Project: Mobile Crane Outrigger Support for Suspended Carpark Slab
Location: Hurstville, NSW
Sector: Commercial Construction
Services Provided: Temporary Works Engineering, Equipment Hire, Installation
Equipment Used: Legend 100T Heavy Duty Modular Propping System, Hardwood Timber Sleepers
Project Overview
A contractor required temporary structural support beneath the outriggers of a large mobile crane positioned over a suspended concrete carpark slab.
With limited structural documentation available for the existing slab, and the crane loads in excess of its original design capacity, the risk of punching shear failure was significant. Legend Hire engineered a heavy duty modular propping solution using the Legend 100T Modular Propping System mounted on stacked hardwood timber sleepers to distribute crane loads safely across the slab and deliver the lift programme without incident.
The Challenge
The crane was positioned in a active carpark outside the construction zone, asuspended concrete slab with no confirmed structural drawings and limited information on the existing reinforcement layout, slab thickness, or original design loads.
This created a compounding set of structural and logistical risks:
Unknown slab capacity: Without verified structural documentation, the slab’s ability to carry concentrated crane outrigger point loads could not be confirmed. Any load above the slab’s actual capacity risked shear failure at the contact points.
Load concentration: Mobile crane imposed high concentrated loads over a relatively small footprint. Without spreading and vertical support, these loads transfer directly to the slab as point loads; the most critical failure mode for suspended concrete structures.
Live carpark environment: The carpark remained in active use throughout the works. Temporary fencing was installed to delineate the work zone, and the propping layout was constrained by existing infrastructure including structural columns, overhead services, and active traffic lanes.
The propping system needed to resolve all three constraints: manage load concentration, compensate for unknown slab capacity through conservative load distribution, and be installed efficiently within a live, occupied environment.
The Solution
Legend Hire’s engineering team designed a load distribution and propping arrangement calibrated to the crane’s outrigger loads and the site’s structural unknowns.
Hardwood timber sleeper pads were constructed in stacked layers beneath each outrigger position to spread the crane’s concentrated point loads across a significantly larger area of the slab surface. The stack height was built up to achieve the bearing level required to reach the slab soffit, with the overall footprint engineered to reduce the unit load intensity delivered to the slab at any single point, directly addressing the punching shear risk on a structure of unconfirmed capacity.
At corner bay positions, a concrete bearing pad was placed between the sleeper pack and the prop bases to provide a level, high-strength bearing surface at the critical load transfer point, and props were installed in pairs to suit the load geometry at the structural boundary.
Legend 100T Heavy Duty Modular Propping System was installed vertically from the sleeper packs to the soffit of the suspended concrete slab, providing direct vertical structural support through the slab to the bearing layer below. The 100T modular system was selected for its high working load capacity, modular configurability to suit the slab height and geometry, and suitability for installation in constrained, live environments.
The combined system created an engineered load path that safely transferred crane outrigger loads through the suspended slab and into the supporting structure beneath, allowing all lifts to proceed within the planned programme.
Engineering Considerations
The primary engineering challenge on this project was the absence of verified structural documentation for the existing slab. Where structural drawings are available, propping design can be calibrated precisely to confirmed slab capacity. Where they are not, the design must adopt conservative assumptions: load distribution must be maximised and vertical support positioned to limit imposed load intensity to levels compatible with the likely capacity range of the structure.
Hardwood timber sleeper perform a critical load-spreading function in this context. By increasing the contact area between the outrigger and the slab surface, they reduce the unit load (kPa) applied to the slab at any single point, providing a safety margin that compensates for structural uncertainty. The stacked layering approach allowed the bearing height and footprint to be adjusted precisely to the geometry and load requirements at each outrigger position.
The Legend 100T Modular Propping System provided the rated vertical capacity required to carry the crane’s outrigger loads, with the modular format allowing configuration to the available headroom and layout geometry within the carpark structure.
For projects requiring suspended slab support during structural modification works, see Legend Hire’s suspended slab propping for column demolition and structural steel installation. For crane operation on suspended slabs using back propping to stable footings, see Legend Hire’s crane back propping on a suspended slab.
Equipment Used
Legend 100T Heavy Duty Modular Propping System, a high-capacity modular propping system configured to suit the slab soffit geometry and crane outrigger load requirements. Selected for its rated 100T working load capacity, modular adjustability across varying heights, and proven performance in constrained, live site environments.
Hardwood Timber Sleepers, built up in stacked layers at each outrigger position to spread crane point loads across a wider slab bearing area. Footprint and stack height engineered at each position to reduce unit load intensity and manage punching shear risk on a slab of unconfirmed structural capacity.
Concrete Bearing Pads, placed between sleeper packs and prop bases at corner bay installations to provide a level, high-strength bearing surface at the critical load transfer interface.
The Outcome
The Legend 100T Modular Propping System, combined with engineered hardwood sleeper load distribution, safely transferred mobile crane outrigger loads through the suspended carpark slab throughout the full lift programme. The contractor completed all lifts without structural incident. The slab was protected from punching shear risk despite the absence of verified structural documentation, and the live carpark environment was maintained safely throughout the works with no disruption to the surrounding tenancies.