cross

section may be cast in one continuous pour, or the cross section has to be cast

in stages.

Specifications:

It can cover a range of spans from 25 m up to the largest non-suspended

concrete decks built; of the order of 300 m. Single box girders may also carry

decks up to 30 m wide. For the longer span beams, beyond about 50 m, they

are practically the only feasible deck section. Below 30m precast beams or

voided slab decks are more suitable while above 50ma single cell box

arrangement is usually more economic.

Single cell box-girder cast-in-situ are used for spans form 40m to 270m.The

box arrangement is done in order to give aesthetic appearance where the web

of box will act as a slender appearance when combined with a slim parapet

profile. Single box arrangements are efficient for both the longitudinal and

transverse designs, and they produce an economic solution for mot medium

and long span structures. This type of deck is constructed span-by-span, using

full-height scaffolding or trusses, or as balanced cantilever using form

travelers. This could be particularly important for medium length bridges with

spans between 40m and 55m. Such spans are too long for twin rib type decks,

and too short for cast-in-situ balanced cantilever construction of box girders,

while a total length of box section deck of less than about 1,000 m does not

justify setting up a precast segmental facility.

Haunches:


6/6

The uprights have to carry the same bending moment as the haunch, but with

the benefit of a compression force due to the weight of the roof. Thus they may

be slightly thinner than the haunches. Haunches are always economical. They

provide the twin benefits of attracting moment away from mid-span and then

providing a greater lever arm to resist this moment economically. Even very

short haunches are valuable in reducing the hogging reinforcement.

Ventajas y Desventajas de Viga Cj

Documents

Transcript of Ventajas y Desventajas de Viga Cj