Hammills Bridge

THE CHALLENGE

Hammills Bridge is a single span, twin-deck, post-tensioned concrete, beam-and-slab structure carrying the A761 Mill Street over the White Cart Water in Paisley. Post-tensioned concrete bridges are particularly vulnerable to corrosion and severe deterioration where internal grouting of tendon ducts is incomplete and moist air, water or de-icing salts can enter the ducting system. As normal inspections do not identify the internal conditions of the post-tensioning system, the level of uncertainty about the internal condition of tendons increases with time. For most forms of construction, defects tend to be relatively easy to interpret and result in ductile modes of failure, where the element cracks and deforms to give advance warning before eventual failure. However, safety critical defects in post-tensioned concrete are typically hidden, very difficult to detect and may result in a brittle mode of failure where the elements can undergo a sudden catastrophic failure without any advance warning. The primary problem with post-tensioned bridges, which sets them apart from other types, is the difficulty in establishing the internal condition of the tendons because outward signs of distress are not generally expected to occur. For this reason, a programme of Post-tensioned Concrete Bridge Special Inspections (PTSI) was undertaken in the UK which involved Special Inspections of the most vulnerable bridges.

SYSTRA’S ROLE

SYSTRA was appointed by Renfrewshire Council to undertake a structural assessment and Special Investigation of A761 Hammills Bridge in Paisley. This involved carrying out a Principal Inspection to determine the condition of the structure followed by a detailed structural assessment of the complex structure to determine its current load capacity.

A Post-tensioned Concrete Bridge Special Inspection was then undertaken to examine the condition of the post tensioned elements and to assess the future risk. All inspections and investigations were carried out in accordance with the guidelines shown in the Department of the Environment, Transport, the Regions Advice Note BA 50/93: Post Tensioned Concrete Bridges – Planning, Organisation and Methods for carrying out Special inspections (superseded by BD54/15 but current at time of initial phases of Special Inspection) and BD 54/15: Management of Post Tensioned Concrete Bridges.

In addition to the Special Inspection, we also specified and supervised site repair works to two of the centre span beams under the central reserve, which had suffered severe corrosion resulting in spalling of almost the entire soffits. A sacrificial anode system was installed within the repaired section of the beams to stop any further corrosion from occurring.