Space Frame Fittings

FIGURE 24.36 Triodetic system.

Handbook Structural Engineering

FIGURE 24.37 Unistrut system.

Circular tube

Circular tube

Hollow steel sphere

Hemispherical shells

Hemispherical shells

Annular diaphragm

Annular diaphragm

FIGURE 24.38 Oktaplatte system.

The allowable bearing strength of hollow spheres can be calculated by the following empirical formulas:

Under compression

Under tension

where D is the diameter of the hollow sphere (cm), t is the wall thickness of the hollow sphere (cm), d is the diameter of the tubular member (cm), [s] is the allowable tensile stress, zc and zt are the amplification factors due to the strengthening effect of the diaphragm, taken as 1.4 and 1.1, respectively, and K is a factor of safety.

24.5.2.6 Unibat

The Unibat System, developed in France, consists of pyramidal units by arranging the top-layer set on a diagonal grid relative to the bottom-layer. The short length of the top chord members results in less material being required in these members to resist the applied compressive and bending stresses. The standard units are connected to the adjacent units by means of a single high-tensile bolt at each upper corner. The bottom-layer is formed by a two-way grid of circular hollow sections, which are interconnected with the apex of pyramidal units by a single vertical bolt (Figure 24.39). Numerous multistory buildings, as well as large-span roofs over sports buildings, have been built using the Unibat System since 1970.

24.5.2.7 Nodus

The Nodus System was developed in England in the early 1970s. Its joint consists of half-casing, which is made of cast steel and has machined grooves and drilled holes, as shown in Figure 24.40. The chord connections are made of forged steel, have machined teeth, and are full-strength welded to the member ends. The teeth and grooves have an irregular pitch to ensure proper engagement. The forked connectors are made of cast steel and are welded to the diagonal members. In the completed joint, the centroidal axes of the diagonals intersect at a point that generally does not coincide with the corresponding intersecting points of the chord members. This eccentricity produces some amount of local bending in the chord members and the joint components. Destructive load tests performed on typical joints usually result in failures due to bending of the teeth in the main half-casing. The main feature of the Nodus jointing system is that all fabrication is carried out in the workshop so that only the simplest erection techniques are necessary for the assembly of the structure on-site.

24.5.2.8 NS Space Truss

The NS Space Truss System was introduced around 1970 by the Nippon Steel Corporation. It originated from the space truss technology developed for the construction of the huge roof at the symbol zone for Expo '70 in Japan. The NS Space Truss System has a joint consisting of thick spherical steel shell connectors open at the bottom for bolt insertion. The structural members are steel hollow sections having specially shaped end cones welded to both ends of the tube. End cones have threaded bolt holes. Special high-strength bolts are used to join the tubular members to the spherical shell connector. The NS nodes enable several members to be connected to one node from any direction without any eccentricity of internal forces. The NS Space Truss System has been used successfully for many large-span double- and triple-layer grids, domes, and other space structures. The connection detail of the NS node is shown in Figure 24.41.

Space Truss System Detail
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