Balanced cantilever construction is suited to precast segmental and cast-in-place bridges. The deck is erected segmentally on each side of the pier in a balanced sequence to minimize load unbalance and longitudinal bending in piers and foundations. The deck is self-supporting during construction, and it also supports erection equipment and construction materials stored on the cantilever. This construction method is particularly advantageous on long spans, in marine operations, and where access beneath the deck is difficult.
Negative bending governs balanced cantilever construction, and this often requires box sections with a thick bottom slab at the root of the cantilever and many longitudinal tendons in the top slab. For a given span length, balanced cantilever erection of precast segmental bridges is often more expensive than span-by-span erection in terms of quantities of structural materials and associated labor. The advantage of balanced cantilever erection is that the segments are handled individually, and the erection equipment is therefore lighter and less expensive than a self-launching gantry for span-by-span construction, which sustains the weight of an entire span of segments during span assembly and application of post-tensioning.
Balanced cantilever bridges typically have box section. The bottom slab of a box girder lowers the cross-sectional center of gravity, which increases the flexural capacity in the negative bending regions. The torsional strength and stiffness of a hollow section assure stability of the cantilevers on the long spans where this construction method is typically employed. This is particularly true when the deck is curved in plan. Single-cell box girders may reach 18-20m of width; transverse ribs in the top slab, diagonal struts propping the edges of the side wings from the bottom web-slab nodes of the cross-section, and combinations of ribs and struts have been successfully used to further widen the top slab of single-cell box girders.
Ribbed slabs with double-T section are simpler to form and cast than box girders and have been used for some balanced cantilever bridges in the past. For a given cross-sectional area, the moment of inertia, radius of gyration, and flexural efficiency of a ribbed slab are smaller than those of a box girder, which ultimately requires more post-tensioning. The center of gravity of a ribbed slab is closer to the top slab, which further complicates the transmission of negative bending. Ribbed slabs comprising solid edge girders and crossbeams are used for balanced cantilever erection of cable-stayed bridges with two planes of stay cables because the stay cables resist most of negative bending and torsion. When the deck is not cable-stayed, balanced cantilever bridges typically have box section.
The deck may have constant or varying depth. Varying-depth precast segmental box girders have been used on 80-180m spans and are mostly competitive on 100-120m spans. The depth of the cross-section, the shear area of the webs and the thickness of the bottom slab diminish along the cantilever to decrease the cross-sectional flexural and shear capacity according to the decreasing demand of cantilever construction. The deck segments close to the pier are deeper than the midspan segments, and their bottom slab is thicker. The segments at the root of the cantilever are therefore shorter than the midspan segments to minimize the weight difference between the segments and assure optimum operation conditions for the lifters used to handle the segments in the precasting facility and at the erection site.
The midspan segments are rarely longer than 3.5m for ground transportation reasons, and the segments of the pier-table are therefore particularly short. When the deck is supported on one line of bearings at the piers, the pier-table assembly typically includes three segments. The central segment over the bearings incorporates a heavy pier diaphragm that transfers shear and torsion from the deck webs to the bearings, and the segment length further diminishes due to the extra weight of the diaphragm. The pier segments are the taller and narrower the longer the span, and on long spans they soon become too tall for ground transportation, and too narrow to assure stability during transportation and handling. Balanced cantilever spans longer than 120-130m are therefore typically cast in-place.
Constant-depth segments are easier to cast and have uniform cross-section, and therefore constant length. Longer segments diminish the transportation cost and accelerate erection with a smaller number of cycles. Constant-depth segments have been used on 30-90m balanced cantilever spans and are mostly competitive on 45-70m spans, especially when two adjacent bridges are erected simultaneously by side shifting the erection gantry to minimize the interference between erection crews and post-tensioning crews and facilitate surveying and geometry control of the cantilevers. On 45-70m spans, balanced cantilever construction of precast segmental bridges competes with incremental launching of prestressed-concrete decks and with steel bridges with concrete slab. Both structural types are often less expensive, but rarely faster to erect. Precast segmental space frames comprising match-cast precast slabs and trussed webs have also been erected as balanced cantilevers.
On 45-60m precast segmental spans, balanced cantilever erection of constant-depth segments also competes with span-by-span erection. Both erection methods address the needs of long bridges with complex geometry and that possibly include thousands of segments. Balanced cantilever erection is slower and requires more post-tensioning and pier-cap stabilization gear when the deck is supported on one line of bearings, but the erection equipment is lighter and less expensive, and the construction method is more fit to urban bridges with different span lengths and varying plan curvature. Multiple spans can be erected simultaneously by combining linear erection with self-launching gantry and ground crane erection when the deck is accessible and the segments can be delivered under the crane. Span-by-span erection requires heavier, more expensive self-launching gantries designed to sustain the weight of an entire span of segments, and linear erection from abutment to abutment can only be accelerated with double shifts of workers, which diminishes the efficiency of production.
On 50-90m spans, balanced cantilever erection of constant-depth precast segmental spans also competes with in-place span-by-span casting with movable scaffolding systems (MSS). Span-by-casting with MSS offers substantial savings in post-tensioning due to:
- The lower demand for prestressing resulting from continuous longitudinal reinforcement. Bridge design standards allow edge tensile stresses in the presence of continuous longitudinal reinforcement that controls crack width in the structural joints, while they require no edge decompression under SLS load combinations at the epoxy joints. For ULS assessment, a cross-section with continuous longitudinal reinforcement has more nominal flexural capacity, the resistance factor of a tension-controlled section is higher than the one of epoxy joints and compression-controlled sections, and as a combined result, the design flexural strength of the same cross-section can be 20-30% higher.
- The use of longer and more powerful tendons that substantially diminish the number of ducts, anchorages and stressing and grouting operations. Spans cast in-place with MSS typically use full-span 31-strand tendons in the webs, which are spliced by crossing and overlapping with the tendons of the previous span in web anchor walls. The top-slab tendons of balanced cantilever bridges rarely exceed 19 strands per tendon and are anchored at every epoxy joint, and the number of anchorages and stressing operations increases substantially.
- No need for expensive plastic duct couplers at the epoxy joints. The duct couplers are a major cost item of balanced cantilever bridges designed for 75- or 100-year service life. The cantilever tendons are embedded into the top slab and are therefore directly exposed to traffic loads and aggressive agents. A 45m cantilever made with 3m segments has 15 epoxy joints, and many grout crossover issues at the joints have proved in the past that the epoxy joints are far from being waterproof. Proprietary plastic couplers are used at the epoxy joints to mechanically splice the embedded ducts and provide full plastic encapsulation of the tendons, which improves quality and life service expectation but increases the cost of post-tensioning substantially. Span-by-span casting with MSS requires a much smaller number of tendons, the ducts are spliced with inexpensive collars within the concrete webs, and the cost of post-tensioning per unit weight of strand further diminishes.
MSS for 50-90m spans mostly achieve 2-week cycles, while a 90m precast segmental span can be erected with weekly cycles with a self-launching gantry. A faster erection rate may be the reason to select precast segmental technology, while avoiding precasting facility and segment transportation, and substantial savings in post-tensioning, may be the reasons to select MSS technology in combination with a time-schedule that accelerates pier erection on the critical path of the MSS. Number of spans to erect and availability of existing precasting facilities also drive the choice between the two alternatives.
Balanced cantilever erection of precast segmental bridges requires a precasting facility, special transportation means, and heavy lifters in the precasting facility and at the erection site, and is therefore addressed to bridges with a great number of spans. The longer the bridge, the more powerful the erection equipment, and heavier and longer segments may be used when ground transportation is not restricted or the segments are delivered on barges. Segment erection with ground cranes, floating cranes or deck-carried lifting frames permits free construction sequences when the deck is accessible and the segments can be delivered under the crane, while the use of self-launching gantries is associated with linear construction from abutment to abutment. Linear construction requires pier availability at the right time, but precast segments and construction materials can be delivered on the deck through the abutment, which accelerates construction and minimizes the impact of ground constraints. The access conditions for the erection crews are also much better.
Precast segmental bridges erected as balanced cantilevers are often supported on one line of bearings at the pier-caps to simplify and accelerate the erection of the pier tables, and structure stability during erection is an important consideration. The segments of the pier-table assembly should have the same weight as the other segments not to require special lifters, and the pier-diaphragm segment is therefore narrow and tall.
Narrow pier-diaphragm segments facilitate self-launching gantry erection, as also the front auxiliary leg of the gantry takes support on the pier-cap during positioning of the segment. Narrow, tall and heavy segments, however, may be unstable during transportation and placement. The area beneath a balanced cantilever bridge is rarely flat, and the need for the hauler to climb slopes and make sharp turns can destabilize the segments. Ground conditions further worsen in wet weather, and the need for a solid road base from the precasting facility to the segment pick-up site can significantly increase the construction cost of the bridge.
Some pier-table segments have been split horizontally to increase their width and diminish their height. Horizontal splitting complicates segment fabrication and delivery but increases stability during handling. The bottom segment is placed on four jacks located at the corners of the pier-cap, the cap segment is placed over it, and the front pier crossbeam of the gantry is moved over the cap segment for completion of self-launch and application of the starter segments of the cantilevers. During these operations, the front auxiliary leg of the gantry takes support on a steel bracket anchored to the front face of the pier. The pier brackets induce significant longitudinal bending in the piers and may trigger specific forms of instability of slender piers, which suggests minimizing the length of the pier-table assembly.
In most cases, the pier tables of decks supported on one line of bearings include three narrow vertical segments connected with longitudinal tendons or prestressing bars. The pier-table assembly is supported on jacks or shim packs while it is bedded on the bearings. If the bearings have dowels on the top plate, recesses are formed into the segments at the dowel locations, and the gap is filled with mortar in-place. If the top bearing plate has no dowels, the plate is grooved and the gap is filled with epoxy mortar.
Hydraulic jacks at the corners of the pier-cap combined with vertical post-tensioning bars that anchor the assembly to the pier-cap provide fast and accurate geometry adjustment and effective initial stabilization of the hammer. The limited flexural capacity of the connection (the bars are typically stressed to 50% of the ultimate tensile strength to be reused 20-30 times) is fit for the load unbalance of only 1 or 2 cantilever segments on either side of the pier.
Tie-down bars may be enough when a self-launching gantry stabilizes the “hammer” (the deck portion including the two cantilevers and the central pier-table assembly) during erection or applies the segments simultaneously on both sides of the pier to minimize load unbalance. When the pier is narrow, steel brackets connected to the pier-cap with prestressing bars and shear keys in recesses may be used to support the jacks and to anchor the tie-downs with a longer lever arm. Higher tie-down forces and flexural capacity can be attained with loop tendons embedded into the pier. All these connection systems are flexible, complicate geometry control during cantilever erection, and may cause discomfort to personnel on the deck.
Temporary pier towers supported on the bridge foundations, tension/compression props on one side of the pier, compression props on both sides, and auxiliary tension ties provide more flexural capacity but are cost-effective only with short piers. Props from foundations are often necessary when a balanced cantilever deck supported on one line of bearings is erected with a straddle carrier shuttling back and forth over the deck because of the large load unbalance of staged construction. For these reasons, straddle carriers and cranes on-deck are typically used on bridges with continuous pier-cap connections.
As an alternative to precasting, the pier tables may be cast in-place at the end of pier erection with special casting cells or within match-cast precast shells. In-place casting of the pier tables diminishes the cost of transportation and erection equipment for the precast segments, which is often governed by the pier-table segments. It also removes complex activities from the critical path, accelerates critical-path operations of cantilever erection, and is the typical solution for continuous pier-cap connections, where a thicker pier diaphragm facilitates developing pier reinforcement into the pier table in seismic regions.
When the bridge includes two adjacent decks, connecting the decks with cast-in-place straddle bents generates transverse frame action in the portal piers that diminishes pier bending under lateral loads and improves the transverse seismic response with double plastic hinging. Cast-in-place pier tables cantilevering out of a central T-pier may also be used when the piers of two adjacent decks are located in the median of existing roads.
Thin non-reinforced wet joints are used between cast-in-place pier tables and the starter segments of the cantilevers to compensate for the geometry irregularities of in-place casting. The use of wet joints lengthens the cycle time of the hammer by a couple of days as the erection equipment holds the segments in-place during curing. Accurate geometry control is indispensable for the cast-in-place pier tables, especially as regards the duct alignment of the top-slab tendons. Additional clearance for duct splicing across the wet joints can be provided by including small blockouts on either side of the joint.
Accurate positioning of the starter segments of the cantilevers is indispensable in bridges with cast-in-place pier tables and continuous pier-cap connections. The deck geometry cannot be corrected by rotating the entire hammer like in the case of decks supported on one line of bearings, and shimming of the epoxy joints is the only possibility of correction of inaccurate initial alignment. When the starter segments of the cantilevers can be delivered at the base of the pier, they may be applied to the pier table with ground cranes or special lifting frames prior to the arrival of the self-launching gantry to move complex activities out of the critical path and accelerate critical-path operations and the span cycle time.
The use of precast shells is a hybrid solution between precasting and in-place casting. One or two outer shells for the pier table are match-cast in the precasting facility against the starter segments of the cantilevers. The precast shell is positioned over rebar protruding from the pier-cap, and the weight saving generated by in-place casting of webs, pier diaphragm and tendon deviators is often enough for positioning of the shell with a ground crane. The shell is placed on four jacks supported on brackets or collars anchored to the pier-cap. After alignment and cage completion, the shell is filled with a continuity pour that uses the outer shell as a portion of the formwork.
Precast shells offer the stability of continuous pier-cap connections and the erection rapidity of match-cast joints with the starter segments of the cantilevers. The drawbacks include the need for accurate positioning of the shells to set a correct alignment for the cantilevers, a large number of rebar couplers for the diaphragms, labor-intensive cage assembly and forming operations in the precasting facility and on-site, and the risk of poor concrete compaction and honeycombs during filling of diaphragm and tendon deviators due to rebar congestion.
The shell is supported on an erection frame anchored to the sides of the pier with stressed bars and shear keys in recesses or supported on through beams crossing the pier for transfer of vertical load. Four jacks are used at the corners of the erection frame to align the shell. The jacks on one side of the frame may be interconnected to generate a torsional hinge for 3-point loading. The shell is very flexible, a four-point support may distort it, and locked-in twist would complicate the epoxy gluing operations for the starter segments of the cantilevers.
Most balanced cantilever bridges have varying depth along the span, and the length of the segments increases along the cantilever to balance their decreasing weight. Length and weight of the midspan segments are governed by handling and transportation requirements. Lengths up to 3.5m are usually transportable on public roads without excessive restrictions; longer segments for marine operations may be delivered on barges.
Ground cranes, floating cranes, ground cranes on barges, on-deck lifting frames, and overhead self-launching gantries typically hoist the segments with a central lifting beam that is permanently connected to the crane. The crane applies the segment to the tip of the cantilever, and hangers and spreader beams are not necessary for balanced cantilever construction. The first-generation lifting beams used four slings anchored in slotted holes to lift the segment with the gradient and crossfall required to align shear keys and gluing bars with the previously erected segment. Pull cylinders applied to the slings for hydraulic adjustment of segment alignment were used to save labor and accelerate the cycle time of segment erection. New-generation lifting beams for balanced cantilever bridges are applied to the crane permanently and do not use slings. They are equipped with tilt cylinders and slew rings that rotate the segment in the horizontal, longitudinal and transverse plane for final alignment.
One or two top-slab tendons per web are anchored at each and every epoxy joint in relation to tendon size, top slab thickness, interference with web stirrups, and anchorage protection. The top-slab tendons of modern balanced cantilever bridges are anchored within the top web-slab nodes of the cross-section to streamline load transfer and avoid the weight, cost and complexity of top-slab blisters. Anchoring the top-slab tendons within the cross-section simplifies segment production and diminishes the labor demand, but has some drawbacks.
- Post-tensioning operations are on the critical path of cantilever erection. After application of a new segment and positioning of the stressing platform, the next segment can be applied to the cantilever only after fabrication and stressing of the tendons anchored in that joint, and removal of the stressing platform. Regardless of the type of crane used to handle segments and stressing platforms, balanced cantilever erection requires extremely efficient post-tensioning operations and rapid removal, storing and re-application of the stressing platforms.
- The segments are short because of ground transportation requirements, and a large number of small tendons is necessary for symmetry reasons when the tendons are anchored at each and every joint. The large number of anchorages and specialized operations (splicing and pressure-testing of HDPE or HDPP ducts, tendon fabrication and stressing from both anchorages, sealing of anchorages, grouting) increases the cost of post-tensioning and lengthens the cycle time of segment erection. Multiple tendons are typically grouted in one operation out of the critical path, which also diminishes the risk of grout crossover at epoxy joints devoid of plastic duct couplers. Some bridges have been designed with top-slab tendons anchored at every other joint, which also offers pros and cons but can accelerate the erection cycle.
- The anchorages of the top-slab tendons are located in regions of the epoxy joints that are directly exposed to traffic. Epoxy joints are structural discontinuities, joint leakage has been reported in multiple occasions, the longitudinal axial compression at the top deck surface is typically very low in balanced cantilever bridges, concrete creep behind tendon anchorages may cause decompression and cracking of the epoxy joints, direct traffic loading of the anchor area may further worsen cracking, and the small concrete cover of the tendon anchor plate from the deck surface may cause concerns in modern bridges certified for 75- or 100-year service life. The use of sealed anchorages and double grouting (within the duct, and of the cavity between sealed anchorage and next segment) is often specified to assure proper durability of tendon anchorages, which further increases the cost of post-tensioning.
The post-tensioning operations are critical-path items in a balanced cantilever bridge, and tendon geometry and the details of anchorages and alignment keys in the top slab are standardized as much as possible. The use of prefabricated tendons to be uncoiled and pulled into the duct in one operation was standard procedure years ago to accelerate tendon fabrication. Nowadays, individual strand pushing has become so fast and reliable that pulling the entire tendon or pushing the individual strands has become a matter of preference and confidence.
Long tendon ducts cross many epoxy joints in top-slab regions directly exposed to traffic. Even if HDPE or HDPP ducts offer higher quality and reliability than the corrugated metal ducts, the watertight containment of the tendons has solutions of continuity at each and every joint between deck segments. Proprietary duct couplers are rarely used at the epoxy joints because of their cost, labor demand, and dimensions. The outer diameter of the duct couplers is 50-60% wider than the diameter of the ducts they couple, which may create geometry conflicts and excessive weakening of the cross-section when all of the ducts are coupled at the joint. The use of duct couplers often requires larger spacing between the ducts, which leads to thicker top-slab haunches and increased weight of the segments.
The longest tendons of a 60m cantilever for a typical 120m span have 35-40 solutions of continuity at the epoxy joints, and pressure-testing operations for the ducts are on the critical path of cantilever erection. Avoiding the use of duct couplers may cause pressure leaks at the joints, which must be located, fixed and re-tested before starting strand insertion into the duct. Delays accumulated during the post-tensioning operations must be recovered during the application cycles for the next pairs of segments, and these operations require surveying and often joint shimming to assure proper alignment of the cantilever. Although modern self-launching gantries with two winch-trolleys can apply three and even four segments per day to each cantilever, so frantic operations may compromise the quality of epoxy joints, top-slab post-tensioning, and the final alignment of the bridge.
Top-slab blisters may be unavoidable for the cantilever tendons anchored at the tip of the cantilever when the midspan closure is too narrow to apply the stressing jacks to the strand tails extending from the anchor plates of the adjoining cantilevers. A few top-slab blisters are also necessary for the top-slab tendons crossing the midspan closure to assure adequate negative flexural capacity in the midspan region. Internal continuity tendons in the bottom slab are always anchored in blisters located at the bottom web-slab nodes within the box cell due to no access to joint faces. The recent trend in balanced cantilever bridges is using external tendons anchored in the pier diaphragms for part or the entirety of continuity post-tensioning.
Temporary prestressing bars anchored in blisters within the box cell are used to hold the segments in-place and press the epoxy joints during fabrication of the new set of top-slab tendons. The gluing bars are designed to hold the cantilever segment in-place so that the crane used to handle the segments can be immediately released for a new erection cycle during tendon fabrication, which assures optimum labor rotation for the erection and post-tensioning crews.
Permanent internal post-tensioning bars in the top slab have been used as gluing bars in wide decks to contrast the effects of shear-lag and the 45° distribution of the tendon anchor forces and assure more uniform axial compression of the top slab. Because of shear-lag, the longitudinal axial compression in the top-slab regions at bridge centerline and at the tips of the side wings is lower than the axial compression at the web-slab nodes. The anchor forces of the top-slab tendons are applied to the top web-slab nodes and provide uniform compression of the deck only 1-2 joints behind the anchor point. The top slab tendons are often designed with frame-element models that cannot depict these effects and their combination, which may lead to partial decompression and opening of curing epoxy joints. Even if the joints may ultimately turn out compressed on hammer completion, the permanent loss of adherence due to poor epoxy curing conditions may cause shear and flexural cracking and spalling in top slab joints directly exposed to traffic.
The segments become lighter toward the tip of the cantilever, and variations in the prestressing load of the gluing bars may be necessary to assure even pressing of the epoxy joints. Uneven pressing may cause varying joint thickness, which can affect the vertical alignment of the cantilever after several segments. Bottom edge compression is maintained after tensioning the top-slab tendons to avoid tensile cracking of cured joints and poor adhesion and gaps in the uncured joints close to the tip of the cantilever due to insufficient weight of the end segments.
Small geometry inaccuracies accumulate when long deck sections are erected, and some correction to segment alignment may be necessary. This is achieved with shims of woven glass fiber embedded into the epoxy to increase the thickness of the joint. Although this technique achieves only a small angle break at the joint, the effect is amplified as subsequent segments are applied and their joints are also shimmed. Joint shimming is a “last resort” since it causes stress concentration and prevents proper joint closure, which may cause voids, insufficient filling, penetration of epoxy into the post-tensioning ducts, and grout crossover among the tendons.
Permanent top-slab tendons are installed for most segments, and temporary gluing bars are therefore necessary only at the 3-4 leading segments of the cantilever during erection. When continuous coupled bars are used for gluing of segments, bars, bar couplers and anchor nuts are removed before applying the last segment of the cantilever as it is often impossible to dismantle them after midspan closure.
Midspan closure pours are used in modern balanced cantilever bridges to provide continuous connections between adjoining hammers. The rear cantilever of the new hammer is secured to the completed bridge with strong-backs bridging the closure gap. The strong-backs are anchored to the top slab of the first two segments on either side of the closure pour with shims and stressed bars crossing the top slab in the lifting holes of the segments. Strong-backs and anchor bars are designed for the weight of forms and concrete for the closure pour, the residual load unbalance, the realignment forces for the cantilevers, the effects of thermal gradients in the deck, and the frictional resistance of sliding bearings at the pier-caps.
The closure pour is typically 0.5-1.5m long, and even if tie-down bars and strong-backs are designed to provide flexural continuity between the adjoining cantilevers, the flexibility of the connection cannot completely prevent midspan rotations. The weight of long closure segments may cause deflections and rotations of the closure joints and pier-cap rotations in the leading hammer. For the precast segments, these deformations are compensated for with the casting curve. Long closure segments may be cast in two phases to take advantage of the stiffness of the first-phase U-segment during casting of the top-slab closure. Mild continuity post-tensioning may be applied to the first-phase U-segment to further stiffen the central connection.
The joint surfaces must be clean, free of laitance, and roughened to expose coarse aggregate. AASHTO requires 6mm roughness, which is achieved by bush-hammering or chipping the joint after sandblasting. Application of bond enhancer may be specified to increase adhesion of fresh concrete. The closure segment is cast directionally from one joint toward the other to avoid joint cracking in the bottom slab.
The midspan closures are weak points of balanced cantilever construction. The precast segments of the cantilevers are post-tensioned and glued with epoxy adhesives that assure some tensile strength when applied and cured properly. The midspan closures are not glued, and reinforcement protruding from the end segments of the cantilevers is rarely used. Even when theoretically not necessary, short top-slab continuity tendons crossing the midspan closure are used in modern balanced cantilever bridges to cope with the effects of thermal bowing of precast segments and other undesired effects.
Long viaducts may require expansion joints to alleviate the effects of thermal deformations and the time-dependent shortening of the deck due to concrete creep and shrinkage. Different solutions are available for the expansion joints, in combination with sliding bearings, monolithic pier-cap connections and the use of leaf piers that provide flexural connection with minimal longitudinal shear stiffness:
- Expansion-joint segments are used at the quarter of the span to transfer shear and to allow longitudinal movement and rotation. These joints rely on their location at the counter-flexure point to minimize the angle breaks under live loads. The two halves of concrete seat-type hinges are blocked together during cantilever erection. Temporary top-slab tendons and concrete shims are installed through the expansion joint to provide sufficient flexural capacity to erect the remainder of the cantilever. After midspan closure and application of continuity post-tensioning, the temporary tendons are removed and the hinge is released. The expansion-joint segments are cast in special molds and are connected to the adjacent segments with wet joints.
- Midspan expansion joints are used to transfer bending, shear and torsion while allowing longitudinal movement. Continuity is achieved with two steel girders that cross the expansion joint within the box cell and are anchored with two lines of sliding bearings on either side. Sliding bearings are used on top and bottom of the steel girders to transfer positive and negative bending and shear throughout the joint. Upon completion of the first hammer, the girders are lifted and inserted into the box cell. Upon completion of the adjacent hammer, the girders are pulled through the expansion joint and secured in their final position. The weak points of this solution are the weight and complication of two diaphragm segments on either side of the joint, the difficult maintenance of bearings and steel girders within the box cell, the presence of structural steel members under an expansion joint in the roadway surface, and the impossibility of replacing the steel girders in case of need.
- Pier expansion joints are used to support the end spans of continuous frames. Continuous frames require long end spans to avoid uplift at the end bearings when live loading is placed on the interior spans. A pier table at the expansion joint is erected first. The two end segments are blocked and secured together on two lines of sliding bearings with concrete blocks in the expansion joint and temporary post-tensioning tendons to create a monolithic pier table, and short balanced cantilevers are erected on either side. After constructing the adjacent hammers and closing the wet joints with the expansion-joint hammer, the prestressing tendons throughout the joint are released and the concrete blocks in the joint are removed to release movement. Pier expansion joints facilitate bearing inspection and maintenance and minimize the weight of the deck, but additional piers and the shorter end spans of the continuous frames disrupt the visual repetitiveness of long bridges.
In 81 pages in full A4/letter format, Balanced Cantilever Construction of Precast Segmental Bridges provides exhaustive coverage of the topic. It explores bridge design, the loads and stiffness interactions to consider for the design of piers and superstructures, the stability of tall bridge piers under the loads applied by self-launching gantries, the midspan closure operations, the design of cable-stayed precast segmental decks, and the expansion joints of balanced cantilever bridges. It also explores loads, kinematics, performance and productivity of the self-launching gantries for balanced cantilever construction, and the different families of lifting frames.
The eBook is indeed a milestone of the bridge engineering eManuals project. Combined with Short Line Match Casting of Precast Segmental Bridges (42 pages), the two monographs:
- cover the entire construction process of precast segmental bridges erected as balanced cantilevers;
- explain how balanced cantilever construction influences bridge design and precasting operations;
- explore bridge design for modularity and the factors that drive the choice between precast segmental technology and in-place casting with form travelers;
- explain 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;
- explain how to generate the casting curve in relation to fabrication sequence, erection sequence and time-dependent effects, the geometry control of short-line casting (inclusive of commercial software programs and how they work) and the progressive correction of geometry errors during short-line casting;
- explore the long-line method for varying-depth balanced cantilevers, the post-casting operations, different organizations of the stockyard, and segment delivery and epoxy gluing at the erection site.
Last but not least, the eManual Construction Cost of Precast Segmental Bridges (134 pages) and the companion estimation spreadsheet explore the construction cost of precast segmental decks. The segment fabrication costs include the setup costs of precasting facilities and the production costs of the short- and long-line method. Segment transportation includes trucking, trains and barges, with or without intermediate staging areas. Segment erection includes span-by-span and balanced cantilever construction and the setup and production costs of the different types of special equipment. The estimation spreadsheet includes 1004 cost items (yes, you have read well: one thousand and four) and three columns for each cost item: construction costs, opportunities (potential of cost savings), and risks (potential of extra costs).
When combined, the monographs of BridgeTech provide 349 pages of exhaustive coverage of all the construction methods and all the types of special construction equipment for precast segmental bridges. If you thought that ASBI Construction Practices Handbook for Concrete Segmental and Cable-Supported Bridges was the international reference for the design and construction of precast segmental bridges, you will be greatly surprised.
The eManuals complement 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.