Medium-span precast segmental bridges can be erected span-by-span and as balanced cantilevers. There are some differences in the segment production for the two types of bridges, but the main differences between the two construction methods are the different costs of post-tensioning and the erection equipment.
Post-tensioning is typically more expensive in balanced cantilever bridges because the deck supports itself and the weight of the erection equipment during construction. Most top-slab tendons are anchored at each and every joint, the average length of the tendons is shorter than in span-by-span bridges, and the cost of tendon anchorages and the labor demand of frequent post-tensioning operations further increase the cost of prestressing.
The erection equipment, however, is less expensive as the cantilevers are self-supporting during construction. As thoroughly discussed in Balanced Cantilever Construction of Precast Segmental Bridges (81 pages), self-launching gantries and lifting frames for balanced cantilever construction are designed for the weight of the individual segments. A self-launching gantry for span-by-span construction supports an entire span of segments before gluing of the epoxy joints and application of post-tensioning, and the gantry is heavier, more expensive, and slower to reposition. Exhaustive information on span-by-span construction is available in the collection Span-by-Span Construction of Precast Segmental Bridges (134 pages).
With balanced cantilever construction, the post-tensioning operations are on the critical path of deck erection. After applying a new segment to the tip of each cantilever, fabrication and stressing of the new pair of tendons must be completed before a new segment erection cycle can start, and the erection gantry cannot be fully utilized. Erecting adjacent balanced cantilever bridges simultaneously by side shifting the gantry from one bridge to the other enhances productivity and diminishes the criticality of operations.
The segments are released as soon as the temporary prestressing bars for joint gluing are stressed, and the gantry can erect a new pair of segments on the second bridge during the post-tensioning operations on the first bridge. Erecting 2+2 segments on two decks every day offers the same productivity as erecting 4+4 segments on one deck, and surveying of the cantilevers is much easier. However, two sets of pier-cap stabilization gear are necessary when the deck is supported on one line of bearings as the gantry cannot be used to stabilize the hammers.
Simultaneous erection of adjacent decks is lowering the entry threshold of balanced cantilever construction to 50-55m constant-depth spans. Compared with span-by-span erection, the lower cost of the self-launching gantry soon offsets the higher cost of post-tensioning in relation to more demanding construction stresses and shorter tendons.
Balanced cantilever construction offers many advantages in urban bridges where restrained pier locations often require irregular spans. A self-launching gantry for span-by-span erection is designed for the longest span of the bridge and is under-utilized when many spans are shorter. A gantry for simultaneous balanced cantilever erection of adjacent decks offers efficient segment handling and post-tensioning operations regardless of the length of the spans, and the most expensive components of the gantry can be reused in other machines with minor or no reconfiguration.
Balanced cantilever erection of adjacent decks has been successfully used on 50-65m constant-depth spans with cycle times similar to span-by-span erection. A 60m balanced cantilever span has 8-segment cantilevers, and segment erection (2+2 segments per day on each deck) requires about 4 days. Adding fabrication of the pier tables, midspan closure and repositioning of the gantry, a 10-day cycle time for two adjacent spans can be achieved within a few cycles and maintained consistently. The cycle can be accelerated further if precast segmental pier tables can be erected with ground cranes.
Span-by-span erection of adjacent 60m spans offers a similar cycle time, but a self-launching gantry for balanced cantilever erection is lighter, less expensive and much easier to reposition than a self-launching gantry for span-by-span erection. Balanced cantilever erection also leads to flexible construction procedures, as hammers on short piers can be erected with ground cranes during gantry erection on tall piers, and multiple erection lines can therefore be set up without major investment.
There are a few disadvantages though. A gantry for balanced cantilever erection needs two winch-trolleys to achieve the same productivity, while an overhead gantry for span-by-span erection carries one winch-trolley only. Additional disadvantages of balanced cantilever erection include:
- Complex segment production due to the presence of internal post-tensioning ducts in the segments. This is a disadvantage only if the segments for span-by-span erection are designed for external post-tensioning.
- Complex geometry control, especially when the deck is erected linearly from abutment to abutment. Geometry control is simpler when adjacent balanced cantilever decks can be erected simultaneously. The two erection methods can be combined when most of the piers of the bridge are parallel but some are not. In this case most of the spans are erected simultaneously by side shifting the gantry, and short portions of the bridge are erected linearly with reverse launch of the gantry until the next pair of adjacent piers is reached to support the wide crossbeams for side shifting.
- More expensive post-tensioning because of shorter tendons and more tendons and anchorages. This is especially true in light-rail transit bridges where spans erected span-by-span are left simply supported. When the spans are stitched together with wet joints to achieve longer continuous units that diminish the number of expansion joints of highway bridges, the span tendons are designed to resist self-weight bending in simply supported configuration, short tendons are necessary in the negative bending regions of the deck, and the cost of post-tensioning of span-by-span erection increases.
- Expensive pier crossbeams and double sets of pier-cap stabilization gear are necessary when the gantry is used for simultaneous erection of adjacent bridges, as the gantry cannot be used to stabilize the cantilevers.
Photo: Courtesy VSL