Stoned again

The Victorians were not great fans of load-bearing masonry. It was they who started the trend for constructing steel-framed commercial buildings faced with non-load-bearing cladding, a trend that lasted for more than 100 years. Indeed, the all-masonry method appeared to have been consigned to the dustbin of history. However, one century on in the City of London, there are signs that a revival is taking place.

Foremost in this is Two Plantation Place, part of a 93,000 m2 development owned by British Land that occupies most of a city block between Fenchurch Street and Great Tower Street. Arup Associates' design for the block is not, however, a traditional brick-on-brick solution; instead, blocks of limestone are stacked above each other as a series of perpendicular panels, giving the facade a contemporary flavour. And the lack of a bond pattern means that some distinctly modern technology was needed to ensure that the building retained its structural integrity.

Perpendicular panels were used because of a factor the Victorians didn't have to contend with – Part L of the Building Regulations. This building was designed after the 2002 revision, and Arup Associates had to work out how it was going to deal with heat-gain through the facade. "You either shade the glass indirectly or replace it with something solid. That is the challenge of Part L," says Graham Goymour, project architect on the building.

The answer came in the form of vertical stone fins projecting from the facade. These have been arranged like a chessboard to shade the windows – the fins repeat in two-floor patterns, interrupted with a stone panel above and below each fin. The loads are carried vertically through the corners of the panels where they intersect – an area just 300 mm square. On the east side of the building, there are wide windows to the north of the fins where the shadow is cast longer, and narrow windows on the south side. This arrangement becomes more random on the south-facing facade, with a greater proportion of solid wall panels, because it faces the sun.

The starting point for this unusual facade was Two Plantation Place's next-door neighbour, also designed by Arup Associates and called, logically enough, One Plantation Place. Arup Associates had split the development into two buildings, partly because of its size and partly because it wanted a public thoroughfare to run through the middle of the development to re-establish the traditional City street pattern. The first building is the larger of the two and had planning permission before Arup Associates' thoughts had turned to the second building.

The first seven storeys of One Plantation Place had Bavarian limestone fins to shade the conventional cladding. These were the starting point for the facade next door. Goymour says: "An important element of Plantation One is the suspended stone fins – they create the appearance of a largely stone building E E when viewed obliquely. Plantation Two takes its cue from this." Once it had decided to use the same stone for the facade of Plantation Two, Arup Associates naturally wanted to continue the innovative approach to facade design. "The way we had used stone had been received very well on Plantation One," Goymour says. "But as well as consistency of materials, the planners wanted to see a separate sense of identity so it didn't become a single huge development on the site." The important difference, of course, was the decision to use stone structurally on Two. Robert Pugh, the project's structural engineer, explains: "It was logical to use the stone as a loadbearing element – it has a compressive strength four times that of concrete and we were keen to get value out of it." Plantation Two had a much lower floor-to-ceiling height than its sister building, so space was at a premium. The all-masonry approach removed the need for columns at the perimeter of the building and so liberated precious lettable floor area.

Pugh adds that there was another compelling reason – once the sums had been added up. "The system was about 10% cheaper than the cost plan allowance for a curtain wall system. Indeed, it had been part of the concept strategy to procure a structural wall from the precast market, rather than the overheated curtain wall market." The proposed solution had to be tested. "There was no precedent for this type of structure. The eccentric loading of the panels was the new bit," says Pugh. Eccentric loading was an issue because each panel consisted of five limestone blocks and, because the loads pass only through the corners of the panels, there could be a tendency for them to splay apart. Arup Associates carried out finite element analysis to predict how the stone would behave, and then built a scale model and subsequently a full-sized mock-up to fully test the panels.

The solution was to pre-tension them by embedding steel tie-bars within the stone. The degree of pre-tensioning is adjusted depending on where in the building the panels are located. Precast stone spandrel panels are used as the device to provide interconnections between the stone elements and the steel floor beams at the edge of the floorplates. These have stainless steel boxes cast-in to connect the panels at the corners and transfer the loads down the building. Steel beams tied back to the steel frame are also linked to the facade via this connector. Because the structural columns are based on a 3 m wide spacing, a transfer beam has been installed at first-floor level. This transfers the loads to a 6 m wide column at ground-floor level, enabling shops to have larger frontages.

Pugh says the main challenge was accurate detailing and careful construction. It certainly seems to have paid off; it may even revive a building method that was viewed as obsolete 100 years ago.