Base Position on a Suspended Slab
A builder's hoist is to be installed at a six-level apartment project. The only position that keeps the landing gates within 900 mm of the floor slabs is on the suspended ground slab over the basement car park. The manual gives a maximum base load of 62 kN on a 2.0 m × 2.0 m base frame. The structural drawings show the suspended slab is rated at 5 kPa imposed load, with no propping below and the nearest column footing 4.5 m away. The foreman says the slab 'holds a concrete pump truck, so it will be fine' and wants the hoist erected tomorrow; the crane and rigging crew are booked for one day only. The hoist will carry renderers and their materials to level six.
1. Work out whether the proposed base position is acceptable, and justify your answer with numbers.
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Calculation
- Base load: 62 kN
- Bearing area: 2.0 m × 2.0 m = 4.0 m²
- Average bearing pressure: 62 ÷ 4.0 = 15.5 kPa
Against a 5 kPa slab rating, the average pressure alone is about three times the allowable. That figure is before any allowance for:
- dynamic/vibration effects from repeated starts and stops;
- eccentric loading (a cage loaded to one side, or the base frame not bearing evenly);
- the overturning moment from wind on a 24 m mast, which transfers into the slab through the base and the ties.
The 62 kN in the manual is a maximum figure that assumes the base is fully and evenly supported - a slab that deflects under load does not provide that.
Verdict: not acceptable as proposed. The position cannot be used until a structural engineer certifies how the load is carried.
2. The foreman argues the slab 'holds a concrete pump truck'. Why doesn't that settle the question?
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Because the two loads are not comparable in the way that matters.
- A pump truck is transient: it is on the slab for hours, and it is normally set up on outriggers positioned over columns or ground beams, so its load is delivered as a few concentrated reactions straight into the structure.
- The hoist applies a permanent, repeated, vibrating load in one spot for the whole build, plus wind overturning moments through the base and every tie, plus the extra load from the ties anchored into the slabs above.
- A truck might also have been the reason the slab was propped at the time, or it might have been placed under supervision - the foreman's recollection does not establish any of that.
'Something heavier has been here before' is an observation, not a structural check. The real questions are what the load path is and what the slab was designed for, and only the structural engineer can answer them in writing.
3. What would you need before erection could proceed at this location, and what are the fallback options?
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Fallback options, in order of preference:
- Relocate the hoist to ground carried on a footing or ground beam, with a landing platform/bridge to reach the slabs (watch the 900 mm gate-to-slab gap).
- Prop the slab from the basement, with the props, soleplates and load path to the footing designed and certified.
- Engineered load spread - grillage or spreader beams sized to get pressure under 5 kPa. That means roughly 12 m² of bearing area before dynamic factors, which is a large footprint on a site where the gates must sit 900 mm from the slab edge.
- Change the access method entirely (scaffold, EWP) and re-sequence the renders.
Documentation needed before erection at this position:
- written certification from the structural engineer for the base support and the tie loads into the structure;
- confirmation the proposed configuration is within the manufacturer's manual (base frame, tie spacing, free-standing height);
- updated rigging/erection plan, revised SWMS, and a briefing to the rigging crew;
- the person with management or control of the site approving the change - not the foreman's assurance.