It is official: Burlington House is very solidly built. Before the Royal Academy’s latest exhibition, a structural engineer was brought in to assess whether the eighteenth-century Palladian mansion’s first-floor galleries would support a weight of 90 tons. He decided they could. Ninety tons is the combined weight of the steel reinforcing bars (rebars) in Straight (2008-12), the main installation in Ai Weiwei’s new solo show (until 13 December). The work’s deceptively simple title hides an ugly truth: the rods of rusted steel composing its elegantly undulating form were reclaimed from the shoddily constructed school buildings that collapsed during the 2008 Sichuan earthquake, killing more than 5,000 children. The walls alongside the seismograph-shaped installation a