A 6 m Mobile Tower on a Sloping Warehouse Floor
You are erecting a mobile scaffold tower inside a warehouse to install sprinkler pipe at 6.0 m above the floor. The tower base is 1.5 m × 1.5 m, the platform is at 6.0 m, and the casters are 200 mm diameter. The concrete floor falls about 1.5% toward a drain, and the doorway on the travel route is 2.2 m high. Sprinkler pipe crosses the route at 5.8 m. The crew proposes to build the tower at the store, push it to the work position with one worker on the platform to steady it, then lock the casters and start work. Outriggers are available but have not been fitted. The warehouse supervisor wants the job finished before the next shift and says the tower 'is only moving 20 metres'.
1. Check the height-to-base ratio and decide whether outriggers are required. Show the calculation.
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Use the platform height and the actual base dimension, not the footprint of the casters alone.
- Without outriggers: 6.0 m ÷ 1.5 m = 4.0, i.e. a 4:1 ratio. For a freestanding mobile tower the usual permitted ratio indoors is about 3:1 (and lower outdoors or where the manufacturer specifies less). 4:1 exceeds that, so the tower as proposed is not stable.
- With outriggers set to give, say, a 2.5 m effective base: 6.0 m ÷ 2.5 m = 2.4:1, which is within a 3:1 limit.
So outriggers are required, fully extended and loaded according to the manufacturer's instructions, with the casters locked and the tower checked for level. The 1.5% floor fall is about 0.86 degrees; it is not steep, but it is enough that the tower must be levelled with the adjustable casters or packing and checked with a spirit level, and the casters must be locked and preferably chocked. If the manufacturer's ratio is lower than 3:1, or the tower is sheeted, the outrigger base may need to be larger.
2. Evaluate the proposed moving method and state the correct procedure for relocating the tower.
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The proposal is unsafe for three reasons: a person on the platform can be thrown off or crushed; the tower's stability margin is reduced while rolling; and an overhead pipe at 5.8 m is only 0.2 m below the platform level, so the worker or the guardrail will strike it.
Correct procedure:
- Clear the platform of tools, materials and people.
- Check the route: floor joints, drains, ramps, door headroom and the 5.8 m pipe. The tower at 6.0 m platform height cannot pass under a 5.8 m pipe, so either the pipe section is installed after the tower is in position, or the tower is partially dismantled and re-erected beyond the obstruction.
- Retract or adjust outriggers only as the manufacturer permits for movement; many mobile towers are moved with outriggers raised. Lock casters before climbing.
- Push at the base, never pull from the platform; use enough people and a spotter.
- At the new position, set outriggers, lock casters, check level, refit guardrails and toe boards, then inspect and tag before use.
Moving 20 m does not reduce any of these requirements.
3. Set out the erection sequence and the checks you would sign off before the platform is used.
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Sequence:
- Inspect components: casters, frames, braces, guardrails, toe boards, outriggers, platform decks. Reject bent or cracked parts.
- Position the base, lock casters, fit outriggers and level the tower.
- Build the first frame and fit horizontal and diagonal braces; do not climb a partially braced frame.
- Add frames lift by lift, keeping the platform at a safe working level; install guardrails and toe boards as each level is completed, not at the end.
- Fit the platform at 6.0 m with a safe ladder or internal stair access; never climb the outside or stand on guardrails.
- Check clearance to the sprinkler pipe and any electrical services.
- Inspect: level, outriggers, casters locked, braces, couplers, guardrail height, toe boards, platform decking, access, and the load on the platform.
- Tag and hand over with a brief to the users on load limits and the prohibition on moving the tower while occupied.