Node Connectors and Their Influence on Free Form Structures
Introduction to Double Layer Structures
There are two main concepts involved in the realization of free-form structures – single-layer structures and double-layer structures. Double layer structure design has been well known for many years. Comprehensive comparisons of node connectors for double-layer structures are published in [EBE75] [LAC77] [OCT02].
The classical node connector for double-layer structures is the ball node connector. This connector type was adopted in space frame systems like MERO, Krupp-Montal, Zueblin, Tuball (Octatube), and others. The design and calculation of this connection type are described in [MER03] [OCT02]. Cladding elements are preferably connected to the ball node elements via point supports like spider connectors with rotules. Spider connectors are also often used as fixings for glazing elements. If cladding elements require linear support, secondary frames or purlins have to be connected to the ball nodes.
The Ball Node Counterpart: The Bowl Node
The complementary element to the ball node in double-layer structures is known as the bowl node. It was developed by MERO GmbH in Wuerzburg, Germany. The bowl node allows for the use of structural members with prismatic cross sections (e.g. RHS) in the outer layer to directly support the cladding elements. The design of this connection type is described in [MER94]. In the last few years, the combined application of ball node and bowl node connectors has enabled the successful realization of several double-layer structures with ambitious or free-form geometries like the Stockholm Globe Arena [KLI89], the Eden Project [KNE01] and the Singapore Arts Center [SAN02].
Node Connectors for Single Layer Free Form Structures
The growing importance of single-layer structures in recent years was influenced by the architectural preference for transparent building envelopes. Node connectors for single-layer structures can be divided into two fundamental groups – splice connectors and end-face connectors. A comparison of node connectors for single-layer structures was done by K. Fischer in [KFI99].
Splice Connectors: What You Need to Know
These node connectors are characterized by the following:
- The contact surface between the node and the connected structural member runs along splice plates in the longitudinal axis of the member.
- The fixing can be realized as a bolted splice with bolts that are shear-stressed or by welding.
In 1988 Schlaich Bergermann & Partner in Stuttgart, Germany, published the basic principles of a reticulated structure with a splice connector [SBP88], whose first implementation SBP-1 is shown in Figures 1 and 2. The node connector consists of two flat plates that are connected by a single central bolt. Simultaneously, a clamp for cable bracings can be connected to the node through the central bolt. Each structural member is connected to the horizontal splice plates by two or more bolts in single shear. The central bolt allows for an easy adjustment of the horizontal angle Ui between the structural members. Vertical angles can be accommodated by folding the splice plates. Twist angles can be adjusted only in the very limited range of imperfections. As a consequence of the small section height of the splice plates, the node connector can transfer only limited bending moments.
This implementation of the splice connector was successfully used in several free-form structures, such as the courtyard roof of the City History Museum in Hamburg and the roof of the indoor swimming pool in Neckarsulm [SBP92a] [SBP92b][SBP03].
Figure 3 shows the subsequent modification SBP-2 of the original splice connector. The modified node connector consists of three flat plates that are connected by a single central bolt as in the previous version. The two outer horizontal splice plates are connected to machined lug fittings at the end of the structural members by two or more double-sheared bolts. The inner splice plate is connected to Figure 3. Splice Connector SBP-2 machined fork fittings at the end of the other structural members by two or more doubled sheared bolts. The limits for the horizontal, vertical and twist angles are the same as for SBP-1. Due to the double shear connection higher bending moments than with SBP-1, it can be transferred. Among other projects, this version of the splice connector was proposed for the roof structure of the railway station in Berlin-Spandau [SBP99].
Examples of Splice Connectors in Architecture
Figure 4 shows the implementation HEFI-1 of a splice connector, which was published in 1999 by Helmut Fischer GmbH in Talheim, Germany [HFI99] [KFI99]. The node connector consists of two flat discs with a circular groove and four holes. The structural members have machined fittings with shear tongues at their chamfered ends. The shear tongues are plugged into the grooves of the two discs. The discs and the structural member are fixed together by bolts.
Horizontal, vertical and twist angles of a structural member at a node can be accommodated by the geometry of the machined fittings at the corresponding end of the member. The splice connector HEFI-1 was applied for the courtyard roof in Berlin Friedrichstrasse no. 1991-1992 and the Hippopotami House of the zoological garden in Berlin [KNA98] [SBP03].
Figures 5 and 6 show the implementation SBP-3 of a splice connector developed by Schlaich Bergermann and Partner in 1996 for the inner court roof of the DZ-Bank in Berlin [GAR99] [SBP03].
The node connector consists of a solid plate with up to six horizontal finger splice plates. The structural members have machined fork fittings at their ends, which are connected to the finger splice plates of the node by two or more bolts in double shear. Horizontal, vertical and twist angles of a structural member at this node can be accommodated to a certain extent by the geometry of the machined finger splice plates.
Figure 7 shows the principal design of a splice connector with vertical splices POLO-1. A similar node design was developed by Polonyi & Fink in Cologne, Germany, for the canopy roof of the railway station in Cologne [WOE88].
This node connector consists of a cylindrical or prismatic core and up to six vertical splice plates. The structural members have vertical fork fittings at their ends, which are fixed to the splice plates by two or more bolts in double shear.
The splice plates can be used as fork fittings – in this case, the structural members will have lug fittings at their ends. Horizontal, vertical and twist angles of structural members at a node can be accommodated by the geometry of the splice plates. Due to the more favourable orientation of the splice joint, higher bending moments can be transferred. A similar node connector was developed by Schlaich Bergermann & Partner for the vestibule roof of the Deutsche Bank building in Berlin [GAR98].
The Spectrum of End-Face Connectors
These node connectors are characterized by the following:
- The contact surface between the node and the end-face of the connected structural member is transverse to the longitudinal axis of the structural member
- The connection can be realized as an end-plate connection with tension-stressed bolts or by welding.
After the development of the Splice Connector POLO-1 came the implementation of SBP-4 of an end-face connector developed by Schlaich Bergermann & Partner for the Schlueterhof courtyard roof of the German Historical Museum in Berlin [SBP03]. The node connector is made of two cross-shaped plates and four end plates that are welded together. The structural members are connected to the node end-faces with butt welds.
During construction, the structural members can be provisionally fixed to the node end-faces by bolts. In the cavity between the two cross-shaped plates, a clamp for cable bracings is connected to the top plate by four bolts. Horizontal angles of structural members at this node can be accommodated only by the prefabricated geometry of the cross-shaped plates. Vertical angles can be adjusted to a certain extent by the geometry of the machined node end-faces. Twist angles can be accommodated only in the limited ranges of imperfections. As a result of the considerable section height of the node end-faces, high bending moments can be transferred up to their full member strength.
After the SBP-4, the welded end-face connector WABI-1 was developed by Waagner-Biro AG, Vienna, Austria, for the courtyard roof of the British Museum in London [WAB00] [WAB01]. The node consists of a star-shaped plate with 5 or 6 arms.
Each arm runs between adjacent structural members. These nodes are made from thick plates by cutting perpendicular to the plate surface. The end faces of the structural members have a double mitre cut to match the corresponding node gap between adjacent arms. The thickness of the node plate is less than the height of the connected structural members. The top and bottom surfaces of the node plates are connected to the members by filet welds. The side surfaces are connected by butt welds. Horizontal, vertical and twist angles of structural members at this node can be accommodated by the geometry of the double mitre cuts at the end of the members. High bending moments, up to the full member strength, can also be transferred.
Another end-face connector OCTA-1 (figure 19), was developed by Octatube Space Structures BV, Delft, Holland, as a modification of the Tuball node system [OCT02]. The End-Face Connector OCTA-1 is a node made from a hollow sphere with openings at the top and the bottom. Each structural member is connected to the node sphere by two bolts, which are mounted from the inside of the hollow sphere. Horizontal, vertical and twist angles of structural members at this node can be accommodated by the geometry of the two bolt holes for each member. Direct support of cladding elements by the members across the node connector is not feasible.
In 1994 MERO GmbH, Wuerzburg, Germany published a series of end-face connectors along with the bowl node, which was called “MERO Plus” [MER94]. One of these node connectors is the cylinder node MERO-1 (figure 20). The node is made from a hollow cylinder with openings at the top and the bottom. Each structural member is connected to the node cylinder by two bolts, which are mounted from the inside of the hollow cylinder. Horizontal, vertical and twist angles of structural members can be accommodated by the geometry of the machined plane surfaces at the node. The connection enables the transfer of relatively high bending moments.
Another “MERO Plus” connector is the block node MERO-2. The node is cut from a thick plate. Each structural member is connected to the block node by one or two bolts, which are mounted from the inside of the structural member. Hence, the member must be a hollow section profile like RHS, SHS or CHS. Alternatively, the members can be welded to the node. Horizontal, vertical and twist angles of structural members can be accommodated by the geometry of the machined plane surfaces at the node. The bending capacity is similar to the capacity of the cylinder node MERO-1.
Another “MERO Plus” connector is the dish node MERO-3 (figure 12). This node consists of a dish, i.e. a hollow cylinder with a bottom plate. The structural members are connected to the node by only one bolt. Horizontal, vertical and twist angles of structural members can be accommodated by the geometry of the machined plane surfaces at the node. The bending capacity of the connection is rather small.
Figures 13 shows the recent implementation MERO-4 * of an end-face connector. This node was developed by MERO for the roofs over the Central Axis and the Service Center of the New Fair in Milan, Italy. Both roofs are free-form reticulated structures. The roof over the Central Axis has a length of approximately 1300 meters and a width of 32 meters. The roof structure is divided into twelve structurally independent parts. Figure 14 shows the first two parts during construction. The structure has approximately 16000 nodes and 41000 structural members. The structural members are T-profiles with a height of 200 millimetres and a width of 60 millimetres. The roof structure is supported by approximately 180 columns. Six spikes at the top of each column connect the column to the roof structure.
The node is made of two dish nodes, one node for the top chord of the structural members, and one for the bottom chord at the end of each member. The structural members are connected to both nodes by two bolts or by welding. Horizontal, vertical and twist angles of structural members can be accommodated by the geometry of the machined plane surfaces at the nodes. The connection is capable of transferring high bending moments.
* Patent Pending
Applicability of Node Connectors for Single Layer Free Form Structures
In summary, there are varied scenarios when it is appropriate to use node connectors for single-layer free-form structures. Generally, most of the splice connectors require geometric and structural optimisation of the free-form structure, while the end-face connectors are geometrically more flexible and usually do not require structural optimisation. However, this does not change the fact that non-optimized free-form structures are more complex and thus more expensive than optimized free-form structures.
If you are interested in learning more about node connectors and free-form surfaces, Novum Structures can provide information and guidance.
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