GB2426987A

GB2426987A - Joist cap

Info

Publication number: GB2426987A
Application number: GB0617882A
Authority: GB
Inventors: Robert Sinclair Hanks; Kenneth Stanley Targett
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-09-12
Filing date: 2006-09-12
Publication date: 2006-12-13
Anticipated expiration: 2026-09-12
Also published as: GB0617882D0; GB2426987B; GB2426987A8

Abstract

A open-bottomed joist cap 10 is used with a joist that has an end portion embedded in a hole through a blockwork wall in order to prevent air leakage through the wall in the event that the joist end shrinks or splits. The joist cap is moulded from resilient plastics material and comprises an end wall portion 14, a pair of side wall portions 12L, 12R projecting from the end wall portion and a top wall portion 16 projecting from the end wall portion 14 and connecting the side wall portions 12L, 12R. When relaxed the side wall portions 12L. 12R mutually converge in a direction away from the end wall portion 14 with an angle of convergence that increases progressively from about zero at the top of the joist cap to a maximum at the bottom of the joist cap. The joist cap therefore grips the joist. A mould for moulding the joist cap has cavity portions for forming the end wall, side wall and top wall portions. The cavity portions for the side wall portions are parallel. The joist cap is formed by moulding in the mould, removal from the mould and allowing the sidewalls to converge naturally.

Description

TITLE

Joist cap and method of manufacture thereof

DESCRIPTION

This invention relates to joist caps and to methods of manufacture of joist caps.

As is discussed in the introduction of patent document GB2420799B (Vaughan and Hennen), joists were traditionally supported by having their ends embedded in holes in the supporting walls. However, if the timber used for the joist is not thoroughly seasoned, the end of the joist can, in time, shrink and split, leaving gaps through the wall, causing draughts and reducing the thermal-insulation, acoustic-insulation and fire-retardant properties of the wall.

More recently, joist hangers have been used. With a joist hanger, the end of the joist is not directly supported by the wall, but instead stops short of the wall and sits in a cradle portion of : the joist hanger, and hook portion of the joist hanger is embedded in the wall. Joist hangers solve the air leakage problem, but they introduce their own problems, as described in detail in * SI* GB2420799B, which can result in a weak structure, or in collapse of the building while it is **. S under construction. To solve these problems, a return to the embedded- end method of joist * : support has been proposed, but additionally using a joist cap that provides a seal in the case * where the joist shrinks or splits. *S**

*:::: A number of joist caps have been proposed, as described in patent documents GB2394234B, GB2420799B and GB2420800B of Vaughan and Hennen, GB2392928B and G82393459B of Simpson Strong-tie and GB2405881A of Manthorpe. Each of these joist caps has: an end wall portion that extends across the far end of the joist: side wall portions that extend along the sides of the end of the joist; and a top wall portion that extends along the top of the end of the joist. The side and top wall portions are embedded in the wall, and even if the joist shrinks or splits, the end wall portion of the joist cap serves to maintain a seal to prevent air leakage through the wall. Some of the joist caps described in the documents mentioned above also have a bottom wall portion that extends along the bottom of the end of the joist, and the joist sits on the bottom wall portion. However, some of the joist caps described in the Vaughan and Hennen patents are devoid of a bottom wall portion. An example of a joist cap 10 without a bottom wall portion is shown in the accompanying Figures 1A and lB. The lack of a bottom wall portion provides a number of advantages: (i) the end of the joist sits directly on the blockwork of the wall; (ii) any moisture in the end of the joist can be absorbed and dispersed by the blockwork of the wall, rather than being trapped; (iii) once the joist has been laid, it is not necessary to lift the end of the joist in order to fit the joist cap; and (iv) the side wall portions of the joist caps can be interdigitated, as shown in the accompanying Figure 2, so as to reduce the space required for storage and transportation.

The joist cap shown in Figures 1A and lB is preferably made from plastics material, such as polypropylene, by injection moulding. A problem with injection moulding such an article is that two wall portions that are to be at right angles in the fmished article need to be moulded at an obtuse angle, unless they are supported by some other portion of the article, because upon release from the mould the angle tends to reduce. In the joist cap 10 shown in Figures IA and 1B, towards the tops of the side wall portions 12L, 12R and the end wall portion 14, these wall portions 12L,12R,14 are held at right angles by the top wall portion 16.

However, towards the bottoms of the side wall portions 12L, 12R and the end wall portion 14, there is no bottom wall portion to hold the wall portions 12L, 12R, 14 at right angles. Therefore, in order to produce a joist cap 10 with side wall portions 12L,12R at right angles to the end * * . S wall portion 14, as shown in the prior Vaughan and Hennen patents and in Figure 1, it is necessary in the mould for the side wall portions mutually to diverge in the direction away from the end wall portion with an angle of divergence that increases progressively from zero at the * S S...

top of the joist cap to a maximum at the bottom of the joist cap. An example of two mould **5*** * S halves 18M,18F having this feature is shown in Figure 3. It will be appreciated that the * 20 toolmaking costs for such a mould 18M, 18F are high because the surfaces 5*5* S. 20L0,2OLI,2ORO,20RJ of the mould I 8M, 1 8F that produce the side wall portions 12L, 12R S...

need to be machined with a twist, rather than simply being planar.

An aim of the present invention, or at least of specific embodiments of it, is to reduce the toolmaking costs of a joist cap of the general type shown in Figures IA and lB and/or to provide an improved joist cap.

In accordance with a first aspect of the present invention, there is provided, a mould for producing a bottomless joist cap, the mould having cavity portions for forming the end wall, side wall and top wall portions of the joist cap, characterised in that the cavity portions for producing the side wall portions are generally parallel. Because of this, the toolmaking costs for the mould are reduced.

In accordance with a second aspect of the present invention, there is provided a joist cap characterised in that the joist cap is moulded from resilient plastics material and, when relaxed, the side wall portions mutually converge in the direction away from the end wall portion with an angle of convergence that increases progressively from about zero at the top of the joist cap to a maximum at the bottom of the joist cap. The side wall portions therefore need to be sprung apart in order to fit the joist cap to the end portion of the joist. However, due to the resilience of the plastics material, the side wall portions of the joist cap will then grip the end portion of the joist so that the joist cap cannot be dislodged so readily from the end portion of the joist before the wall is built around the joist cap.

A specific embodiment of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1A is an isometric view from the front, right and top of a joist cap of the type envisaged in patent GB2420799B; Figure lB is an isometric view from the rear, left and below of the joist cap of Figure 1A; Figure 2 is an isometric view of five joist caps of Figures 1A and lB with their side wall : ::* portions interdigitated; * * * Figure 3 is an isometric view of a pair of mould halves for forming the joist cap of a..

Figures 1A and 1B; *.S's* * . Figure 4 is an isometric view of a pair of mould halves used in an example of the method ** * of the first aspect of the present invention for forming an embodiment of joist * ::::* cap of the second aspect of the present invention; Figure 5A is an isometric view from the front, right and top of a joist cap produced by the mould halves of Figure 4; and Figure SB is an isometric view from the rear, left and below of the joist cap of Figure 5A.

Referring to Figure 4, a mould 20 used in the example of the invention comprises male and female mould halves 22M,22F which can be closed by movement of the male mould half 22M in the direction of the arrows 24 and can be opened by movement in the opposite direction.

The male mould half 22M has a base portion 26 with a generally cuboidal moulding projection 28, i.e. all of the moulding surfaces of the moulding projection 28 are generally parallel or generally at right angles to each other. The female mould half 22F comprises a block 30 having a generally cuboidal moulding recess 32, i.e. (subject to the following sentence) all of the moulding surfaces of the moulding recess 32 are generally parallel or generally at right angles to each other. Around three sides of the mouth of the recess 32, a rabbet 34 is formed which produces a flange 42 in the moulded article. When the mould 20 is closed, the bottom surface 36B of the moulding projection 28 engages the bottom surface 38B of the moulding recess 32.

However, the side surfaces 36L,36R, top surface 36T and outer surface 360 of the moulding projection 28 are spaced from the side surfaces 38L,38R, top surface 381 and inner surface 381, respectively, of the moulding recess 32, for example by a distance of 2.5 mm. A very slight tapering may be provided in order to facilitate release of the article from the mould.

Accordingly, when molten polypropylene is injected into the closed mould 20 via an injection port 40, the polypropylene takes on the shape of the joist cap 10 shown in Figures 1A and lB.

However, when the polypropylene cools, the mould halves 22M,22F are opened and the joist cap is ejected, the joist cap 10 relaxes to the shape shown in Figures 5A and 5B. That is, the side wall portions 12L, 12R converge towards each other in the direction away from end wall portion 14, the angle of convergence increasing from zero at the top of the joist cap 10 to a maximum at the bottom of the joist cap 10.

* ** In a polypropylene prototype of the joist cap 10 of Figures 5A and 5B, the wall thickness of the joist cap 10 was 2.5 nun, the spacing ST between the side wall portions 12L,12R at the top of the joist cap 10 was 50 mm, the height H of the joist opening was ****, 227 mm, and the depth D of the joist opening was 65 mm. In its relaxed state, the spacing SB *SS* between the side wall portions 12L,12R at the bottom of the joist cap 10 was 35 mm. *

Furthermore, the force F that was needed to be applied between the side wall portions 12L,12R ** 20 about two-thirds of the way down from the top of the joist cap 10 in order to render the side wall portions 12L,12R generally parallel was about 22 Newtons.

The joist cap 10 of Figures 5A and 5B is used in a similar fashion to that described in patent GB2420799B, except that the side wall portions 12L, 12R are prized apart in order to fit the joist cap 10 to the end of the joist. Once fitted, the side wall portions 12L, 12R grip the sides of the joist and thus reduce the chances of the joist cap 10 being accidentally dislodged before the wall is built up around the end of the joist. The flange 42 is embedded in the mortar of the wall part-way through the thickness of the wall in the manner described in patent GB2394234B.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many modifications and developments may be made thereto within the scope of the present invention.

Claims

1. A joist cap for use with a joist that has an end portion to be embedded in a hole through a blockwork wall, the joist cap comprising an end wall portion arranged to cover the end of the joist and substantially to seal the hole through the blockwork wall, a pair of side wall portions projecting from the end wall portion to receive the end portion of the joist therebetween and to be received between the blockwork of the wall to either side of the hole, and a top wall portion projecting from the end wall portion and joining the side wall portions, characterised in that: the joist cap is moulded from resilient plastics material and, when relaxed, the side wall portions mutually converge in a direction away from the end wall portion with an angle of convergence that increases progressively from about zero at the top of the joist cap to a maximum at the bottom of the joist cap.

: *.*

2. A mould for producing a bottomless joist cap for use with a joist that has an end portion S. to be embedded in a hole through a blockwork wall, the joist cap comprising an end wall * .S* portion arranged to cover the end of the joist and substantially to seal the hole through the blockwork wall, a pair of side wall portions projecting from the end wall portion to receive the * end portion of the joist therebetween and to be received between the blockwork of the wall to either side of the hole, and a top wall portion projecting from the end wall portion and joining * . the side wall portions, wherein: **** the mould has cavity portions for forming the end wall, side wall and top wall portions of the joist cap, characterised in that: the cavity portions for producing the side wall portions are generally parallel.

3. A method of manufacturing a joist cap, comprising the steps of: injecting plastics material into a mould as claimed in claim 2; removing the resulting joist cap from the mould; and allowing the side wall portions naturally to converge.

4. A joist cap produced by a mould as claimed in claim 2 or a method as claimed in claim 3.

5. A building having a joist end embedded in a wall with a joist cap as claimed in claim 1 or4.

6. A joist cap, mould, method of manufacture or building, substantially as described with reference to Figures 4, 5A and SB of the drawings. * .. * * . *..S * * S... * . *...

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