Like any epic journey, turning Birmingham’s 1960s reinforced concrete railway station into a futuristic transport hub, would be full of twists and turns. “We were running four design programmes in parallel, all at different stages and each affecting the building and that was quite complex,” says Stephen Ashton, Atkins’ engineering director on the £750 million project.
Starting in February 2008 Atkins’ original contract was for detailed design of the station redevelopment for client Network Rail. This would open up the dark and gloomy underground platform complex and increase passenger capacity in the station through the creation of a new concourse, improve vertical access to the platforms which sit below ground level, and deliver an enlarged station building. All of which was to be enveloped in an iconic futuristic roof structure.
The glass roof had been replaced with a transparent polymer called ETFE (Ethylene Tetrafluoroethylene) which was to sit on a 30m long structural steel arched trusses. “We had this whole new loading system coming into the building with new roof structure and cladding along combined with the removal of the central part of the existing reinforced concrete frame. This means we are changing how the building moves, how loads were being applied and therefore we needed to analyse the building to ensure that the end product was stable,” says Ashton. From this the Global Stability Analysis (GSA) tool was born. This digital model enabled the team to mimic the impact of local design changes across the whole structure, running computational analysis as major changes were made.
Ashton says that throughout the project there were six key construction stages where the model was used as a construction tool to ensure stability through the build and advice on temporary works design. In the case of the new roof the team sought to ensure that loading was carried vertically through the existing reinforced concrete columns.
“As a result of all these loading changes we ended up strengthening 52 columns with concrete jackets of 100-200mm thick depending on the loads,” says Ashton.
Even more challenging was that as contractors started onsite it became apparent that the limited available as-built drawings did not accurately reflect the true condition of the building. “We had requested intrusive surveys for the start of the detailed design but the client was not able to undertake a significant number of these because it would mean closing parts of the station and Pallasades Shopping Centre which could not be done. So the decision was made to undertake those intrusive surveys during construction,” says Ashton.
This meant making a lot of design assumptions about existing building from the size of columns to percentage of reinforcement. Although the team made conservative estimates to be sure of the structural integrity they also assumed that the building was in relatively good condition which unfortunately turned out not to be the case. In some areas poor construction work on the 1960s building meant that the concrete contained a lot of voids, various concrete elements had deteriorated and rebar became exposed. Original movement joints for the building were not where the plans said they were.