Harmonisation in bridge design: designers in Europe are now obliged to work with Eurocodes; luckily much software has already been adapted.

The mandatory pan-European switch from design using local design codes to design using Eurocodes is now imminent. The availability of software for the new design codes has always been acknowledged as a key factor in the implementation of the change and Bestech Systems has been working for more than four years adapting its Sam analysis and design software. Most of the modules are now available for Eurocodes.

Bestech has carried out a comparison to give some insight into the changes that practising engineers will encounter. The example covered a simple concrete bridge deck which was designed first to the British design code BS 5400 and then to the Eurocodes using the UK National Annex. The differences have been evaluated, both in terms of the resulting design and the design process.

The bridge deck example comprises two 20m-long spans with a 25[degrees] skew, made continuous over its central support. It carries a single carriageway, and is constructed with UK-standard Y3 beams at 1m centres.

One of the aspects to be compared was the serviceability limit state bending design. In terms of actions, the BS 5400 beam is designed for a live load sagging moment of 384kNm and hogging moment of 328kNm. The Eurocodes beam is designed for a variable load characteristic sagging moment of 511kNm (383kNm frequent) and characteristic hogging moment of 387kNm (289kNm frequent).

There are two principal differences in the design process for this bridge. Firstly, using BS 5400 it is necessary to check the design both for combination 1 and for combination 3 (thermal effects included) whereas the traffic and thermal effects are checked to the Eurocodes in a single process. Although the process itself is more complex, the use of software makes it simpler overall, says Bestech.

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The second principal difference is that concrete properties such as strength at transfer, creep and shrinkage are all calculated within the program from expressions given by the code, rather than estimated and specified as data. This introduces a lot of additional of calculations, but software tools mean the only impact for the engineer is the time to review the calculations that are output.

In this case there are 420 lines of calculations for the BS design, and 550 lines for the Eurocodes design. Using the software tools, the design process is very similar and takes a similar amount of time despite the added complexity.

As the physics are independent of the design code, the SLS still controls the bending design for this type of bridge deck. However, in BS 5400 the tension limit is controlled by stress, while in the Eurocodes the tension limit is controlled by either decompression or crack width. An additional complication is the fact that a lower level of loading is applied for these checks than for the compression limit.

In both cases, traffic loading controls the design rather than thermal effects. For the BS 5400 design, 19 tendons are required and 17 for the Eurocode design. Each tendon contributes approximately 0.65MPa to the average concrete stress in this example. The difference in the number of tendons arises from the increased jacking force allowed by the Eurocodes, and from the differences in default values for creep and shrinkage suggested by BS 5400. When these parameters are matched, both designs give the same number of tendons. This suggests, as might be expected, that the BS 5400 default values are conservative.

The principal area where engineers new to the Eurocodes will find most unfamiliarity in use is in the combinations of actions using EN 1990 and EN 1991-2. "Most experienced engineers coming to this fresh will most likely spend a week getting to understand it, " says Bestech Systems managing director Barry Skinner. Even a half day of training can result in a far better understanding.

Skinner also recommends making responsible use of the embedded knowledge and training help that is available in software. Every item of data should be appraised, using on-line help which gives references to the relevant code clauses. Similarly, every line of the calculations printout which also makes use of the copious references to the code clauses--should be critically reviewed.

Bestech

www.bestech.co.uk