GB2399829A

GB2399829A - Load bearing span

Info

Publication number: GB2399829A
Application number: GB0307042A
Authority: GB
Inventors: Robert Osborne
Original assignee: PREN
Current assignee: PREN
Priority date: 2003-03-27
Filing date: 2003-03-27
Publication date: 2004-09-29
Also published as: GB0307042D0

Abstract

A load bearing span comprises a plurality of lengths of timber 10, said timbers being held together in a laminated structure by a mechanical retention means (Figs 2a, 2b), where spacers (18, Figs 2a, 2b) are provided between adjacent timber lengths to maintain a gap between the timber lengths. Preferably there is an arcuate support member 34 associated with the span. The laminated timber spans may be used as deck sections for bridging purposes as shown by Figure 5. Further defined is a load bearing span where the span members can be joined together end-to- end.

Description

Load Bearing Structures This invention relates to load bearing structures

and, more particularly, to a load bearing span such as a bridge or roof of a building constructed primarily of timber, and a method of constructing such a load bearing span.

It is well known to construct products such as furniture and the like using hardwood timber, such as oak. The timber used to construct such products is of a high quality and therefore relatively expensive. However, a substantial quantity of lower grade timber is produced every year, for which there is currently an insufficient market.

The use of timber in construction of bridges has been proposed in the past. In most cases, a bridge requires a deck which is substantially stronger than a single length of timber, and it is for this reason that stress laminated timber decks have been proposed. A conventional stress laminated deck is constructed by laminating together pieces oftimber which have been placed on edge, until a solid deck ofthe required width is achieved. The laminating is achieved either by gluing the individual timber members together, or by compressing them through application of a post-tensioned prestress in the transverse direction. The latter system serves a dual purpose in providing a deck that is both the support for the wearing surface and also the structural system for transferring the imposed loads and forces through the substructure. The compression force due to prestress allows transfer of vertical shear between the laminates through friction and also resists transverse bending which is induced by the loads, provided the bending tensile stress is less than the initial compressive stress. The effective "stiffness" of the deck is a function not only of the timber depth and modulus of elasticity, but also the level of prestress applied to the deck.

In order to enable random lengths of commercially available timber to be used in the construction of any required length of deck, lengths of such timber may be joined end-to-end by means of conventional butt or finger joints together, in order to create effective single lengths of timber of any required length.

The advantages of employing timber in structural engineering are numerous, and include: À timber is durable and long-lasting - with modern treatments, timber structures may be expected to last for at least 50 years; À timber structures are of simple construction - construction usually demands low skills and simple equipment, and maintenance is usually equally simple; À the structures may be made using prefabricated components, which may be entirely factory made thereby ensuring good quality; À timber has a high strength-to-weight ratio, which saves on foundations and gives confidence in reuse of old foundations, such that crane loadings are reduced and money is saved; À timber is relatively inexpensive, and at least small span rural bridges can be built in timber at a significantly lower cost than if steel or concrete is used; À timber structures are generally more aesthetically pleasing than those made of, for example, concrete or steel; À timber is chemically stable and tends not to be affected by materials such as de-icing salts, which adversely affect materials such as steel or concrete; À timber does not expand and contract as much as concrete or steel with changes in temperature, such that a road over a timber span can be made continuous, without the need for troublesome joints; À timber structures are renewable and sustainable, which is obviously advantageous to the economy; À timber removes carbon from the atmosphere and locks it on the ground, which is obviously highly advantageous for the environment.

This known method of construction using stress laminated timber is described in more detail in a paper entitled "Development of Limit States Design Procedures for Stress Laminated Timber Bridge Decks" by Keith Crews, University of Technology, Sydney - Australia.

However, although the above-described method of construction has been found to be a useful method of constructing durable decks for bridges to be employed in locations which are subjected to substantially constant climate conditions, there are a number of disadvantages which make it unsuitable for use in constructing load bearing spans for use in more changeable climates. For example, external bridges used in the United Kingdom and other countries having similar climate conditions, are subjected to alternating periods of damp, cold and wet weather and then warmer, dry weather. The timber is caused to contract in cold weather and expand in warmer weather, which expansion and contraction in the abovementioned stress laminated timber deck, puts the individual lengths of timber under considerable stress and eventually causing breakage (of the timber and/or the glue holding the timber members together) which permits entry of water into the laminated structure, causing rot and reduction in strength of the overall structure.

We have now devised an arrangement which seeks to overcome the problems outlined above.

Thus, in accordance with a first aspect of the present invention, there is provided a load bearing span comprising a plurality of lengths of timber, said lengths of timber being held together longitudinally relative to each other in a laminated structure by means of mechanical retention means, and spacer means being provided between at least two adjacent lengths of timber so as to maintain a space between said at least two lengths of timber within said laminated structure.

Also in accordance with the first aspect of the present invention, there is provided a method of constructing a load bearing span, the method comprising the steps of providing a plurality of lengths of timber, connecting said plurality of lengths of timber together longitudinally relative to each other to form a laminated structure and providing spacer means between at least two adjacent lengths oftimber in said laminated structure, and securing said plurality of lengths of timber in said laminated structure by means of mechanical holding means, said spacer means maintaining a space between said adjacent lengths of timber when they are secured within said laminated structure.

The provision of spacer means, which is facilitated by the use of mechanical means to hold the timber members together (as opposed to glue or the like), enables spaces to be maintained between the individual timber members of the laminated structure, which spaces permit expansion and contraction of the timber members and, more importantly, provide an effcient means of ventilation of the timber members to facilitate drying during and after damp climate conditions, which helps to prevent rotting of the timber members.

Another issue to be considered in the construction of a load bearing span using timber is that, the longer the length of a single unsupported deck, the lower the its load bearing capacity will be. Therefore, timber load bearing spans constructed using the conventional method described above are only suitable in circumstances where only a short span is required or where the load bearing capacity requirement ofthe span is relatively low. This is another reason why timber is not generally used in the construction of load bearing spans, and such are more commonly constructed using concrete and/or steel.

We have now devised an arrangement which seeks to overcome the additional problems outlined above.

Thus, in accordance with a second aspect of the present invention, there is provided a load bearing span comprising a plurality of laminated timber load bearing span members joined together end-to-end.

The span members are beneficially joined together such that the net resultant force between adjacent span members is a compressive force therebetween, which strengthens the overall structure and provides the required stability.

Also in accordance with the second aspect ofthe present invention, there is provided a method of constructing a timber load bearing span, comprising the steps of providing a plurality of laminated timber load bearing span members and joining said span members together end-to- end.

The individual load bearing span members are preferably in accordance with the first aspect of the present invention. -s -

Thus, the second aspect of the present invention provides a modular construction for timber load bearing spans, which may be used individually for short spans up to, say, 11 metres, or in multiples to form, for example, cable stayed spans of up to and in excess of 66 metros.

The load bearing span of the first aspect of the present invention, or each load bearing span of the second aspect of the present invention, preferably additionally comprises a truss, preferably of bowstring configuration, i.e. an arch member, in each parapet. The arch member is beneficially predominantly in compression, with load being applied from the load bearing span or deck via the parapet structure. In a cable stay arrangement, a bottom tie or bracket may not only be used to restrain the end of the arch member, but may also be subjected to compression forces.

Accordingly, an aspect of the invention provides a modular mechanically laminated load bearing span, such as a bridge, which may utilise relatively short lengths of timber of relatively small cross-section, with medium to low structural strength grading. Individual lengths of timber can be end-finger or butt jointed, for example, to achieve the necessary lengths, which lengths can then be laminated according to the method of the first aspect of the present invention, to form a composite structure having the required structural dimensions.

Finger jointing in particular easily accommodates variable length sections, resulting in negligible waste, whereas mechanically laminating members according to the first aspect of the present invention, as opposed to conventional methods, provides a robust solution to exposed timber, allowing shrinkage and local distortion to occur without compromising structural integrity.

The primary elements may comprise of, for example, lOOmm wide x 50mm thickness oak laminates bolted together to form fully composite structural members. It will be appreciated that structural grade oak or other hard wood, in particular, is economically produced by and readily available from woodland improvement sources in relatively short lengths of small cross-section, although many other types of timber may be used in the present invention.

The mechanical retention means may comprise one or more bolts, extending through each of the timber members, with a nut or similar securing means being provided at one or both ends of the bolt to secure the laminated structure together. In this case, a hole must be provided at a corresponding position through each of the timber members to receive the bolt. More preferably, the mechanical retention means may comprise a sheath, belt or clip, preferably of metal, which fits around or over the outside ofthe laminated structure to secure the individual timber members together. This method avoids the need for precise drilling of holes through each of the timber members.

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: Figure 1 is a partial side view of a laminated timber load bearing span according to an exemplary embodiment of the present invention; Figure 2a is an exploded schematic cross-sectional view of a portion ofthe structure illustrated in Figure 1, illustrating the configuration ofthe timber members and the manner in which they may be fixed together; Figure 2b is a schematic diagram illustrating another method in which the timber members of Figure 1 may be fixed together; Figure 3 is a schematic plan view illustrating the manner in which two lengths of timber may be joined together to form a longer length of timber; Figure 4 is a schematic side view of a bridge structure according to an exemplary embodiment of the present invention; Figure 5 is a schematic side view of a modular bridge structure according to an exemplary embodiment of the present invention; and Figure 6 is a schematic end view of a truss arrangement for use in the bridge structure of Figure 5.

Referring to Figure 1 of the drawings, a laminated timber load bearing span according to an exemplary embodiment of the present invention comprises a plurality of lengths 10 of timber, such as oak, rayed one on top of the other to form a laminated structure. Each of the lengths of timber 10 consist of a plurality of individual timber members 12 which are joined together end-to-end (by means of a butt joint or, more preferably, a finger joint 16, as illustrated in Figure 3) to form a length oftimber ofthe required length. A stainless steel end plate 14 may be provided at the end ofthe span to aid in affixing the end ofthe span to a fixed structure, in use.

Referring more particularly to Figure 2a ofthe drawings, in one embodiment ofthe invention, each length 10 of timber is stacked one on top of the other, with one or more metal plate spacers 18 (or similar) between each length. A hole 20 is drilled through the entire stack of timber, and a bolt 22 having a head 24 is pushed through the hole 20; A portion of the end of the bolt 22 protrudes from the hole, over which end a bolt 26 can be placed to secure the bolt and fix the lengths 10 oftimber together.

In an alternative embodiment, as shown in Figure 2b of the drawings, instead of drilling a hole through the laminated structure and inserting a bolt therethrough, a metal clip 28 may be provided around the laminated structure. The clip 28 may be held in place by compressing it or otherwise causing it to contract to fit around the stack of timbers, or providing some form of screw means or other securing means (not shown).

Referring to Figure 4 of the drawings, a bridge structure 30 according to an exemplary embodiment of the present invention, comprises a deck 32 constructed using the laminated timber structure illustrated in and described with reference to Figure 1 of the drawings. An arch member 34 is provided between each end ofthe deck 32, the arch member 34 preferably also being constructed using the laminated timber structure described with reference to Figure 1 of the drawings. Brackets 36 are provided at spaced apart intervals on each side and along the length of the deck 32, which brackets retain respective upright support members 38.

Horizontal beams 40 extend between each pair of adjacent support members 38, the beams 40 together defining a handrail for the bridge. The area between the beams 40 and respective upright support members 38 is provided with a lattice or trellis arrangement 42 to complete the parapet and provide additional support for the handrail 40. Connection members 44 are provided at each end ofthe deck 32 for connection ofthe structure to other, similar structures, or for securing the structure to fixed members, as well as connecting the ends of the arch member 34 to the deck 32. Such connection may be made with two metal splices which are joined by a channel which takes the arch thrust in end contact. The vertical component ofthe load is transferred directly by bearing shoes onto the abutment and the horizontal component is resisted by a bolted connection to the bottom tie or bracket 36.

Referring to Figure 5 of the drawings, a cable sway bridge arrangement comprising six ofthe structures 30 of Figure 4 connected together is illustrated. Thus, as shown, the bridge is of a modular construction, comprising six bridge sections 30 according to the present invention connected together end-to-end. The resultant force between adjacent bridge sections 30 is compression, which provides support and adds stability to the overall construction. As stated above the illustrated bridge has a cable sway support arrangement comprising a truss 46, from the top of which extend a plurality of cables 48 which are connected at the connection members 44 to each end of the bridge sections 30.

Referring to Figure 6 ofthe drawings, the truss 46 consists oftwo poles 50 which are arranged relative to each other so as to provide a tapered profile from the bottom of the structure to the top. A substantially horizontal deck supporting member 52 extends between the poles 50, close to the bottom thereof, for supporting the deck(s) 32 ofthe bridge section(s) 30. The truss 46 is mounted on a plinth 52 or the like, as shown, and cables 48 extend from a position close to the top of the truss 46. It can be seen in Figure 6 that the parapet 54 of the bridge section may be provided with an additional support member or outrigger 56, if required. l

Claims

CLAIMS: 1. A load bearing span comprising a plurality of lengths of

timber, said lengths of timber being held together longitudinally relative to each other in a laminated structure by means of mechanical retention means, and spacer means being provided between at least two adjacent lengths of timber so as to maintain a space between said at least two lengths of timber within said laminated structure.
2. A load bearing span according to claim 1, comprising a support member extending between positions at or adjacent said deck.
3. A load bearing span according to claim 2, wherein said support member is an arched member.
4. A load bearing span according to claim 2 or claim 3, comprising one or more parapets, a support member being provided in the or each parapet.
5. A load bearing span according to any one of the preceding claims, wherein at least some of said lengths of timber comprise two or more shorter timber members joined to each other at their ends by means of, for example, a finger or butt joint.
6. A load bearing span according to any one of the preceding claims, wherein said mechanical retention means comprises one or more bolts, extending through each of the lengths of timber, with a nut or similar securing means preferably being provided at one or both ends of the bolt to secure the laminated structure together.
7. A load bearing span according to any one of claims I to 5, wherein the mechanical retention means comprises a sheath, belt or clip, preferably of metal, which fits around or over the outside ofthe laminated structure to secure the individual lengths of timber together.
8. A load bearing span substantially as herein described with reference to the accompanying drawings.
9. A method of constructing a load bearing span, the method comprising the steps of providing a plurality of lengths of timber, connecting said plurality of lengths of timber together longitudinally relative to each other to form a laminated structure and providing spacer means between at least two adjacent lengths of timber in said laminated structure, and securing said plurality of lengths oftimber in said laminated structure by means of mechanical retention means, said spacer means maintaining a space between said adjacent lengths of timber when they are secured within said laminated structure.
10. A method of consulting a load bearing span substantially as herein described with reference to the accompanying drawings.
11. A load bearing span comprising a plurality of laminated timber load bearing span members joined together end to end.
12. A load bearing span according to claim 11, wherein the span members are joined together such that the net resultant force between adjacent span members is a compressive force therebetween.
13. A load bearing span according to claim 11 or 12, comprising a plurality of load bearing span members according to any one of claims 1 to 8.
14. A method of constructing a timber load bearing span, comprising the steps of providing a plurality of laminated timber load bearing span members (preferably according to any one of claims 1 to 8) and joining said span members together end -to-end.