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Located at latitude N22.2° and longitude E114.1°, the Tsing Ma Suspension Bridge has a total span of 2,132 m and carries a dual three-lane highway on the upper level of the bridge deck and two railway tracks and two protected carriageways on the lower level within the bridge deck. The main span across the Tsing Yi Island and the Ma Wan Island is 1,377 m long, as shown in Figure 2.9, making this bridge the longest of its type. The angle between the bridge longitudinal axis and the south is 73°. The bridge was designed by Mott MacDonald of Croydon (Mott MacDonald Group, 1990; Beard, 1995).

2.5.2.1 The Bridge Towers and Piers

Two reinforced concrete bridge towers, the Tsing Yi tower and the Ma Wan tower, are located in shallow water and stand on a ship impact protection island. The two towers are of almost identical geometric and structural configuration except that the topmost portal beam in the Ma Wan tower is 150 mm higher than the counterpart in the Tsing Yi tower but the top level of both towers is the same: 206.4 m with respect to the base level (Figure 2.10). The breadth of each tower leg changes from 9 m at its top to 18 m at the base level with a constant width of 6 m.

Figure 2.9 Configuration of the Tsing Ma Bridge (unit: m) 7.000 36.000 8.000 6.000 6.000 +152.000 +107.000 10.000 +63.886 Rain Level at tower

12.000 +54.500 +2.000 0.000 PD 40.000 +5.000 Front View +191.500 Tsing Yi +191.650 Ma Wan Side View +200.859 Tsing Yi +201.009 Ma Wan 3.000 2.800 3.500 3.400 +70.000 4.200 +2.000 18.000 2.359 13.000 9.000

Figure 2.10 Configuration of the bridge towers (unit: m)

Each tower leg is designed with two rectangular hollow shafts that start from around 15 m above the base level to the topmost portal beam. The two legs in each tower are inclined towards each other at a slope of 1 in 100 and connected by four deep pre-stressed concrete portal beams. The dimensions of the beams vary from locations and they are the smallest of 7×2.8 m2 _{(height × width) at the top and the}

40 23 76.5 355.5 Ma Wan Tower SPAN = 13.77 Tsing Yi Tower 300 72 72 72 72 Z 206.4 206.4 78 X Anchorage M1 M2 Ma Wan Island T3 T2 T1 Archorage Tsing Yi Island

largest of 12×4.2 m2 _{at the bottom. The four portal beams are centrally hollow with}

a steel truss cast in the concrete enclosing a narrow corridor. The narrow corridor of the size of 2×1 m2 _{is equally constructed in each portal beam for passage}

purpose. Underneath the tower legs are two heavily RC foundation bases which are laid on suitable rock.

The two side spans on the Ma Wan side and Tsing Yi side are supported by two and three piers, respectively, as seen in Figure 2.9. The two piers supporting the Ma Wan approach span, namely piers M1 and M2, are at a distance of 355.5 m and 432 m, respectively, measured from the Ma Wan tower. For the Tsing Yi approach span, the three piers are located at an identical distance of 72 m from each other starting from the Tsing Yi tower in a sequence of piers T3, T2 and T1. All supporting piers in the side spans are reinforced concrete structures. Piers M1, T2 and T3 are free-standing piers of similar design. Pier M2 provides lateral restraint to the bridge deck against lateral loads and carries two saddles at its top above the deck. These two saddles deflect the main cables through a small angle. Pier T1 is part of the approach road and slip road structure on the Tsing Yi side. It also provides lateral restraint to the bridge deck. Piers M1, M2, T2 and T3 are hollow for most of their height whereas pier T1 is almost a solid wall. Out of the five piers, piers M2 and T1 are in different structural forms whereas the remaining piers (M1, T2 and T3) share a common structural layout except that the height of the three piers is unequal.

2.5.2.2 The Bridge Deck

The bridge deck is a hybrid steel structure consisting of Vierendeel cross-frames supported on two longitudinal trusses acting compositely with stiffened steel plates that carry the upper and lower highways. The stiffened plates acting with two longitudinal diagonally braced trusses at 26 m centre provide the vertical bending stiffness of the bridge deck. Transverse shear forces are carried by the steel plates together with the plane bracing systems that join the plates at both upper and lower flanges and span vent openings. The mixed plane bracing-plate systems enable the longitudinal trusses to provide lateral bending stiffness. At the main span and Ma Wan side span, the deck is suspended from the main cables at an 18 m interval. Near the Ma Wan and Tsing Yi bridge towers, the bridge deck changes to incorporate two additional inner longitudinal trusses to share forces with the main trusses, and the deck plates extend over the centre to cover the full width of the bridge without vent openings. The cross section of the bridge deck also changes in the Tsing Yi side span and in the area near the Ma Wan abutment, where the deck is supported by piers rather than suspenders.

The bridge deck at the main span is a suspended deck and the structural configuration is typical for every 18 m segment. Figure 2.11 illustrates the configuration of the suspended deck module consisting of mainly longitudinal trusses, cross frames, highway decks, railway tracks, and bracings. Two outer longitudinal trusses, spacing at 26 m, link up the cross frames along the bridge longitudinal axis and act as the main girders of the bridge.

Intermediate cross frames Main cross frame Longitudinal inner truss Cross bracing (top centre)

Longitudinal outer truss Orthotropic deck

plate (bottom)

Orthotropic deck plate (top)

(a) Isometric view of the deck module

Carriage

way Carriageway Carriage way Carriage way Airport Railway 13.000 13.000 13.400 7.100 41.000 18.000 2.350 13.000 7. 643 South North

(b) Main cross frame

Figure 2.11 Configuration of the deck at the main span (unit: m)

Each longitudinal truss is comprised of upper and lower chords and vertical and diagonal members. In the 18 m deck module, there are one main cross frame and four neighbouring intermediate cross frames with two on each side of the main cross frame. The five cross frames are 4.5 m apart from each other and connected by the two outer longitudinal trusses. Each cross frame (see Figure 2.11b) is comprised of upper and lower chords, inner struts, outer struts, and upper and lower inclined edge members. Through suspender units connected to the intersections of edge members of the main cross frame, this deck module is suspended to the main cable. Two pairs of sway bracings are connected from the suspension points at the main cross frame to the outer ends of the upper chords of the two adjacent intermediate cross frames to strengthen the structural stability.

Two symmetrical bays of top highway deck plates are supported by the upper chords of cross frames and longitudinal trusses. Between them is a row of central cross bracing stretching from neighbouring cross frames. Regarding the bottom deck, there are two railway tracks laid on the central bay with one row of central cross bracing and two rows of outer cross bracings added to brace the bottom chords of cross frames. Two bays of deck plates in the bottom deck on two outer sides are supported on the bottom chords of cross frames and longitudinal trusses. This deck module is symmetric to the middle vertical plane, with a width of 41 m (= 2×20.5), a lateral distance between the two suspension points at the main cross frame of 36 m (= 2×18), a height of 7.643 m and an inner clearance of 5.35 m in the middle.

The structural translational movements at the Ma Wan abutment are restrained in three translational directions. At the Tsing Yi abutment, the vertical (z-axis) and lateral (y-axis) movements of the bridge deck are restrained while the deck can move freely along the longitudinal direction (x-axis).

2.5.2.3 The Bridge Cables and Suspenders

The cable system of the Tsing Ma Bridge consists of two main cables, 95 pairs of suspender units and 95 pairs of cable bands.

The two main cables 36 m apart in the north and south are accommodated by the four saddles located at the top of the tower legs. Each main cable consists of 91 strands of parallel galvanized steel wires in the main span and 97 strands in the side spans. The number of wires per strand is 360 or 368 with the wire diameter of 5.38 mm. Each cable has a cross sectional area of 0.759 m2 _{consisting of 33,400}

wires in the main span and 0.800 m2 _{consisting of 35,224 wires in the side span.}

The main cables were formed by the traditional aerial spinning technique. The resultant cable has an overall diameter of approximately 1.1 m after compacting.

The two parallel main cables are seen as horizontal sagged cables fixed at the tower saddles and moving together with the towers and transferring the loadings into the anchorages. The Tsing Yi side span cables are seen as inclined sagged cables with the top ends fixed at the tower saddles and the lower ends fixed at the main anchorage. The Ma Wan side span cables are also inclined sagged cables with the top ends fixed at the tower saddles and with the lower ends fixed at the main anchorage through the saddles on pier M2. On each main cable, there are 19 suspender units within the Ma Wan approach span and 76 suspender units in the main span. In the Tsing Yi approach span, there are no suspender units. Each suspender unit is made of a pair of wire ropes of 76 mm diameter that passes over the cable bands on the main cables and is then attached to the chords of the bridge deck by steel sockets. There are thus four strands in each suspender unit, held apart by spacer clamps. The distance between two suspender units is 18 m along the longitudinal axis of the bridge deck.

2.5.2.4 The Bridge Anchorages and Foundations

On the Tsing Yi side, the main cables are extended from the tower saddles to the main anchorage on the ground. On the Ma Wan side, the main cables extended from the Ma Wan Tower are fixed first by pier saddles at the deck level and then by the main anchorage saddles on the ground. The two anchorages are gravity

structures which are integral with the deck abutments. At Tsing Yi the anchorage is largely below ground in a 290,000 m3 _{rock excavation. In contrast, the Ma Wan}

anchorage is only partially buried. Figure 2.12 shows the Tsing Yi anchorage. The foundations are generally simple spread footings laid on suitable rock, expect the Ma Wan tower is laid on caissons in approximately 12 metres of water.

Figure 2.12 The Tsing Yi anchorage