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Giant step for concrete

A large scale infrastructure project to ease traffic congestion west of Kuala Lumpur involves bridging a live motorway. David Bennett reports

The Greater Kuala Lumpur Traffic masterplan provides a system of ring roads and radial roads to ease traffic flow in and around the city.

However, these new roads cannot function effectively if the feeder network is unable to cope. The Western Kuala Lumpur Traffic Dispersal Scheme project, SPRINT, is designed to relieve congestion west of the city and cater for the increased commercial traffic from planned developments in the Damansara, Penchala and Sungai Buloh districts.

The Ringgit 650M (US$171M) SPRINT project provides three important links to the radial and ring road system - the Kerinchi, Damansara and Penchal links - providing 26km of six lane highway that will allow motorists to travel more quickly and safely west of the city. The 11 ramped interchanges will eliminate traffic signals and improve the dispersal of traffic at intersections with existing roads.

The project which started on site in December 1998 must be complete by December 2001. GMMJV the government appointed main contractor, has sub-let parts of the work to ensure that the project is finished on time.

Road construction is a conventional design incorporating base course and wearing course layers laid over a well prepared granular sub-base. The majority of the ramped interchanges - be they viaduct or flyover - are being built using standard post-tensioned U beams that can span up to 30m between column supports.

At interchanges 4 and 13 at Jalan Duta and Kerinchi however, the ramped viaducts have to span up to 65m to bridge the Federal Highway and North Klang Valley Expressway. The standard prestressed beam and column solution was uneconomic and just not feasible for such long spans, particularly when trying to plan the work over a live motorway.

GMMJV originally asked its consultant to consider the alternative of a composite steel box girder for the spans because it was lightweight and slim. But when the work was put out to tender it proved to be too expensive. 'Malaysia has to rely on imported steel. The exchange rate crisis of the late 1990s inflated the price by the time subcontractor Persys wanted to procure the steel for fabricating the box girder sections, ' says Afshin Forouzani of Robert Benaim & Associates (Malaysia).

Persys approached Rober t Bena im & Associates (RBA) to redesign and cost an alternative box girder viaduct in concrete. Having fixed the original design in steel, the concrete solution had to match closely the shallow depth of the steel section to maintain the headroom over the live motorway. An exact match was an impossibility - the dead weight of concrete being much too much of a handicap - but with some smart engineering analysis, one of the slimmest concrete box girder sections to be built over a motorway by balanced cantilever construction, was conceived.

'The haunch section near the supports was the same depth as the steel option, ' says Forouzani. 'In fact we managed to feather the section depth down to 3.2m at the haunches which is a lot less than 3.8m for a more conventional concrete box design'. Material quantity dictates the price of box girder construction, so the lighter you make the box the more competitive the price. The new design saved the client in excess of 20% on the original steel box girder option.

'To maintain the shallowest depth we had to use a lot more prestressing and specify a grade 55 concrete for the precast segments and carefully profile the tendon layouts to match the moment envelope, ' says Forouzani. 'We controlled the secondary moments by defining the stitching and stressing sequences very carefully and adjusting them to suit the applied load conditions.'

But that was not all. Many engineering hours were spent running design checks using specially developed in-house spreadsheet calculations for a number of load conditions. Hundreds of combinations of tendon layouts and stressing stages were investigated until the optimum solution was found. In addition to the usual live loads these bridges had to support an abnormal 430t vehicle load and this increased the complexity of the design.

The competitiveness of the design was helped by the availability of suitable precast moulds from a segmental bridge that Persys had recently completed.

The resulting concrete segmental viaduct is a single cell, splayed box section, with a soffit width of 3.3m and web walls 400mm to 500mm thick. For the maximum span of 65m the segment depth varies from 2.3m at mid-span to 3.2m at the piers, and supports either a 6.5m or 8.5m wide carriageway above it .

'One other complication in the design was the tight curvature of the viaducts.

The 93m radius of the curvature precluded the use of external prestressing and affected both the permanent and temporary works design, ' says Forouzani. 'In particular the transverse stability of the structure during the free cantilevering stage.'

This meant casting 29 unique segment types, each with a different profile and different reinforcement detail. The segments were cast in 3m long lengths and lifted into position by crane in a staged sequence to maintain the balance of the cantilevers. The segment weight was kept to below 60t to ease transportation and handling operations in the precast yard and during erection.

The two interchanges involve 1.4km of ramped box girder viaducts with spans ranging from 30m to 65m. Work over the live motorway takes place at night when traffic flow is minimal. The pier segment is erected first and aligned prior to grouting the pot bearings. A temporary steel propping structure on one side of the pier stabilises the cantilever during erection.

The first segment is placed on one side of the pier support on the temporary props. The segment is stabilised by tie-down prestressing bars that clamp the segment to the props. Subsequent match cast segments are then positioned alternately, one on each side, to maintain the equilibrium of the cantilever. As each new segment is positioned it is glued to the adjacent one with epoxy resin and held in place by temporary steel prestressing rods.

The permanent prestressing cables are fed through the ducts in the segment box and stressed after two pairs of balancing segments have been erected. The temporary prestressing rods are then removed. When the span is nearly complete, the gap between the cantilever arms is bridged with an insitu concrete stitch before the continuity prestress is applied across the entire span.

'To complete a 65m span for example, it will take two weeks working through the night. All the stressing operations are carried out during the day while erection of the segments over live roadways is done by night, ' says Forouzani.

'Such a span will require two pier segments and 20 span segments to complete it. The design allows cantilevers to be erected independently of one another, so that work can be progressed concurrently by the contractor to maintain construction speed.'

Four of the interchange ramps have six spans and one has seven, the overall length varying between 256m and 330m. To ensure a smooth ride for vehicles over the viaducts, the segmental ramps have been designed as a continuous structure with expansion joints only at the abutments. This arrangement will also help minimise maintenance work.

Persys, in association with RBA, was responsible for the box girder superstructure of two interchanges, while other contractors were responsible for building the Y shaped pier supports and the bored piled foundations for the interchange ramps.

Project details

Client: Sistem Penyuraian Trafik KL

Main contractor : GMMJV/ Gamuda

Main consultant: Zaidun-Leeng

Segmental bridge ramp contractor: Persys

Segmental bridge consultants: Robert Benaim & Associates with Lan Konsult

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