Glenstone, the bucolic art museum 15 miles outside of Washington, D.C., is building a new pavilion to house one of Richard Serra’s most recent large-scale steel sculptures. The 4,000-square-foot structure is expected to open next spring or summer. Its design is a collaboration between Serra and architect Thomas Phifer, whose New York firm Thomas Phifer and Partners expanded the private Maryland museum in 2018, adding 12 concrete pavilions clustered around a central water court. The complex, embedded in the property’s verdant hillside, represents only a sliver of Glenstone’s 230 pastoral acres that have been enhanced over the years with 8,000 new trees and other plantings, by Adam Greenspan, design partner of the Berkeley-based PWP Landscape Architecture.