The Cambered Forecourt
Two operatives need to replace a floodlight using a tower with a 6.0 m platform height. The tower is 1.0 m wide and 2.5 m long and is to stand on a tarmac forecourt that falls about 1.5% towards a drain. One adjustable leg would sit directly over a 600 mm square manhole cover. The forecast is 25 mph gusting 35 mph, and the tower will be up for 'only 30 minutes'. Outriggers are available, but there are no spreader pads. The site manager says stabilisers can be left off because the job is short, and a colleague suggests winding the legs down fully to level the tower and packing the manhole leg with a piece of timber. The operatives have a spirit level but no manufacturer's instructions on site.
1. Can the tower be erected in the forecast wind? Explain the decision.
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The tower should not be erected until the manufacturer's maximum permitted wind speed has been confirmed. Many mobile towers have a relatively low permitted wind speed when erected, often around 17 mph (about 7.7 m/s) unless the manufacturer states otherwise, and a forecast of 25 mph gusting 35 mph is likely to exceed that limit. The fact that the tower will be up for only 30 minutes does not remove the risk: a gust can act on the tower during that short period, and an unplanned event such as a door opening or a sheet being carried past can increase the load. The correct decision is to postpone the job, relocate the work to a sheltered area, or use a different access method. If the manufacturer's instructions state a higher limit, the operatives still need to monitor conditions and stop if the wind increases. Without the manual, the safe assumption is that the tower is not suitable for the forecast.
2. What are the problems with the ground and the proposed levelling method?
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A 1.5% fall is not automatically unacceptable, but the tower must be erected vertical and within the manufacturer's adjustment limits. Winding one or more legs down fully to compensate for the slope can overload the legs or reduce the tower's stability, so the operatives must not exceed the marked adjustment range. Packing a leg with timber is unsafe because timber can crush, split or slip, and it does not provide a firm, continuous support. The manhole cover is also unsuitable: it may not be designed for the concentrated load from a tower leg, it may move, and bridging it with a single leg concentrates load on one point. The correct approach is to relocate the tower so that all legs bear on firm, level ground, use spreader pads to distribute the load, and adjust the legs only within the permitted range. If the slope cannot be accommodated, the tower should not be used on that spot.
3. What should be done about the outriggers, stabilisers and moving the tower?
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Outriggers or stabilisers are not optional extras to be left off because the job is short. At a 6.0 m platform height, the manufacturer is very likely to require them, and the manual will state the configuration, the maximum height for each configuration and any ballast requirements. The operatives should fit them exactly as specified, check that all locking devices are engaged and ensure the feet are on spreader pads where needed. The tower should not be moved with people, tools or materials on the platform, and the manufacturer's procedure for moving must be followed, including retracting or adjusting outriggers if required. If the manual is not available, the tower should not be erected because the operatives cannot confirm the correct stabiliser configuration. The site manager's 'short job' argument does not override the product instructions or the stability requirements.