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Works on bridge structural beams gaining momentum

Wednesday, June 20 2018, Kuching: With construction works gaining momentum at all 11 works packages of Phase 1 since the Pan Borneo Highway Sarawak project started in 2015, it is worth noting that works on the bridge structural beams have gained significant foothold too.

This is an important milestone in road and bridge construction works as it proclaims the culmination of the messy and time consuming sub-structure works at the bridge locations, enabling the commencement of the superstructure i.e. the top road surface of the bridge, further paving the way for continuity of the road works at both ends of the bridge.

The structural bridge work is formed with a composition of cement, stones and sand, otherwise called concrete, and steel. Absolutely no soil and earth involved.

This combination of ingredients has a minimum specified time requirement to attain strength before any further load is imposed on it.

In this aspect, “time elapse” inherently becomes obligatory to keep bridge works apart from the general road works.

In the latter’s case, earth, stone and bitumen materials are involved with hardly any consideration for time to gain strength.

As such, it is quite understandable that with 115 bridges involved in Phase 1 of the Pan Borneo Highway Sarawak project, where most of them to be built in duels (one bridge each for a single carriageway), the critical most segment is bridge works. Thus, the beams launching is an important event.

An example of bridge works actively underway is at the Sungai Kua Bridge-Sungai Arip Bridge works package (WPC08). The works package had recently completed launching of beams for the 3-span Sg Basai bridge and at the 3-span Sg Kua Bridge. Works are on way to launch beams for the 3-span Sg Duat bridge.

Beams launching activity is currently in vogue across all works packages, with launching works almost completed at all 4 bridges in the Telok Melano-Sematan (WPC01) works package.

Beams are long linear structural components, required in a bridge to help span and allow a freeway underneath, specifically, to give way for water flow of river, vehicle flow or various other movements underneath.

These beams, usually long enough to cater to the span requirement, cannot be cast in their place of final seating for various constraints - firstly, due to water flow deep below, height of the bridge, and most of all due to the longer time constrains required for the concrete to attain strength.

In order to avoid these constraints and gain advantage of not having to wait for strength-gaining of concrete while in place, these beams are cast at a place other than the bridge itself.

Beams are usually cast in a yard, and sufficiently in advance to allow it to gain strength before the bridge substructure is ready to receive them.

From a structural adequacy point of view, to withstand higher loads eventually for larger span widths, these beams are additionally arranged with high tensile steel wires along the length, embedded within hollow tubes pre-embedded within the concrete mass to a designed pattern.

It is after the concrete has gained the requisite strength that these steel wires will be pulled by jacks (called pre-stressing) and released back to give a horizontal compression thrust to the concrete, thus enabling the beam to take extra loads.

It is also advised by consultants that after this pre-stressing, the beams should be allowed to wait in place for an additional 60 days to allow for shrinkage and creep to occur, giving further reasons that beams have to be cast beforehand and separately in a yard.

Since these beams are cast in yard, the next challenge comes in transporting, handling and launching the beams at the final bridge location.

Such activities require perfect experience and technical skills. A little flaw at any step can crack a beam and render it useless, causing the whole process to get jammed up for want of a single new beam or two.

As such, a perfectly designed method statement is always deemed mandatory. This statement contains minute details of extreme significance, such as mechanism of lifting the beam at the yard, one beam at a time, condition and load capacity of lifting cranes, transportation mode, condition of access approach to yard, relative positioning of the cranes and trailers (containing the brought in beam) at the launching position, radius of swing and load capacity of the cranes.

All these are done by agencies experienced enough to handle the whole process of ensuring the bridge beams are launched perfectly in place.