S. Withycombe, Halcrow Group Ltd, Swindon, UK
Firstly I would like to congratulate the authors on their excellent paper describing what is clearly a well engineered and successful project. I am curious though to learn more about the early development stages and the issues that were faced in changing from the illustrative design for a concrete box girder, to the alternative composite solution. For works procured through design, build, finance and operate (DBFO) or design and build (D&B) methods, constraints imposed by planning and approvals granted prior to tender based on the illustrative design are often a major barrier to being able to offer alternative and better value solutions at tender stage. The paper makes brief mention of the pier positions being fixed by the constrained pareto optimality (CPO), but little else on this aspect. I wonder if the authors would be able to expand on the issues they faced and the process they went through in changing from the illustrative design, in particular the change from a concrete to a steel composite deck, and how this affected the tender costs and programme, and risks for the project team.
Author's reply
We welcome Stuart Withycombe's question and recall serious consideration of the constraints to which he refers during the tender stage. While the initial tender information indicated that all previous conceptions of the bridge had been using concrete, and strict criteria were imposed regarding the bridge form based on this, the tender documents and briefings were clear in that the use of steel was not prohibited. It would, however, be understandable for bidders to interpret the controlling criteria as eliminating steel as an option that might attract the interest of the client. Despite this, early options and value engineering workshops convinced our team that steel–concrete composite was right for this site with the stated employer's requirements. Key technical influences were
the highway vertical geometry with steep approach gradients needed to achieve headroom clearance over the swale. Steel offered more favourable span/depth ratios
the (almost) linearly variable spans and correspondingly variable structural depth with little repetition appeared to us to attract the flexibility offered by modern automated steel fabrication processes
use of steel with permanent formwork and precast parapets maximised opportunity to minimise wet trade activities on site when it was clear that the site programme would require at least two winters.
Despite the unanimous support within the team there remained an underlying concern that other bidders might promote a more conforming option and so parallel exercises without the main team were conducted to ensure that opportunity was not being lost.
The tender process also required regular scrutiny by the client's team of the development of designs for tender and an open approach was adopted throughout to ensure that the proposals would not be rejected for technical reasons. We recall discussing quite intricate details of the steel designs with the client from an early date, but no outright rejection of the principle of the use of steel was ever made. We took care to explain why we thought ours was the right solution (including the influences above) and it seems that the information provided must have helped to develop confidence within the client's team.
Note: with regard to the constantly varying span/depth (i.e. tapering elevation referred to in (b) above) we would be interested to hear of any reader's knowledge of such an arrangement being adopted elsewhere. More commonly long estuarial viaducts tend to have short end spans and constant length approach spans with longer ‘anchor’ spans either side of the longest main span. Also would Stuart Withycombe care to express a view as to how a tapering structure of similar spans might best be accomplished with a concrete deck?
