Edinburgh Trams provided some technical details when first considering the overhead system and these are produced (in summary) below.
The use of wall fixings on tramways is well precedented and has been in use for over 120 years on tramways throughout the world. Similar fittings are also used on trolleybus systems. There are approximately 380 tramway systems currently in operation, of these 87 have been built in 25 countries since 1978, there are 29 more to be completed in the next 2 years. There are approximately 400 trolleybus systems currently in operation.
Trams are usually supplied with electric traction power from overhead line systems at a voltage not exceeding 750 Volts direct current nominal. Tram Overhead Line Equipment [O.L.E.] can be supported by poles with cantilever arms. It can also be supported by span wires between poles or building attachments and this minimises the amount of equipment placed in the street. The height of the contact wire that supplies the electriccurrent to the tram, or any other live part of the overhead electric traction supply system must not be less than 5800mm above the surface of any carriageway except where lower headroom is necessary beneath existing bridges over the tramway. Higher clearances may be used where a road is designated as a High Load Route, or there are technical or aesthetic advantages to be gained by so doing. At other places accessible to the public, the position of the contact wire or any other live part of the O.L.E. must not be less than 5200mm above the ground.
The insulation of the live overhead equipment from the span wire attached to a building is achieved either by the insertion of at least two insulators into a metal span wire, or by the use of non conducting synthetic material for the span. Both systems are currently in use on tramways within the British Isles and meet, or exceed, the requirements set out by HMRI (The Railway Inspectorate) and the appropriate Electricity Regulations.
The use of buildings to support electric traction systems is well established, the approach was used in many of the ‘first generation’ electric tram and trolleybus systems and is used at some point on the majority of the ‘second generation’ systems throughout the World.
The span ‘wire’ (which can be made of metal or a non metallic and non conducting material) is anchored to the buildings on either side of the street by means of a wall fixing. These usually consist of an expanding bolt that is inserted into the façade of the building from the outside. This is set in a grout material in the drilled hole and is expanded to grip the sides of the hole. In the majority of cases the tramway undertaker does not need to gain access to the interior of the building to fix, or maintain, the fitting. Fittings of this type have been used with success on brick, stone and mass concrete buildings.
Fixings cause no damage to a building as the stress applied to the building façade is within the reserve strength of normally sound building. A waterproof seal can be achieved between the fixing and the building and no damage from water penetration or corrosion staining would be expected, given minimal maintenance by the installer.
Indeed, many buildings in Glasgow still have wall fixings from the ‘first generation’ tramway – often over 40 years since they were last used or maintained.
Where buildings incorporate pre-stressed or reinforced concrete beams and columns, integrity of the beam must not be compromised by drilling into it to provide the fixing. In this case, a band around a structural element has been used successfully to support the wire. Where a building has an external glass skin, completely enclosing the structure, a flexible grommet may be fitted around a small hole drilled in the glass, allowing the span wire to pass through the outer skin and to be anchored to the building frame.
The powers to attach equipment to a building can be found in section 15 of the Edinburgh Tram Line One and Edinburgh Tram Line Two Acts. Where a Listed Building is specified in part 2 of Schedule 10, no equipment can be attached to that building without first obtaining Listed Building Consent.
Edinburgh Trams will have powers (subject to the prior approval procedure of the
local planning authority – vide s.70 of Bill) to affix equipment to any building
provided that the promoter gives 28 days notice to the relevant property owners of their
intention so to do.
There would be an initial structural survey of the building, as required, before a
rosette was attached to the building as part of the O.L.E.installation programme.
The height of the contact wire or any other part of the overhead electric traction supply system should not be less than 5800mm above the surface of any carriageway. It may be higher (up to approximately 6.2m) where a road is designated as a High Load Route, or there are technical or aesthetic advantages to be gained by so doing. The span wire will adopt a catenary shape across the road, with the ends where it attaches to the buildings being higher than the point where it supports the contact wire to accommodate the sag of the span wire. The increase in height will depend on the width of the street, but it can be assumed that for most streets the span wire rosettes will be fitted at about second floor level in a typical building.
The span wire will either be fitted with insulators to isolate it from the live overhead wire, or it will be made of a non-conducting material. This means that the span wire cannot become live. The live conductor wires are situated over the tram tracks, the insulators (where they are required) will be provided close to the conductor wire. This means that anyone leaning out of a building will be beyond touching distance from any live electric conductors.
The overhead line equipment generates very little noise. The only contact between the moving tram and the wire is made by a self lubricating sliding contact on the top of the collector equipment mounted on the roof of the tram. This generates minimal noise as it moves along the wire. The overhead wire system is flexible and tends to dissipate any noise and vibration. The insulators serve to break any metal-to-metal path for noise to pass into the building. During operation of the tramway the quantity of acoustic energy transferred through the overhead line equipment will be very small and will not lead to noise or vibration levels that will affect the users of the building.
If building maintenance work needs to be undertaken close to the live equipment a requirement to obtain an agreement from the tramway operator as to a safe method of working. This safe method of working may require the electrical isolation of adjacent tramway electrical equipment and hence the work may have to be done out with normal tramway operating hours.
In the case of Listed Buildings no equipment can be attached to that building without Listed Buildings Consent, in other cases of buildings with architectural, townscape or historic significance the Edinburgh Tram will take advice from the City of Edinburgh Planning Department and from all other relevant authorities as to the precise location and design of wall rosettes. Wall fixings have been applied sympathetically to many historic buildings elsewhere in Europe
The new type of wall fixing
The wall fixing consists of 3 main components; a threaded stud for insertion in the wall, a screw on cap and attachment ring and a locking nut. The span ‘wire’ – which may be made of metal and fitted with two insulators, or may be made of non-conducting material. It is terminated with a loop and attached to the attachment ring by a fixing link. After a suitable period to allow the grout to mature the fixing is subject to a pull-out test. This test applies a load to the stud of approximately 14kN; this is 200% of the normal working load of 7kN on the attachment. If the fitting passes this test it is ready to support the overhead line equipment which will be fitted at a later date. The individual components of the fixing are designed to provide a factor of safety of 20kN, i.e. almost three times the normal maximum working load.
The precise location of the fixings and the distance apart depends upon the local ground conditions and the degree of curvature of the road – and hence the track – in both the horizontal and vertical plane. Generally speaking the wire will need support and location every 20-40m.
The benefits of reducing street clutter and possible obstruction to building facades by use of building fixings in place of support columns is obvious. The impact that tramway building fixings will have on the maintenance regime for buildings is no greater than that created by the provision of overhead power supply lines, overhead telephone lines or the use of wall mounted street lighting apparatus.
Below are photographs of some of the Rosettes in place between Haymarket and York Place. This is the only area, apart from the Gyle A8 underpass, to have Rosettes by August 2016.
The images can be viewed individually or as a slide show.
The second page can be accessed by following the link at the end of this page or by clicking here.
R I B E produced this design, based on a modern replacement for the original Rosette, for possible use on Edinburgh Trams.
Image from the Aberdeen Tram Rosette Appreciation Society.
Overhead line fixings as seen in the A8 road underpass.
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