Though permanently moored among the docks of London’s Canary Wharf, Crossrail Place has a central role in the city’s bustling transportation and trade networks. Designed by Foster + Partners (F+P), the long, tubular structure contains one of nine new railway stations for London’s East West Rail network, due to begin operating in 2018, as well as a shopping center and a public park. In all, the enormous, ship-like building includes more than 100,000 square feet of retail and restaurant space and 45,000 square feet of green space across seven stories, four of which are below grade and submerged underwater.
Topped by an arched lattice roof with a tessellated ETFE (ethylene tetrafluoroethylene) skin that responds to climatic conditions in real time, the futuristic form and materiality of Crossrail Place are a direct response to the area’s maritime history; its barrel vault of crisscrossing glulam spruce and billowing, triangular ETFE cushions recall the dozens of wooden clippers that once docked in this part of the city.
The lattice roof spans the concrete box structure’s 112-foot width and 1,017-foot length (longer than three football fields) and comprises 1,418 glulam beams and 564 custom steel nodes, 348 of which are unique in shape. The diagonal glulam members tie into the reinforced-concrete slab that supports the center’s elevated park, while the glulam beams running parallel to grade are spaced every 6 meters (19.7 feet). Where the vault roof appears to continue below the park level, the timber suspends from the slab and is structurally isolated from the roof. Fourteen diamond-shaped openings, each nearly 40 feet wide, cluster along the roof’s apex.
According to Austrian timber contractor WIEHAG, Crossrail Place’s roof is the largest timber project in the U.K. Its success, says Ben Scott, F+P’s partner-in-charge for the project, is the result of an extraordinary collaboration between the architects and the builders, including Seele, also in Austria. Through a series of intensive work sessions, both in the U.K. and in Austria, the team developed a specialized method for the exchange of 3D information. “All the nodes, beams, cushions, and flashings were designed and fabricated as one parametric family,” says F+P computational designer Jonathan Rabagliati. “This permitted the exchange of data sets and geometric rules facilitating the gradual refinement of the design through successive digital and physical prototypes.”
Although the vault roof is curved, the individual glulam beams are straight. F+P explored two possibilities: double-curved beams with simpler steel-node connections or straight beams with more complex nodes. “The question was really where to put complexity and where to put simplicity,” Rabagliati says. WIEHAG made the case that the latter option would be more cost effective, noting that the production of straight glulam beams could be almost completely automated.
Even then, the dimensions of the timber are hardly standard, ranging from 13 feet to almost 25 feet in length. The diagonal beams in the roof lattice are 33 inches deep, while the horizontal beams are half as deep, at approximately 18 inches. Both average 10 inches in thickness. At the east and west ends of Crossrail Place, a double-curved steel ring beam and two pairs of curved timber members help create the roof’s dramatic, approximately 100-foot cantilever over a series of restaurant terraces.
Still, the steel node connections were by far the most challenging aspect of the project, Scott says. The nodes connect up to six glulam members, each bolted to a pair of steel plates, and position the timber at the angles necessary to create the roof’s overall curvature. To design the nodes, the architects built 3D prototypes and, eventually, a full-scale mock-up. In general, the nodes resemble two toy jacks fused together, but they vary as well in size, shape, geometry, number of connections, and more.
The nodes also provide an attachment point for the aluminum mullions, which secure and suspend the inflated ETFE cushions 7 inches above the glulam shell. A series of tubes, cleverly hidden between the beams and mullions, circulate air through the individual cushions at a low pressure, which can fluctuate based on wind patterns. “If winds are high, the pressure inside the pillows increases,” Scott says. “The whole skin is reacting to its environment by becoming stronger.”
Like the structure, the elevated public park treads the line between past and present, filled with palms and other exotic species originally brought to London centuries ago on merchant ships. The diamond-shaped openings in the roof lattice and the translucency of the ETFE combine to create a microclimate that allows the tropical plants to thrive. Those openings might have complicated the structural design of the roof, but Scott says this was one challenge that resolved itself. A cable tie replaces the horizontal beams across the open span, while the voids also reduced the roof’s structural loads. “Where there’s no ETFE, the key load cases have gone,” he says. “There’s no wind load [or] snow load, so it was not actually as difficult as we were expecting.”
Although trains aren’t scheduled to run through the tunnels of Crossrail Place for several more years, the development’s retail portions and park opened to fanfare in May 2015. Rabagliati says the project’s visibility has undoubtedly raised the profile of timber construction in the city—and changed some perceptions of the firm. People “normally associate Foster + Partners with ‘high-tech’ architecture,” he says. “What they don't appreciate is that engineered timber is probably more high-tech than steel.”