Boom tip contact with a low bridge during travel
You are travelling a 100 t all-terrain slewing mobile crane between two work areas on a public road within a construction site. The route passes under a temporary services bridge with a clearance of 5.4 m. The crane is travelling with the boom lowered and retracted, but the boom tip is still 5.9 m above ground because the boom rest is not engaged and the boom has not been fully stowed. The route was checked by a labourer on foot, who measured the clearance at the bridge's lowest point but did not account for the crown of the road or the crane's suspension travel. The site speed limit is 20 km/h and the crane is being driven by a licensed operator. A banksman is walking ahead but is not in radio contact.
1. Identify the hazards and explain why the route check was inadequate.
Read the worked answer
The primary hazard is boom tip contact with the bridge, which could damage the bridge, the boom, or the crane's hydraulic and electrical systems, and could cause the crane to stop or slew unexpectedly. Secondary hazards include striking the bridge with the boom's underside, and the crane becoming wedged under the structure.
The route check was inadequate for three reasons. First, the labourer measured the bridge's lowest point but not the crane's highest point along the whole route; clearance must be checked at the point of minimum clearance and at the crane's maximum height, which may occur elsewhere due to road crown or suspension travel. Second, the road crown means the crane's effective height changes across the carriageway, and the crane may not travel on the centreline. Third, suspension travel means the boom tip moves vertically as the crane crosses uneven ground, so a static measurement understates the risk.
A 5.9 m boom tip under a 5.4 m bridge is a 0.5 m deficit before any of those factors are added. The boom must be fully retracted and stowed on the boom rest, or the route changed.
2. What controls and communication arrangements should be applied before the crane moves?
Read the worked answer
The crane must be configured for travel: boom fully retracted, lowered and secured on the boom rest, with the boom rest pin engaged if the manufacturer requires it. The operator must confirm the travel configuration against the manufacturer's manual, because some cranes require the boom to be fully retracted and stowed for road travel and others allow a lowered boom.
The route must be re-surveyed by a competent person who measures the actual clearance at the bridge and records the crane's travel height in the stowed configuration, including suspension travel and road crown. If clearance is marginal, the route should be changed.
Communication: the banksman must be in radio contact with the operator, with an agreed set of signals and a clear stop command. The banksman should walk ahead on the driver's side where visibility is best, and the operator should stop if contact is lost. The site speed limit should be observed, and the crane should approach the bridge slowly and stop before it if there is any doubt. The move should be planned and briefed, with a spotter at the bridge and the area barricaded to other traffic.