Case study snippet
Amey is undertaking a comprehensive structural assessment of the Kincardine Bridge on behalf of Transport Scotland. The 822m long structure, constructed in 1932, has 33 spans of differing structural forms and carries road and rail traffic over the River Forth. The swing-span bridge was designed to pivot to 90 degrees to allow shipping to pass through but was permanently closed to shipping in 1988.
Here we focus on Amey’s assessment of the swing-span section of the bridge where cutting edge analytical tools were used to give a detailed and accurate picture of the bridge and its capacity. These findings will inform a targeted repair and strengthening strategy, helping to prolong the life of this historic structure.
The challenge
Key technical challenges included modelling complex global articulations and assessing steel buckle plate flooring.
The 111m swing-span has a self-weight of around 1,500 tonnes. It comprises two riveted steel Warren-type truss girders with supporting vertical hangers and bracings at the centre bay. The truss supports cross girders, longitudinal stringer joists, a steel buckle plate deck with concrete infill and a disused control room that had been used to operate the swing-span.
The footways are formed using curved steel cantilever brackets supporting a flat plated steel deck with concrete infill. A double webbed girder at the bottom of the swing-span rests on 60 steel rollers arranged on a circular runway which formerly enabled the pivoting motion.
The bridge's bearing articulation was complicated by its former pivoting capability. Uplift forces had been applied historically via rolling wedges at the ends of the swing-span to avoid "chatter" (deflections) under the worst loading conditions. The inducement of these wedges to cause uplift effectively added bearings which modified the bridge's articulation from balanced cantilever with free ends to a continuous structure. This resulted in the stress distribution from traffic actions differing from the stress distribution from permanent actions.
Amey’s analysis of the swing-span assessed all conceivable global articulations and any amalgamations of these. This approach was designed to capture possible extremes of exhibited behaviour. Amey's additional sensitivity verifications also accounted for sliding bearings seizing when subjected to temperature fluctuations and uncertainties in the magnitude of uplift induced by the rolling wedges.
Our approach
Amey’s comprehensive structural assessment is in line with the newly implemented DMRB CS 454 standard. The quantitative analysis encompasses the entire superstructure, substructure and foundations and the assessment will account for reported deterioration to some of the critical structural elements observed in recent inspections.
- Global analysis approach
For the swing-span section, Amey developed a 3D finite element model using Midas Civil 2020 software (See Figure 3). The cantilever footways were assessed locally, with reactions applied to the global model. Dummy members simulated the buckle plate decking which was also separated out from the global model for a localised analysis. - Efficiency in assessment
Midas Civil 2020 software add-on features were used meet recently implemented CS 454 and CS 458 Traffic Action assessment protocols. These simulate almost countless iterations of vehicle loading configurations in quickly and efficiently – taking only seconds - to demonstrate the most onerous action effect envelopes. This software also optimised the application of the dynamic effects of vehicles as they crossed the swing-span section, as well as the positioning and factoring of traffic lanes. - Assessing complex behaviours
The bridge uses buckle plate deck systems. These exhibit complex behavioural characteristics and are a largely outdated structural form. Consequently, the availability of present-day technical literature about buckle plates is limited.
The buckle plates used in the Kincardine Bridge have a span greater than 1.2m which renders the simplified assessment approach in CS 456 inappropriate. Instead, Amey analysed action effects using a 3D space frame model comprising a grid of beam elements.
Elementary checks on actions, support reactions, deformed shape and deflections were undertaken to validate the model behaviour as much as reasonably practical. In addition, load effects were proportioned between the concrete infill layer and the buckle plate based on their relative axial and bending stiffnesses.
Amey also used advanced analytical techniques such as a buckling analysis using 3D shell models and 2D line beam models to more accurately analyse and model the actual properties and structural behaviour and improve the associated plate and column buckling slenderness parameters which increased the capacity.
The advanced assessment approach undertaken by Amey significantly increased the load carrying capacity compared to the previous assessment (completed by others) and helped avoid unwarranted remedial or strengthening works.
Achievements
Amey’s assessment work on Kincardine Bridge played a pivotal role in the publication ‘Kincardine Bridge – an engineering triumph 85 years on’ in the ICE Bridge Engineering journal. This publication emerged from a collaborative effort between Amey and their client, Transport Scotland.
Moreover, Amey’s assessment approach, coupled with the integration of advanced analysis techniques to ensure structural safety while minimising the need for remedial/strengthening measures, gained internal recognition through the acquisition of the Outstanding Design Award & the Owen Williams Award in 2022. Externally, the project was ‘highly commended’ in the 2023 CIHT Infrastructure Award.
Benefits
Amey’s use of the latest and most sophisticated analysis techniques to optimise structural behaviour will inform a targeted repair and strengthening strategy and ultimately prolong the life of the bridge.
Our approach successfully met significant challenges including accounting for historic stress and assessing the complex behavioural characteristics associated with buckle plate systems.