Notes -Bridge Design - Box Girder Bridges

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Aalto University

School of Engineering

Department of Structural Engineering and Building Technology

Rak-11.3001 Design of Bridges 17.10.2012

Box Girder Bridges

Prayash Gomdenn

Binesh Puliyelil Joy

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A box girder bridge is a bridge in which the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete. It is typically rectangular or trapezoidal in cross-section. Box girder bridges are commonly used for highway flyovers and for modern elevated structures of light rail transport. The box girder can also be part of portal frame bridges, arch bridges, cable-stayed and suspension bridges of all kinds.

Box girder decks are cast-in-place units that can be constructed to follow any desired alignment in plan, so that straight, skew and curved bridges of various shapes are common in the highway system. Because of high torsional resistance, a box girder structure is particularly suited to bridges with significant curvature.

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3 1.1 Types of Box girder Bridges

The Box-Girders can be of different forms and geometry, but we can characterize three basic sections, that are represented below:

1.1.1 Based on Geometry

1. SIMPLE MONOCELLULAR BOX GIRDERS:

Fig: Single Cell box girders

2. MONOCELLULAR BOX GIRDERS WITH RIBS OR STRUTS:

Fig: Single Cell Box girders with inclined struts 3. DOUBLE-CELL BOX GIRDERS:

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4 4. COMPOSITE MULTIPLE BOX GIRDERS:

Fig: Composite Multiple Box girders 1.1.2 Based on Materials used

a) Concrete Box Girder:

Fig:Typical single cell Concrete Box Girder cross section b) Steel Box Girder:

Fig: Steel Box girder cross-section

c) Composite Concrete Box Girder:

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5 1.1.3 Based on reinforcement

a) Reinforced concrete box girders b) Pretentioned concrete box girders c) Posttentioned concrete box girders d) Segmental box girders

2 Distinctive Features

· A box girder has high torsional stiffness and strength, compared with an equivalent member of open cross section. This is of particular advantage in structures curved in plan like highway ramps, but in straight structures this will also contribute to the efficient support of eccentric loads and to the effective distribution of load in transverse direction. · Box girder will have large flange widths. Increased flange widths make it possible to use

large span/depth ratios. This is an advantage if construction depth is limited. Also it can lead to more slender structures which are mainly considered more aesthetical.

· The space enclosed within in the girder may be used for the passage of services such as gas pipes, cables, water mains etc. In some cases bottom flange can also be used as another deck that accommodates traffic.

· Maintenance of a box girder can be easier, because the interior space can be made directly accessible. Alternatively, this space may sealed providing a noncorrosive environment. · Box girders are generally aesthetic. This is due slender form and also a result from the

uncluttered undersurface. Trapezoidal box girders are more pleasing to the human eye. · The shape of the box girder can vary a lot. This makes them easier to design for

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Fig: Typical components of a box girder bridge with composite concrete deck Shear keys

A shear key is a shaped joint between two prefabricated elements that can resist shear through the geometric configuration of the joint.Shear keys are provided in the webs and flanges of precast segments of a Box-Girder. They serve two functions. The first is to align the segments when they are erected. The second is to transmit the shear force between segments during construction.

Diaphragms

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. By the provision of diaphragms, transverse bending stresses caused by the moments, resulting from differential deflection of top and bottom slabs are eliminated. The use of diaphragms at supports which are at definite locations of concentrated loading significantly diminishes the differential deflections near the supports and should always be provided.

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7 1.3 Advantages

· High stiffness against torsion compared to normal plate girders · Good option when building bridges that curve in plane

· Smaller economical construction depth when compared to plate girders

· It has high structural efficiency and torsional strength which minimizes the prestessing force required to resist a given bending moment. Prestressing is not required for spans up to 60m.

· Their closed shape makes them more efficient in corrosion resistance than plate girders, as the shape drastically reduces the exposed surface area

· The upper flange can act also as a part of the deck structure

· Bracings can be hidden in the space available inside the girder and the structure becomes more aesthetic from outside.

1.4 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.

· Box girders are more expensive to fabricate, and they are more difficult to maintain, because of the need for access to a confined space inside the box.

· Girders are not efficient as trusses in resisting loads over long spans. · Typically higher construction costs compared to I-section steel girder · Design part is complex

Box girder bridges are reasonable to use when:

· The spans range from 12 m to 300 m (Reference Barker and Puckett, Design of Highway Bridges)

· In girders curved in plan where there is higher torsion · Fairly low depth is required

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8 1.5 Examples of Box Girder Bridges

Example a) Swanport bridge over River Murray, near Murray Bridge in South Australia

Example b) Box girders curved in plan,Fossedyke Bridge, Lincoln 1.6 Brief History of Development of Box Girder Bridges:

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.

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Fig: Evolution of Box Girders

The development of high strength prestressing steel made it possible to span longer distances. The first prestressed concrete bridges, most of I-cross sections were built towards the end of the 1920’s.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.

The box girder bridge was a popular choice during the road building expansion of the 1960s and many new bridge projects were in progress simultaneously. A serious blow to this use was a sequence of three serious disasters, when new bridges collapsed in 1970 (West Gate Bridge and Cleddau Bridge) and 1971 (Koblenz Bridge). Fifty-one people were killed in these failures, leading in the UK to the formation of the Merrison Committee and considerable investment in new research into steel box girder behavior.

Most of the bridges still under construction at this time were delayed for investigation of the basic design principle. Some were abandoned and rebuilt as a different form of bridge altogether. Most of those that remained as box girder bridges, such as Erskine Bridge , were either redesigned, or had additional stiffening added later. Some bridges were strengthened a few years after opening and then further strengthened years later, although this was often due to increased traffic load as much as better design standards. The Irwell Valley bridge of 1970 was strengthened in 1970 and again in 2000.

(Source:http://en.wikipedia.org/wiki/Box_girder_bridge)

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10 Longest Box girder Bridges

Name Country main span

1 Jacques Cartier Bridge Canada 334.35 m

2 Stolma Bridge Norway 301 m

3 Raftsundet Bridge Norway 298 m

4 Sundoya Bridge Norway 298 m

5 Pont du Bras de la Plaine France 280.773 m

6 Ujina Bridge Japan 270 m

7 Neckar Viaduct Germany 263 m

8 Gateway Bridge Australia 260 m

9 Ponte Deputado Darcy Castello de Mendonça

Brazil 260 m

10 C & O Bridge United States of

America

259.08 m

(Source: http://en.structurae.de/structures/stype/index.cfm?id=6167)

(Fig: Girder Footbridge located in City of Westminster (Bridge of Aspiration), London) (Source: http://en.structurae.de/structures/data/index.cfm?id=s0003479)

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2 Structural Behaviour of Gox Girder

A structure will be safe when subjected to a design loading if the forces and stresses calculated throughout the structure are in equilibrium with each other and with the design loading, and if they do not anywhere exceed the yield strengths of the material.

When box girders are used, two additional effects must be considered, torsion and distortion due to eccentricity in loading. A general loading on a box girder, such as shown in fig 2,1 for single cell box, has components which bend, twist, and deform the cross section. If the torsional component of the loading is applied as shears fig 2,1 (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 2,1 (c), there are also forces acting on the plate elements fig 2,1 (f), which tend to deform the cross section. As indicated in fig 2,2 the movements of the plate elements of the cross section cause distortion stresses in the transverse direction and warping stresses in the longitudinal direction

Torsional warping Distortion Fig:2,1 Behaviour of box girder subjected to eccentric loading

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For a simple uniform box section subject to pure torsion this warping is unrestrained and does not give rise to any secondary stresses. But if, for example, a box is supported and torsionally restrained at both ends and then subjected to applied torque in the middle, warping is fully restrained and torsional warping stresses are generated. Similar restraint occurs in continuous box sections which are torsionally restrained at intermediate supports.

Fig 2,2 : Distortional warping stresses

2.1 Analysis

There are many methods for analysis

· Simple line analysis or beam analysis · Grillage analysis[ref]

· BEF Analysis (Beams on elastic foundation) · Space frame analysis

· Finite element method

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Fig 2,4 : Bending moment and stresses due to eccentric loading (Ref Bridge deck behavior, E C Hambly)

2,2 Design of super structures as segmental box girder

a) when beam theory is used, single box cells shall not should not be more than 12m wide including cantilevers. For wider bridges, multiple cells shall be used

b) for maximum economy, the span to depth ratio for constant depth should be 18 to 20.variable depth structre have 18 to 20 at supports and 40 to 50 at midspan for continuous span)

c)a shallow box girder that is too wide begin to behave as a slab. So width to depth ratio should also be considered

2.3 Cross sectional restraints

Cross sectional restraints are more often used in steel box girders than concrete girders. They are recommended due to distortion effects. The main functions of cross sectional restraints are

· to preserve the shape of the box against distortion

· to transfer an externally applied torque to the box walls through shear flow · to provide transverse support to longitudinal stiffeners under compression · to support traffic loads directly (from an orthotropic deck)

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Fig 2,5: internal stiffeners in steel box girder The main functions of cross sectional restraints are:

· to preserve the shape of the box against distortion

· to transfer an externally applied torque to the box walls through shear flow · to provide transverse support to longitudinal stiffeners under compression · to support traffic loads directly (from an orthotropic deck)

· to transmit forces from box walls to the supports.

All-welded construction permits the simple cross section shown. This consists of a web plate, fillet welded to a single thick plate at each flange. A change in flange thickness is achieved by tapering the thickness at the end of the thicker plate, and butt welding it to a thinner plate. An alternative is to use multiple-flange plates, with widths successively reduced to the outside, permitting longitudinal fillet welds to be provided between the plates at the stepped edges. All-welded construction permits the simple cross section shown. This consists of a web plate, fillet welded to a single thick plate at each flange. A change in flange thickness is achieved by tapering the thickness at the end of the thicker plate, and butt welding it to a thinner plate. An alternative is to use multiple-flange plates, with widths successively reduced to the outside, permitting longitudinal fillet welds to be provided between the plates at the stepped edges.

3 ConstructionMethods 1. Segmental Construction

The bridge deck is constructed sequentially beginning from the pier, one section at a time. In pre-cast segmental bridge, the precasted concrete segment is constructed at the factory, then transported to the site and hoisted into place. As the new segment is suspended in place by the crane or gantry, workers install steel reinforced that attaches the new segment toproceeding segment. Each segment of the bridge is design to accept connections from both proceeding and succeeding segment. This process is repeated until the span is completed.

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Fig 3,1: Segmental Construction 2. Span-by-Span

An entire span of segments is supported by a temporary truss orgirder which sitting on the pier until it is post-tensioned and self-supporting.The truss or girder is launched to the next adjacent pier, and the process is repeated.

Fig3,2: Precast span-by-span construction with overhead truss (the Spaghetti Bowl in Las Vegas, Nevada)

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Fig3,3: Cast-in-place span-by-span construction with self-launching forms (Rosario Victoria Bridge in Argentina)

3. Progressive Placement.

The erection of multiple span bridges proceeds in one heading, often with temporary supports to limit stresses in the structure during erection.This method is particularly well suited to sites with severe access limitations or where environmental issues limited contractor access to the work site. Its first cast-in-situ application was in Finland in 1967.

4. Incremental Launching.

A segment is attached to the previously completed bridge superstructure, and then the entire completed bridge is launched outward before the subsequent segment is assembled. This method was first applied on the Rio Caroni Bridge in Venezuela in 1963.

Fig 3,4: Incrementally launched deck for Neckarburg Arch Bridge in Germany, 1977 (Juan and Joseph, 2004)

(Incremental launching: http://www.youtube.com/watch?v=S3Kf9e6JgF4) 5. Segmental Cast-in-place with Traveler Formworks

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This method of construction still utilizes the segmental method of construction and balanced cantilever but the segment is casted in-situ. Thetraveler formworks which is hang onto the previously casted segment is used to form the segment. After the segment has been casted, after three to four days, the temporary stressing will be applied to hold the segment to it place. The formworks then will be push forward to the next segment and the same process repeat until the middle of the span on both directions, creating a balanced cantilever structure. The figure below shows the construction by traveler form using balanced cantilever method.

Fig: Vietnam Veterans Memorial Bridge - cantilever construction of a segmental box girder using traveling forms (Juan and Joseph, 2004)

Fig: Classification according to construction method, span length, total bridge length and construction progress

(Source:

http://books.google.fi/books?id=2-UmAsMxsFEC&printsec=frontcover&hl=fi&source=gbs_ge_summary_r&cad=0#v=onepage &q&f=false)

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18 4 Conclusion

· box girders are used because of their good resistance to torsion

· box girders can be designed to have a good aerodynamic shape. So can be used even above 200m with cable stayed or suspension bridges ref

· the clear external surfaces and the use of inclined webs gives a good appearance · shear lag (ref) and the stability of the compression flange both require consideration in

wide flanges

· design of webs is generally similar to that for plate girders · distortional effects must be considered

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19 5 Sources

Literature

· Bridge Deck Behavior (Second Edition) E.C. Hambly

· Barker, Puckett, 2007 (Design of Highway Bridges, an LRFD Approach) · Theory and Design of Bridges, Petros P. Xanthakos

· http://www.fgg.uni-lj.si/kmk/esdep/master/wg01b/toc.htm