架桥机毕业论文外文翻译.doc

大学毕业设计1附录 A 外文文献参考翻译A1 外文文献BRIDGE ERECTION MACHINESMarco Rosignoli HNTB Corp., USAContents1. Introduction to Bridge Construction Methods2. Main Features of Bridge Erection Machines3. Beam LaunchersSummaryBridge industry is moving to mechanized construction because this saves labor, shortens project duration and improves quality. This trend is evident in many countries and affects most construction methods. Mechanized bridge construction is based on the use of special machines.New-generation bridge erection machines are complex and delicate structures. They handle heavy loads on long spans under the same constraints that the obstruction to overpass exerts onto the final structure. Safety of operations and quality of the final product depend on complex interactions between human decisions, structural, mechanical and electro-hydraulic components of machines, and the bridge being erected.In spite of their complexity, the bridge erection machines must be as light as possible. Weight governs the initial investment, the cost of shipping and site assembly, and the launch stresses. Weight limitation dictates the use of high-strength steel and designing for high stress levels in different load and support conditions, which makes these machines potentially prone to instability.Bridge erection machines are assembled and dismantled many times, in different conditions and by different crews. They are modified and adapted to new work conditions. Structural nodes and field splices are subjected to hundreds of load reversals. The nature of loading is often highly dynamic and the machines may be exposed to hundreds and strong wind. 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 **大学毕业设计2inconceivable in permanent structures subjected to such loads.The level of sophistication of new-generation bridge erection machines requires adequate technical culture. 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 risks of wrong operations are not always evident in so complex machines, 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 fatalities and huge delays in the project schedule. 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.1. Introduction to Bridge Construction MethodsEvery bridge construction method has its own advantages and weak points. In the absence of particular requirements that make one solution immediately preferable to the others, the evaluation of the possible alternatives is always a difficult task.CoMParisons based on the quantities of structural materials may mislead. The technological costs of processing of raw materials (labor, investments for special equipment, shipping and site assembly of equipment, energy) and the indirect costs related to project duration often govern in industrialized countries. Higher quantities of raw materials due to efficient and rapid construction processes rarely make a solution anti-economical.Low technological costs are the reason for the success of the incremental launching method for PC bridges. CoMPared to the use of ground falsework, launching diminishes the cost of labor with similar investments. CoMPared to the use of an MSS, launching diminishes the investments with similar labor costs. In both cases launching diminishes the technological costs of construction and even if the launch stresses may increase the quantities of raw materials, the balance is positive and the solution is cost effective.The construction method that comes closest to incremental launching is segmental precasting. The labor costs are similar but the investments are higher and the break-even point shifts to longer bridges. Spans of 30-50m are erected span-by-span with overhead or underslung launching gantries. Longer spans are erected as balanced cantilevers: self- launching gantries reach 100-120m spans and lifting frames cover longer spans and curved bridges.Heavy self-launching gantries are used for macro-segmental construction of 90-120m **大学毕业设计3spans. Span-by-span erection of macro-segments requires props from foundations. Balanced cantilever erection involves casting long deck segments under the bridge for strand jackin。