Project: Back propping for mobile crane outriggers on a suspended concrete slab
Location: Lidcombe, NSW
Sector: Commercial construction
Client: Harris Air Conditioning
Services Provided: Temporary works design, back propping system supply and installation
Equipment Used: Legend 100T Modular Propping System, Multiprops, hardwood timber sleepers
Project Overview
Harris Air Conditioning engaged Legend Hire to provide a back propping solution for a mobile crane operating from a suspended concrete slab at a shopping centre in Lidcombe, NSW, allowing new air conditioning units to be lifted into position. Mobile crane outrigger reactions arrive as concentrated point loads that routinely exceed the design capacity of a suspended slab. Legend Hire engineered a back propping arrangement using the Legend 100T Modular Propping System, Multiprops and hardwood timber sleepers to transfer those loads through the slab to a competent bearing level, with all propping installed inside a loading dock that remained in operation.
The Challenge
The crane was required to set up on a suspended concrete slab to reach the plant positions. Outrigger reactions are highly concentrated point loads, well beyond what a suspended slab is designed to carry in service. Without back propping, those loads would have been imposed directly on a structure never intended to receive them.
The constraint that shaped this job was operational rather than structural. The propping had to be installed and maintained in a live loading dock during operating hours. The dock is a working service area for the centre with vehicle movements, waste handling and trade access continue regardless of construction activity. Every propping position had to be located clear of trafficked routes, physically protected against impact, and set out without closing the dock.
Overhead conditions added a further constraint. The soffit carries services, and the available headroom is limited, so prop configurations had to suit the actual clear height at each position rather than a nominal figure.
Engineering Considerations
- Load path continuity. The governing principle in any back propping design is that load must travel in an unbroken vertical path from the point of application to a competent bearing level. Propping was set out directly beneath the confirmed outrigger positions so that load was not reintroduced into slab spans not designed to carry it.
- Load distribution. Concentrated outrigger reactions were spread through a combination of modular propping at the primary load points and Multiprops arranged around them, reducing localised demand on the slab at each position.
- Bearing pressure. Hardwood timber sleeper cribbing distributed prop reactions into the supporting surface and provided the level, stable base required for accurate prop installation on a slab with fall to drainage.
The Solution
Legend Hire designed a back propping arrangement to receive the crane outrigger loads and distribute them safely through the structure.
The system combined three elements:
- Legend 100T Modular Propping System: deployed at the primary load points beneath the confirmed outrigger positions, providing high-capacity vertical support with a verified load path.
- Multiprops: installed in a surrounding configuration at each position to distribute load into the slab over a wider area and reduce localised punching demand at the propping head.
- Hardwood timber sleepers: cribbed beneath each prop cluster to spread the reaction into the supporting surface and provide a level, stable founding platform.
Each propping cluster was founded on hardwood sleepers and barricaded with warning tape on installation, keeping the props clearly delineated from vehicle and pedestrian movement for the duration of the works. Positions were set out clear of marked traffic lanes so the dock could continue operating around the temporary works.
Legend Hire coordinated the design and set-out with Harris Air Conditioning and the crane contractor so that propping positions matched the confirmed crane set-up and outrigger footprint before installation commenced.
The Outcome
The engineered back propping system carried the full outrigger reactions of the mobile crane through the suspended slab to a competent bearing level. The crane completed all scheduled lifts, placing the new air conditioning units into position.
- Suspended slab protected from outrigger loads exceeding its design capacity
- Continuous vertical load path maintained beneath each outrigger position
- All air conditioning units lifted and placed as scheduled
- Loading dock remained operational throughout the propping period
- Zero incidents, no structural damage and no impact to temporary works recorded
- Programme maintained
Frequently Asked questions
Yes, provided the outrigger loads are supported by an engineered back propping system. Suspended slabs in car parks and loading docks are designed for distributed vehicle loading, not for the concentrated point loads a crane outrigger applies. The crane’s confirmed outrigger reactions are assessed against the slab’s capacity, and propping is designed to carry the difference down to a competent bearing level.
A vertical propping system installed directly beneath each outrigger position. High-capacity modular props carry the primary load, additional adjustable props distribute it into the slab over a wider area, and hardwood timber sleeper cribbing spreads the reaction into the supporting surface below.
Yes. Back propping is engineered temporary works, designed using the crane contractor’s confirmed outrigger reactions together with structural information for the supporting slab. Legend Hire designs and supplies back propping for mobile crane operations across Sydney, Brisbane and Melbourne.