7.24 The chord marking should indicate the available headroom over a width of not less than 3 metres. The height indicated on the warning signs, determined in accordance with paras 7.8 to 7.12, should be the lowest headroom between the chord limits. On a narrow bridge where the headroom decreases rapidly away from the centre, a 3 metre chord marking is appropriate. On a wide bridge with only a gradual reduction in headroom it might be possible to increase the chord width to 6 metres or more without excessively restricting the signed height, thus maintaining two-way operation for all vehicles that can pass under the bridge. In these circumstances, the warning sign to diagram 531.1 is unlikely to require the supplementary plate to diagram 531.2. Where 16'-6" (5.03 m) headroom is available over much of the carriageway width, but not all of it, diagram 532.3 or 532.3A should be used with the central signs varied to 16'-6" (5.0 m) and the signs on the side chords indicating the clearance available at the near side channel (see paras #7.10 and #7.11 for measurement of clearances). Black and yellow markings shown in diagram 532.2 or 532.2A should then be used over those parts of the carriageway where the clearance is below 16'-6", to give greater emphasis to the restricted height. It is recommended that the yellow parts of the marking should be retroreflective; they may also be fluorescent (see para 7.6). The chord markings shown in diagrams 532.3 and 532.3A may be used in circumstances other than described above, particularly on wide roads where there are two lanes in one or both directions, with the side chord positioned over the left hand lane. Side chords are also useful where vehicles with a height well below that shown for the centre of the road might strike the bridge if keeping close to the left hand side of the carriageway.
COMPOSITE BRIDGES
7.25 Some bridges originally built as arches have been adapted with the addition of girders or beams. Where the arch is the lowest part, the whole structure should be signed as an arch bridge. Black and yellow striped plates (to highlight the profile of the arch) should be suspended from the bridge beam, together with further plates on the arch itself.
7.26 Where the beam is lower than any part of the arch, the whole structure should be signed as a non-arch bridge and plates with black and yellow markings to diagram 530.2 suspended from the arch at the height of the beam. Similarly, where two adjacent beam bridges have different headroom, the plates should be located on the higher bridge at the height of the lower one.
7.27 Experience has shown that these suspended plates will themselves be struck from time to time and that rigidly-mounted aluminium substrates are not suitable. Rubber or other flexible material should be used for the backing, suspended by means of chains or hinges fixed securely to the bridge structure by a method agreed with the bridge owner. The plates should not be fixed rigidly by screws or bolts to the face of the bridge, as there is a greater risk than with flexibly-suspended plates of them being dislodged and falling onto vehicles on the road beneath. The use of rubber-backed plates will help to avoid annoyance to nearby residents from the noise of hanging metal plates striking the bridge structure in wind or vehicle slipstream. It is recommended that the yellow parts of the marking should be retroreflective; they may also be fluorescent (see para 7.6). When the signs are lit, the plates should also be lit whenever practicable. This is particularly helpful where a girder bridge is followed by a more restrictive arch bridge.
OVERHEAD POWER CABLES
7.28 For guidance on signing overhead power cables, see paras 21.9 to 21.12.
OVERHANGING BUILDINGS
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530 Available headroom with imperial and metric units shown on separate signs
May be used with diagram 530.1, 530.2, 572 or 573. The height may be varied (see Appendix C). The metric sign may be omitted or placed to the right of the imperial sign, but must not be used alone
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530A Single sign showing available headroom in both metric and imperial units
May be used with diagram 530.1, 530.2, 572 or 573.
The height may be varied (see Appendix C)
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530.1 Building overhanging part of carriageway
May be used only in combination with diagram 530 or 530A. A distance, an arrow or both may be added.
"building" may be varied to "buildings" or to "structure"
7.29 Where an overhanging building or other structure causes a restriction over part of the carriageway, the plate to diagram 530.1 should be used with diagram 530 or 530A. A distance may be added. Where the building is in another road, an arrow may be added, pointing horizontally to the left or to the right. Details are given on the working drawing (see para 1.8).
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