WO1990006409A1

WO1990006409A1 - Arches

Info

Publication number: WO1990006409A1
Application number: PCT/GB1988/001092
Authority: WO
Inventors: Ibrahim Madhi Al-Khattat
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-06-17
Filing date: 1988-12-08
Publication date: 1990-06-14
Anticipated expiration: 1991-06-08
Also published as: GB2206365B; GB2206365A; EP0415927A1; GB8814387D0; GB8714224D0

Abstract

This invention comprises a design and construction method whereby various engineering structures are easily assembled from a series of segments (1) alternating with connector units (2). Especially for arch-shaped structures, slide-fitting segment members to the connectors (2) provides, through forces of contact, a self support mechanism during and after the erection process. By tensioning at least one strand (5) that runs through it, the structure is held firmly together under compression. In addition to providing the load carrying capacity of the structure by neutralising internal bending tensile stresses, tensioning the strand provides, via the connectors (2), a simple and effective inter-segmental jointing mechanism which simplifies assembly and dismantling of the structure. Since compression, rather than flexure, is the predominant mode of internal loading of segmental members, materials that are otherwise unsuitable, like small-diameter timber logs, become useful structural materials.

Description

ARCHES

This invention relates to a method of design and construction of engineering structures, especially arch- shaped ones, which are made up of: (a) a series of segments, (b) a series of jointing members (connectors) adapted to engage, especially by slide-fitting, the ends of the segments, (c) at least one tensioning strand passing through the series of segments and connectors, and (d) means for anchoring and tensioning the strand in order to compress the segments and connectors together thereby configuring them into a stable structure. The slide-fit contact forces between the connectors and ends of segments provide a self- support mechanism for an arched structure being assembled by successively mounting the segments and connectors, working from both ends towards the peak of the arch. The completed structure is also self-supporting even before installing and tensioning the strands.

Presently, to take examples from the construction industry, metal girder and truss structures are usually built up by bolting, welding, or riveting together of their primary members, producing heavy units that are costly to manufacture, transport and erect. Timber truss structures are assembled by analogous methods, many of which are not convenient or reliable enough to ensure good load transfer between two jointed members. Laminated timber arches and beams are also in use, but they are relatively expensive and also have most of the drawbacks of other conventional systems. Indeed, lack of satisfactory jointing methods is a major problem in the design and use of structural timber in general. In all cases, the jointing method usually imposes a limit on the types of usable structural materials and on achievable economies in the cost, time, effort, equipment, and skills needed to manuf cture, transport, and erect a load-bearing structure. By combining segmentation with prestressing and with the use of simple short-term and permanent jointing techniques, this invention provides a superior and cheaper alternative to current structural design, jointing, and erection methods in numerous cases. Prestressing, an established technology especially relevant to large span concrete structures, is applied to a structure made of a series of segments alternating with inter-segmental connectors, by tensioning at least one multi-wire strand passing through it. As in prestressed concrete mechanics, tensioning the strand imposes a reactive balancing internal compression in the structure. This compression is superposed on the tensile/compressive regime of bending stresses resulting from bending moments due to the transverse loads carried by the structure . The effect of the superposition is to neutralise the tensile stresses and, b^y a careful choice of strand profile, produce an optimum compressive stress regime. Moreover, according to the method of the invention, this compressive stress regime furnishes a permanent jointing technique (by pressure adhesion) that can simplify not only the assembly of the structure but also the dismantling process if the need arises .

Current methods and concepts of prestressed concrete design and practice can be used, with discretion, in conjunction with the method of this invention. For example, the necessary prestressing force to hold the structure stiffly together and to support safely any external loads it is intended to carry can be calculated. One or more multi- wire strands or single wires may be used, with profiles defined in the design process. An extra merit of the method of the invention over prestressed concrete applications is the relatively small magnitudes of prestressing forces required, due to the lighter dead weights of the structures involved.

For arched structures, e.g portal frames used in large- span buildings, this invention also provides a short-term jointing method during assembly that employs slide-fit connector-segment contact forces as will be later explained through an example.

The nature of the jointing methods of the invention and the resulting regime of predominantly or totally compressive internal stresses in the segments enable the use of materials otherwise structurally unsuitable, especially under bending. A good example is small-diameter round timber logs (SRT) , a perpetually abundant and largely wasted natural resource, presently categorised commercially as a scrap material. Unlike sawn timber, the axisymmetric fibrous growth ring structure is intact and gives SRT members relative strength and better performance predictability. A particularly important application of SRT through this invention is in the construction of arch- shaped portals and other structural units as used in building and bridge construction. SRT segment members act as struts rather than beams exposed to bending and, consequently, to sagging.

Large-span arch-shaped structures made by the method of this invention can be easily transported as small pieces that are assembled on site with little or no need for heavy lifting equipment like cranes.

As simple changes in the geometry of the connectors and/or the relative lengths of segment members can produce an unlimited number of profiles, greater design freedom is available to the architect and engineer via the method of this invention.

The segments can be of many materials , shapes and forms. For example, segment members may be straight or curved. In addition to having an open section made up of several solid, hollow, or bundles of solid or hollow members, a segment may consist of a single hollow section housing the tensioning strands. Especially in this latter case, the ends of a segment may be so formed as to engage, especially by slide-fitting, the adjacent segments directly, achieving the desired profile of the structure, and providing passage for the prestressing strands. This arrangement eliminates the need for independent connectors but still provides the slide-fit contact necessary for self- support of arched structures during and after assembly. The segment members may slide-fit and/or be otherwise fixed or bonded to a single connector. Alternatively, twin connectors may be used which fit back-to-back and are keyed- in together to ensure good load transfer across segments ends. This arrangement is useful where the structure needs to be dismantled and relocated as complete segments, several times during its useful life, or if minimum assembly work on site is preferred.

The connectors act as a means of joining adjacent segments and any other members that need to be attached to the unit frame, of producing the desired profile, and of supporting the tensioning strand(s).

A structure made according to the method of this invention may additionally be in the form of a straight beam or column, a closed curve or polygon, in two- or three- dimensional forms. An example of the latter forms is a dome structure. This suggests applications in the aerospace and automotive industries, as structural skeletons of vehicles or vessels. Here, it may be possible to simplify the manufacture and fabrication of complicated structures by producing small modular segments that are easily assembled and prestressed.

Apart from the internal structure and jointing methods, the function and versatility of application of a structural element made according to this invention are similar to those of the various equivalent forms presently in use. In what follows, one method of carrying out the invention is described in text and drawings, by way of example only. The described frame constitutes a structural unit of a large-span rectangular building, e.g a swimming pool, a warehouse, or a large domestic garage/workshop. Figure 1 is a side elevation of a frame unit 11, approximating the semi-circular arch profile indicated by a dashed line. A segment comprises four parallel straight timber log (SRT) members 1 arranged in a rectangular open section with ends slide—fitting into connectors 2. Cross beams (purlins) which carry the roof and wall cladding 4 fit into housings 3 attached to the connectors 2. These housings are optional pieces which are in this case welded to the connectors 2 to provide a rigid end fixing for purlins . In this example, the SRT logs are also used as purlins to emphasize the usefulness of SRT as a structural ^"material, and also for simplicity. The connectors are, in this case, single units made from mild steel, and consist of a suitably stiffened central diaphragm (in this case a simple plate) to which short lengths of pipe are welded on either side in the required numbers, spacings, and inclinations in order to house the ends of segment members. As shown in Figure 1, there are two types of connectors: straight and angular, according to the profile angles they produce. For angular connectors, short curved pipes are welded to the diaphragm as one form of providing a smoothly curved passage for the high-tensile steel tensioning strand 5. Also for angular connectors, a filler material (e.g. sand-cement grout) is used to fill circular wedges at the bottoms of the short pipes, so as to provide even seating for SRT log ends. Figure -2 is a perspective sketch of an angular connector indicating the strand's passage 10.

The single 7-wire high-tensile steel strand used here is anchored at both base-connectors 6 of the arch which are suitably stiffened to take the anchorage loads. Any suitable prestressing and measurement techniques may be used to impart the tensioning force to strand 5 and to measure this force respectively.

Figure 3 depicts an example of a simple mechanical method by which the tensioning force may be applied and/or later adjusted. Tightening bolts 8 which surround the strand 5 and pass through base-connectors 6 pushes down the strand grip assembly 9, thus increasing the tension in the strand 5. Standard pretensioning grips are used.

Figure 4 is cross—section A-A indicated in Figure 1, displaying the structurally efficient rectangular open- section arrangement of the segment members, providing easy access to the tensioning strand. As the strand is tensioned, the compressed segment members naturally tend to increase in diameter slightly, enhancing the slip-fit contact and hence the bearing of SRT log ends against the connectors' pipe sockets.

As contact with the strand is confined to short smoothly curved passages through the ^" angular connectors, tensioning friction losses are much less than in comparable concrete structures. The assembly process starts by securing each of the base-connector units 6 of a frame to the two bolts (not shown) on each foundation plinth 7. Structurally, this is designed as a two-pin arch or frame . The two halves of the arch are built up as overhanging arched cantilevers by successively slide—fitting segment logs and connector units into position. The cantilevers, anchored down by the foundation bolts plus any additional anchorage for assembly purposes dictated by the structural design, are self- supporting by virtue of the slide-fit contact forces between the logs and the short connector pipes. However, there is enough flexibility in the cantilevers to enable their ends to be eased in together at the peak of the frame. As the design loads are usually much larger than the weight of the structure itself, there is always a large margin of safety when an arched cantilever forming part of a frame is being assembled. The completed frame, and indeed the whole skeleton, are stably self-supporting even without the prestressing strands which are installed and tensioned afterwards. As no connector unit or segment member is too heavy for one person to handle, no lifting equipment is needed at all to install this particular structure.

Alternatively, each frame may be assembled on the ground, prestressed, and lifted into position.

Figure 5 is a schematic general arrangement diagram of the load carrying skeleton of the building, comprising the frames 11 described in Figure 1 and the cross beams (purlins) 12. Another possible arrangement is that the skeleton is tied down to tops of columns or walls above ground, thus forming only the roof support rather than the complete structural frame of the building. In all cases, the usual structural engineering provisions have to be made, e.g. counteracting the spreading thrust of the portals under load, and providing lateral bracing.

Claims

1. A structure, especially an arch—shaped one, which is made up of: (a) a series of segments, (b) a series of jointing members (connectors) adapted to engage, especially by slide-fitting, the ends of the segments, (c) at least one tensioning strand passing through the series of segments and connectors, and (d) means for tensioning the strand in order to compress the segments and connectors together thereby configuring them into a stable structure. The slide-fit contact forces between the connectors and ends of segments provide a self-support mechanism for an arched structure being assembled by successively mounting the segments and connectors, working from both ends towards the peak of the arch. The completely assembled structure is also self- supporting even before installing and tensioning the strands.

2. A structure as claimed in claim 1, wherein segments (a) are made up of solid, hollow, or bundles of solid or hollow elements arranged in an open section, spanning the length of the segment and engaged at the ends, especially by slide-fitting, by the adjacent connectors.

3. A structure as claimed in claims 1 and 2, except that segments (a) are made up of a single hollow section, accommodating the strands (c) .

4. A structure as claimed in claims 1 to 3 , wherein the connectors (b) are formed as single units, or twin units mounted back-to-back, and keyed in together.

5. A structure according to any preceding claim, wherein timber logs are used as segments or segment members.

6. A structure as claimed in claims 1 to 3, wherein the connectors (b) are formed as integral parts of one or both ends of adjacent segments (a) .

7. A structure according to any preceding claim, which is shaped in a linear, arched, closed curve or polygonal profile, in two or three dimensions,

8. A structure according to any preceding claim, wherein methods adapted from prestressing technology and practice may produce variations in the nature, number, or manner of application of tensioning strands and method of stressing them.

9. A structure according to any preceding claim, adapted to be reversibly transformed, from a flexible into a stiff assembly capable of withstanding the intended loads, by tensioning the strands.

10. A structure according to any preceding claim, adapted for use as a structural skeleton in bridge and/or building construction.

11. A structure according to any preceding claim, adapted for use as a structural skeleton for a vehicle or a vessel .

12. A structure according to any preceding claim, adapted for use as a scaled model or a toy.

13. A method of design and construction of engineering structures which comprises the use of segments , connectors , and tensioning strands as defined in any preceding claims.

14. A structure substantially as described in the accompanying text and drawings.