Author: Michael Slevin BEngCEng MIEI, senior structural engineer, Banagher Precast Concrete
Prestressed concrete bridge beams are firm favourites in the short and medium span bridge market - of up to 50 metre spans - and have been in successful use for the past 60 years. The bridges, for which the industry and Banagher Precast Concrete manufacture beams, require considerable expertise in design.
In the past few years, the design regulations have been harmonised throughout Europe as part of the Common Market for goods and services with the introduction of a suite of eurocodes and European product standards. Each member state of the EU has the responsibility for structural safety and the eurocodes therefore come with a national annex in which national preferences with respect to safety factors and so on are given.
Banagher Precast Concrete has just introduced its bridge beam design manual. The designs in this manual have used the UK national annexes which are very similar to the Irish national annexes. Where the Irish regulation would be different, it is noted in the text. The coherence of the new regulations allow, with the use of harmonised product standards, a common approach which is used as the basis of the CE marking of bridge beams, in turn enabling them to be part of an open pan-European market.
This manual gives Banagher Precast Concrete's view of the new design process for prestressed bridge beams to the eurocode and it is hoped will promote more use of bridge beams and a common and agreed understanding of the many new clauses in the codes. The manual considers the design of the innovative Banagher W beam in a typical two-span road bridge.
The internal beam is designed in detail and as the loading code is also new, the approach to the use of this code is also demonstrated. Other areas of the whole bridge design are commented upon, particularly where they can have an important influence on beam design. These are mentioned at appropriate places where they would naturally occur. Suggestions for further reading are also given.
The format of the manual has the commentary and the design information followed by the example calculations. This design manual should be read in conjunction with our ‘Bridge Beam Manual’, which includes our full range of precast prestressed bridge beams and their associated span tables along with all other relevant information required by a bridge engineer in choosing a precast section.
Ribbed fibre reinforced concrete
The design is for a two-span integral bridge, with each span having a length of 30.75m from centre of abutment to centre of pier giving an actual beam length of 30.50m. The bridge carries a 6.0m wide carriageway with 1.5m wide footways on either side.
The superstructure consists of six Banagher Precast Concrete prestressed W11 beams with a 230mm structural in-situ reinforced concrete deck slab 200mm over the top of the beam cast on ribbed fibre reinforced concrete (FRC) semi participating permanent shutter. There are in-situ diaphragms at the abutments and pier.
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Abbey Bridge and Viaduct replacement in Evesham, Worcestershire[/caption]
A 500mm gap between the precast beams is used at the pier to allow projecting links from the crosshead. There is a 1500mm wide precast crosshead spanning between two precast columns which makes up the pier.
The bridge beams span from abutment to crosshead with 500mm bearing/embedment and without the need for temporary support.
The foundations for the abutment are modelled as strip foundations 1m deep and 4m wide. The sizing of the foundations is taken from experience and for this example will suffice as the W11 beams are all that is being looked at in detail not the supporting structure.
The precast columns are fixed into place with dowel bars projecting up from the foundation below. The columns will need to be propped and the sleeves filled with non-shrink grout which will then need to be left to strengthen before the placing of the precast crosshead.
Bending moment
Currently there is no design manual on the market that gives a bridge engineer such a detailed worked example. The manual describes how to choose the correct beam and perform all of the checks required to the new eurocodes. Where a calculation is carried out a reference to where it can be found in the codes is given. This is to help the bridge engineer cut down on the time taken to rifle through all of the various codes.
The manual not only has calculations but it also contains sections, elevations, exposure class and cover diagrams, a 3D model of the bridge, screen shots showing bending moment and shear force diagrams, prestressing tendon layouts and much more.
The integral bridge is modelled using MIDAS Civil, a 3D software analysis package. There is also a full tutorial video online describing the full process.
The manual is free to view on our website
www.bancrete.com I have presented the manual to design houses and colleges in Ireland and the UK over the past number of months and am available on request for same.