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A new approach

Contractors and sign manufacturers could soon be invited to start building a new generation of elegant, easy to maintain and install motorway gantry. The following pages explain how the new designs have emerged and look at the results of an ideas search launched by the Highways Agency in February and March.

Over the last few months motorway gantry design has undergone a major rethink. The Highways Agency is this month publishing a new standard for its next generation of signing structures and has drawn on engineering and architectural expertise to make designs more efficient and aesthetically pleasing.

Driving the change is the fact that the Agency wants to erect more gantries around the motorway network as it simplifies signing and extends the use of variable speed limits and other techniques to make more efficient use of trunk roads.

Such a large increase in the number of gantries has focused the Agency's thoughts on the appearance and cost of building and maintaining these structures. Traditionally gantries have been designed to carry just about anything from directional and electronic signs to speed cameras and maintenance personnel. They also have to be capable of withstanding high impacts from errant lorries without collapsing on the live motorway.

As a result the structures are heavy and have been criticised for their ugliness. They are also cumbersome to erect and around £230,000 of the average £400,000 cost of one of these structures is attributable to delays to traffic resulting from lane closures during installation particularly of the foundations.

One reason why the Agency is now moving away from such heavy designs is that it has come to realise that individual gantries often carry too much information. Research carried out for the Agency by consultant Mouchel in 1995 shows that motorists faced with combinations of directional and electronic signing cannot take in all the information.

As a result the Agency now wants to separate directional signs from signalling information and variable speed limit messages. This and other revisions of existing design standard BD51/94 should allow the development of quicker to erect, more aesthetically pleasing and easier to maintain structures.

Further impetus for a design rethink has come from abroad, where the Agency has studied the more elegant, lighter gantry structures spanning motorways in France, Germany and the Netherlands.

The design rethink gained impetus in February when Mouchel was appointed to carry out an ideas search for new designs among architects and consulting engineers. 'We didn't want people who knew everything about gantries,' says Jon Wallsgrove the architect/planner in the Agency's Structures Design Group. 'We wanted fresh ideas.'

The ideas search concept was to encourage innovative ideas, either to implement on the M25 or to develop for the future depending on the designs produced.

The Highways Agency and Mouchel chose four teams - two led by architects and two led by civil engineering consultants - to work up new design ideas. They were:

Yee Associates plus Brown & Root

John McAslan & Partners plus Antony Hunt Associates

MSP-Maunsell plus Norman Foster & Partners and Pearce Traffic

Symonds Travers Morgan plus Atelier One and David Marks Julia Barfield.

Team members were paid for five days work spread over the three weeks following their appointment in late February. At the end of March proposals were presented to a panel of four assessors, two from the Highways Agency and two from Mouchel. The assessors judged designs on their appearance, functionality, adaptability, maintenance, installation, foundations and costs.

All of the teams had to produce designs for a single gantry spanning four lanes of motorway. This would be capable of carrying an electronic message board and controlled motorway indicators plus speed cameras and maintenance workers.

The Agency also sought proposals for advanced directional sign gantries to span a single four lane carriageway or a dual four lane motorway. Teams were also given the option of designing cantilevered motorway signal supports and were also invited to produce their own 'blue sky' gantry innovations.

To help them the Agency supplied the teams with a draft of the new gantry standard BD51/98. This contained several key modifications. The most significant of these was a clarification of lower impact loading requirements for roadside gantry supports if they are set back from the edge of the carriageway. The new standard also permits reduced load bearing and stiffness where there are only signs and no walkway.

Impact requirements in existing standard BD51/94 are one of the main reasons for prevailing heavy designs. The current standard assumes that a collision will impose two sets of parallel forces on a gantry support, one at 750mm above road level and one at between 1m and 3m above road level. These assumptions have constrained designs in the past, resulting in designs described by Mouchel as 'massive' and 'intrusive'.

Under the new draft standard gantry supports can be moved away from the edge of motorways, space permitting. This in turn should reduce the need for high impact loadings creating opportunities to improve gantry appearance.

The Agency was encouraged by the proposals developed by the four teams, all of which produced much lighter structures. 'The Ideas Search has produced some innovative and useful ideas which, if implemented, will substantially reduce the costs of gantries in terms of materials and fabrication costs, erection costs and the cost of disruption caused by road closures during erection. Furthermore they will result in a radical improvement in the appearance of gantries in this country,' says Mouchel's final report.

The judges said that all four designs had unique strengths. They also suggested that three of the design teams link up with contractors to prepare their ideas for immediate construction. The most

forward looking proposal - produced by the Symonds Travers Morgan team was not considered ready for construction but won praise for its efforts to push the boundaries of advanced electronic

message technology and foundation design.

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