Structure

Details Structure Structure

2002-01-07

2004-12-21

Friedman, Carl D. (Department: 3635)

Static structures (e.g., buildings)

Intersection of a cast stonelike component

Cast in situ material at module juncture

C052S745070, C052S745060, C052S082000, C052S081200, C052S090100

Reexamination Certificate

active

06832455

ABSTRACT:

The present invention relates to a structure, notably to a system of concrete beams for forming a dome-like structure.
BACKGROUND TO THE INVENTION:
This invention relates to skeletal frame systems which support fibreglass or other flexible, semi-rigid or rigid roofing sheets, panels or glazing to provide a weatherproof or other cover or roof over large open spaces. Typical examples of where such systems find use are sports fields, stadia, velodromes, athletic tracks, assembly buildings, stockpile covers, leisure facilities, aircraft hangers, open plan factories, train or bus stations, vehicle garages or stores and the like. For convenience, such potential uses for the systems will be denoted herein in general by the term covered open spaces.
Such systems usually require that the roof structure should not be supported by internal pillars, since these would interfere with the free use of the internal space of the covered open space and, in some cases, would obstruct the view of spectators.
Large span structures, that is structures which have a clear span length of 50 meters or more, have traditionally been constructed using steel as the structural material. Large span steel structures which are substantially planar require out of plane strutting to provide stability and to enable the structure to be self-supporting and to carry its design load. This can result in a mass of secondary steelwork, which is not only expensive, but is also visually unattractive. The present invention relates to the construction of three-dimensional structures, for example domes and the like, where the roof structure extends in three dimensions over an open space. Such structures comprising discrete members rigidly joined together are commonly known as space frames. Typically, the radially outward periphery of the frame subtends an angle of 10° or more (preferably, 10° to 30°) to the horizontal. However, such frames may progressively flatten towards their apex so that the angle subtended to the horizontal decreases, often to zero or near zero at the apex of the frame.
A space frame is a structure in which a plurality of individual components are linked together via rigid joints to form the overall structure. For convenience, the term node joint will be used herein to denote one of the points within the overall structure at which a plurality of the components, for example tubes, rods, bars or beams, are interconnected or jointed together to form the structural framework of the space frame. The node joints are substantially rigid so that they can transmit moments from one component of the structure to another. A space frame is typically a three dimensional structure.
A typical form of such a space frame comprises steel bars having screw threaded ends, which are interconnected at the node joints by means of machined steel blocks with threaded holes into which the threaded ends of the bars engage. It will be appreciated that by their very nature the bars and blocks have to be made to close tolerances. Furthermore, in order to minimise sagging of the metal bars and to keep their transverse thickness within acceptable limits, the bars have to be comparatively short. This requires the use of a large number of joints to achieve a large span structure. This makes such structures complex and expensive.
Another form of a space frame uses tubular steel members welded together at the node joints. The individual tubular members are usually cut from lengths of tube and require the formation of complex shapes at the ends thereof to provide a good fit of the components upon one another at the node joints where typically three, four or more tubular members engage one another. Whilst such members and the node joints could be manufactured off site, it is usually necessary to finalise the exact shape and dimensions of the ends of the tubular members on site as construction of the space frame proceeds. In addition, the individual components have to be assembled into the space frame on site, which requires the use of high towers to support the assembled components in position and subsequent welding of the components at each node joint to form the overall structure. This is complex, costly and often hazardous for the construction operatives.
A fundamental characteristic of a space frame is that all joints at the nodes have to be rigid in the structural sense, i.e. capable of transmitting moments and shears in the X, Y and Z axes and to resist twisting about any of these axes. Traditional designs of such structures using conventional fabrication methods can only achieve this three-dimensional rigidity with cumbersome and expensive joints. Welding of tubular components, or threading of solid bars into solid jointing blocks does achieve this rigidity, but only at a high cost and using complex fabrication techniques.
In steel space frames the dominant cost element will be the joints, whether welded tubular joints or machined and screw threaded blocks. However, reducing the number of joints by increasing the length of the individual bar or beam members is often not possible since the compressive stresses which may be permitted in steel drop off rapidly as the members increase in length, due to buckling considerations. This limits the maximum length of the members which can be used in any given case and the number of joints which can be omitted is thus limited.
Concrete is known and used as a structural material. However, concrete which has not been pre-stressed has, even with reinforcement, hitherto been considered suitable only for short span beams and to be too heavy and weak for large span structures. Established engineering practices have limited the use of concrete in long span structures to the use of pre-stressed concrete. However, where the tensioning wires or rods in a pre-stressed concrete beam or other member are tensioned after the member has been cast, problems are encountered due to in situ corrosion of the wires or rods leading to structural failure of the components. Such materials are not advocated for use in structures which are to be exposed to the elements and where a long operating life is required. Where the tensioning is introduced into the rods or wires before they are incorporated into the concrete during casting of the concrete component, the problem of corrosion is not evidenced. However, the cost of fabrication of such components becomes excessive for large and one-off components and it is impractical to fabricate large pre-stressed components on site without expert operators and complex equipment. Furthermore, difficulties and excessive costs may arise in transporting large pre-stressed concrete components from the site of manufacture to the site of use. Accordingly, pre-stressed concrete is not considered a viable material for use in fabricating a large span space frame.
The use of steel as the material from which large span space frame structures are built is currently accepted as the only practical alternative. This is despite the limitations on the length of the individual components, the complexity of the jointing techniques required to achieve three dimensional rigidity and need to maintain the structure against rust, for example by painting the exposed steel work. This often requires that the covered open space enclosed by the space frame structure be taken out of use during the painting operation.
There thus exists a need for a structurally and commercially viable alternative to the use of steel to construct large span space frames.
The major loading on individual components in a large span space frame structure, especially one made from concrete, is a dead load due to the weight of the components. We have found that, in a three dimensional structure such as a dome, this load is converted to high axial forces within the beam members of the space frame structure. We have found that these forces can be used to advantage in a large span structure to act upon concrete beams which have not been pre-stressed and to simulate the effect of pre-stressing the concrete beams. This discovery enables a con

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