AU726399B2 - Lignocellulose edge-to-edge joints - Google Patents

Description

P/00/0oII Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE

SPECIFICATION

STANDARD

PATENT

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Invent ion Title: LIGNOCELLULOSE EDGE-TO-EDGE

JOINS

Applicant: TOP STAIRS STAFF PTY LTD The following statement is a full description of this invention, including the best method of performing it known to me: -2- LIGNOCELLULOSE EDGE-TO-EDGE JOINS This invention relates to a method of joining lignocellulose sheets or planks edgeto-edge to extend length or width, under circumstances wherein the join will be subjected to loading. Whilst the invention is particularly useful for joining of medium density fibre board, for example as sold under the trade mark "craftwood", it is not limited to such fibre board and can be used for timber and plywoods of suitable characteristics.

Materials such as medium density fibreboard (MDF) have gained a significant popularity in many industries requiring the use of wood products due to its relatively low cost. However, due to the structure of MDF type materials, they have generally been considered unsuitable for use as structural members.

Generally, MDF materials are produced in planks that comprise relatively strong outer surfaces with relatively weak inner regions. This is due primarily to the 15 manufacturing process for these types of materials, the materials comprising relatively short loose fibres that are compressed and subjected to heat and the application of a binding agent. As a result of this process of reconstituting timber, the outer surfaces, which are subjected to greater levels of heat and binding agents, are generally of greater density, and hence strength, than the inner 20 regions of the reconstituted product.

There is frequently encountered a need for joining planks or sheets of such material end to end, but the conventional joins are unsatisfactory in that they will not provide the necessary mechanical strength for heavy loading. This invention seeks to provide a join superior to that disclosed in the prior art, which join will be capable of Withstanding heavy loading under severe conditions, having a strength which may or may not be equal to the original material, but which nevertheless is much greater than has been achieved for example with traditional dovetail type joins, finger joins or butt joins as used in kitchen tops, aided by a simple mechanical fix.

Joining two sections of MDF to each other to form a single unitary piece has generally been achieved in the prior art by cutting the MDF without due consideration of the relative strength and weaknesses inherent in the material. The -3joining of members end to end presents particular difficulties with respect to the structural integrity of the resulting member.

The most popular means of joining sections known to the applicant involves the use of a simple mechanical fix comprising a metal thread and two threaded retainers. The joining of two portions of MDF with such a device involves the creation of recesses into the boards of a sufficient depth to enable the insertion of the component parts and the affixing and tightening of the threaded retainers to the metal thread thus creating a tight join between the boards. This type of join is illustrated in Figure 1.

As will be noted from Figure 1, the recesses in the face of the MDF extend into the inner regions of the board and hence into the weaker region of the material.

Indeed, loading tests of this type of join have generally revealed that the join is significantly weaker than a unitary piece of material.

15 In attempting to produce a join of superior strength as compared with the prior art, the applicant sought to produce a join that was cognisant of the relative inherent strengths and weaknesses of MDF type materials.

It will be appreciated by those skilled in the art that the worst condition which can be encountered in the joining of two work pieces of MDF is a 20 combination of bending and shear loads, and it is an object of this invention to provide a satisfactory join and method of producing such joins, which will resist such loading better than previously known joins.

'While the invention is applicable to sheets which may be subjected to bending loads which are usually imparted normal to the plane of the joined sheets, the invention is even more applicable to load bearing members wherein the load is applied to the edges. For example, the stringer of a staircase is an application wherein the invention is particularly useful. For that reason the embodiment described hereunder is applied to and is generally .discussed in relation to a staircase stringer.

Giving due consideration to the relative strengths and weaknesses of MDF type materials, the applicant attempted to produce a join between planks of material using conventional types of joins including dovetail, finger and butt type joins. As these types of joins do not require recesses to be cut into the surface of the planks, it was believed that they would be of greater structural integrity.

The applicant soon discovered that joins such as dovetail and finger joins are highly impractical in terms of their manufacture and further found that they result in a significantly weak join. In addition to their impracticality, these types of joins result in an unsightly finish in the join region. This necessitates application of a suitably decorative finish to cover the join or considerable sanding of the join region in preparation for the application of paint.

In attempting various modifications, the applicant discovered some success with a tongue and groove arrangement wherein the tongue section comprised an extended neck portion with a generally circular head or termination in profile.

Wishing to improve upon the initial success of this particular type of tongue and groove arrangement, the applicant sought to find the optimum dimensions of the tongue and groove portions to enable the greatest possible structural integrity.

15 However, the applicant discovered that there were no established relationships that provided an indication as to the likely result on the exerted forces due to the variation of the dimensions of the tongue and groove portions.

Failing to discover any such relationships, the applicant developed a 0 method which results in a set of such relationships enabling the effect of varied dimensions to predict the resultant force interactions in the join.

Accordingly, the present invention provides a method of determining the dimensions of tongue and groove portions forming a join between two substantially planar members, wherein said tongue includes a projecting neck portion and a generally circular termination, the method including the steps of: a) establishing the moment capacity of the join in terms of the dimensions of the tongue and groove portions on the basis that the tensile strength of the tongue neck portion is greater than the shear capacity of the material of the two planar members in the region of the tongue termination; b) establishing the moment capacity of the join in terms of the dimensions of the tongue and groove portions on the basis that the tensile strength of the tongue neck portion is less than the shear capacity of the material of the two planar members in the region of the tongue termination; c) selecting an initial value for the various dimensions; d) calculating the moment capacity resulting from the relationship established at step a); e) calculating the moment capacity resulting from the relationship established at step b); f) calculating the difference between the moment capacities of step d) and step and 15 g) varying the values of the dimensions of the tongue and groove portions and repeating steps d) to e) until the difference calculated at step f) is an S_ acceptable value and if the difference calculated at step f) is acceptable then the last set of dimensions are those to be used.

The invention also provides a novel type of join that comprises a structural integrity greater than that known in the prior art.

The further description of the various aspects of the invention will now be provided with reference to the accompanying drawings. However, it is to be appreciated that the following description is not to limit the generality of the above description.

In the drawings: Figure 1 represents a plan view of a joining means that is prevalent in the prior art; and Figure 2 represents a plan view of a tongue and groove arrangement of the present invention.

With reference to Figure 1, a join between two generally planar members and 7 comprises a simple mechanical fixing device 8 which itself comprises a threaded rod 9, two retaining members 12 and two nuts 13. To insert the fixing -6device 8 and to tighten it to secure a firm join between the two members 5 and 7, generally circular recesses 15 are cut into the surface of the members and are adjoined by a further recess 17 which communicates between the two recesses The recesses are generally of only a sufficient depth to enable the insertion of the fixing device 8 such that it resides generally in the plane of the two members and does not protrude beyond the surface of the members.

Even though the recesses may not penetrate the members, for materials such as MDF these recesses are sufficient to significantly reduce the structural integrity of the MDF.

Figure 2 represents a plan view of a tongue and groove arrangement of the present invention and details the various dimensions and force planes relevant to the method of the invention, and necessary for the further illustration and understanding of the invention.

As was previously noted, this invention is particularly relevant to stringers 15 for staircases and as such, the two members 18 and 20 will be referred to as stringer sections.

The forces which must be considered in such a join are the shear forces due to direct loading by a force in the plane of the stringer sections, and the tensile forces which must be resisted by the tongue necks and also by the material 20 adjacent the recesses which retain the tongues.

Figure 2 details a typical tongue and groove arrangement consisting of a member 18 with two protruding tongue portions and another member 20 with two matching grooves. Although the invention is not limited to the configuration as detailed in Figure 2, this configuration has been chosen to illustrate the method of determining the optimum dimensions.

For the purposes of illustration, a downward force is assumed to be exerted on the joint.

On the left hand side of Figure 2, is sketched a compression/tension diagram that illustrates the parts of the members that will be in compression and those that will be in tension as a result of the assumed downward acting force.

For simplicity, the compression/tension diagram is modelled as two simple right angled triangles with the crossover from compressive forces to tensile forces half way down the member.

Figure 2 also details the shear planes of the most likely shear failure point 22 and 23. These shear planes also have a corresponding set of shear planes on the opposite side of the tongue portion located symmetrically about the plane of symmetry through the tongue. For the purposes of the method disclosed herein, it is only necessary to consider one set of these shear planes.

In addition to the shear planes, Figure 2 also details the most likely plane at which a tensile failure would occur as item Based upon the assumptions in respect of the compression/tension diagram, the centre of compression is assumed to be located at a position one third the distance along the compression triangle from the point of maximum compression. As the crossover from compression to tension is assumed to occur at a distance half way across the members, this results in the centre of compression being assumed to be located at a distance equal to one sixth of the width of the members.

15 Figure 2 details the centre of compression at a distance d/ 6 from the top edge of the members where d represents the width of the members. In Figure 2, the centre of compression is also aligned with the centre of the upper tongue portion. However, this is not necessary and in fact, it is expected that in most cases, this alignment would not occur.

20 The main objective of the method is to determine values for the dimensions x, t 1 tz, yl, y2, di and dz such that the moment capacity of the join is approximately the same for either a failure due to shear failure or due to tensile failure of a tongue and groove portion. The set of values that result in this situation are those dimensions that will provide the greatest structural integrity for the join.

The moment capacity of the join will consist of the shear resistance of the tongue and recess portions, the tensile strength of the tongue portion and the adhesion force of the glue which cements together the two contiguous surfaces of the tongues and recesses. The moment capacity of course will depend finally on the actual mode of failure of the join.

If the shear capacity across the planes 22 and 23 is less than the tensile strength of the minimum of the tongue neck, then the moment capacity is determined by: -8- Mcap Fpad 2 FRldl Madn where FR1 thickness of member. yi Fs and "thickness of member" is the known thickness of the material comprising the member, yi distance from join to tongue portion edge along tangent formed in line with the force FRI, di distance from the centre of compression to the projected outer edge of the tongue portion, and Fs K K 4

K

5

K

6 Fs, where Fs 1 shear capacity of material, K load duration factor, iK4 partial seasoning factor, Ks seasoned timber factor, 15 and Kg temperature variation factor, and "Madn" represents the adhesion force of the glue which cements together the two contiguous surfaces similarly FR2 thickness of member. Y2 F, *5 9 where Fs is the same value as would be calculated for FRI "thickness of member" is the known thickness of the material comprising the member, Y2 is the distance from the point of minimum width of tongue portion to the point of intersection between the edge of the tongue and a projection from the point of minimum tongue width parallel to the plane of symmetry of the tongue portion, and d 2 is the distance from the centre of compression to the projected inner edge of the tongue portion.

FR2 and FRI are the forces that must be resisted, and are illustrated in Figure 2. Although this embodiment is restricted to two tongue and groove -9portions, it will be recognised by those skilled in the art that this may be extended to any number of tongue and groove portions along the length of a join.

Values for K 1

K

4

K

5 and K6 can be determined experimentally and will depend upon the environment of the structural member and the seasoning of the timber. As a first approximation, values of unity are generally adopted.

If on the other hand the tensile strength is less than the shear capacity across the planes 22 and 23, then the moment capacity Mcap =T.dd2 +Madn where "Madn" represents the adhesion force of the glue which cements together 10 the two contiguous surfaces, dl represents the distance from the centre of compression to the projected outer edge of the tongue portion, dz represents the distance from the centre of compression to the projected inner edge of the tongue portion, and represents the tensile strength of the tongue neck and can be determined by S. "T Ft x Area Ft i thickness of member, tl Swhere Ft' tensile capacity of material, the thickness of the member can be measured and ti the minimum width of the tongue neck portion.

The tensile capacity of the material may be determined experimentally.

However, in most cases, the manufacturer of timber products can supply this information.

With respect to the adhesion moment, if the tensile strength of the glue is Flt, then the tensile force allowable is equal to F 1 t multiplied by the area of application in the tensile region, excluding some of the re-entrant area of the neck of each of the tongues.

It will be appreciated that the compression strength of the fibre board is likely to be somewhat greater than either the tensile or shear strength, and therefore the compression forces are not herein considered.

However, the adhesion moment is of importance, and since the contiguous surfaces of the edges of the recesses which accept the tongues and the tongues themselves can be produced to a high degree of accuracy, there is a potential to provide an adhesion between those surfaces which approaches the cohesion of the fibre board itself. The adhesion moment can therefore be considered, and that can be determined by e 5d d i d Madn=T dI+ 6 2

+T

2 d 2 d -2x-2t2 2 4 where Ti and T 2 are the forces which must be resisted as illustrated in the drawings and can be determined by T F'tg A 1 SFtg x b x (5d/ 6 d) and

T

2 Fltg. A 2 SF'us. b. (d-2x-2 t 2 a« where F' tensile strength of the glue,

A

1 area over which force T 1 acts,

A

2 area over which force T 2 acts, b thickness of the members d width of the members dl the distance from the centre of compression to the projected outer edge of the tongue portion X minimum distance from edge of member to edge of tongue portion and t 2 maximum width of tongue portion.

A1 and Az represent the areas over which the adhesive force of the glue will act which results in the forces T 1 and Tz. The length of the members over which -11 the glue acts resulting in force T 2 is A2/b and similarly, the length of the members over which the glue acts resulting in force T 1 is Al/b. Although these lengths appear to be approximately the same in Figure 2, in general, it is considered highly unlikely that these distances would be equivalent.

By calculating the moment capacity for each failure mode and comparing the result, an appropriate set of values can be found for the dimensions of the tongue and groove portions that will result in the moment capacities being approximately the same. Stated differently, the closer the difference between the moment capacities for the two different failure modes is to zero, the stronger the resulting join. As the calculation of the set of values that results in the difference between the moment capacities being close to zero is an iteration, it is more likely that an acceptable value will need to be chosen at which the iteration may cease.

The choice of this acceptable value will determine how optimal the set of values for the dimensions will be and how close the resulting join will be to the strongest 15 possible join.

It will be noted that the necks of the tongues are of a curved shape to provide a gradation of force resistance and avoid the possibility of stress concentration.

The strength achieved by the above embodiment greatly exceeds strength 20 by other means known to the applicant.

Preliminary test results comparing the strength of various types of joins have been conducted by the applicant to provide some quantitative indication as to the improvement in the structural integrity of the join of the instant invention.

Initially, an MDF stringer with no join was tested and was found to fail at approximately 6.2kNm.

A specimen joined with a metal threaded retainer substantially as illustrated in Figure 1 and glued at the adjoining faces was then subjected to the same test.

In this instance, the join was found to fail at approximately 15% of the material strength of a stringer with no join.

Numerous specimens joined with an improved tongue and groove arrangement substantially as depicted in this specification were also subjected to the same test and found to fail at approximately 30 to 50% of the material strength of the stringer. In addition to the improved structural integrity of the join, it was -12more robust and less likely to be damaged during handling as compared with the specimen joined with a metal threaded retainer.

The above embodiment was limited to a stair stringer or risers but if members are to be joined, obviously the configuration shown can be duplicated over the entire length of the members. It can be seen that the characteristics of this joining method are applicable to many different widths and thicknesses of medium density fibre board and not limited to stair stringers. Indeed, this method can also be applied to timber and plywood of suitable characteristics.

The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims.

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Claims (7)

1. 2. A method according to claim 1 wherein the moment capacity of the join in terms of the dimensions of the tongue and groove portions on the basis that the tensile strength of the tongue neck portion is greater than the shear capacity of the material of the two planar members in the region of the tongue termination is determined by Mcap FR 2 d 2 FRldi Madn where FRI thickness of member. yl Fs and "thickness of member" is the known thickness of the material comprising the member, y 1 distance from join to tongue portion edge along tangent formed in line with the force FRi, 15 d, distance from the centre of compression to the projected outer edge of the tongue portion, and Fs= K1.K4.K5.K 6 .Fs 1 where Fs' shear capacity of material, K, load duration factor, 20 K 4 partial seasoning factor, K 5 seasoned timber factor, and K 6 temperature variation factor, and "Madn" represents the adhesion force of the glue which cements together the two contiguous surfaces similarly FR2 thickness of member. y 2 Fs where Fs is the same value as would be calculated for FRi "thickness of member" is the known thickness of the material comprising the member, y 2 is the distance from the point of minimum width of tongue portion to the point of intersection between the edge of the tongue and a projection from the point of minimum tongue width parallel to the plane of symmetry of the tongue portion, and d 2 is the distance from the centre of compression to the projected inner edge of the tongue portion.
2. 3. A method according to claim 1 wherein the moment capacity of the join in terms of the dimensions of the tongue and groove portions on the basis that the tensile strength of the tongue neck portion is less than the shear capacity of the material of the two planar members in the region of the tongue termination is determined by Mcap T.d+d2 d Madn 2 :where "Madn" represents the adhesion force of the glue which cements 15 together the two contiguous surfaces, dl represents the distance from the centre of compression to the projected outer edge of the tongue portion, d 2 represents the distance from the centre of compression to the projected inner edge of the tongue portion, and represents the tensile strength of the tongue neck and is determined by 9* T Ft x Area Ft 1 thickness of member. tl o.:oo where FtI tensile capacity of material, the thickness of the member can be measured and tl the minimum width of the tongue neck portion.
3. 4. A method according to claims 2 or 3 wherein the adhesion moment is determined by -d l Madn 6 +T d2 -d -2x-2t2 2fl ]4 Li r i /j -16- where T 1 and T 2 are the forces which must be resisted as illustrated in the drawings and is determined by Ti Fltg. A, Fltg b x 5 d/ 6 d) and T2 Fltg. A 2 =F'ttg b d 2x-2t2 2 where Fltg tensile strength of the glue, A 1 area over which force Ti acts, A 2 area over which force T 2 acts, b thickness of the members 15 d width of the members dl the distance from the centre of compression to the projected outer edge of the tongue portion X minimum distance from edge of member to edge of tongue portion 20 and t 2 maximum width of tongue portion. o .o
4. 5. A join formed between two substantially planar members including at least one correspondingly shaped tongue and groove portion wherein the dimensions of the tongue and groove portions are determined according to the method of any of the preceding claims.
5. 6. A stringer for a staircase made from at least two substantially planar members, said planar members being joined in accordance with the method according to any one of claims 1 to 4.
6. 7. A method substantially as hereinbefore described with reference to Figure 2 of the drawings. A LI.,. -17-
7. 8. A join or a stringer substantially as hereinbefore described with reference to Figure 2 of the drawings. DATED: 14 August 2000 PHILLIPS ORMONDE FITZPATRICK Attorneys for TOPSTAIRS AND STAFF PTY LTD S Ot 00 a pV