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A complete worked design of pretensioned NU girders for a two-span, continuous highway overpass, from section properties through serviceability, camber and ultimate limit state checks.
Precast, pretensioned NU girders made continuous over an interior pier are a common choice for highway overpasses in Alberta, but their design depends on tracking how stresses change as the structure moves from a simply supported girder to a composite, continuous and integral frame. This course follows a single worked example of a bridge with two equal 48 m spans crossing a divided highway. Learners see how material properties, transformed composite section properties, strand layout with deviated strands and hold-down points, and load distribution are established before any checks are made, all in line with the Canadian Highway Bridge Design Code (CSA S6).
The course then works through each construction and service stage in turn. Learners calculate prestress losses immediately after transfer and over the long term, check serviceability limit state (SLS) stresses at transfer, during deck construction and in service, and quantify restraint moments at the pier caused by creep, differential shrinkage and thermal gradients. A practical method for estimating camber at transfer, at deck placement and at the end of service life is presented. The example closes with ultimate limit state (ULS) flexural resistance using strain compatibility and shear design at the abutment and pier, giving bridge engineers a clear, reusable calculation framework.
What Will Be Covered:
Design basis, materials and transformed composite section properties for an NU girder
Strand layout with straight and deviated strands, hold-down points and hold-down force limits
Construction staging from simply supported girders to a continuous, integral structure
Dead, construction and live load effects, including live load distribution by the simplified method
Short-term and long-term prestress losses
SLS stress checks at transfer, during deck construction and in service
Creep, shrinkage and thermal gradient restraint moments at the pier
Camber and deflection estimates from transfer to end of service life
ULS flexural resistance by strain compatibility
ULS shear design at abutment and pier using the general method
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