Engineering

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Code Rules and their application

Various codes are used in bridge assessment. Highway bridges in the UK are dealt with using the latest version of BD21(2001) and BA16(1997). These are the Departmental Standard and the Advice Note. The Highways Agency has set itself against further updates of these documents despite manifest errors.

For Railways, the relevant code is Network Rail's Line Code of Practice NRGNCIV025.

New highway bridges will be designed to BD91/04 (UNREINFORCED MASONRY ARCH BRIDGES). There are conspicuous discrepancies between BD21, BA16 and BD91.

2010 sees the introduction of Eurocodes which will allow much greater freedom to the engineer and may allow us to sweep aside some of the worst stupidities of (particularly) the Highways Agency codes.

Loads

Two aspects of Loads are covered here, the way they are specified and the way they are modified before they reach the arch.

Specification

Vehicles: The vehicles specified for analysis are the AWR regulations (see clause 6.23 disguised under the heading Wheel Load Dispersal on P 6/8). They are represented by axle patterns listed in Appendix A of BD 21/01. Perhaps surprisingly, the requirement is to analyse for a bogie rather than for the full vehicle. This is because the worst loads are around 1/8 to 3/8 span so if the load is spread over more than ¼ span it tends to reduce the effect.

Axles and Bogies: Most arches are of relatively short span so that the loads from a single axle or group of axles represent the worst load cases. Archie-M has groups of load files to deal with the old C&U regulation vehicles, the modern EU regulation ones and a set of AWR loads to include restricted weights. Note that the load files which do not include impact should not normally be used in assessment. They are provided for the user to explore more general cases if necessary.

HA Loads: For larger bridges, a combination of a distributed load and a knife edge load is specified. This can be achieved by creating two separate loads and applying them together.

HB Loads: These were intended to represent special vehicles and are based on a trailer which went out of use in about 1955. For modern purposes it will be better to check a bridge for a specific vehicle, though an HB rating may be required for some assessments. We have provided a table of loads stepped in 5unit increments and applying only 2 axles. The HB axle is 3m wide and the wheels extend beyond that. The minimum lane width to accommodate an HB vehicle is at least 3.5m. It will, therefore, be necessary to modify the default lane width of 2.5m when considering HB loads. From Version 2.4 the minimum lane width is included as part of the load and prevents this error.

 

Railway Loads: For railway loading we have used standard 25tonne axles,  appropriately spaced. Because railway loading requires the application of centrifugal effects, we have built wheel load patterns which should be applied to a narrower bridge width of 1.7m.

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