The design of precast pre-stressed bridge beams differs significantly from standard reinforced concrete design and steel design – it contains elements of complexity not found in these structural calculations, writes Steve Lowe
News

 

Author: Steve Lowe, senior design engineer, Shay Murtagh Precast Ltd

Precast pre-stressed concrete bridge beams are frequently the preferred option in Ireland and the UK for road bridges, rail bridges, footbridges and other structural applications incorporating long spans. A pre-stressed bridge beam can be used in beam-and-slab, solid slab and voided-slab deck types and are suitable for simply supported, continuous and integral spans.

Precast pre-stressed beams substantially reduce and simplify in-situ concrete construction while simultaneously ensuring a high quality, robust and durable structural core. In addition, pre-stressed concrete bridge beams are an environmentally friendly solution that ensures low maintenance and a low whole-life cost.

The design of precast pre-stressed bridge beams differs significantly from standard reinforced concrete design and steel design – it contains elements of complexity not found in these structural calculations. Stress-limit checks form the core of pre-stressed concrete design and typically govern the selection of the pre-stressing strand layout for bridge beams.

The said stress-limit checks carried out in accordance with the Eurocodes require the simultaneous satisfaction of 12 inequalities. These inequalities are not taken directly from a design code, but are rather derived from first principles used in accordance with the requirements of the relevant design standards. Until significant experience in pre-stressed concrete design is acquired, it can be time consuming and potentially frustrating to carry out a preliminary or detailed design of pre-stressed concrete bridge beams. Optimising the design of pre-stressed bridge beams requires a higher level yet of expertise.

Shay Murtagh Bridge Beam Technical Manual


Bridge beams

Types of bridge beams

The Shay Murtagh Bridge Beam Technical Manual was introduced as a working tool to assist with both the preliminary and detailed design of precast pre-stressed bridge beams and the optimisation of the same. The manual covers a wide range of beam types as indicated (right).

Each beam has a comprehensive range of span tables to assist the bridge engineer in choosing the optimal beam type, size and centres for a specific structure. It should be noted that the span tables are generic and, as such, incorporate a number of conservative assumptions regarding support conditions, bridge width and concrete grade.

If any of these assumptions are not true for a given bridge (the assumptions are noted within the technical manual), the Technical Department at Shay Murtagh can be directly contacted to assist with further optimisation of the beam sizes and centres. The U-beam span table below is typical for precast pre-stressed bridge beams used in beam-and-slab type decks.

U-beam span tableThe MY-beam span table below is typical for in-fill slab type decks.

MY-beam span tableThe Shay Murtagh Bridge Beam Technical Manual is also a valuable tool for the detailed design of precast pre-stressed concrete bridge beams. The section properties, cross-sectional dimensions, available pre-stressing strand locations, typical shear-link details and other typical structural details associated with each beam type that are required to carry out the detailed design of a pre-stressed bridge beam are contained with the technical manual. Several of these items relevant to U-beams are given below as an example.

U-beam strand positionsU-beam properties

U beam

Standard section of typical shear reinforcement for U-beams

New features in the third edition


The third edition of the manual has recently been released. The Span Tables for TY, TYE, Y and YE beams have been updated following a more rigorous series of design estimates. Details relating to strand layouts and cross-sectional dimensions have also been enhanced for the same beam types. The ‘Designer’s Checklist’ was revised and expanded. However, the primary improvements of the third edition relate to building information modelling (BIM) and mobile access.

  • The power of BIM

BIM is quickly becoming the mainstay of the bridge-building industry. This approach seeks to streamline the entire design process while enabling cross-channel communications quickly and easily. This is primarily accomplished through the use of a centralised computer model, as opposed to disparate sets of traditional architectural drawings. Shay Murtagh has embraced BIM within the third edition of the Bridge Beam Technical Manual, which now contains a ‘BIM Library’. This comprehensive and technically rich database includes all models of precast pre-stressed bridge beams alongside their unique specifications.

  • BIM Library

The addition of an easy-to-access BIM Library to the technical manual is a particularly advantageous development due to its modular nature. All of the precast pre-stressed bridge beams currently supplied by Shay Murtagh are presented in Revit models. Thus, they can be easily downloaded and used within conceptual models. Not only will this help bridge engineers to convey and other bridge professionals to understand the unique requirements of each bridge, but the ability to implement these objects in tandem with an existing technical design can significantly reduce job completion time and more adequately address required levels of safety.

  • App for mobile access

With most bridge engineers and other bridge professionals now accessing the internet via smartphones and tablets, the third edition of the Bridge Beam Technical Manual can now be downloaded as an app. This app is available for iPad and Android users (both accessible via either a tablet or a smartphone). It can be procured at either the Play Store or the Apple Store as a free download.

Employing the synergy of streamlined access with additional technical amenities and the significant potential of the BIM Library, this latest edition of the Shay Murtagh Bridge Beam Technical Manual was launched in May 2014.

For more information or to download your copy of the Bridge Beam Technical Manual, see www.shaymurtagh.co.uk.

http://www.engineersjournal.ie/wp-content/uploads/2014/11/100_1508.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/11/100_1508-300x300.jpgDavid O'RiordanNewsbridges,construction
  Author: Steve Lowe, senior design engineer, Shay Murtagh Precast Ltd Precast pre-stressed concrete bridge beams are frequently the preferred option in Ireland and the UK for road bridges, rail bridges, footbridges and other structural applications incorporating long spans. A pre-stressed bridge beam can be used in beam-and-slab, solid slab and...