When assessing for structural integrity of lighting columns, a site’s classification as ‘exposed’ can be a critical factor. But, as recent work on the M5 Avonmouth Bridge has highlighted, understanding site exposure can also mean having to unpick the nuances of the standards.
By David Lodge
Occasionally, I get asked for advice on managing assets in some interesting situations which are a little out of the ordinary. Avonmouth Bridge on the M5 is a good example.
This article explores the management of lighting columns at exposed sites, using the M5 Avonmouth Bridge as a case study. It explains the definitions of ‘normal’, ‘exposed’, and ‘very exposed’ sites as outlined in PD6547:2023 and CD354, highlighting some of the inconsistencies I feel there are between these guidelines.
The objective is to inform lighting designers and asset managers on how to recognise these site classifications and the importance of seeking expert advice when making changes to lighting columns in such environments.
INTRODUCTION
The M5 Avonmouth Bridge, spanning the River Avon, is a vital infrastructure component that faces unique challenges due to its exposed location.
The bridge’s lighting columns, mounted on a deck 30m above the river, are subject to harsh environmental conditions, including high winds and coastal exposure. Therefore, there needed to be some important additional considerations for managing the lighting columns at such an exposed site, which this article will aim to unpick.
The management of lighting columns primarily focuses on structural integrity, wind loading, and environmental exposure.
Documents, including BS EN 40, PD6547 and DMRB CD 354, provide guidelines for designing and assessing lighting columns. However, I would argue there is a gap in the practical advice and case studies that illustrate these standards in real-world scenarios.
When assessing Avonmouth Bridge and the structural integrity of its columns, the site’s classification as ‘exposed’ was a critical factor.
The combination of a coastal terrain, elevation, and the bridge’s mounting structure placed it firmly outside what is considered a ‘normal’ site under standard design assumptions. But, first, what actually do we mean by an ‘exposed’ site?
UNDERSTANDING AN ‘EXPOSED’ SITE
The definitions of exposed sites are slightly different across the different documents used to assess lighting columns. Understanding whether a site is classified as ‘normal’, ‘exposed’, or ‘very exposed’ is crucial for understanding the risks associated with the site and affects the method used for assessing the structural integrity of lighting columns.
- ‘Normal’ sites (all documents). These are areas where wind conditions are typical and not significantly influenced by local topography or other factors. Lighting columns mounted at ground level, within inland locations having a site altitude of 250m above mean sea level (aMSL) or lower and not subject to topography or local wind funnelling.
- ‘Exposed’ sites (as defined in PD6547:2023). Lighting columns must be:
- at site altitude exceeding 250m aMSL;
- mounted in elevated positions (for example, on bridges, overpasses, or the top deck of a multistorey car park); and
- with significant topography or local wind funnelling.
These sites require special consideration in design and assessment because of the increased wind loads and environmental exposure.
However, the defining criteria ignores locations that are not covered by the rationalised wind loading method presented in PD 6547. For example, the Shetland Isles, where windspeeds are likely to lead to wind pressures exceeding ‘extra heavy’. It also does not include exposed coastal locations, for which, again, a design method is provided within PD 6547.
- ‘Very exposed’ sites (as defined in DMRB CD 354). These encompass minor structures:
- at site altitude exceeding 250m aMSL;
- coastal sites within 5km from the coast; and
- sites subject to significant local wind funnelling.
These sites require extra consideration where CD 354 is specified and wind loadings at very exposed sites will be derived using Eurocode 1 (BS EN 1991-1-4). However, while this calculation method has the ability to more accurately calculate the wind pressures including the more exposed phenomena, the assumptions made in BS EN 40 for terrain categories are not followed and the resulting wind pressures are likely to produce lower (less conservative) designs than applying the wind loading methods in BS EN 40-3-1.
In sum, it appears that an overarching definition of exposed sites would be useful here, which could be used in combination with BS EN 40-3-1 and PD 6547:2023. The criteria for exposed sites and very exposed sites from above would produce the following identification criteria:
DEFINING EXPOSED TERRAIN
Terrain categories reflect the relative roughness of the ground caused by permanent features (buildings, forests, hedges, and so on) present within the terrain. They are generally categorised as: Terrain Category (TC) 0 – sea, I – coastal, II – country, III – town/suburban, IV – city.
As the wind passes from sea, to coastal, to country terrain, the wind speed adjusts to the increased roughness by slowing down.
This effect is most pronounced close to the ground, with roughness affecting the windspeed less and less as height above ground level increases.
The majority of windspeed adjustment to terrain changes can take place in as little as 5km but can take as much as 100km. The effect of multiple changes of terrain roughness within this distance is to cause variations in windspeeds both up and down as the wind passes across the area. Windspeed is therefore quite site specific.
Coastal terrain can be defined as sites within 5km of the coast, and this can be easily measured on a mapping website by selecting the site location and then scanning in all directions to see whether the coastline crosses a virtual 5km radius circle.
One issue with the practical application is that ‘the coast’ is not clearly defined as a term. In practice, a good definition would the high-water mark. This is of course the boundary line between the land that is washed by the high tide and the land which remains dry.
For natural coastlines, for example beaches, mud flats and tidal marshland, this is the inland edge of the tidal zone where there is no large vegetation, hedges, trees or other obstacles, providing increased roughness.
This coastline should be seaward of any permanent sand dunes. For man-made coastlines, the line of the dock wall, promenade, sea wall, or other building or structures, ignoring small or isolated features like piers, pontoons or markers within the tidal zone.
Applying this approach makes selecting the location of the coast on the satellite map layer relatively easy.
However, in thinking about site roughness there are other scenarios which can lead to confusion. Large inland bodies of water, for example including many of the lochs in Scotland and those in the Lake District, provide an over-water fetch longer than 5km and these can enable windspeeds to increase due to the reduced terrain roughness.
For this reason, minor structures sited on the downwind lakeshore can be more exposed than those on the upwind lakeshore.
Considering the likely change in terrain category that would result, for inland water bodies greater than 5km in length the terrain category would decrease (numerically and in roughness).
In practice, these locations should be classified as TC I – coastal, lakes, or areas with negligible vegetation and without obstacles or more simply perhaps TC I – coastal and lakeside.
Clearly, the selection of correct terrain category requires an understanding of the wind loading standards method of calculation and some experience of assessing terrains in line with those standards.
If in doubt, it is better to select the lower terrain category/roughness for a site as this will give conservative results, However, bear in mind this may make the design, checking and approval process for the structural design or assessment more onerous and there would be benefit in taking expert advice in these circumstances.
PROPOSED DEFINITION FOR EXPOSED SITES
The follow list combines the identification criteria from PD 6547 and CD 354 giving a comprehensive list of conditions where a site should be considered exposed.
In general, these are locations, where the rationalised wind-loading factor method in PD 6547:2023 is not applicable and site-specific design loads are calculated to BS EN 40-3-1 (where CD 354 is not applied) or to BS EN 1991-4 (for CD354). Exposed sites include minor structure locations:
- In the Shetland Isles where basic wind speeds are very high.
- With site altitudes exceeding 250m aMSL for normal exposure sites.
- With TC I – coastal or lakeside (within 5km of the coast as defined by the high-water mark, adjacent to large lakes, or areas with negligible vegetation and without obstacles).
- Mounted on elevated structures (over 2m above ground level).
- With significant topography (orography) features.
- where significant impacts from local wind funnelled are present.
M5 AVONMOUTH BRIDGE
When considering these issues in relation to the M5 Avonmouth Bridge, the structural assessment of the bridge lighting columns revealed several key findings:
- Exposed location. The bridge’s coastal terrain and elevation (30m above the River Avon) classify it as an exposed site.
- Wind loading. Calculations were completed accounted for the coastal location and the elevated mounting position, checking the columns to ensure they could withstand the applied loads.
- Ageing columns. The age of the existing column meant there was a risk that the columns would be unsuitable for the new wind loading conditions.
Following an initial assessment of the columns using a desktop survey to collate existing information about the columns, a structural assessment was completed that showed the columns would be unsuitable based on the limited information and conservative assumptions made.
This was a significant setback, as the replacement of these lighting columns was expected to be both challenging and expensive given their location on the bridge, beyond the safety barriers, and 30m over the water surface of the river.
Studying the structural assessment calculations results showed the critical section of the columns to be the door opening.
Information from site indicated that the columns benefited from internal door reinforcement to strength the sides of the door opening. During the planned site visits to complete structural condition surveys, the contractor was able to collect detailed information including the door reinforcement dimensions.
Together, the condition survey and the structural dimensions provided sufficient information to enable a more accurate and realistic structural assessment calculation, including the door strengthening. As a result, the existing lighting columns were found to be acceptable for the new wind loading.
CONCLUSIONS
The example of the M5 Avonmouth Bridge underscores the importance of understanding the environmental exposure conditions when managing lighting columns.
The definitions of normal, exposed, and very exposed sites are crucial in this context. The Avonmouth Bridge exemplifies an exposed site because of its coastal location and elevation.
The structural assessment and approval process demonstrated the need for detailed calculations and expert advice to ensure the safety and durability of the lighting columns.
The inconsistencies identified between PD6547 and CD354 highlight the need for a unified approach to defining exposed sites. The proposed new definition aims to provide clearer identification criteria, addressing the gaps between existing guidance and specifications.
In sum, lighting designers and asset managers are encouraged to recognise these site exposure classifications at the earliest opportunity and seek expert advice when making changes to lighting columns in challenging environments.
David Lodge is owner of David Lodge Lighting and chair of the B/509/50 British Standards Committee for Lighting Columns
This is an abridged version of the article that appears in the July-August edition of Lighting Journal. To read the full article, simply click on the page-turner to your right
Image: David Lodge Lighting
