The Nodus System was developed in England in the late 1960s and early 1970s by the British Steel Corporation. This system was commonly used for flat double-layer-grids (FDLGs). The node is made of two hemispherical cast iron casings, clamped with a high strength bolt. When the bolt is tightened, the threaded end connectors of the chords are clamped between the two half-casings. The toothed connectors are placed in between the casings. The bolt head is placed on the half casing such that a flush surface is provided so that cladding could be attached directly to the members. The mating half casing has four lugs to connect the bracing members. A sealing gasket is inserted between the half casings before securing the assembly with a bolt placed at the center. A steel pin is placed through the forked end connectors of the bracing. Forked connectors are welded to the ends of the bracing members and connected to the lugs with a steel pin. The top layer members can be made of round or rectangular hollow sections allowing the roof deck to be directly placed on them and eliminate the need for separate roof supporting members (purlins).
The benefit of the Nodus System is that most of the units are shop fabricated with high accuracy and only the simplest parts are erected in the field. The joints are also designed such that they can not be assembled incorrectly, preventing construction errors.
The Nodus System was designed such that the centerlines of bracing members and the top/bottom layer members did not meet each other at a common point within the node, which results in a small secondary bending moment. A decrease in the angle between the bracing and the top or bottom layer members, results in an increase in the eccentricity and larger secondary bending moment and a less efficient system.
Since the Nodus joint system was made with only two possible lug orientations, the grid configurations are also limited to square-on-square, square-on-square offset, and square-on-diagonal layouts.
Since the purlin system can add by as much as 20% to the structure’s weight, how the decking is supported became important. Therefore, to simplify and reduce the costs for this system the decking was directly connected to the chord (top layer) members. This required that the joints to be designed so that they are flush with the level of the main chord members. Because the chord members act as purlins, they were designed and detailed as continuous through the joint. It was also required that the joint strength approach the members' strength which they jointed to. Since the module size and depth can vary widely, the bracing members' angle can also change. Therefore, it is only practical that the bracing members be pinned.
When the grid is supported around its periphery, the bracing member forces are only a small percentage of the chord (top or bottom layer) member forces. However, when the supports are placed only at the corners of square grids, the bracing member forces can be equal or even greater than the maximum chord forces for some grid configurations.
Six different size of joints have been manufactured with round tubes having outside diameters equal to 2.4 in., 3 in., 3.5 in., 4.5 in., 5.5 in., and 6.6 in. and square tube sizes of 2.5 in., 3 in., 3.5 in. , 4.5 in., 6 in., and 7 in..
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