Motorola Interchange, Malaysia
	Designer:		BBR Systems Ltd.
	PT Contractor:		BBR CS (M)
	Years of Execution:		1997 to 1998
	Main Span:		85 m
	Construction Method:		Cantilever
	Cable System:		BBR DINA

Introduction

Outside Kuala Lumpur in Malaysia, the Lebuh Raya Damansara – Puchong (in short LDP) project is a 40-kilometre 6 lane free-flow expressway stretching from Sri Damansara in the north to Putra Jaya in the south. It is the western link of the Middle Ring Road II (MRR II), part of the Greater Kuala Lumpur Masterplan, which has been designed to improve traffic movement by providing a ring road with fourteen multi-level interchanges, allowing traffic to bypass busy city centre streets.

This requirement for multi-level interchanges led to the design of a new bridge over the existing bridge at the Motorola Interchange and in the same North-South direction. Many bridge types were considered in the evaluation phase before construction of this "bridge over the bridge" could start, but all were rejected because the traffic flow on the lower bridge was not allowed to be interrupted during construction and the existing number of traffic lanes had to be maintained.

Thus a Stay Cable Type bridge emerged as the best alternative combining not only the advantage of its construction method but also imposing, due to is sleek lines, a characteristic mark on the local landscape.

Geometry

The northern and southern approaches to the Stay Cable bridge consisted of 2 ramps built with prefabricated beams. The LDP Cable Stayed Bridge has one main span of 85m and two side spans of 36.125m each. Both pylons were made of concrete with a total height of 34.2 m each, of which 24.2m are above the deck level and 10.2m below. The bridge has been designed basing on the loading as specified in the British Standard BS 5400: Part 2: 1978.


Foundations

The design of the foundations gave some problems because the core sampling showed a sharp transition of the geology at foundation level. It was found that a line passing practically between the legs of the southern pylon separated the limestone with big cavities up to a depth of 50m with good quality granite.
In addition to the difficulties imposed by the soil at foundation level, there was an underground cycle tunnel crossing the bridge axis, as well as ducts carrying water and telephone lines.

All these external parameters could only be dealt by designing a foundation on piles, thus requiring unavoidable traffic interruptions on the lower bridge during the period when the pile boring machine was used.

The piles used were:
  •   Micropiles of 270 mm dia., for the left leg of the southern Pylon, and
  •   Bored piles of 1.20m and 1.50m dia., for the right leg of the southern Pylon

In order to avoid possible buckling of the casing in the area of the cavities and to reduce to a minimum the settlement of the existing abutment, as a precaution, additional boreholes were drilled to grout up the cavities.

For the foundation of the northern pylon, since works were considerably delayed with the problems encountered on the southern side and due to the solid granite found here, it was decided to install steel H-piles.

The pile caps of both legs of each pylon were connected with a post-tensioned concrete tie-beam sized 1.2 X 1.2m to counteract the axial force developed due to the inclined legs of the pylon.

The supports at each end of the side spans were placed on transition piers, which carried on one side the reaction forces at the end span of the stay cable bridge and on the other the reaction forces of the approach ramp. 8 high tensile MACALLOY bars with dia. 40mm each, transferred the tensile forces of the end span through the transition pier to the ground. In order to counteract the high horizontal splitting forces introduced by these vertical tensile bars, the diaphragms on both ends of the stay cable bridge were made of concrete and horizontally post-tensioned with 6 BBR CONA 1905 tendons (i.e 19 strands dia.0.5") placed on the top and bottom of the diaphragm.

Pylons

The legs of the Pylons were inclined approx.12° to the vertical until the level of the cross beam, which was situated at a height of 24.7m. After this level they went up vertically for the last 9.5m. Their construction was executed by the climb form system from DOKA. An important part in the construction of the superstructure played a multipurpose temporary portal frame made of steel. It acted as:

• Stabilisation for both pylon legs during construction before reaching the cross beam level.
  
  
• Supporting the cross beam before final positioning.
  
  
• Supporting the formwork traveller for the first construction stage.
  
  
• Stabilising the deck against side sway during construction by connecting with vertical shear pins the lower part of the frame with the deck slab through prepared openings.

Stay Cables

The anchoring of the 20 BBR DINA stay cables on top of each pylon was accomplished by placing steel inserts into it, which were afterwards concreted with the top portion of the pylon. In order to save weight these steel inserts were designed to work together with the concrete forming a composite structure, therefore they were equipped with shear connectors.

The BBR DINA stay cables were shop manufactured with 73 to 163 wires. They were delivered to site ready assembled on reels in order to permit immediate installation. The time required for the installation of each stay was very short, between 2 to 3 hours. Their capacity was between 5'000 kN and 10'000 kN. Grouting was performed on site in ducts coloured in bright grey. Stressing was performed from the deck slab.

Superstructure

The four lane traffic deck, made of concrete, has a total width of 18.7m and consists of two longitudinal edge beams sized 1.60m wide by 1.40m high, and the deck slab in-between with a width of 15.5m and a thickness of 0.26m. Between the edge beams there are transverse steel diaphragms spaced every 4.25m, which have also been equipped with shear connectors on both ends as well as on the top flange in order to act as composite members.

In order to accommodate the conflicting demands for a cost effective design combined with an extended durability, the edge beams were longitudinally post-tensioned. At every construction joint of the edge beams the 10 longitudinal tendons placed there, were coupled and stressed. There were in total 8 tendons of type BBR CONA 1206 (i.e 12 strands dia.0.6") and 2 tendons BBR CONA 706 (i.e 7 strands dia. 0.6").

For the construction of the deck, besides the already mentioned portal truss frame, a horizontal formwork traveller attached by means of vertical steel bars from the portal truss was required. In order to avoid placing additional temporary stay cables in front of each edge beam element, the construction sequence was organised in such a way that casting of the edge beams was always one stage in advance of the casting of the deck slab between them.

Starting from each pylon the casting proceeded advancing symmetrically. Before casting the closing pour between the two bridge halves, the end span element on top of the transition pier was cast.

Bearings

A particular feature of this bridge is that there are no bearings under the superstructure in the pylon area. All longitudinal and transverse forces are supported from bearings placed only on the transition piers, at the ends of the side spans. This bearing layout has the pleasant side effect, that a person driving his car on the lower bridge has the feeling that the bridge above him is literally suspended in the air. After completion of the pavement and the expansion joints the Motorola Stay Cable Bridge is scheduled to open for traffic at the end of 1998.