Lights and Shadows of Mechanized Bridge Construction

Lights and shadows of mechanized bridge construction

The technological aspects of construction influence the modern bridge industry from the very first steps of design. Entire families of bridges such as the launched bridges, the span-by-span bridges and the balanced cantilever bridges take the name itself from the construction method. The full-span method so frequently applied in high-speed railway projects is another example.

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 equipment.

Beam launchers and shifters are used to erect precast beams. Self-launching gantries and lifting frames are used for span-by-span and balanced cantilever erection of precast segmental bridges. Form travelers and movable scaffolding systems (MSS) are used for in-place casting of segments and entire spans of prestressed-concrete bridges. Forming carriages are used for segmental casting of the concrete slab of composite bridges. Portal carriers with underbridge and span launchers fed by tire trolleys are used for transportation and placement of precast spans. Lifting platforms are used to hoist macro-segments for suspension bridges. Alternative configurations of machines are also available for most construction methods.

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 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 erection 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 erection 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 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 subject to such loads.

The level of sophistication of new-generation bridge construction equipment requires adequate technical culture in 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 risks of wrong operations are 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 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.

In a perfect world, bridge construction equipment would be purchased to meet clear performance requirements, would be designed according to international standards and project-specific technical specifications, would be subject to independent design checking, would be fabricated and commissioned within quality-control procedures, and would be operated by experienced supervisors and well-trained crews according to procedures issued by the manufacturer.

We are not in a perfect world though. Bridge construction equipment is often purchased by procurement personnel that have just a vague idea about what they are buying and tend to recommend decisions to the management based on the only parameter they can compare: the "cost". The final cost for the contractor is often higher than the figure written at the end of the offer, and the overall value of two apparently similar machines may also be pretty different. Inspections may clarify if the machine is in good conditions or is a freshly-repainted pile of rust; however, other aspects influence the value of a machine. The labor and crane demand of site assembly, for example, may be a bitter surprise if hundreds of field splices are designed with friction bolts and lap plates instead of shear pins or longitudinal stressed bars.

Because of its weight and dimensions, bridge construction equipment is assembled on-site. A rule of thumb is that site assembly may take 7-10 hours of labor per metric ton of steel, and two cranes for the entire period. A lot of money, so the contractor frequently takes care of assembly labor and cranes to save supplier's overheads. Identifying what should have been assembled in the workshop (paid with the cost of the machine) and what should be assembled on-site (paid by the contractor) may lead to interesting discussions if not specified in the contract. Most of us would guess that primary hydraulic systems should be assembled and tested in the workshop and only the hoses should be applied on-site. I saw building all the hydraulic systems of an 800t MSS on-site (from bending cold drawn steel tubing to painting) because the contract did not explicitly list that work within the workshop tasks. In addition to the extra cost for the contractor and a two-month delay in the project schedule, a delay penalty was turned into an accelerated delivery bonus...!

The absence of comprehensive design standards further complicates the situation. Although construction is the most critical moment in the lifetime of a bridge and poor workmanship may irremediably affect quality and durability, bridge construction equipment does not receive any attention or research funding. Ruling bridge design with state-of-the-art standards is just the first step: assuring quality and durability of the final product requires similar levels of attention and control also during construction.

If one adds up the cost of delays in the project schedule, forensic investigations, civil proceedings and possible criminal charges, the cost of failure of bridge construction equipment may be much higher than the cost of the equipment itself. Despite this, "it is an axiom among some designers that for temporary works during construction, lower safety factors may be applied than for completed permanent structures. In light of the uncertainties with temporary structures and their history of failures, this is an inappropriate and even dangerous attitude if applied indiscriminately. If we put aside the notion that "these things" are temporary and generally less important, then the design loads should perhaps be even more severe, the allowables even lower, and the calculated safety factors even higher than those for permanent structures." Forensic Structural Engineering Handbook (Robert T. Ratay, 2nd Edition, Mc Graw Hill, 2010).

Safety is another hidden problem. "Safety of temporary structures is the concern of the designer, the contractor, the building official, and the insurer, as well as the workers on the job and the general public. Yet, this most important component of the construction process is still not a "field" of practice but a neglected stepchild, at times claimed, at other times disclaimed by both designers and contractors, and almost totally neglected by researchers and educators. Although increasing in recent years, still surprisingly little guidance exists in the civil engineering profession for their design, erection, maintenance, and removal. A tacit attitude seems to prevail in the design-construction industry: "these things" are temporary and generally less important, therefore greater risks are acceptable than in permanent structures. Certainly, less care and control is exercised with temporary than with permanent structures. Possibly as a result, far more failures and loss of life occur during construction than in completed projects." Forensic Structural Engineering Handbook (Robert T. Ratay, 2nd Edition, Mc Graw Hill, 2010).

In some industrialized countries, the loss of lives during bridge construction is one order of magnitude higher than the loss of lives due to failure of bridges in operation. Since bridge construction takes a few years and bridge service covers decades, the risks for the workers are two orders of magnitude higher than the risks for the users.

In an imperfect world, also operations are uncertain. A machine too complex to operate may be so slow to force the supervisor to risky shortcuts to keep the schedule. It may even be so slow to force the contractor to purchase a second machine. Being vaguely optimistic on the performance may market a second machine in the next future. Binding the supplier to performance, productivity and labor demand is therefore in the best interest of the contractor.

If the machine is brand-new, procurement may be even more complex. Who will identify performance requirements and technical specifications for such an expensive piece of equipment? Who will review the suppliers' proposals? Who will be the independent design checker? Where will the machine be fabricated in response to heavy custom duties? Who will audit the manufacturer's quality-control processes during fabrication? Who will supervise load testing and site commissioning? Who will inspect the machine during operations?

Mechanized bridge construction is widespread all over the world. When comparing different proposals, one often notices large cost differences between apparently similar machines. Many aspects should be considered in the comparison: different average quality of steelwork in different countries, different weight resulting from steel grade and structural efficiency, degree of mechanization and access, durability and energetic efficiency, modularity and easy reconditioning for future reuse, and easy shipping and site assembly.

Decisions on bridge construction equipment are trans-disciplinary in nature. Safety is the first concern, performance and productivity govern planning and investments, structure-equipment interaction affects the design of bridge and special equipment, risk mitigation is a major issue for contractors and insurance carriers, and the quality-control of design, fabrication and operation are strictly related.

Few has been written on these machines in spite of their fundamental role in the modern bridge industry. These are the rationale, mission and values of the eManuals of BridgeTech. The project conveys trans-disciplinary information and guidance on modern bridge design and construction technology to engineering professionals interested in mechanized bridge construction.

  • Bridge owners and designers will find comprehensive information on how the bridge will be built and will interact with special equipment during construction.
  • Educators will find a solid technology basis for theoretical courses of bridge engineering.
  • Contractors will find information on procurement, operations, performance and productivity of special equipment and a guide to value-engineering, time-scheduling, risk analysis, bidding, safety planning and the formation of management and site personnel for the risks and the advantages of mechanized bridge construction.
  • Designers and manufacturers of special equipment will find comprehensive information on design loads and load combinations, calibration of load and resistance factors, design for robustness and redundancy, numerical modelling and analysis, out-of-plane buckling and prevention of progressive collapse, human error, failure of materials and systems, repair, reconditioning and industry trends.
  • Erection engineers, resident engineers, inspectors and safety planners will find information on operations, casting cycles, cycle times, loading and structure-equipment interaction.
  • Forensic engineers will find numerous case studies on failed equipment.

The courses of modern bridge design and construction technology that Dr. Rosignoli teaches for the Continuing Education Program of the American Society of Civil Engineers and on-demand in the offices of bridge owners, designers and constructors explore new and emerging bridge technology and modern construction methods. The eManuals of BridgeTech further expand and integrate the courses for an exhaustive coverage of the topic.