A method of constructing a 3-dimensional, upwardly convex frame on a supporting surface using a plurality of substantially identical longitudinally extending frame members,,,, each of the frame members,,, having a pair of middle holes adjacent the middle of its length and a single hole adjacent each end of its length; and whereby the method comprises the steps of: positioning a first frame member on a supporting surface; placing a second frame member on the supporting surface and elevating the first frame member sufficiently to enable an overlapping arrangement of the first frame member and the second frame member; positioning the second frame member with one of its middle holes positioned below one of the end holes of the first frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the first frame member and the second frame member together, whilst permitting some relative movement between the first frame member and the second frame member; placing a third frame member on the supporting surface and elevating the second frame member slightly to enable an overlapping arrangement of the second frame member and the third frame member; positioning the third frame member with one of its middle holes positioned below one of the end holes of the second frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the second frame member and the third frame member together, whilst permitting some relative movement between the second frame member and the third frame member; placing a fourth frame member on the supporting surface and elevating the third frame member slightly to enable an overlapping arrangement of the third frame member and the fourth frame member; positioning the fourth frame member with one of its middle holes positioned below one of the end holes of the third frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the third frame member and the fourth frame member together, whilst permitting some relative movement between the third frame member and the fourth frame member; positioning one of the end holes of the fourth frame member above one of the middle holes of the first frame member, wherein the relative movement afforded by the loose fitting pins facilitates the overlapping positioning of the first and fourth frame members,; placing a relatively loose fitting pin through the overlapping holes, to fix the fourth frame member and the first frame member together, whilst permitting some relative movement between the fourth frame member and the first frame member;and wherein elevating the overlapping regions of the frame relative to the supporting surface by twice the vertical thickness of the frame members begins to introduce an upwardly convex curvature into the frame. positioning a first frame member on a supporting surface; placing a second frame member on the supporting surface and elevating the first frame member sufficiently to enable an overlapping arrangement of the first frame member and the second frame member; positioning the second frame member with one of its middle holes positioned below one of the end holes of the first frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the first frame member and the second frame member together, whilst permitting some relative movement between the first frame member and the second frame member; placing a third frame member on the supporting surface and elevating the second frame member slightly to enable an overlapping arrangement of the second frame member and the third frame member; positioning the third frame member with one of its middle holes positioned below one of the end holes of the second frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the second frame member and the third frame member together, whilst permitting some relative movement between the second frame member and the third frame member; placing a fourth frame member on the supporting surface and elevating the third frame member slightly to enable an overlapping arrangement of the third frame member and the fourth frame member; positioning the fourth frame member with one of its middle holes positioned below one of the end holes of the third frame member; placing a relatively loose fitting pin through the overlapping holes, to fix the third frame member and the fourth frame member together, whilst permitting some relative movement between the third frame member and the fourth frame member; positioning one of the end holes of the fourth frame member above one of the middle holes of the first frame member, wherein the relative movement afforded by the loose fitting pins facilitates the overlapping positioning of the first and fourth frame members,; placing a relatively loose fitting pin through the overlapping holes, to fix the fourth frame member and the first frame member together, whilst permitting some relative movement between the fourth frame member and the first frame member;and wherein elevating the overlapping regions of the frame relative to the supporting surface by twice the vertical thickness of the frame members begins to introduce an upwardly convex curvature into the frame.