But traffic jams have become a common occurrance on these two bridges during rush-hours, so the third Macau-Taipa Bridge, which opened last month, will make it easier to travel between the two.

There only two fixed links between Macau and the district of Zhujiang River Delta - one is at Custom Office in the north of Macau, which connects to the mainland, the other is the Lotus Bridge which connects to the Beijing-Zhuhai Highway and 105 national highway. This is restricting the development of Macau's economy, so the Chinese government is planning to build a railway network to connect some of the main cities of the Delta district, Macau and Hong Kong. The third Macau-Taipa Bridge which has been designed to carry road and rail traffic, will eventually form part of this network.

The new bridge is located at the Radar Channel - the centre of which has been relocated 250m further south through dredging. The navigation clearance is 28m high on a 150m-long main navigable span.

The bridge is located in the fault region of the Zhujiang Delta, and there is a probability of more than 10% that an earthquake with a dynamic force peak value of 0.1g will take place within the next 50 years.

Soil conditions at the site can be divided into three layers which each have different compaction characteristics and plastic states. The top layer, which is 8-10m thick, consists of mud, flowing plastic clay and loose sand; the middle 6m-thick layer is soft plastic clay or low-density sand, and the lower layer is mid-dense to dense sand and hard plastic or stiff clay. This layer ranges in thickness from 10m to 40m. The underlying granite bedrock can be found at 22m to 61m depth, dipping from north to south.

The bridge has three vehicle traffic lanes in each direction on the upper deck and two lanes in each direction on the lower deck, which is intended to be converted into double-track light rail at a later date.

Two water service pipes with a diameter of 800mm and seven-layer cable ducts 600mm wide are also included on the lower deck. The main bridge is a prestressed concrete cable-stayed bridge with a main span of 180m and two side spans each 110m. The prestressed concrete box girder approach bridges consist of a total of 22 spans, each 60m long, and with a constant girder depth.

Vertical navigation clearance for the main bridge is 28m at high tide, with a navigation channel of at least 150m wide. The bridge is designed to accommodate vessels of 4000DWT with a velocity of 8m/s. Special impact protection has been designed to protect the piers on both sides of the navigation channel.

In order to simplify construction and provide reasonable transverse force for the section, each bridge is designed as a single cell box girder. Each 13m-wide upper deck carries three 3.5m-wide traffic lanes, along with a 1m-wide pedestrian walkway: the lower deck, which is 8m wide, is designed to ultimately carry a single traffic lane and a 4m-wide light rail track. A gap of 3.1m is left between the two boxes, in order to allow the installation of the pylon for the main span. However, in order to provide sufficient room to anchor the diagonal cable, the anchorage zone is enlarged, at which point the bridge is almost 16m wide.

The main bridge is a cable-stayed concrete structure with its cables in a parallel plane, harp arrangement. Each side span has a longitudinal gradient of 5%, connected in the middle at a vertical curve of radius 3500m. The main girder of the deck is concrete, is 6.13m deep and is suspended from cables anchored at 10m intervals.

The side spans are supported on vertical movable bearings, while vertical and lateral bearings have been installed between the girder and the towers. The longitudinal movement of the cable-stayed bridge is restrained by a longitudinal 'floating' system which is designed on a number of pretexts. Because of the short span length, the horizontal movement that a seismic event is expected to generate would be 150mm at its maximum - movement which can be easily absorbed by the expansion joints at the ends of the spans. So the floating system allows the two towers to absorb horizontal seismic force between them, with the result that none of the piers is subject to excessive seismic force. This system can also reduce the thermal forces caused by temperature fluctuation.

There is no elastic tendon as is commonly used in cable-stayed bridges, due to lack of diaphragm plate in the box girder.

The box girder of the main bridge is designed with a thin-walled section 6.13m deep, with 1% crossfall on the bridge deck. To improve the strength of the box girder, the bottom plate is 1.4m thick at the bearing point. Accordingly, vertical diaphragms are arranged within the web plate. On the underneath of the top plate, there will be a cross-strut to prevent warp and distortion stress of the box girder. The diaphragm is 2m thick at the bearing point on the tower pier and decreases to 1.2m at end support and cable anchorage positions.

To resist the tensile stress imposed by the traffic loads on the upper and lower decks, the top and bottom plates of the box girder are reinforced by lateral prestressing steel strands. And to minimise the main tensile stress on the box girder, the web plates are strengthened by vertical prestressing bars and tendons along the bridge.The anchorages of lateral prestressing steel strands, vertical bars and longitudinal tendons are located where the web plates and bottom plates of the box girder joint.

There is a risk that the prestressing ducts might weaken the box section; to prevent this, outside the bottom and web plates of the box girder there is a U-shaped 'brim' where ducts can be arranged for prestressing lateral steel strands across the bottom plates. The stay cable is anchored in the block beneath the web plates of the main girder.

Because traffic is also carried within the box girder, ventilation, fire protection and fire-fighting facilities must be provided inside the box. Vertical headroom is restricted, hence full use must be made of the lateral space. The width of the bottom slab is so sensitive to the forces on the box girder that it has had to be minimised as far as possible to provide sufficient headroom. Therefore the box girder adopts a tilted web plate which will simplify transmission of cable forces to allow for better distribution.

The height/span ratio of the bridge is 1/29.4, demonstrating the heaviness of the structure. Outside the box girder, a 'brim plate' is designed for the purpose of improving stratification, and in the centre of the web plate near the neutral axle, 0.8m-diameter openings are arranged every 5m along the bridge in order to ventilate air and smoke from the tunnel.

The main girder is rigid enough, but by comparison, the main towers are a little soft. From the point of view of its structural forces, the bridge can be defined as a partial cable-stayed bridge, on which the cable force is only used to improve the stress condition of the main girder. For this particular bridge, a range of cable spacings at 6m, 7m, 8m, 10m and 12m has been calculated, all of which satisfy the requirements. The chosen cable spacing of 10m has been adopted for a number of reasons.

Firstly, the wider spacing reduces the number of segments, resulting in a shorter construction period and hence quicker progress. Having a wider spacing also provides a clearer view and improved visual effects for motorist. It gives benefits in terms of wind resistance, and allows the cables to work to their full capacity, making the structure more efficient.

Making the spacing too large would have required the construction of large segment, which would have made construction more difficult, and more expensive. There are 96 cables in total.

The concrete cable tower rises 85m above the pile cap and 48m from the bridge deck. It is formed of three columns, forming an 'M' shape; two columns stand beside the girder and the middle column stands between the two boxes, symbolising the first letter of Macau. Four cross-beams complete the tower structure - two upper and two lower beams.

Construction of the new bridge began in October 2002 and was completed in January of this year.

Owner: Macau Government GDI (Gabinete para o Desenvolvimento de Infrastructures)

Contractor: CTMB joint venture (Chon Tit (Macau) Investment & Development Company; Zhongtie Major Bridge Engineering Group; Major Bridge Reconnaissance & Design Institute)

Consultant: Pengest International/Civil Engineering Consultants Company

Quality control: Civil Engineering Laboratory of Macau

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