BEHAVIOUR OF BOX GIRDER BRIDGES

GUIDED BY

Dr.G.RamaKrishnan

Submitted By

R.Chandrakumar

Registration No.:2831756037

B.Tech Final Year Civil

GIRDER

A girder is a support beam used in construction

Girders often have an I beam cross section for strength, but may also have a box shape, Z shape or other forms

Girder is the term used to denote the main horizontal support of a structure which supports smaller beams

A girder is commonly used many times in the building of bridges

BOX GIRDER

A girder that forms an enclosed tube with multiple walls, rather than an I or H beam.

It can also be known as a tubular girder.

Originally constructed of riveted wrought iron.

They are now found in rolled or welded steel, aluminium extrusions or pre-stressed concrete.

BOX GIRDER BRIDGES

It is a bridge in which the main beams comprise girders in the shape of a hollow box.

The box is typically rectangular or trapezoidal in cross-section.

The box girder is an efficient form of construction for bridges because it minimizes weight, while maximizing flexural stiffness and capacity.

Commonly used for highway flyovers.

TYPES OF BOX GIRDER BRIDGES

1. Concrete

2. Structural steel

3. Composite (steel and RCC)

BEHAVIOUR OF BOX GIRDER BRIDGES

A general loading on a box girder, such as shown in fig 1 for single cell box, has

components which bend, twist, and deform the cross section. Thin walled closed section girders

are so stiff and strong in torsion that the designer might assume, after computations based on the

elemental torsional theory, that the torsional component of loading in fig 1(c). has negligible

influence on box girder response. If the torsional component of the loading is applied as shears

on the plate elements that are in proportion to St. Venant torsion shear flows, fig 1 (e), the

section is twisted without deformation of the cross section. The resulting longitudinal warping

stresses are small, and no transverse flexural distortion stresses are induced. However, if the

torsional loading is applied as shown in fig 1 (c), there are also forces acting on the plate

elements fig 1 (f), which tend to deform the cross section. As indicated in fig 2 the movements of

the plate elements of the cross section cause distortion stresses in the transverse direction and

warping stresses in the longitudinal direction.

Figure 1

Figure 2

 

.

FLEXURE:

A vehicle load, placed on the upper flange of box girder can occupy any position,

transverse as well as longitudinal. This load is transferred transversely by flexure of deck to the

webs of box girder.

For understanding the various stresses generated, initially consider that the webs of box girder

are not allowed to deflect. The structure resembles a portal frame. The flexure of deck would

induce transverse bending stresses in the webs, and consequently in the bottom flanges of the

girder. Any vehicle load can thus be replaced by the forces at the intersections of deck and web

as shown in fig 3.

Figure 3

WARPING OF CROSS SECTION:

Warping is an out of plane on the points of cross section, arising due to torsional loading. Initially considering a box beam whose cross section cannot distort because of the existence of rigid transverse diaphragms all along the span. These diaphragms are assumed to restrict

longitudinal displacements of cross sections except at midspan where, by symmetry the cross section remains plane. The longitudinal displacements are called torsional warping displacements and are associated with shear deformations in the planes of flanges and webs.

In further stage assume that transverse diaphragms other than those at supports are removed so that the cross section can distort. It results in additional twisting of cross section under torsional loading. The additional vertical deflection of each web also increases the out of plane displacements of the cross sections. These additional warping displacements are called distortional warping displacements/

Thus concrete box beams with no intermediate diaphragms when subjected to torsional loading, undergo warping displacements composing of two components viz, torsional and distortional warping displacements. Both these give rise to longitudinal normal stresses i.e. warping stresses whenever warping is constrained. Distortion of cross section is the main source of warping stresses in concrete box girders, when distortion is mainly resisted by transverse bending strength of the walls and not by diaphragms.

SHEAR LAG:

In a box girder a large shear flow is normally transmitted from vertical webs to horizontal flanges, causes in plane shear deformation of flange plates, the consequence of which is that the longitudinal displacements in central portion of flange plate lag behind those behind those near the web, where as the bending theory predicts equal displacements which thus produces out of plane warping of an initially planar cross section resulting in the “SHEAR LAG". Another form of warping which arises when a box beam is subjected to bending without torsion, as with symmetrical loading is known as “SHEAR LAG IN BENDING”.

Shear lag can also arise in torsion when one end of box beam is restrained against warping and a torsional load is applied from the other end fig 7. The restraint against warping induces longitudinal stresses in the region of built-in-end and shear stresses in this area are redistributed as a result which is an effect of shear deformation sometimes called as shear lag. Shear distribution is not uniform across the flange being more at edges and less at the centre fig 9.

FIGURE 7

In a box beam with wide, thin flanges shear strains may be sufficient to cause the central longitudinal displacements to lag behind at the edges of the flange causing a redistribution of bending stresses shown in fig 8. This phenomenon is termed as “STRESS DIFFUSION”.

The shear lag that causes increase of bending stresses near the web in a wide flange of girder is known as positive shear lag. Whereas the shear lag, that results in reduction of bending stresses near the web and increases away from flange is called negative shear lag fig 12. When a cantilever box girder is subjected to uniform load, positive as well as negative shear lag is produced. However it should be pointed out that positive shear lag is differed from negative shear lag in shear deformations at various points across the girder.

At a distance away from the fixed end in a cantilever box girder say half of the span; the fixity of slab is gradually diminished, as is the intensity of shear. From the compatibility of deformation, the negative shear lag yields. Although positive shear lag may occur under both point as well as uniform loading, negative shear lag occur only under uniform load.

FIGURE 8

It may be concluded that the appearance of the negative shear lag in cantilever box girder is due to the boundary conditions and the type of loading applied. These are respectively external and internal causes producing negative shear lag effect.

FIGURE 9

REFERENCES:

STEEL CONSTRUCTION INSTITUTE, Design Guide for Composite Box Girder

Bridges-second edition, march 2007.

Sherif El-Tawil and Ayman M. Okeil, Behaviour And Design OF Curved Composite Box

Girder Bridges, October 2002.

Zakia Begum, Analysis And Behaviour Investigations of Box Girder Bridges,2010.