Project: Back propping for mobile crane outriggers on a suspended concrete slab
Location: Greystanes, NSW
Sector: Commercial construction
Client: Western Sydney Construction
Services Provided: Temporary works design, equipment hire, installation
Equipment Used: Legend 100T Modular Propping System, hardwood timber sleepers, adjustable steel props
Project Overview
Western Sydney Construction engaged Legend Hire to provide back propping beneath mobile crane outriggers operating over a suspended concrete slab at Greystanes, NSW. Crane outrigger reactions arrive as concentrated point loads that routinely exceed the in-service design capacity of a suspended slab. Legend Hire supplied the Legend 100T Modular Propping System, founded on hardwood timber sleepers, to carry outrigger loads from the slab soffit down to a competent bearing level. Restricted access to the propping positions governed the installation methodology. All scheduled lifts were completed with structural loads managed within design limits.
The Challenge
Concentrated outrigger loads on a suspended structure. The crane was required to lift from a suspended concrete slab. Without back propping, those reactions would have been imposed directly on a slab never designed to receive them, with punching shear the governing failure mode.
Restricted access to the propping positions. The propping locations sat within a confined section of the structure basement corridor. Mechanical handling was not possible at the prop positions. Every component of the propping system, base plates, modular sections, head jacks and timber sleepers had to be manually deployed and assembled in place. Component weight and manoeuvring clearance therefore became design constraints, not just logistical ones.
Working around live construction activity. Propping had to be installed and maintained without obstructing ongoing works or the crane’s confirmed set-up footprint.
The Solution
Legend Hire supplied a back propping arrangement configured for both the load demand and the access constraint.
- Legend 100T Modular Propping System was installed vertically from the sleeper base to the slab soffit at the primary load points beneath the confirmed outrigger positions. The modular format was decisive here: the system assembles from discrete sections that can be carried into position individually and built up in situ, which a single-piece prop of equivalent capacity could not achieve in the available access.
- Hardwood timber sleepers were laid beneath each prop base to spread the reaction into the supporting surface and establish a level, stable founding platform.
- Screw jack head assemblies at the prop head provided fine height adjustment to close the gap to the slab soffit and pre-load the prop, ensuring the load path was engaged before the crane took load.
- Adjustable steel props were installed at adjacent positions to distribute load into the slab over a wider area.
Legend Hire coordinated set-out with Western Sydney Construction and the crane contractor so that propping positions matched the confirmed outrigger footprint before installation commenced.
Equipment Used
- Legend 100T Modular Propping System: high-capacity modular propping rated to 100 tonnes, assembled in situ from individually handleable sections
- Hardwood timber sleepers: load spreading at the prop base and level founding platform
- Screw jack head assemblies: fine height adjustment and pre-loading at the slab soffit
- Adjustable steel props: secondary load distribution
Frequently Asked Questions
Where propping positions cannot be reached by plant, a modular propping system is used so that components can be carried in by hand and assembled in position. Base plates, modular sections, head jacks and timber sleepers are handled individually, allowing full rated capacity to be built up in a space too confined for single-piece props or mechanical handling.
Prop capacity is determined by the confirmed outrigger reactions supplied by the crane contractor, together with the number of props sharing the load at each position. Heavy mobile crane outriggers commonly require props rated to 100 tonnes, which is why the Legend 100T Modular Propping System is specified for this application.
Sleepers spread the prop base reaction across a wider bearing area, reducing unit bearing pressure on the supporting surface. They also provide a level, stable founding platform on uneven ground, which is required for accurate vertical prop installation.