A complex temporary support structure has been installed inside a west London church as part of a major remodelling project. NCE reports.
St Jude’s Church, in London’s Earl’s Court, was once renowned for having the “richest congregation in the country”. Built in the 1870s by wealthy landowner Robert Gunter, the church was certainly very grand: its 30m long nave was built with an extra wide transept, giving a maximum width of 23m, reflecting the ambitious scale of the building, and giving it the capacity to accommodate a congregation of up to 1,600 people.
“It required very accurate setting out because we wanted a pile directly beneath each of our props without the need for big chunky pile caps”
Lawrence Smith, Mabey Hire Services
Today St Jude’s is being given a new lease of life, not as a parish church but as the headquarters for St Paul’s Theological Centre, the Church of England’s newest and fastest growing theological college, linked to the global phenomenon of the Alpha Course of Christianity studies.
The building’s transformation from parish church to theological college involves some extensive - and technically adventurous - modification. It has been remodelled by HMDW Architects to provide accommodation over three levels, including the church’s original basement and undercroft. This labyrinth of tiny cellars, with barely 2m of headroom, is being enlarged by excavating a new full-height basement beneath the nave, a task that requires careful temporary support for the delicate masonry superstructure.
The roof of St Jude’s is supported by ornate brick arches that run either side of the nave and above the aisles. These are, in turn, supported by 10 slender cast iron columns resting on masonry plinths on top of brick piers set into the basement floor.
Excavating the new basement meant removing these foundations and everything below the columns. Clearly some method of temporary support was required.
Main contractor Mansell Construction Services asked specialist support equipment supplier Mabey Hire Services (MHS) to suggest an economical solution to this delicate problem.
MHS responded with a design that entailed supporting the entire structure at high level via the brick arches, leaving the cast iron columns hanging.
Its solution involved building a support structure made up of Mabey’s in-house designed Mass 50 props to be erected at mid-span of the arches and supported at ground level on a series of temporary, augered concrete piles installed beneath the existing floor.
The St Jude’s design involved a major departure from the tender documents and initially met with resistance from Mansell’s planner and estimator. Nevertheless, it was to prove decisive: the firm was awarded the main contract largely on the strength of the propping design, according to Palmer.
The design was not without its technical challenges though, and piling tolerances were critical. “It required very accurate setting-out because we wanted a pile directly beneath each of our props without the need for big chunky pile-caps,” explains MHS project team leader Lawrence Smith.
The piles were installed by Hampshire-based contractor Neil Foundations using a compact continuous-flight auger (CFA) rig tracked into the church through the front door. Mansell erected a scaffold platform to support the machine once it was over the threshold, explains Smith. “The suspended timber floor had by then been removed, so inside the door was a yawning void.”
Once inside the door, the piling rig was lifted off the platform with a hoist hanging from an overhead runway beam.
The challenge of placing the 300mm diameter CFA piles accurately was overcome by using precast concrete “doughnuts” - heavy slabs with a central hole just large enough to take the auger - which were carefully positioned in shallow depressions and held in place with a weak cement grout. Once in place, all the piling rig had to do was insert the auger into each doughnut and drill to the required depth.
When the reinforced concrete piles had been installed they were cut to about 50mm below their required level and a smooth, level top of high-strength grout formed on top using a simple cardboard tube. When these had reached their required strength, MHS resin fixed bolts into the tops and started building its support system.
In preparing the masonry arches to receive the temporary supports, Mansell discovered that the ornate “bronze” capitals between the slender iron columns and the arches were in fact hollow and made of sheet copper.
Underneath was a less attractive - but far more practical - cast iron column head. In the light of this information it was decided to remove the copper fascia and to support the arches using fabricated steel beams and stools that fitted perfectly into the column head stiffeners.
These allowed MHS to prop the top of the iron columns directly instead of propping the masonry arches, which meant that the load path from the arches remained unaltered and the risk of cracking was much reduced.
Assembling the support structure was done entirely by hand, bay by bay. “It was a bit like building a tree-house - as each vertical panel of the tower was completed it was boarded out and used for access to build the next section,” explains Smith.
Between the towers, the span was too great for a single Mass 50 beam to support the cast iron column load.
Here, instead of using heavy universal beams, MHS built up laminated beams, comprising two Mass 50 members, as each component was light enough to allow the structure to be assembled manually.
Using electronic distance measurement (EDM) equipment, Mansell monitored deflection of the church arches as the floor was excavated beneath the iron columns.
Smith had anticipated up to 4mm of deflection, but the inevitable over-estimation of the building weight meant that actual deflection was less than 3mm.
The use of four 50t capacity vertical members per tower meant that they were able to carry the 30t column loads between them without the need for hydraulic pre-loading.
Excavation is currently under way and, once this is completed and the basement floor slab cast, Mansell will extend the cast iron columns with new permanent steel columns that will be bolted into position with low-profile hydraulic “flat-jacks” placed under their bases. The steel columns will then be jacked up to take the load from the temporary support framework and then filled with grout to provide permanent support.
The MHS structure will then be removed, the temporary piles demolished and a new ground floor installed on steel beams spanning the basement.
The project is due for completion at the end of the year. From the outside, St Jude’s will have barely changed, and to the casual observer, there will be no clue as to the transformation that has taken place inside.