New-generation bridge construction machines are complex and delicate structures. They handle heavy loads on long spans under the same constraints that the obstruction to overpass exerts on the bridge. Safety of operations and bridge quality depend on complex interactions between human decisions, structural, mechanical and electro-hydraulic components, PLC control systems, and the bridge being erected.
Despite their complexity, these machines must be as light as possible. Weight governs the initial investment, the cost of shipping and site assembly, the self-launch stresses, and sometimes even the cost of the bridge. Weight limitation dictates the use of high-grade steels and designing for high stress levels in different load and support conditions, which makes these machines potentially prone to instability.
These machines are assembled and decommissioned many times, in different conditions and by different crews. They are modified, reconditioned and adapted to new work conditions. Connections and field splices are subject to hundreds of load reversals. The nature of loading is often highly dynamic, the equipment may be exposed to strong wind, and the full design load is reached multiple times and sometimes exceeded. Impacts are not infrequent, vibrations may be significant – most machines are actually quite lively because of their high structural efficiency – and the stiffness interactions with the bridge are difficult to assess and typically neglected by the bridge design standards.
Movement adds the complication of variable geometry. Loads and support reactions are applied eccentrically, the support sections are often devoid of diaphragms, and most machines have flexible support systems. Indeed such design conditions are almost inconceivable in permanent structures subjected to such loads.
The level of sophistication of new-generation bridge construction machines requires adequate technical culture to bridge owners, designers and constructors. Long subcontracting chains may lead to loss of communication, the problems not dealt with during planning and design must be solved on the site, the risk of wrong operations is not always evident in so complex structural systems, and human error is the prime cause of accidents.
Experimenting new solutions without the due preparation may lead to catastrophic results. Several bridge erection machines collapsed in the years with a heavy tribute of fatalities, wounds, damage to property, delays in the project schedule, and legal disputes. Technological improvement alone cannot guarantee a decrease in failures of bridge construction equipment, and may even increase them. Only a deeper consciousness of our human and social responsibilities can lead to a safer work environment. A level of technical culture adequate to the complexity of mechanized bridge construction would save human lives and would facilitate the decision-making processes with more appropriate risk evaluations.
The courses of bridge launching and mechanized bridge construction that I teach for the American Society of Civil Engineers and face-to-face in the offices of bridge owners, designers and constructors explore new and emerging bridge technology and modern construction methods. The e-books of the Bridge Engineering eManuals project integrate the courses to provide exhaustive coverage of the topic.