Slab Bridge

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Aalto UniversitySchool of Engineering

Department of Structural Engineering and Building Technology

Rak-11.3001 Design of Bridges 10.10.2012

Slab Bridges

Teemu Ahonen

Martina Jurigova


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Contents

1 Introduction 2

1.1 Present day definition 2

1.2 General definition 2

2 Classification 3

2.1 Main parts of the slab bridges 3

2.2 Types of slab bridges (material) 3

2.3 Types of slab bridges (structure) 3

2.4 Another method of classification 4

3 Cross sections 5

3.1 Different cross section 5

3.2 Different types of cross section 5

4 Structural behaviour 5

4.1 Design phases 6

4.2 Analysis .6

4.3 Reinforcement concrete T-beam bridges 7

4.4 Construction 8


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1 Introduction

Slab bridge is probably the most common bridge type. It is mainly used incrossings and over waterways and also as an underpass for walkways. Slab bridge

has two fundamental features compared to other bridge types. First its superstructureis substantially wide compared to the height of the slab. Secondly by reason of theformer, slab bridges can divide loading to a very broad area.

Slab bridges have usually a span length of 10…20 meters and it would begood to make the slab continuous over more than one support. Structurally it wouldbe economic if all middle span length proportions would be equal and edge spanswould be 80% of this length.

1.1 Present day definition

A bridge with a superstructure that is composed of a reinforced concrete slabthat is either singular, constructed in place, or a series of narrow, precast slabs.

This slab is parallel with the roadway alignment and spans the space betweenthe supporting abutments or other substructure parts.

1.2 General definition

A bridge with a superstructure that is composed of a slab that is eithersingular, constructed in place, or a series of narrow, precast slabs.(The material often

used in early times are stones and timber) A short- bridge consisting span of a reinforced-concrete slab resting on

abutments.

Fig 1. . Štefánikův Bridge in Prague


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2 Classification

2.1 Main parts of teh slab bridge

Fig 2. . Main parts

2.2 Types of slab bridges (material)

• Timber slab bridges• Stone slab bridges• Steel slab bridges• Reinforcement concrete slab bridges

2.3 Types of slab bridges (structure)

• Slab bridges: Structure comprises a slab, supported at it ends. Usually madeof concrete(with reinforced),also possible in laminated timber.

• Beam-slab bridges(Slab girder bridges): Structure comprises one or morebeams. Commonly used for simple bridges with a span of up to 200 metres.The beam height is roughly 1/20th of the span, made of steel concrete ortimber.


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Fig 3. . General types

2.4 Another method of classification

Slab bridge is the most common form in small span structures, and also canbe divided into two types: overall structure and assembly structure (hollowcore slaband solid slab).

Fig 4.


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3 Cross sections

3.1 Different cross section

The height of the cross-section is determined by the longest span length. The

height can be calculated with the equationh = 0,22 + 0,04·lmax ,

where lmax is the length of the longest span.The equation is very simple and of course the height depends also of the

width of the slab, the number of supports and the obliquity of the bridge.

3.2 Different types of cross section

Fig 5. Types of cross section

4 Structural behaviourThe structural analysis of a slab bridge is based on different computer

software and the software use mainly difference method or element method. Aloaded slab bends in longitudinal and in transverse direction. The calculation ofstresses caused by bending is a very complex mathematic problem and it is possibleonly in simple cases.


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Fig 6. Bending in a slab due to loading

4.1 Design phases

Design is a two-phase process:

• The first phase involves determining the design loads and their effects interms of moment and shear forces

• The second phase involves selecting members that have sufficient strength to

resist the effects of the intended loads on the bridge. Before considering thedesign process complete, the failure modes (lateral buckling, excessivedeflection, end bearing, and so forth) as well as moment and shear must bechecked

•

4.2 Analysis

The analytical method only applies to slab bridges with the main reinforcementrunning parallel to the direction of traffic. The slab acts as a one-way slab in thedirection of traffic .

Area above the neutral axis acts in compressionthe reinforcing steel in the bottom of the slab carries all of the tension and the

concrete carries no tensionOnly the moment capacity is determined for the slab since shear generally will not

control in thin, reinforced-concrete membersOnly a one-foot-wide strip of slab at the midspan should be considered.The longer the slab is in proportion to its width the more the slab is similar to a

beam. In long span slab bridges the slab can be calculated as a beam and only indetails the influence of the slab can be taken into account.


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Fig 7.

4.3 Reinforcement concrete T-beam bridges

T-beams are used to obtain longer span lengths than those allowed by slabbridges

Fig 8. Reinforcement


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4.4 Construction

A slab bridge can be built as in-situ or prefabricated and the material in slabbridges is in actual fact always reinforced concrete. In-situ process is usually used incantilevered slab bridges. Cantilevered slab bridges are typical bridges inunderpasses when span length is 8…10 meters and over waterways with a span

length of 10…18 meters. The economy of a cantilevered slab bridge is based on thatthe bridge type has no massive abutments.

A prefabricated reinforced slab bridge is used in crossings, underpasses and overwaterways with a span length of 4…10 meters. A prefabricated slab bridge is built upby using foundation, abutment and superstructure elements.

Fig 8. Aleksanterinkatu Street bridge in Porvoo


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Literature

[1] Paavola, H. Loikkanen, P. Jutila, A. Sillanrakennustekniikan perusteet. First Edition.Otakustantamo. 1979. ISBN 951-671-201-0

[2] Hambly, E.C. Bridge Deck Behaviour. Second Edition. Taylor & Francis. 1998.[3] Suomen Rakennusinsinöörien Liitto RIL ry. RIL 179 Sillat. Hangon Kirjapaino Oy. 1989.

ISBN 951-758-196-3

[4] http://alk.tiehallinto.fi/sillat/suunnit2.htm#tuottajat

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Slab Bridge