Erection Day Wind and the Short Jib
A 60 m jib crane is being erected with a 45 m jib configuration for the first phase. The climbing frame and the first mast sections are up; the jib and counter-jib are on the ground ready for the mobile crane. Forecast is 30 km/h mean with gusts to 45 km/h at 10 m, and the exposed site is on a hill. The erection supervisor says the manufacturer's chart allows erection in 'wind up to 12 m/s' and wants to continue. Your anemometer at the mast top reads 9.8 m/s mean with peaks of 13.5 m/s. The mobile crane's chart for the pick radius is at 88% of capacity for the jib section. Two riggers are new to tower crane erection and are being directed by the supervisor by radio only.
1. Assess whether erection should proceed, identifying every factor that bears on the decision.
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Erection should not proceed. The supervisor's reading of the limit is wrong in a way that matters.
- Wind measurement point. A limit expressed in m/s is normally referenced to a defined height and averaging period, and the manufacturer's manual states which. My mast-top reading of 9.8 m/s mean with 13.5 m/s peaks already exceeds a 12 m/s limit on gusts, and the 10 m figure understates what is happening at jib height on an exposed hill. Gusts, not means, govern the loss of control of a suspended jib section.
- Mobile crane margin. At 88% of chart capacity there is no allowance for wind-induced dynamic load, for the jib section catching the wind as a sail, or for the two-crane/rigging interaction. A pick that tight is not a pick to make in gusty conditions.
- Competence and communication. Two riggers unfamiliar with erection, directed by radio only with no line of sight confirmed, is a control failure independent of wind. Radio-only direction during a critical pick removes the ability to see a developing problem and stop it.
- Configuration change. Erecting a 45 m jib on a crane designed for 60 m changes the counterweight and load chart; the manual's erection sequence and wind limits must be followed for that configuration, not the generic figure quoted.
Decision: secure the crane and the ground-level components, lower the jib section to its stands, and wait for conditions inside the manual's limit with a margin. Rebrief the rigging team and establish line-of-sight signalling before restarting.
2. What must be verified about the crane's stability and structure before the jib is pinned, and why does it matter on this site?
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Before the jib is pinned I need to confirm:
- Foundation and ballast as designed for this configuration. The counterweight and ballast schedule for a 45 m jib differs from 60 m. If the wrong ballast is fitted, the crane is either over-stressed at the mast or has insufficient moment resistance. Check against the manufacturer's configuration table, not memory.
- Climbing frame and mast plumb/bolt torque. Every mast section connection must be torqued and recorded, and plumb checked, before a long cantilever (the jib) is added. An out-of-plumb mast under a jib load increases bending on one side.
- Slew brake and temporary restraints. During erection the crane is not yet a balanced machine; the jib is a cantilever until the pendant bars are tensioned. The slew must be free or restrained as the manual requires, and the jib must be supported until pendants are connected.
- Site-specific ground. On a hill with exposed ground, the crane base and the mobile crane's outriggers sit on material whose bearing capacity and slope stability must be confirmed. A pad that was adequate in dry weather may not be after rain.
The reason this matters here is that the wind is loading a structure that is temporarily in its least stable state. The exam is testing whether you understand that the crane's rated capacity assumes a completed, correctly ballasted, plumb machine - not one mid-erection.