Assessing the Strength of Existing Road Bridges

It is estimated that there are approximately 160,000 bridges in the UK, 90,000 of which are owned and maintained by Central or Local Government and the remainder belonging to Network Rail, the Canal and River Trust (formerly British Waterways) and Sustrans.

In 1987, the Department for Transport introduced a national bridge assessment programme aimed at identifying those bridges on the national highway network that were in need of strengthening or replacement prior to the introduction of heavier (40 tonne) HGV weight limits in 1999 to comply with European legislation. Two years later, the local authorities in the UK started their own complementary programme, resulting in a massive programme of inspection, assessment, repair and replacement of bridges. Although all of the trunk road structures have now been assessed, there is still a significant amount of bridges that have not been assessed under the original programme.

In addition, the promotion of renewable energy generation has seen a proliferation of wind farm developments, resulting in a significant increase in the transportation of ‘Abnormal’ Vehicles (i.e. those with axle load/spacing that is beyond the scope of normal highway loading standards). This means that all of the highway structures (frequently on minor rural roads) on the route to the proposed site need to be assessed for the abnormal wind farm vehicles. This has been a major work stream for the SYSTRA bridges team in the last few years.

To assess the condition of a bridge, Principal Inspections are undertaken every 6 years, supplemented with less detailed General Inspections every two years in between. A review of the original assessment is undertaken at the same time to ensure that there has been no further deterioration of the structure, change in operational load carrying requirements or indeed change to assessment standards that may affect its load capacity. If any of this criteria has changed then the structure is re-assessed.

Occasionally there are no as-built records available for the structure and an intrusive investigation is undertaken to determine the structural elements and material strengths.

The Design Manual for Roads and Bridges (DMRB) has evolved to include standards and guidance for the assessment of highway bridges and, perhaps more importantly, gives guidance on appropriate strategies for monitoring structures that fail the assessment in the interim period until remedial action can be undertaken. Given the constraints on public spending, this provision is frequently used by bridge authorities to justify the delay in strengthening structures that fail assessment but are deemed suitable for monitoring.

Following the assessment, a strategy is determined by the bridge maintainer to prioritise strengthening and reconstruction works, taking into consideration of a number of factors such as location, route designation, traffic flows, local conditions, availability of alternative routes and environmental problems.

Historically, bridge design standards did not require the consideration of accidental loading on footways and parapets caused by an errant vehicle and consequently it is common for structures to have sufficient capacity under the carriageway but not under the footways or parapets. In this scenario bridge authorities often use ‘partially effective barriers’ (e.g. trieff kerbs or temporary metal barriers) to protect non-carriageway parts of the structure in the interim period.

SYSTRA has extensive experience in undertaking inspections, assessments and checking of existing bridges in the UK and overseas. Our scope of services in this area includes:

• Inspection of bridges to both Principal Inspection and Inspection for Assessment levels;
• Supervision of specialist investigation contractors to determine structural data not available in the record information;
• Monitoring of structures during abnormal vehicle movements;
• Special Inspections of post-tensioned concrete bridges to BD54/15;
• Assessment of bridges to current standards to determine current load capacity taking into account the condition of the bridge;
• Recommending remedial measures to improve the capacity of sub-standard bridges; and
• Preparation of design and specification of remedial strengthening measures

For further information please contact Walter Smyth wsmyth@systra.com