composite girder

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Aalto UniversitySchool of Engineering

Department of Structural Engineering and Building Technology

Rak-11.3001 Design of Bridges 28.11.2012

Composite Girder

Timo Lahti


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Contents

1 Introduction 2

1.1 Introduction to composite structures 2

1.2 Advantages and disadvantages of composite girders 2

1.3 Types of composite girders 4

2 Structural Behavior 5

2.1 Calculation basics 5

2.2 Shear connectors .6

2.3 Carbonfiber composite structures 7

3 Building of composite bridges 8

3.1 Building phases 8

3.2 Building platforms 11

4 Sources 13


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1.2 Advantages and disadvantages of composite girders

Advantages:

• Efficient use of materials due to composite action

• Lower chance of buckling compared to steel bridges

• Slender structures

• Lower dead weight due to cross-section

• Repair methods with carbon plates

Disadvantages:

• Involves more factors to be considered than normal concrete bridges

• Inefficient on areas with negative moments

• Many different states has to be calculated, for e.g. erection phase

Fig 2. Ten Mile Creek bridge.


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1.3 Types of composite girders

Composite bridges may be made of multiple types. They may also be either simple beam or

continuous bridges. They may be beam or box girder. Other options are also possible, but

they are not usually considered that economical.

Fig 3. Composite box girder with rectangular steel box sections[3].

Fig 4. Composite box girder bridge with open-topped trapezoidal steel sections [3].

Composite structures may be created as bridges and decks. The deck structure may also

include steel form, which may also act along the composite structure. This reduces the amount

of work that has to be done during the construction. For spans between 45m to 100m multiple

girders are typically used.

Fig 5. Welded shear studs[2]


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2 Structural Behavior

2.1 Calculation basics

Stiffness of the materials has to be determined to calculate the load distribution. It is generally

assumed that the cross-section works completely as a composite structure.

To determine the bending stiffness of the structures, some components are required. The

effective width of the concrete together with the steel structure has to be determed. The

effective width of the concrete depends on the shear connectors and the position of the beam.

The variation of the materials in different states has also to be considered such as creep.

Changes in the elasticity of a material affects the whole structure and its behavior. Different

load cases have to be considered.

Fig 6. Variables used in the calculation of composite structures[2].


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Axial stiffness of the concrete and steel is calculated. After it we can determine the composite

action.

Slipping is not usually considered on the composite calculations. Instead it is assumed that the

structure works completely as a composite structure. The composite steel effect is calculatedfrom the following formula:

After composite action is known, we can calculate the bending stiffness of the composite

structure:

2.2 Shear connectors

An important part of any composite structure is the shear connector. Shear connector is thepart between the selected materials that transfers the shear force through them. The behavior

and strength of the connectors is vital to the whole bridge structure.

Fig 1. Typical shear planes in a steel-concrete structure[1].

The type of the connector is selected according to the loads and materials. Shear connectors

are either flexible or rigid. Flexible connectors allow more slip between the materials, which

causes more deformation and loss of strength in the structure. Flexible connectors are,

however, often more economical or practical choice and are therefore used.

Typical connectors in steel-concrete structures are headed studs. They act as flexible

connectors. Headed studs allow significant movement or slip at the ultimate limit state, whichshould be taken into account when calculating the structure..

For larger shear flows with concentrated sudden changes in magnitude, rigid plates are more

appropriate. For possible tensile forces, hoops are practical. Also steel bars can be used as

rigid connectors.

Shear connectors are calculated by dividing the longitudinal shear force by the capacity of a

connector. Depending on the used calculation code this formula can also have some

coefficient. The coefficient may also vary between the ultimate limit state and the

serviceability state.


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Fig 7. Shear stud failures[2].

2.3 Carbonfiber composite structures

One of the newer materials used in composite bridges is carbon. Carbon can be glued to a

beam, which will begin to work as a composite structure with the concrete beam. This technic

is mainly used as a repairing method, not when building new bridges. Carbon plates have

tensional strength, which varies mainly between 2000-3000 MPa. This makes it as a useful

choice for tensional parts.

Use of carbon plates require special know how. One of the available instruction manual is a

publication on glued strengthening on bridges published by the finnish road agency. Other

codes or publications concerning carbon plate strengthening are german codes such as DIN.

Fig 1. Maximum length between two glued panels in a concrete structure[6].


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Factors that have to be considered when designing a carbon composite structure:

• Strain of the carbon plate

• Strain of the steel at the moment of gluing

•

Shear force

• Anchoring of the glued surface

• Climate effects

• Mixing and working of the glue

• Testing of the glue

3 Building of composite bridges

3.1 Building phases

Composite bridges may be cast-in-situ or prefabricated structures. Cast-in-situ steel-concrete

composite bridges follow mostly the following construction phases:

• Installing of the steel beams

• Formwork for the deck

• Installing the reinforcement

• Casting the concrete

• Removing the formwork


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Fig 8. Open trapezoidal composite box girder during construction[3].

Fig 9. Bridge concrete deck formwork[2].

In case of prefabricated composite bridges, the prefabricated concrete elements are installed

directly on top of the steel beams. The share connectors are completed by pouring concrete

after the element installation to the connectors connecting the concrete elements and the steel

beams.


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The steel beams could be theoretically welded to the prefabricated concrete elements, in case

the concrete elements have welding plates installed on them.

Fig 10. M20 Newington Bridge[3].


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Fig 11. Avenues Walk Flyover bridge.

Fig 12. Avenues Walk Flyover bridge under construction.

3.2 Building platforms

Building platforms are developed for cast-in-situ composite girders. These platforms aim to

make installation cheaper and faster. The platform is supported from the steel beams and is

moved during the work onwards. All work on the concrete is done from the platform, which

itself can be weather protected.


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Fig 13. Composite forming carriage [5].

Fig 14. Composite forming carriage [5].


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Literature[1] David Collings. Steel-concrete composite bridges. Thomas Telford publishing, 2005

[2] Jonathan R. Hatlee. The viability of steel-concrete composite girder bridges with

continuous profiled steel deck. Virginia Polytechnic Institute. 2009

[3] http://www.steelconstruction.info/Box_girder_bridges

[4] Schneider Bautabellen. Werner Verlag. 2010

[5] http://www.doka.com/web/products/system-groups/doka-load-bearing-systems/bridge-formwork/composite-forming-carriage/index.en.php

[6] Liimausvahvistamisohjeet. Tiehallinto. 2007


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