Project: Back propping for 450T mobile crane on suspended concrete slab
Location: Kirrawee, NSW
Sector: Commercial / residential construction
Client: CIP Constructions
Services Provided: Temporary works design, back propping system supply
Equipment: Used Legend 100T Modular Propping System, Multiprops, hardwood timber sleepers
Project Overview
CIP Constructions engaged Legend Hire to design and supply a back propping system allowing a 450T mobile crane to operate from a suspended concrete slab in Kirrawee, NSW. Mobile crane outrigger reactions are 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 outrigger loads through the slab and down to a competent bearing level. The crane completed all scheduled lifts from the suspended slab with structural loads managed within design limits.
The Challenge
A 450T mobile crane was required to carry out lifts from a suspended concrete slab. Crane outrigger reactions arrive as highly concentrated point loads, well beyond what the slab was designed to carry in service. Without a back propping system, those loads would have been imposed directly on a structure never intended to receive them.
The site geometry added a further constraint. Access conditions were difficult, and several propping locations sat above a lower suspended level rather than above ground bearing. At those positions, a single level of propping would simply have relocated the problem one floor down. Two levels of propping were required to carry the load through to a competent bearing level, with the upper and lower props aligned so that the load path remained continuous and vertical through the structure.
Propping also had to be installed and maintained within an active construction site, without obstructing ongoing works or the crane’s own set-up footprint.
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 directly beneath the outrigger positions, providing high-capacity vertical slab support with a verified load path.
- Multiprops: installed in a surrounding configuration to distribute load into the slab over a wider area and reduce local punching demand at the propping head.
- Hardwood timber sleepers: cribbed beneath the props to spread load into the supporting surface and provide a stable, level founding platform on an uneven site.
At locations sitting above a lower suspended level, propping was installed across two levels, with the lower level positioned to maintain vertical continuity of the load path down to a competent bearing level.
Legend Hire coordinated the design and set-out with CIP Constructions and the crane contractor so that propping positions matched the confirmed crane set-up and outrigger footprint before installation commenced.
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. Where props are offset between levels, load is reintroduced into a slab that was not designed to carry it. Set-out alignment between propping levels was therefore critical to this design.
Load distribution. Concentrated outrigger reactions were spread through a combination of modular propping at the primary load points and Multiprops around them, reducing localised demand on the slab.
Bearing pressure. Hardwood timber sleeper cribbing distributed prop reactions into the supporting surface and provided the level, stable base required for accurate prop installation.
Equipment Used
- Legend 100T Modular Propping System: high-capacity modular propping for concentrated structural loads
- Multiprops: adjustable steel props for load distribution and secondary support
- Hardwood timber sleepers: load spreading and founding platform
The Outcome
The engineered back propping system carried the full outrigger reactions of the 450T mobile crane through both propping levels and into a competent bearing level. The crane completed all scheduled lifts from the suspended slab with structural loads managed within design limits.
- Suspended slab protected from outrigger loads exceeding its design capacity
- Continuous vertical load path maintained through two levels of propping
- All scheduled crane lifts completed from the suspended slab
- Zero incidents / no structural damage recorded
- Programme maintained
Frequently Asked Questions
Crane back propping is a temporary works system installed beneath a suspended slab to support the concentrated loads applied by a mobile crane’s outriggers. It transfers those loads through the structure to a competent bearing level, preventing the slab from carrying loads beyond its design capacity.
Back propping is required whenever crane outrigger reactions exceed the design capacity of the slab the crane is set up on. Because outrigger loads arrive as concentrated point loads, this is the case for most mobile cranes operating over suspended structures. The crane contractor’s confirmed outrigger reactions are assessed against the slab’s capacity to determine what propping is required.
Loads are transferred through a vertical propping system installed directly beneath the outrigger positions. Props carry the load from the slab down to the level below, and timber sleeper cribbing spreads the reaction into the supporting surface. Where the level below is also suspended, propping continues through multiple levels until a competent bearing level is reached.
Back propping systems are designed as engineered temporary works, using confirmed outrigger reactions from the crane contractor and structural information for the supporting slab. Legend Hire designs and supplies back propping systems for mobile crane operations across Sydney and NSW.