Short-Line Match Casting of Precast Segmental Bridges

Short-line match casting of precast segmental bridges

Segmental precasting of prestressed-concrete bridges is a well-established construction method that offers many benefits on suitable projects. The advantages include economies from industrialized, repetitive construction procedures that reduce costs and construction time. Factory production enhances quality and facilitates quality control. Rapid erection, minor site disruption and easy maintenance of highway and railway traffic at the erection site are additional advantages. Segmental bridges are easily adaptable to curved alignments and may have pleasant aesthetics. The disadvantages include the high cost of a precasting facility and the special equipment necessary to handle, transport and erect the segments, and the great number of structural joints in the structure.

Precast segmental bridges may be erected with four construction methods: span-by-span erection with self-launching gantry; balanced cantilever erection with ground cranes, lifting frames or self-launching gantry; progressive placement with a cable-stayed system or temporary piers; and incremental launching. Incremental launching of precast segmental bridges is typically more expensive than launching long deck segments cast in-place behind the abutment, and the combination of incremental launching and precast segmental technology is therefore addressed to very specific cases.

Span-by-span erection is the most common, simplest, and often most cost-effective construction method for precast segmental bridges. It is typically addressed to long bridges with a great number of 25-45m spans with large plan radius, and it is compatible with simply-supported and continuous spans. The maximum gradient of the bridge governs the design of the erection equipment, but a 4-5% gradient is rarely a major problem. The segment weight ranges between 30t and 150t, and the span weight ranges between 200t and more than 2000t. Although external post-tensioning facilitates segment production and enhances strand protection in the non-infrequent case of leaking joints, many span-by-span bridges have been post-tensioned with internal tendons.

The span-by-span method has reached 60m spans; however, for spans longer than 45-50m or very wide, this method loses much of its appeal due to the cost and complexity of self-launching erection equipment designed to sustain the weight of an entire span of segments. Another factor that discourages from span-by-span erection of long spans as an alternative to in-place span-by-span casting is the extra cost of post-tensioning resulting from the need to prevent edge tensile stresses at the epoxy joints between segments. The extra cost of post-tensioning may be particularly significant when the spans are lifted from barges or released as simply supported to accelerate the cycle; both solutions require a large number of small internal tendons and special anchor segments at the ends of the temporary span assemblies.

Long spans are typically continuous in highway bridges, and span-by-span erection has been successfully extended to 60-65m spans by locating the wet joints between precast segmental assemblies at the quarter or fifth of the span to support the erection gantry on the leading pier of the span to erect and on the front cantilever of the completed bridge. Negative bending in the self-launching gantry due to precast segments cantilevering beyond the leading pier diminishes the weight of the gantry and the rear support reaction applied to the completed bridge, and design of span post-tensioning is simpler and more cost-effective. The continuous units are typically as long as possible to minimize the number of expansion joints in the deck; 10-span units are often a lower-bound target in bridges devoid of particular geometry anomalies.

The balanced cantilever method can also be used on 40-60m constant-depth spans, though it is best suited to longer varying-depth spans. The segments are erected individually or in pairs with cranes on the ground or barges, lifting frames or cranes on deck, and self-launching gantries. Convertible self-launching gantries erect long balanced cantilever spans and shorter span-by-span approach units. Adjacent bridges may be erected simultaneously with side-shifting gantries that apply balanced cantilever segments to one hammer during critical-path post-tensioning operations on the adjacent bridge. Balanced cantilever spans longer than 110-120m are typically cast in-place with form travelers because of the height, weight, and poor stability of precast pier-table segments. Lifting frames and derrick cranes are the typical solution for long precast segmental cable-stayed bridges.

Progressive placement with cranes or lifting frames is the most time-consuming erection technique for precast segmental bridges because of the single work location. The erection equipment can be particularly inexpensive, especially when ground cranes erect the segments throughout the length of the bridge and temporary towers support the front cantilever during erection. Progressive placement has been applied or considered for 30-90m spans in environmentally sensitive locations where construction access was restricted to one or both ends of the bridge.

Three construction methods for prestressed-concrete bridges compete on the 40-60m span range: span-by-span erection of precast segments, span-by-span in-place casting with MSS, and incremental launching. Incremental launching is often the most competitive solution in medium-length bridges with simple geometry because of the lower cost of specialized construction equipment. Segmental precasting and in-place casting with MSS address longer bridges that can benefit from systemization and automation of repetitive, labor-intensive tasks and productivity optimization. Compared with segmental precasting, in-place casting with MSS offers simpler geometry control and a much smaller number of structural joints, requires less post-tensioning as many design standards allow edge tensile stresses in service conditions, and avoids precasting facility and segment shipping and handling.

Incremental launching construction of 50-60m spans with half-span segments is typically based on weekly segment cycles and by-weekly span cycles with one shift per day. Simulation engines may be used to optimize the design, construction and level of prefabrication of launched bridges. Some MSS achieve weekly span cycles with two shifts per day on 40-50m spans with simple and repetitive geometry, and productivity increases. Span-by-span erection of precast segmental spans with a self-launching gantry may reach 3-day cycles in short simply-supported spans, while longer continuous units often require weekly cycles. The erection can rate can be accelerated with double shifts.

The difference in productivity may be a major advantage in very long bridges with short piers. In shorter bridges, or when the piers are tall, the project’s critical path often includes pier construction, and the fast-track erection of a precast segmental deck with a self-launching gantry may have to be delayed until a sufficient number of piers are available to avoid disrupting gantry operations after a few span cycles. In-place casting with MSS and incremental launching may start as soon as the abutment and a few piers are available, and the impacts of a longer span cycle time on project duration can therefore be minimized.

Although the segmental construction concept is relatively simple, the level of technology involved in the design and construction of precast segmental bridges is more demanding than other types of bridge construction. Deep understanding of the technology requirements is necessary to facilitate construction, to avoid problems encountered in the past, and to reduce delays and costs related to concerns on non-critical issues or lack of understanding of critical issues. The same conclusions, however, apply for incremental launching and in-place span-by-span casting as well.

Precast segmental construction is all about standardization: same segments, as few variations as possible, and yet enough flexibility for the contractor. Several studies have been made in relation to the bridge length necessary to amortize the investments of segmental precasting. Deck surfaces around 20.000sqm may warrant feasibility analyses, although the availability and cost of skilled labor and the presence of local precasting facilities are often the real discriminators. In some countries, labor is so inexpensive that in-place casting on ground falsework is hardly beatable when the area under the bridge can be disrupted.

A second main factor is bridge modularity. Large-scale infrastructure programs that promote the same configuration (span distribution, standardized cross-section geometry, deck erection methods that meet contractor’s expertise and existing equipment) for multiple bridges are good candidates for precast segmental technology, as the casting cells and specialized erection means for the first bridges can be reused in new bridges or resold to other contractors after their use. In addition to cost savings, the reuse of existing bridge designs and construction equipment in design-build pursuits mitigates risk and accelerates project delivery with minimized lead time and shorter learning curves. Other factors that may influence the decision between segmental precasting and other construction methods are:

  • Distance of the precasting facility from the bridge erection site and logistic restraints on the segment delivery routes. This may be the most critical factor when multiple bridges are erected at different locations.
  • Height of the bridge on the ground and deck accessibility with ground cranes.
  • Ground constraints. Long bridges over water are more likely to be precast than bridges over land; however, incremental launching construction and span-by-span in-place casting with MSS may be more competitive on bridges of medium length. Bridges with short piers in accessible areas are likely to be erected with ground cranes rather than by self-launching gantry. Bridges with tight plan radii or in congested urban areas can be difficult for gantries and ground cranes that obstruct traffic and may require the use of articulated overhead or underslung gantries.
  • Segment delivery on the deck, on the ground or both. On-deck delivery is compatible with span-by-span and balanced cantilever erection with self-launching gantry and requires erecting the spans in sequence from abutment to abutment, which implies higher risk profiles, lengthens the span cycle time, and may prevent finishing work on the bridge. Ground delivery provides more flexibility and the area under the bridge may be used for segment storage. Ground delivery, however, is affected by ground constraints, disrupts the area under the bridge, requires more expensive hoists in gantries and lifting frames, and may lengthen the span cycle time when the piers are tall due to the longer cycle time of segment hoisting.
  • Variability of the cross-section. Varying width or bifurcations add complexity, lengthen the span cycle time and complicate or prevent the use of most types of underslung self-launching gantries that support box girder segments under the side wings. Span length variations in varying-depth bridges also affect segmental precasting operations, especially when the webs are inclined and the width of the bottom slab is therefore narrower in deeper segments.
  • Plan curvature. The casting cells for short-line match casting with tight plan radii are more expensive and complex to operate. Erection of curved spans may require telescopic overhead gantries or underslung self-launching gantries with articulated trusses, and so specialized machines add costs and complexity. Varying deck crossfall also complicates the use of overhead self-launching gantries and lifting frames.
  • Vertical curvature affects the operations of both types of self-launching gantry for span-by-span erection, particularly during repositioning.

Precast segmental bridges are used for spans ranging from 30m to 120m. Below 40-50m the use of precast beams and cast-in-place top slab is often less expensive, even in very long bridges. When site constraints prevent crane erection of precast beams, a beam launcher or a beam shifter is also lighter and less expensive than a self-launching gantry for span-by-span erection of precast segments. Spans of 160m have been reached with lifting frames for balanced cantilever erection. They are the upper bound of precast segmental technology due to the depth and weight of the pier-table segments and the cost of cantilever post-tensioning; longer precast segmental spans are achieved only in cable-stayed bridges.

Most box girders for highway bridges have single-cell arrangement with side wings. The top slab width varies between 6m and 21m with bottom slab width ranging between 3.5m and 9m. Bi- and multi-cellular box girders may be wider but two or more core forms complicate the design and operation of the casting cells, and single-cell solutions are therefore preferred with very wide decks as well.

Wide single-cell box girders often include diagonal struts anchored to the bottom web-slab nodes and that prop the top slab at the tip of the side wings and at the center of the deck. Precast struts and transverse trusses are sometimes combined in cable-stayed bridges with one central plane of stay cables supporting twin precast segmental box girders, while they are rarely necessary with twin box girders supported at the edges with two planes of stay cables. Precast segmental technology, however, is rarely applied to modern cable-stayed bridges designed for long service life because of the weight of these structures and the huge number of longitudinal and transverse construction joints in the top slab, which are directly exposed to traffic and aggressive chemicals and are therefore a potential weak point in terms of durability.

The precast segments for wide decks have large width-to-length ratio, which makes them potentially prone to thermal bowing during short-line production. On relatively short spans, wide decks are also prone to shear-lag effects that diminish the longitudinal compression generated by post-tensioning in the top slab regions far from the webs. Compression deficiencies due to thermal bowing and shear-lag effects may cause premature failure of top slab epoxy joints directly exposed to traffic and may lead to premature deterioration of precast segmental bridges, especially when the top slab is not protected by waterproof membranes and asphalt layers.

Handling and transportation requirements govern the length of the segments. Segments up to 3.6m-long may often be transported on public roads without major restrictions and are the typical choice for narrow single- or dual-track LRT bridges. When the precasting facility is close to the bridge and the segments can be transported within the right-of-way of the project or with barges, the segments are made as long as practical to accelerate construction and diminish the number of joints. When the deck is wide, however, the segments are rarely very long to limit their weight. Segments weighting up to 70-80t are usually within the lifting capacity of cranes available locally, while heavier segments suggest the use of special erection means.

With the diffusion of 75- and 100-year service-life design requirements, match casting and epoxy gluing of precast segments have acquired a critical role in assuring quality and durability of precast segmental bridges and avoiding premature joint failure.

Short-Line Match Casting of Precast Segmental Bridges introduces span-by-span, balanced cantilever and progressive construction of precast segmental bridges and explains how the erection method influences bridge design and precasting operations. Precast segmental bridges are all about standardization, and the eManual explores bridge design for modularity and the factors that drive the choice between precast segmental technology and other construction methods.

The eManual provides exhaustive coverage of the short-line method, the operations of casting cells for constant- and varying-depth segments, and the geometric design of the deck for standardized production of atypical segments and geometry correction with the typical segments.

With extensive illustrations, the eManual explains how to generate the casting curve for the segments in relation to casting sequence, erection sequence and time-dependent effects, the geometry control of short-line match casting (inclusive of commercial software programs and how they work) and the progressive correction of geometry errors during short-line casting.

The eManual also explores the long-line method for varying-depth balanced cantilevers and the dual-track U-segments of light-rail transit bridges, the post-casting operations common to all types of segments, different organizations of the stockyard, and segment delivery and epoxy gluing at the erection site.

The eManual is an indispensable gateway to the numerous monographs of the Bridge Engineering eManuals Project on span-by-span and balanced cantilever construction of precast segmental bridges and the corresponding families of self-launching gantries and lifting frames. Even bridge owners, designers and constructors with extensive experience of precast segmental technology will find fresh information in the eManual, and extensive feedback from the international bridge industry.

The eManual complements Precast Segmental Bridges, the 1-day course that Dr. Rosignoli teaches on-demand in the offices of bridge owners, designers and constructors. The bridge courses of Dr. Rosignoli originated within the ASCE Continuing Education Program. For more than 40 years, the American Society of Civil Engineers has ensured high-quality professional development and the latest innovations for bridge engineers. The ASCE Continuing Education Program is accredited by IACET to meet the premier benchmark for adult learning and undergoes review by a committee of professionals to select instructors that are authorities in their field, have decades of practical experience, and provide an outstanding level of expertise, in-depth training, and dedication to engineering.

The courses that Dr. Rosignoli teaches for the ASCE Continuing Education Program and on-demand in the offices of bridge owners, designers and constructors are true learning experiences to train bridge teams in modern bridge design and construction technology while meeting continuing education objectives. The courses foster personal research, innovation and professional development; promoting technical culture is indeed an excellent way to motivate, train and retain staff.

Learning is very effective with small groups of bridge professionals; 10-30 attendees is often the best compromise between interaction and the time constraints of hundreds of slides, although a 2010 seminar for the IABSE gathered 162 attendees in Singapore with excellent results. Richly illustrated with hundreds of photographs, the courses are constantly top-rated for material and presentation.