Introduction to Mechanized Bridge Construction

Introduction to mechanized bridge construction

The bridge industry is moving toward mechanized construction because this saves labor, shortens project duration and improves quality. This trend is evident in many countries and involves most construction methods. Mechanized bridge construction is based on the use of special equipment.

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 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; control systems, and the bridge being erected.

In spite of their complexity, the bridge construction machines must be as light as possible. Weight governs the initial investment, the cost of shipping and site assembly, the self-launching stresses, and the loads applied to bridge decks, piers and foundations. Weight limitation dictates the use of high-grade steel and designing for high stress levels in different load and support conditions, which makes these machines potentially prone to instability.

Bridge construction 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 subject to hundreds of load reversals. The nature of loading is often highly dynamic, and the machines may be exposed to unanticipated impacts 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 inconceivable in permanent structures 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-site, the risk of wrong operations is 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 construction 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 facilitate the decision-making processes with more appropriate risk evaluations.

Every 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 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 an efficient and rapid construction method rarely make the solution anti-economical.

Low technological costs are the reason for the success of the incremental launching method for prestressed-concrete (PC) bridges. Compared with the use of ground falsework, launching diminishes the cost of labor with similar investments. Compared with the use of a Movable Scaffolding System (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 some quantities of raw materials, the balance is amply positive and the solution is competitive.

The construction method that comes closest to incremental launching is segmental precasting. The labor costs are similar but the investments are much higher (precasting facility, segment transportation, self-launching gantry erection) and the break-even point shifts to longer bridges. Spans of 30-50m are erected span-by-span with overhead or underslung self-launching gantries. Longer spans are erected as balanced cantilevers: self-launching gantries may reach 100-120m spans, and lifting frames cover longer spans and curved bridges.

Self-launching gantries are also used for macro-segmental construction of 90-120m spans. Span-by-span erection of macro-segments requires propping the deck from foundations, while balanced cantilever erection involves casting long deck segments under the bridge for strand jacking into position. Both solutions require high investments for specialized equipment that is difficult to reuse with other construction methods.

On shorter bridges, prefabrication is limited to the girders and the deck slab is cast in-place. Precast beams are often erected with ground cranes. Sensitive environments, inaccessible sites, tall piers, steep slopes and inhabited areas often require assembly with beam launchers, and the technological costs increase.

Light-rail transit (LRT) and high-speed railway (HSR) bridges with 30-40m spans may be constructed with fully-precast spans. The investment in technology required by full-span precasting is so high that the threshold of cost effectiveness is reached with hundreds of spans. The precasting facility delivers two to four spans per day for fast-track construction of large-scale projects. Optimized material quantities and low labor costs add to the high quality of factory production. Ground cranes may erect four single-track LRT spans every night, while portal carriers with underbridge and span launchers few by tire trolleys are needed to handle the heavier spans of HSR bridges.

Medium-span PC bridges may also be cast in-place. For bridges with more than two or three spans it is convenient to advance in line by reusing the same formwork several times, and the deck is built span-by-span. Casting occurs in either fixed or movable formwork. The choice of equipment is governed by economic reasons, as the labor cost associated with a fixed falsework and the investment requested for an MSS are both considerable.

Starting from the forties, the original wooden falsework has been replaced with modular steel framing systems. In spite of the refined support structures, labor may exceed 50% of the construction cost of the span. Casting on falsework is a viable solution only with inexpensive labor and small bridges; obstruction of the area under the bridge is another limitation.

An MSS comprises a casting cell assembled onto a self-launching frame. MSS are used for span-by-span casting of long bridges with 30-70m spans. If the piers are not tall and the area under the bridge is accessible, 90-120m spans can be cast with a 45-60m MSS supported on a temporary pier in every span. Repetitive operations diminish the cost of labor, the quantities of materials are unaffected, and quality is higher than that achievable with a falsework.

Bridges crossing inaccessible sites with tall piers and spans up to 300m are cast in-place segmentally with form travelers, as balanced cantilevers. When the bridge is short or the spans exceed 100-120m, the deck supports the form travelers. Overhead travelers are preferred for in-place casting of PC box girders, while underslung travelers are used in cable-stayed decks and cable-supported arches. With long bridges and 90-120m spans, two longer casting cells may be suspended from a self-launching girder that also balances the cantilevers during construction.

The bestselling eManual of BridgeTech explores new and emerging bridge construction technology and modern construction methods for bridge owners, designers and constructors looking to save time, labor and costs, reduce risk, and increase the value and quality of bridge projects through mechanized construction.

The eManual explains the reasons for the industry trend toward mechanized construction, discusses its principles, means and methods, and explores the different types of bridge construction machines and their influence on the design and construction of the bridge.

With extensive illustrations, Introduction to Mechanized Bridge Construction introduces launchers and shifters for precast beam bridges, the different families of self-launching gantries for span-by-span construction of precast segmental bridges, and the MSS for span-by-span casting of box girders and ribbed slabs with double-T section.

The eManual also explores the form travelers for balanced cantilever casting of segmental decks and arches, the telescopic MSS for balanced cantilever casting of macro-segments, the self-launching gantries and lifting frames for balanced cantilever erection of precast segmental bridges, the self-launching gantries for span-by-span macro-segmental construction, and the span carriers with underbridge and the span launchers fed by tire trolleys for full-span precasting of high-speed railway bridges.

Extracted from the bestseller Bridge Construction Equipment (2013, ICE Publishing) and Chapter 6.37.40 Bridge Erection Machines in the UNESCO Encyclopedia of Life Support Systems, the eManual is an excellent starting point of remote learning programs of bridge construction technology, and an indispensable gateway to more specialized monographs of the eManuals project.

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