Photo: Rear end of the U-beam found laying on a completed span while the front end of the U-beam had sunk into the river
A bridge with 8 spans where each span length measured 37.5 meter is under construction. Each span is able to fit in 3 units of precast U-beam and previously 6 units of precast U-beam had been successfully installed on the piers. The laying of the precast U-beams is by self-launching beam launcher with a capacity of 160 tonnes and consisting of 3 main components- 2 units of A-truss frame structure, 8 units of support leg and 2 units of overhead travelling crane with a lifting capacity of 100 tonne/ winch. Normally, the precast U-beams with the size of 37.5 m x 2.5 m x 1.25 m weighing 180 tonne will be casted at a nearby casting yard and once completed, the U-beams will be transported to the bridge site by a prime mover.Then both rear end and front end of each U-beam will be hooked onto the overhead travelling cranes. On the day of the accident, while transporting a U-beam towards the next span, the front-end of the U-beam which was hooked on to one of the overhead travelling cranes slowly dropped. The front-end of the U-beam then entirely sunk into the river while the rear end dropped on the completed span and hit a steel plate measuring 4’ x 6’ x 1” causing it to flipped towards 3 nearby workers. One worker died after directly hit by the steel plate while the other 2 workers were badly injured. Investigation shows that the brake system which controls the movement of the wire rope drum and a ratchet used as an extra safety device to prevent the downward movement of the drum can be the 2 main factors contributing to the accident.
Below are several recommended safety measures to be taken by relevant parties while using the self launching beam launcher for a bridge construction :-
- Provide a systematic inspection and maintenance plan to ensure the self-launching beam launcher is always in a good and proper condition especially on the components of the overhead travelling crane which mainly functioned during beam uplifting and transporting to the next span
- Prepare a comprehensive risk assessment document (HIRARC) for bridge construction works especially in the operation of the self-launching beam launcher and immediately formulate a suitable safe operating procedure (SOP) and conduct special training sessions for all persons involved in the operation of the self-launching beam launcher
- Main components such as brake pad, main rope sheave, wire rope, gear box and the hydraulic system must be tested and thoroughly inspected and examined to detect any serious physical defects which are able to reduce the brake and gear capacity and affects the safe movement of the wire rope. The contractor shall not operate the beam launcher until all the defective components are totally replaced with new parts.
- Electrical system such as control panel button and electric cables connecting the control panel to its mechanical parts such as overhead travelling cranes and trolleys are always thoroughly inspected to ensure its good condition before carrying out any beam lifting and beam transportation towards the next span
- The personnel assigned to control the movement of the beam launcher must be adequately trained and highly experienced especially in communication skills and techniques.
- The ratchet which is said to function as a stopper to prevent the drum movement and also act as a additional safety feature in the event of brake failure while transporting the beam towards the next span must be confirmed and inspected in terms of its straightness and its ability to be fitted properly into the drum sprocket slot.[Refer to the original manufacturer on the actual function of the ratchet, whether its intended purpose is for stopping the drum during beam transportation or it is actually used strictly for maintenance purposes only.
- The braking capacity from both external gripper and internal gripper acting simultaneously to stop the drum movement has been significantly reduced by totally deactivating the internal gripping system and causing the brake to be heavily dependent on the external gripper to stop the drum movement.
- In order to legally operate the overhead travelling cranes and carrying out the bridge construction activities, the persons involved must obtain a valid certificate of fitness for both cranes, approved design of the beam launcher system and a written permission from DOSH to carry out the construction activities. In the case of any repair and modification works on the overhead travelling cranes or other aspects of the beam launcher system, the owner must engaged a certified competent firm and obtain a written approval from DOSH.