The boom in high speed rail projects in China has led to the use of launching gantries for bridge construction becoming commonplace. But with contractors habitually reusing this equipment, greater emphasis is needed on maintenance and machine reconfiguration to reduce risks and improve reliability. Yabin Liu and Fuchun Yuan explains.
Over the last 15 years the expansion of the high-speed rail network in China has been rapid. Since construction of the first high-speed passenger railway between Qinhuangdao and Shenyang began in 2000, launching machines have been widely used, especially for the full span viaduct construction method. In 2006, the first 900t-capacity gantry was used for full-span bridge construction on the Beijing-Tianjin high speed railway, and since then a whole range of launching machines has been developed for the full-span erection of prefabricated bridges, such as the launching gantry with longer underbridge, launching gantry for double box beam, special launching carrier with underbridge for travelling through tunnels and so on.
Launching machines are always customised to suit the specific bridge design, construction conditions, structural dimensions of beams and so on and the machinery supplier can only begin the design after receiving these details from the client. As a result, launching machines differ depending on performance, dimension, lifting capacity, functions and so on; even if a bridge project needs multiple launching machines, the supplier may design different erection solutions and the launching machines may vary in structure, operation and controls.
When launching gantries were first used, many owners amortised the machine’s value for a single project, hence machines were not generally reused because there was little chance that they would be suitable for other bridges, in particular in terms of schedule, time period and location. Usually only the salvage value of the material would be assessed.
But since the boom in bridge construction in China, many owners now want to reuse their launching machines, and the equipment represents a fixed asset in the finances of the project. Many machines have been reused in the last decade on different job sites in China, including machines in which parts or components have been replaced or upgraded for the new jobs. Some machines have been reused for up to five different projects.
Due to the size of the machine, it must be transported in separate components and assembled for commissioning at the new job site. When a launching machine is to be reused, a number of items have to be considered before it is moved; the technical analysis and cost evaluation for reuse; checking and repair of parts and components of the used machines; safety evaluation and review of the safety system, and re-commissioning.
Evidence over the last few years demonstrates that many of the accidents involving launching machines were due to lack of attention to one or more of these aspects. Preparing a launching machine for reuse may be more complex than designing a new one, and it needs particular attention when the machines are to be reused by different clients or the machines come from various suppliers.
More than a hundred launching gantries have been ordered for use in China over the last 15 years and these have been reused multiple times. Manufacturer Beijing Wowjoint Machinery Company compiled statistics relating to 52 launching gantries it supplied or was involved with over this time.
The reuse of launching machines is ubiquitous in China; more than 70% of the 52 investigated had been used twice or more; 37% had been used at least three times. Most of the remainder were already in their second use or due to be so in the near future.
On average, launching gantries erect around 730 spans in a lifetime, with almost 40% of gantries erecting more than 1,000. More than 70% of launching gantries have a service life of more than four years, suggesting that the majority of machines must be evaluated and require an intensive inspection for maintenance and safety.
Of these, the vast majority - more than 80% - have a capacity of between 800t and 950t, and are mainly used for construction of high speed rail viaducts.
Meanwhile just 18% of launching machines are estimated to have been reconfigured for reuse, indicating the need for technical analysis and recalculation for the reuse of launching machines.
Analysis and checks must be carried out before equipment is reused, in order to identify any technical problems and analyse the machine’s applicability for the new site. Timely maintenance, technical upgrades or reconfiguration for new working conditions are necessary to avoid problems on the new project (see examples at the end of the article).
Special considerations may include construction conditions, the climate, and specific bridge conditions such as working next to existing lines. Professional design personnel must be employed to carry out technical analysis so that solutions to these special conditions can be identified.
Most of Beijing Wowjoint’s launching equipment is designed to European standards, with some designed according to GB/T 26470-2011 General specification for launching machines and GB/T 3811-2008 Design rules for cranes.
Based on the working conditions of launching machines, the service period for any machine is limited to 15 years. Generally the designer selects a theoretical service life for mechanical equipment of about 1,600 working hours during the 10-year service period. The working class selected overall is A3 with the mechanism working class M4 being applied to the machine.
But in a single mechanism, different parts may be designed for different service lives; for example, the design service life of bearings in the reducer is normally one or two classes lower than that of gears. Therefore in theory, the bearings of the reducer will have to be replaced once or twice in order to achieve an equivalent service life to that of the gears during the same period of use of the reducer.
Launching machines generally fall into two categories; the special launching carrier and the launching gantry, depending on the function of the structure and the launching procedure.
The working cycle of a launching machine normally follows this procedure; the machine launches itself into position; the beam is fed from the rear deck; the beam is positioned onto the piers; the machine launches itself again for the next construction cycle.
During erection of each span, each part of the machine is brought into use at least once, but during their operation, the working time for similar mechanisms during different types of operation can differ between each cycle.
One example would be a special launching carrier which is carrying out three jobs in once cycle; lifting the span at the yard, carrying the span to site, and erecting it at the job site. Some parts such as the beam derrick and pulley block of the main launching mechanism, lifting mechanism and beam lifting winch are in operation for more than three hours during each erection cycle. As a result, the service life for these parts during this type of operation is less than 2,400 spans.
However in the case of a launching gantry which would be combined with a beam transporter to work on the erection of bridge beams, the main launching mechanisms will be at work for just two hours during each working cycle. In this case, the service life of the main mechanism will be more than 2,400 spans.