Project: Temporary column support for lift shaft construction
Location: Surry Hills, NSW
Client: Buildcorp
Temporary works engineer: RBG Temporary Works Engineers
Services provided: Temporary works design, equipment supply, hydraulic jacking and pre-loading
Equipment used: Through-bolted steel column clamps, fabricated steel needle beams, 100T hydraulic jacks, Durapac P-Series two-speed hand pump, modular props, raking props, cross bracing, temporary concrete footings
Project Overview
Buildcorp needed two live-loaded concrete columns supported in Surry Hills, NSW, so their footings could be undermined for a new lift shaft. With poor ground conditions on site, Legend Hire and RBG Temporary Works Engineers designed a through-bolted steel clamp and needle beam system, pre-loaded with 100T hydraulic jacks onto temporary concrete footings.
The Challenge
A new lift shaft required excavation directly beneath the footings of two existing concrete columns. Once the footings were undermined, the columns would lose their load path to ground. The full column load therefore had to be carried by a temporary structure before any excavation started.
Ground conditions made this harder. The excavation passed through clay over weathered sandstone, and props could not bear directly on the existing ground without risk of settlement. Any movement in the temporary works during load transfer could cause the columns, and the structure above them, to drop. The support system also had to stay clear of the lift pit footprint so excavation could proceed.
The Solution
Legend Hire worked with RBG Temporary Works Engineers to design an engineered temporary works system that transferred each column’s load into needle beams and down to temporary footings.
Each column was drilled through and clamped with a stiffened structural steel clamp, through-bolted across the column. The clamp transferred the column load into a pair of deep fabricated steel needle beams, one either side of the column. Each beam end bore on a 100T hydraulic jack founded on a new concrete pad cast into the existing slab. Adjustable cross braces, pin-connected to cleats beneath the needle beam flanges, tied each beam pair together. Raking props restrained the assembly laterally during jacking and excavation.
With the clamp, needle beams and bracing installed, Legend Hire pre-loaded the system with the 100T hydraulic jacks, operated from a Durapac P-Series two-speed hand pump. The pump’s high-flow first stage advanced the jacks quickly to take up the gap. Its high-pressure second stage then gave fine, incremental control as the column load transferred off the existing footings and into the temporary works. Pre-loading was completed before any excavation began beneath the columns.
Below the slab, modular props with screw jack bases were installed on temporary concrete footings benched at the edge of the lift pit excavation. These footings spread the prop loads into the poor ground while excavation continued alongside.
The temporary works were coordinated with Buildcorp’s excavation sequence. Undermining began only once the system was fully installed and pre-loaded.
Engineering Considerations
- Load transfer. The load path runs from the column, through the through-bolted steel clamp and needle beams, into the hydraulic jacks, and down to the temporary concrete footings.
- Clamp connection. The clamp was designed to carry the full column load through the bolted connection, without relying on friction against the concrete.
- Controlled pre-load. The two-speed hand pump allowed rapid take-up followed by fine, incremental jacking, so load came onto the needle beams gradually rather than in one step.
- Lateral stability. Deep needle beams on jacks need restraint against lateral movement. Cross bracing and raking props provided it throughout jacking and excavation.
- Bearing capacity. Temporary concrete footings spread concentrated jack and prop loads into clay and weathered sandstone, limiting settlement.
- Excavation clearance. The temporary works were arranged to keep the lift pit footprint clear for excavation.
Equipment Used
- Through-bolted stiffened steel column clamps
- Fabricated steel plate needle beams
- 100T hydraulic jacks
- Durapac P-Series two-speed hydraulic hand pump
- Adjustable aluminium cross bracing
- Raking props for lateral restraint
- Modular props with screw jack bases
- Temporary concrete footings
The Outcome
Both columns were supported throughout the undermining works. The column loads were transferred into the engineered temporary works under controlled hydraulic pre-load, and Buildcorp could excavate and construct the new lift pit beneath the existing structure.
Legend Hire has delivered related temporary works for Buildcorp in Surry Hills, including back propping a concrete stair landing for masonry demolition. Similar needle beam approaches have been used for needle beam wall support in Ultimo and cantilever needle propping in Neutral Bay. For another project involving propping during structural column replacement, see Rouse Hill.
Explore Legend Hire’s temporary works engineering, modular propping systems and hydraulic jacking equipment.egend Hire has delivered related temporary works for Buildcorp in Surry Hills, including back propping a concrete stair landing for masonry demolition. Similar needle beam approaches have been used for needle beam wall support in Ultimo and cantilever needle propping in Neutral Bay. For another project involving propping during structural column replacement, see Rouse Hill.
Frequently Asked Questions
Column needling is a temporary works method in which steel needle beams are clamped or bolted to a column and supported on props or jacks. The column load can then be carried while its footing or lower section is altered or removed.
Pre-loading transfers the structural load into the temporary support before the permanent support is removed. This controls deflection and settlement during load transfer.
The column load is transferred into an engineered temporary structure, such as a steel clamp, needle beams, jacks and temporary footings, before excavation begins beneath the existing footing.
Where ground conditions can’t take concentrated prop or jack loads, temporary concrete footings spread the load and limit settlement.