Span by span is a relatively new construction technique historically associated with cantilever construction but the advancement in external prestressing has enabled its own potential use to grow. Today it is considered to be the most economic and rapid method of construction available for long bridges and viaducts with individual spans upto 60m.
Span by Span Casting method of Bridge Construction
Decks are begun at one abutment and constructed continuously by placing segments to the other end of the bridge. Segments can be positioned by either a temporary staying mast system through more commonly using an assembly truss.
Before segments are placed the truss with sliding pads is braced over two piers. Depending on the bridge location the segments are then transported by lorry or barge to the span under construction. Each segment is then placed on the sliding pads and slid into its position. Once all segments are in position the pier segment is then placed.
The final stage is then begun by running longitudinal prestressing tendons through segments ducts and prestressing entire span. Deck joints are then cast and closed and ducts grouted. When the span is complete the assembly truss is lowered and moved to the next span where construction cycle begins until the bridge is complete.