Load Chart Dispute on a Rooftop Deck Pour
A 24 m boom is set up on a suspended concrete deck to place concrete for a plant room 18 m away. The deck is rated at 5 kPa. The boom's rear outriggers sit over a beam line; the front outriggers sit at mid-span between beams. The structural engineer's drawing shows the deck can carry the pump but does not give outrigger reaction figures. The operator proposes to use the machine's maximum outrigger reaction from the manual as a conservative figure. The pour is 60 m³, and the deck has a 1.2 m high parapet along the edge nearest the pour. The site is 12 km from the nearest hospital and the crew has one first-aider.
1. Evaluate the operator's proposal to use the maximum reaction figure and explain what the engineer actually needs.
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Using the maximum outrigger reaction from the manual is conservative in the sense that it will not understate the load, but it is not sufficient for a suspended deck assessment. The engineer needs the reaction at each individual outrigger for the actual planned configuration: boom radius, boom angle, load in the line, counterweight, and the position of the boom relative to each leg. Reactions are not equal across the four legs, and the critical case is usually the leg nearest the load at maximum radius, not the machine's nominal maximum.
A single maximum figure applied to all four legs may also overstate load on some legs, leading the engineer to reject a workable setup or to specify unnecessary propping. Either way, the assessment is not accurate.
The correct input is a reaction envelope: the maximum and minimum reaction at each leg across the planned range of boom positions, plus the dynamic allowance the manufacturer specifies. The engineer then checks the deck for the critical leg, considering the beam line versus mid-span positions, and specifies any propping or load spreading required.
2. Identify the additional risks created by the deck edge, the parapet and the remote location, and how they change your planning.
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The 1.2 m parapet limits how close the boom can be positioned and restricts the operator's view of the pour area, so the operator will be working by signal rather than by direct sight. That makes the signal system critical: a designated signal person with a clear line of sight, agreed signals, and a radio backup. The parapet also creates a fall hazard for anyone working near the edge and a trap for the boom if it swings low.
The deck edge means the outrigger nearest the edge is the one with the least structure beneath it and the highest consequence if the deck fails; the engineer's assessment must specifically cover that leg. The remote location and single first-aider mean the emergency plan must account for a response time of at least 12 minutes plus scene access, so the plan needs a nominated person to meet and guide emergency services, a clear access route, and first-aid equipment appropriate to crush and fall injuries. I would confirm the plan with the crew before the pour and check that the first-aider is not the pump operator, who cannot leave the controls.
3. What would you do if the engineer's assessment arrives but does not cover the front mid-span legs?
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I would not proceed. An assessment that covers only the rear legs over the beam line leaves the front legs, which sit at mid-span where the deck is weakest, unverified. Mid-span is precisely where the deck has least capacity, so those are the legs most likely to control the design.
My action is to request a supplementary assessment for the front legs at mid-span, giving the engineer the reaction envelope for those legs and the plan position of each pad. In the meantime, I would consider whether repositioning the machine so all four legs sit over or near beam lines is feasible within the required radius, since that may avoid the supplementary assessment entirely. If neither is possible before the pour, the pour is postponed.
I would record the request and the postponement, and inform the PCBU and the concrete supplier so the delay is managed rather than absorbed silently.