scope:

INTRODUCTION TO STANDARD DESIGNS AND DETAILS

The development of the Standard Designs and Details (SDD) has been undertaken by Network Rail to improve safety, asset reliability and increase efficiency. Their development is linked to Network Rail's overall business objectives to improve reliability of the railways and reduce the funding requirements for the on-going management and maintenance. The basis of the SDD focuses on two main areas that derive from these issues:

• Ensuring that the design meets Network Rail's requirements

• The design is as successful as the designs of previous railway engineers, who built and maintained bridges that have given good service for approaching 150 years.

Network Rail's Requirements

Network Rail's requirements are split between two areas - cost effective delivery, and function:

Delivery Requirements

The SDDs have been taken to a stage where Form As and Bs for each aspect covered (overbridge design, ancillaries etc.) have been submitted and approved. This leads to the following benefits:

• A reduction in the design development timescales and costs.

• Minimising contractor and sub-contractor costs associated with uncertainties in detailing requirements.

• Streamlining the technical approval process for commonly used designs and details.

Functional Requirements

The SDD have been designed to ensure satisfactory performance of the asset under both normal operations and abnormal operations (both planned and unplanned). A further consideration has been Network Rail's requirement to reduce the volume of maintenance and management costs through the adoption of good practice. This leads to a number key design drivers including:

• Failure modes - critical failure modes should give warning, and alternative load paths should be provided for potential local failures.

• No hidden details - all main structural elements should be visible from at least one side.

• Robustness - it is desirable for elements of the structure to have a degree of robustness so that they are not damaged by unforeseen events disproportionate to the cause.

• Capability to support load

• Acceptable deformations

• Structure gauge requirements - the overbridges have been designed to cater for a range of positions of the structure gauge allowing their wide use.

• Safe working environment - the bridges have been designed to minimise the risk to people on or about the bridge.

These and other functional requirements are defined in Network Rail standard NR/L2/CIV/020 (Draft 012) and the requirements therein have been met in designing the Modular Overbridges.