“Once we had the big, basic premise that there was a solid piece for the galleries, which we started to call the icebergs, and then the glass sails for the garden, we started to work them independently,” Mr. Gehry said. “Merging the two would not work, because you couldn’t have curvy galleries, and you can’t hang paintings on glass.” The diaphanous sails, supported on an acrobatic armature of wood and steel, project outside the iceberg.