Dismantling Sequence Error and Assist Crane Set on a Vault
A 55 m tower crane is being dismantled. The assist crane is a 100 t truck crane set on compacted fill over a basement vault. At the planned 30 m radius, its load chart allows 12.5 t. The first jib section weighs 4.2 t and the lifting gear weighs 0.6 t. One outrigger is positioned over the vault, and the mats are 1.2 m by 1.2 m. Wind at the jib is 18 mph. The dismantling crew plans to remove the jib tie bars first because 'the jib is counterbalanced and the assist crane is not yet hooked up.' The manufacturer's dismantling sequence requires the assist crane to be attached and the line tensioned before tie bars are released. The crew also plans to use hand signals because the radios are charging. Tasks follow.
1. Identify the sequence error and explain the structural consequence. What must happen before any tie bar is released?
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The crew has reversed a critical step. The jib tie bars are part of the support system that holds the jib in position. Removing them before the assist crane is attached and the line is tensioned can leave the jib unsupported or partially supported, allowing it to drop, buckle, or swing uncontrollably. The statement that 'the jib is counterbalanced' does not make it self-supporting after the tie bars are released; counterweight balances the crane as a system, not the jib alone during dismantling. The manufacturer's sequence is the controlling procedure and should be treated as a structural instruction, not a suggestion. Before releasing tie bars, the assist crane must be rigged to the jib at the specified point, the line must be tensioned to take the jib load, and the competent person must confirm that the rigging and assist crane are holding. Only then should the tie bars be released in the specified order. If the crew is already committed to a different sequence, work must stop and the manufacturer or a qualified engineer must approve a revised procedure. Dismantling should generally follow the reverse of the approved erection sequence.
2. Evaluate the assist crane capacity, ground support, and wind. What would you require before the pick?
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The suspended load is 4.2 t plus 0.6 t of gear, or 4.8 t, which is well below the 12.5 t chart value at 30 m. Capacity is not the immediate governing issue, but the chart must be verified for the actual boom length, configuration, radius, and outrigger setup, and the rigging weight must be included. The more serious issue is the outrigger over the basement vault. An outrigger reaction can be much higher than the suspended load because it depends on the crane's geometry and counterweight, and a vault roof may not be designed for that concentrated load. The 1.2 m by 1.2 m mats may not be adequate without an engineer's evaluation. The ground must be evaluated by a qualified person, and the outrigger may need to be relocated, bridged, or supported from below. Wind at the jib is 18 mph; the manufacturer's dismantling wind limit must be checked, and the pick should not proceed if gusts exceed it. Before the pick, I would require a written lift plan, verified chart, engineered outrigger support, a level check, and a pre-lift meeting with the assist crane operator and signal person.
3. What communication and exclusion-zone controls are needed, and how should the crew handle the radio shortage?
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Dismantling involves blind or partially blind picks and personnel near suspended loads, so communication must be positive and reliable. The crew should not rely on hand signals from a signal person who is not in clear view of the assist crane operator, and a single signal person should have exclusive control of the crane. If radios are charging, the work should wait until charged radios and a backup are available, or a qualified signal person should be positioned in direct view with an agreed hand-signal set. The signal person must be qualified for the signals used and must know the stop signal. The exclusion zone should keep all non-essential personnel out from under the jib, the assist crane load, and the planned drop area. Tag lines should be used from safe positions, and the descent path should be barricaded. A pre-dismantling briefing should cover the sequence, the stop authority, the wind limit, and the radio channel. If communication fails, the assist crane operator stops. The competent person should confirm the zone and communication before the first pick, and the crew should not improvise around a missing radio.