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The bridge industry is moving toward mechanized construction because this saves labor, shortens project duration and improves safety and quality. This trend is evident in many countries and involves most construction methods. Mechanized bridge construction is based on the use of special construction equipment.
Beam launchers and shifters are used to erect precast beams. Self-launching gantries and lifting frames are used to construct precast segmental bridges. Movable scaffolding systems (MSS) and form travelers are used for in-place casting of spans and segments of prestressed-concrete bridges. Forming carriages are used for segmental casting of the concrete slab of steel girders. Portal carriers with underbridge and span launchers fed by tire trolleys are used for full-span precasting of railway bridges. Lifting platforms are used to hoist deck macro-segments for suspension bridges. Alternative configurations of machines are also available for most construction methods.
New-generation bridge construction equipment is complex and delicate. It handles heavy loads on long spans under the same constraints that the obstruction to overpass exerts onto the bridge. Safety of operations and quality of the final product depend on complex interactions between human decisions; structural, mechanical and electro-hydraulic components; control systems, and the bridge being erected.
In spite of 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.
Bridge construction equipment is assembled and decommissioned many times, in different conditions and by different crews. It is 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, and most machines are actually quite lively because of their high structural efficiency.
Movement adds the complication of varying 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 for structural systems subject to such loads.
The level of sophistication of new-generation bridge construction equipment requires adequate technical culture to all parties involved. 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 and sophisticated 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 construction 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 assure 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.
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