18 March 2003
The building now arriving at Gate 42 ….
Simon Penny of Buro Happold looks at the operational benefits and other cost savings associated with pre assembly buildings.
In principle, pre-assembled buildings can be applied to all construction projects, but the greatest benefits are achieved in busy, congested and high revenue sites such as railway stations, airports, motorways and shopping malls. Indeed the railway industry has been using bridge moving techniques for decades to avoid disruption to railway routes. These principles are now spreading to the aviation, road and retail industries as the costs of disruption escalate.
Quite simply, in a transport as in any other environment, it is to allow the main business of the operator to continue as normally as possible so as not to damage the revenue stream this provides. Pre-assembly allows more of a building to be built away from the airport, station, shopping mall or motorway, hence not disrupting the passenger, shopper or driver.
Additional benefits include the reduction of so-called ‘dirty costs’. These are the direct costs to the project associated with any construction environment, such as security costs, local transport costs and delay costs. Other benefits accrue too, such as better quality control and more programme predictability.
It is money (or the saving of) which makes it possible to pre-assemble. Nowhere is this more critical than in an airport environment. The costs of building ‘airside’ are high – typically double the cost of building in a city centre location. Hence, within reason, any effort to pre-assemble off airport will show savings to the project.
The items to be pre-assembled have to be identified very early on in the design process. To be transported by road places a severe constraint on module size to such an extent that if a building is not designed around this constraint from the start, the pre-assembly opportunities are substantially reduced.
Obviously a pre-assembly facility is required. This can either be completely remote from the airport (for instance to take advantage of cheaper labour costs) or on the airport perimeter. This suffers some of the ‘dirty costs’ mentioned above, but has the big advantage that the severe road transport size constraints can be avoided.
The final but most important factor in making it possible, is a client and project team interested and committed to maximising pre-assembly opportunities. It requires considerable effort throughout the design process to realise the goal of constructing a building away from its final location.
Among many pre-assembly features incorporated on this project, two stand out: a passenger link bridge and a series of pre-fabricated toilet modules.
A passenger link bridge, 50m long by 10m high, was required to connect the new gate lounge to the end of the existing pier. To reduce the ‘dirty costs’ - in this case mainly the cost of night working over a busy airside road - the project team decided to pre-assemble the bridge on the airport perimeter. This allowed the bridge to be built during the day, releasing an aircraft stand at the final bridge location for its intended use for six extra months and keeping the road fully operational throughout. Once the bridge was fully built and fitted out, it was transported a mile across the runway and taxiways to its final location. The lighting within the bridge was operational an hour after installation. Even with the expense of hiring the transporter vehicles, a saving of £250,000 was made to the project cost.
As part of the same project the structural and services design was altered to allow for the installation of fourteen toilet modules (4.1m x 6.9m x 3m). These had been completely fabricated and fitted out in Yorkshire, some 200 miles from their final location, and were delivered to the project and lifted into position in two days. A particular design feature, common to all pre-assembled projects, is the issue of tolerances and positioning. To assist this and to spread the point loads from the modules, steel ‘skids’ or running beams were cast into the floor plate. These were set out very accurately and allowed the modules to be positioned less accurately (at night in the rain).
Both the above examples were completed successfully and, with other smaller initiatives, meant that over 50% of the assembly happened away from the project site.
As a result of increased security awareness over recent years all piers taking passengers from terminal buildings to aircraft loading points are required to physically segregate arriving from departing passengers. For new piers this is fairly straightforward. However, to upgrade existing piers to a fully segregated status is far more difficult. To achieve full segregation another level of passenger route or corridor has to be provided, typically on the roof of the existing pier.
At an early stage in the project it was decided that a principal aim would be to ‘chunk’ the new corridor small enough so that it could be fabricated and transported by road. This restricted a ‘chunk’ to approximately 4m x 3.5m x 7m. One such application for the modular corridor was a project requiring nearly 300m (or 75 modules) of corridor. The complexities of weather proofing 75 joints between corridors 12m above the ground was one of the issues to be addressed.
The solution adopted was to separate the corridor into 20m long ‘base’ units with five 4m ‘top-hat’ units fabricated and placed on top, either at the site or at a sub-assembly facility at the airport. A production facility was set up remote from the airports to assemble the corridor modules in a dry, controlled environment away from the ‘dirty costs’ at the airside location.
Both the structural and services designs had to recognise the modularisation of the corridor. The structural design had to incorporate temporary bracing to keep the ‘top–hat’ units rigid until they were in their final position as well as resolving several tolerance/positioning issues. The services design provided for a 4m services module which could be pre-installed at the production facility and connected to its neighbour on the pier. In this way only the travelator machinery is installed on site with all other items such as carpet, buggy rails and lighting being pre-assembled.
As part of the segregation project a vertical circulation core had to be provided at each aircraft gate to get passengers up to the corridor level. Although this core unit started with the aim of full off-site fabrication, this was not achieved due to site constraints. The new cores had to ‘wrap around’ an existing smaller building and meant that tolerance and positioning issues became more difficult to resolve satisfactorily. It was decided to revert to a more traditional ‘in-situ’ build sequence for this element. It has been mentioned here to show that in many cases the benefits of pre-assembly do not outweigh the costs and disadvantages. Pre-assembly is a means to an end, not an end in itself.
As part of several clients’ ongoing development plans, Buro Happold is looking at further projects, yet to be realised, which will utilise more and more pre-assembly. However, a particular constraint is cranage of pre-assembled components. A combination of weight and reach means modules quickly become more expensive to crane into position as the size increases. To address this, we are looking at the use of composite plastics for the module structure to reduce the dead load of modules requiring moving. There could be a trend here towards aircraft design where every component is specifically engineered for minimum weight. The link bridge mentioned earlier had an aluminium floor system specified partly to reduce weight but also to eliminate an insitu concrete floor which would have been labour intensive and intolerant to being transported.
Another project has involved investigating moving a new sports grandstand weighing 4500 tonnes over 50m. To build this insitu would disrupt premier sporting events so, despite the costs of transportation involved, the savings in maintaining the existing revenue stream could make the off-site manufacture option attractive.
There is an increasing trend industry wide towards off-site manufacture and assembly of buildings. The advantages are even more obvious in an airport or similar environment as the costs associated with working in these areas consistently increase.
Although relatively commonplace in the rail industry, there is an increase in the use of these techniques in aviation and road transport as congestion increases. Looking to the future the retail and communications industries may look to these advantages as they review the damage to their business caused by building insitu.
Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976 offering civil and structural engineering, mechanical and electrical engineering, quantity surveying, building services and environmental engineering, health and safety management, infrastructure and traffic engineering, ground engineering, façade engineering, fire engineering, computational fluid dynamics analysis, disability design consultancy, project management, urban design and a range of specialist CAD services.
