Road bridge strengthening and Improving the load bearing capacity
Stone road bridge strengthening
Built around 1850, the Smithy Bridge is a stone construction, with a single span of 6.7m, crossing Agden Dyke which flows from the Agden Reservoir. Following a structural assessment, the bridge was restricted to a gross vehicle weight of 10 tonnes and so did not meet the current required load bearing standards for highway bridges
Design challenges within a conservation area
Goldhawk was contacted by Amey engineers via Helifix to undertake a survey of the bridge and establish whether it could be strengthened sufficiently to increase its capacity from 10 tonnes to 40-44 tonnes in order to meet current standards. As it is in the Low Bradfield conservation area, which lies within the Peak District National Park, it was also important that any repairs preserved the bridge’s aesthetics.
The Goldhawk Bridge Restoration solution
- A full structural assessment of the bridge was carried out using the ASSARC computer software program, enabling an optimised repair and strengthening scheme to be devised by the MARSYS computer package.
- The bridge strengthening grid pattern was marked out on the underside of the arch and narrow longitudinal and radial slots cut into the masonry.
- Stainless steel HeliBars were bonded into the slots using HeliBond cementitious grout and interlocking CemTies installed at right angles to the HeliBars at the grid intersections. Normal structural movement was accommodated with minimal disturbance to the retained original masonry.
- The entire concealed reinforcement was encapsulated with an elastic structural adhesive, a durable polyureide resin with high bond strength, which can be colour matched to leave the bridge virtually unchanged.
Preparing the site
A working plaform is setup under the bridge and the working area segregated from the public. The highway remains open during the works.
Damage and previous repairs
Damage and previous temporary repairs are clearly visible on the underside of the arch. These are made good where appropriate.
Safety and protection of the area
The working area is made safe and plastic sheeting restricts debris from falling into the water.
Slots cut into the masonry
Recess slots are cut into the masonry in a grid pattern in readiness for the installation of the helibar cage that forms the structural element of the MARS system.
Interlocking CemTies and Helibars
Holes are drilled 300mm into the arch rings and hooked CemTies installed to bind the cage to the arch.
Encapsulation of the structure
Once installed, the cage is then encapsulated with a polyureide resin (MARFLEX) and stricken off to 5mm below the surface ready for pointing.