His subsequent collection, the more difficult "Houseboat Days" (1977), reinforced Ashbery's reputation, as did 1979's "As We Know," which contains the long, double-columned poem "Litany." In 1988, his "Bridge Poem" was installed, using metal letters, on the 375-foot-wide Irene Hixon Whitney Bridge in Minneapolis; the poet was selected by the bridge's architect, artist Siah Armajani, and commissioned by the Walker Art Center. By the 1980s and 1990s, Ashbery had become a central figure in American and more broadly English-language poetry, as his number of imitators attested.