Widening and strengthening of Kingston Bridge

• Multi-span masonry arch bridge
• 2D modelling of new arch structures
• Local transverse analysis to determine forces in spandrel walls

Detailed inspections of Kingston Bridge in Kingston upon Thames, Surrey, revealed numerous defects in the 1914 structure and load assessments resulted in vehicles of over 38 tonnes being prohibited from crossing the bridge.  To allow strengthening of the bridge for increased loadings a permanent widening scheme was chosen to allow traffic flows to be maintained during strengthening works and to help reduce the environmental impact during the construction work. It also provided the opportunity to introduce permanent facilities for buses and cyclists. Designers Symonds Group Ltd used LUSAS Bridge to model the spans of the widened structure for its client the Royal Borough of Kingston.

The bridge widening adds 6.6 m to the previous width of 17.5 m.  New piers comprise reinforced concrete stems founded on a reinforced concrete pile cap with bored cast in-situ concrete piles. The piers are faced with natural Portland Stone.   Precast concrete arch units were used for constructing the spans of the widened bridge to minimise the disruption to river traffic during construction. The precast units forming the spans of the widened bridge are made continuous over the crown using coupled reinforcing bars with in-situ concrete. These precast units are faced with masonry and brick to match the existing arch profile and finishes. An in-situ lightweight reinforced concrete saddle is cast on top of the precast units to form a composite section. Starter bars project from the extrados of the precast units to provide continuity with the saddle.

The spans of the widened bridge were analysed using a 2-dimensional LUSAS Bridge model.  Each span was idealised as being restrained at each end by the piers. To accommodate shrinkage and thermal effects movement joints are incorporated at the end of each arch.  The transverse stiffness of the arch was ignored in the analysis. A plane stress analysis of unit width was therefore carried out using plane stress elements. In addition the stiffening effects of the spandrel walls were not taken into account in the design of the composite reinforced concrete arch barrel.  A local transverse analysis was carried out to determine the forces in the spandrel walls.   End support conditions were modelled to simulate the degree of fixity provided during the construction of the bridge and also in the permanent situation.  The analysis therefore considered the three pinned, two pinned and fixed situations.

The superstructure was modelled globally as a five span masonry arch using another analysis program. This analysis was used to investigate the out of balance forces in the piers and hence to check their stability during the construction phases and when strengthening is completed.  The analysis of the lightweight reinforced concrete saddle and brick arch composite behaviour was based on a nonlinear analysis of the central river span. The material model for the existing brick arches assumed no tensile stresses. The reinforced concrete saddle was modelled using linear elastic properties based on uncracked section behaviour.  The behaviour of the finite element model was benchmarked against a single span model by comparing the order and position of hinge formation under a point load positioned close to midspan.

For more details of the widening and strengthening project for this bridge visit the London Bridges Engineering Group website.