Slimming down and speeding up
Architect Barry Kendell from Sheppard Robson was keen to use stone on the new build element of the redevelopment of No1 Southampton Row in London but also wanted the floor-height mullions to be elegantly slender. To satisfy both requirements, the stone was produced as post-tensioned columns, which also had the added advantage of speeding up the construction of the stonework on site. With sandstone and limestone used in the retained facade, the planners did not insist on any particular stone being used and Cadeby magnesian limestone from Yorkshire was suggested by the stone contractors, Grants of Shoreditch, and selected. The columns stand on transoms of the same stone that transfer the weight back to the floor plate via corbel plates.
The latest use of post-tensioned stone columns can be seen at Southampton Row in the Bloomsbury Conservation Area of London. They are in a Sheppard Robson-designed new-build section of a £40million development, which also includes retained facades, to provide 10,000m2 of retail and office space on five floors.
Lee O'Connor’s contracting company, Grants of Shoreditch, carried out the work on site for main contractors Bovis Lend Lease. Grants were supplied with stonework in Cadeby Brown Permian oolitic limestone from Yorkshire stone suppliers Cadeby Stone.
The post-tensioned mullions are connected to the transoms that are tied back to the floor plates at each level, taking the weight back to the main structure.
Post-tensioning stone involves drilling through the centres of several pieces of stone, threading a steel rod through them and then tensioning the rod by tightening nuts on the ends, spreading the load through a steel plate between the nut and the stone.
Stone is strong under compression. The compressive strength of a sedimentary building stone is usually in the order of 40-50N/mm2 (Cadeby is 51N/mm2) and the tension applied using the steel rod would be unlikely to be more than 5-10% of that, rising to perhaps 20% with the application of lateral loads.
Post-tensioning of stone was developed in 1927 by Eugene Freyssinet, a French engineer. He saw pre-tensioned reinforced concrete beams being made by pouring concrete on to steel under tension. This gives them great strength when lateral loads are applied so they can be used as lintels or beams without cracking or breaking. It is a process still used today.
Freyssinet realised the same principle could be applied to stone by applying the tension after steel had been threaded through the stone (hence post-tensioning as opposed to pre-tensioning). The benefit of adding tension to the stone is that it makes it more capable of supporting lateral loads (from roofs, for example), so slimmer sections can be used. It also means larger components are delivered on site, rather than individual stones, which makes fixing considerably quicker than hand setting each stone.
At Southampton Row it took three weeks to fix the stone. Hand setting would have taken eight weeks.
Although the concept of post-tensioning has been around a while, it has not been widely used and where it has been used it has tended to be on landmark buildings for load-bearing stone elements.
The Michael Hopkins Partnership notably used it in 1995 for the 28 Ketton stone columns that hold up The Queen’s Building at Emmanuel College, Cambridge. Without the tensioning the slender columns would simply be pushed out by the weight of the roof.
Five years later Michael Hopkins used the system again for the Birchover sandstone and DeLank granite columns on the new parliamentary building at Westminster, Portcullis House.
But at Southampton Row the system has been used purely for aesthetic reasons to keep the mullions slender, although it also made fixing quicker.
The architect from Sheppard Robson was Barry Kendall. He told NSS he chose post-tensioning for the stone because “it was the only way we could see it achieving this slenderness and this slenderness was always part of our design”.
He says they had looked at Portland limestone for the project originally and that Cadeby was an option presented by Grants. “The retained facade from last century uses sandstone and limestone, so the need to use Portland wasn’t critical to the planning authority,” says Barry.
He says the stonework was quite a challenge because of its slenderness and because it stands forward of the curtain walling. The detail of the stone design was carried out in conjunction with Grants’ designers.
“The results on site are very good,” says Barry. “You still appreciate the subtlety of the natural stone in the environment. The end result has proved the contractors were certainly worthy of the project.”
One of the requisites of post-tensioning is that the joints between the stones in the columns are precise, because any high points will mean all the load is transferred to that point, which can break the stone.
Improvements in sawing technology, with CNC controls and laser positioning, have made it easier to achieve that level of accuracy. Even so, something is needed to space the stones and take out any minor imperfections that may appear on the surface.
At Emmanuel College lime mortar was used, which presented its own problems. At Southampton Row the solution was 3mm stainless steel spacers with 2mm of neoprine either side to separate the five pieces of stone in each mullion. The spacers were set back 25mm from the surface so the joints could be grouted on site to give the impression of conventional construction.
The new build section of the project that the post-tensioned columns have gone into is designed so that each of the five floors is distinct from the others. Four floors use the post-tensioned stone in two sizes, 72 of the smaller sizes and 16 of the larger.
Nick Bristow, the quarry manager at Cadeby, where the post-tensioned columns were cut and assembled, said it was the first time they had had to produce post tensioned columns. They assembled some samples out of waste stone as test pieces before moving on to the stone selected for the project. But, he says, it proved straight forward.
The stone was honed three sides, with the fourth side being left so that any variations on site could be accommodated. The stones were drilled and threaded with 18mm stainless steel with an M16 thread at either end. Steel plates were put into rebates in the end stones and the nuts tightened to tension the stone.
On site, project manager Mick Laws also reported that the system was easy to use. The columns arrived in specially made crates they called ‘coffins’ to protect them in transit and while being unloaded. They were lifted into place by crane using the steel tensioner as the lifting point and fixed using a three part adjustable bracket into the pre-drilled transoms that were fixed on corbels coming off the floor plates.
The stone was not on the critical path of the project because the building envelope had already been sealed by the glass curtain walling and the stone went in front of that. There were around 80m3 of Cadeby stone in the whole project, including internal columns and the portico.