Node structures for lattice frames

1. A node structure for connecting a member of a lattice frame to one or more other members of the frame, the node structure comprising:

a hollow brace having opposed walls that converge outwardly at an acute angle in cross-section toward a central plane to connect at an outer edge, the outer edge defining a solid curved surface at an outer periphery of the brace, the solid curved surface defining a portion of an outer surface of the brace; and

at least one root portion being a root end of a member of the frame or being for attachment to a member of the frame, which root portion has a central longitudinal axis extending outwardly in the central plane of the brace, wherein the root portion has an inner end cut away at opposite sides around the central plane to leave a joining surface that engages the outer surface and the solid curved surface of the brace without penetrating through the brace so as to extend into contact with both of the opposed walls on opposite sides of the outer edge and thereby embrace an outer region of the brace extending inwardly from the outer edge, wherein the root portion is joined to the brace at the engagement between the joining surface and the outer surface.

2. The node structure of claim 1 , wherein an intersecting edge of the joining surface is concave-curved when viewed on an axis parallel to the central plane where opposite sides of the root portion are cut away, and convex-curved when viewed on an axis orthogonal to the central plane.

3. The node structure of claim 1 , wherein the brace supports first and second root portions with respective outwardly-extending central longitudinal axes diverging outwardly in the central plane of the brace for alignment with respective members of the frame.

4. The node structure of claim 1 , wherein an inward thickness of the brace is greater than a diameter of the root portion and an outward thickness of the brace is less than the diameter of the root portion.

5. The node structure of claim 1 and having a cut-out interrupting the outer edge of the brace, defined by an outwardly-facing concave wall extending between the opposed walls of the brace.

6. The node structure of claim 1 , wherein the opposed walls of the brace are also connected by at least one internal bulkhead spaced from the outer edge.

7. The node structure of claim 6 , wherein the bulkhead comprises a tube extending between the opposed walls on an axis substantially orthogonal to the central axis of the, or each, root portion.

8. The node structure of claim 1 , wherein the opposed walls of the brace have ellipsoid or conical surfaces.

9. The node structure of claim 1 , wherein the brace is in the form of a disc or an ellipse, or a portion of such a disc or ellipse, comprising opposed dished walls that converge outwardly in radial cross-section to connect at the outer edge, which is convex-curved.

10. The node structure of claim 9 , wherein the central axis of the, or each, root portion intersects an axis of curvature of the outer edge of the brace.

11. The node structure of claim 9 , wherein the brace is in the form of a disc portion defined between the convex curvature of the outer edge and chord-like inner edges of each dished wall extending between opposite ends of the outer edge.

12. The node structure of claim 11 , wherein the inner edges of the dished walls together define a seat recess shaped for joining the node structure to an elongate member that has a central longitudinal axis lying on the central plane of the brace.

13. The node structure of claim 12 , wherein the node structure further comprises an elongate cylindrical member for end-on alignment with, and joining to, one or more other members of the lattice frame, the cylindrical member being located in a seat recess of matching shape and having a central longitudinal axis that lies on the central plane of the brace.

14. The node structure of claim 13 , comprising first and second braces angularly spaced about the cylindrical member.

15. The node structure of claim 12 , wherein the segment-shaped disc portion of the brace has an axis of revolution intersecting and orthogonal to the central longitudinal axis of the member for which the seat recess is shaped.

16. The node structure of claim 1 , wherein the node structure is fabricated from metal parts, wherein the opposed walls are formed from sheet or plate metal and any root portion is cut from a metal tube.

17. A method of fabricating a node structure for a lattice frame, the method comprising:

providing a hollow brace comprising opposed walls that converge at an acute angle in cross-section toward a central plane to connect at an outer edge, with the outer edge defining a solid curved surface at an outer periphery of the brace, the solid curved surface defining a portion of an outer surface of the brace;

engaging the outer surface and the solid curved surface of the brace with a joining surface defined by an inner end of at least one root portion being a root end of a member of the frame or being for attachment to a member of the frame, wherein the root portion does not penetrate through the brace and thereby extends into contact with both of the opposed walls on opposite sides of the outer edge so as to embrace an outer region of the brace extending inwardly from the outer edge; and

joining the root portion to the brace at the engagement between the joining surface and the outer surface.

18. A method of fabricating a lattice frame, the method comprising:

providing a unitary hollow brace comprising opposed walls that converge at an acute angle in cross-section toward a central plane to connect at an outer edge;

forming at least one hole through the unitary hollow brace such that the at least one hole defines a closed-loop hole edge that is located at the outer edge and also extends into the opposed walls of the unitary brace;

inserting a root portion, which is a root end of a member of the frame or for attachment to a member of the frame, into the at least one hole such that the root portion penetrates through the brace; and

joining the root portion to the brace around the hole edge.

19. The method of claim 17 , comprising joining a pair of opposed walls to form the brace.

20. The method of claim 19 , comprising forging sheet or plate metal to form the opposed walls.

21. A lattice frame comprising at least one node structure as defined in claim 1 .

22. A wind turbine installation comprising the lattice frame of claim 21 .