GB2506691A

GB2506691A - A floor joist with an increased depth top chord

Info

Publication number: GB2506691A
Application number: GB201218020A
Authority: GB
Inventors: Douglas Wilson Cullen; Martin William Milner
Original assignee: Intelligent Wood Systems Ltd
Current assignee: Intelligent Wood Systems Ltd
Priority date: 2012-10-08
Filing date: 2012-10-08
Publication date: 2014-04-09
Also published as: GB201218020D0

Abstract

A joist 10 and a method of timber floor construction which mitigates collapse in the event of a fire for an increased time is formed with top 12 and bottom 14 chords wherein the top chord 12 has a greater depth than the bottom chord 14. Embodiments as an I joist or an open-web joist are described.

Description

JOIST

The present invention relates to building construotion and in particular, though not exclusively, to a joist providing increased load bearing properties in the event of a fire.

I joists and open web joists are typically used in floor building construction. A joist comprises a top chord and a bottom chord, having a web located there between. An I joist is illustrated in Figures 1 (a) and (b) . The joist A has a top chord B and a bottom chord C which are typically (?) identical rectangular solid timber flanges extending the full length of the joist. A channel is routed on the inner faces D,E of the flanges to accept a timber panel F, such as OSB. Thus in the I joist the web is formed of a timber panel. Open web joists also exist in which the web is formed by a series of V1 shaped connectors joining the top and bottom chords. These open web joists typically use metal connectors, but more recently all wooden open web joists have become available.

In a building construction, a joist is located on two columns or end posts G (only one shown) separated by the length of the joist. In this way, the joist is supported at each end.

Further columns may extend above the ends C so that, in use, the joist is under compression, from above and below, acting perpendicularly to the length of the joist, while also under tension, occurring along the bottom chord in parallel to the length of the jcist. This is why the stronger tensile and compressive material is in the chords at the outer edges of the joist and thus the central material i.e. the web can be reduced in size as it carries very little of the bending forces. This is illustrated in Figure 1.

However, a disadvantage in this arrangement of a joist is that, in the event of a fire, the central portion of the bottom chord will burn through taking the web guickly with it.

As a result, the tension is lost across the bottom chord and the remaining compression, now applied downwardly upon the top ohord, guickly causes the joist to fail and collapse.

US 4,669,243describes a fire proteotive system and method for a support structure suoh as a truss having top and bottom chord members, web members extending between the top and bottom ohord members and oonnector plates for connecting the respective ends of the web members to the top and bottom chord members. Fire protection is provided by an elongated metal member having a base portion and a pair of flanges projeoting upwardly from the base portion to form a channel. The bottom chord member is received within the channel so that the flanges extend at least partially upwardly along respective opposite sides of the bottom chord member and are in contact therewith. The web members are preferably comprised of V-shaped metal webs having a central toothed connector plate, a pair of elongated arms diverging outwardly from the central connector plate and a pair of toothed connector plates integrally formed on respective ends of the arms opposite the central connector plate.

The elongated metal member having a base portion and a pair of flanges projecting upwardly from the base portion to form a channel into which the bottom chord is located, both provides the fire protection and, as the member will extend the length of the truss or joist, will support the joist under compression when the bottom chord is destroyed by fire.

Compared with current joists, this fire protective joist is more expensive as it has addinional metal material and will take longer to manufacture as the metal member must be shaped and fitted to the bottom chord. Additionally, as current trends seek environmentally friendly building materials, the additional metal parts do not meet this environmental need.

Further joist arrangements have been proposed which are designed to resist compression on the top chord when the joist is used with top hung hangers. W020101l8531 is to a wcoden joist. The joist comprises a wooden top chord, a wooden bottom chord, and a main section ccmprising at least one cf wcoden boards and wooden webs adhesively connected to the wooden top chord and to the wooden bottom chord to form at least one of an I joist subsection and an open-joist subsection along the main section of the wooden joist. An end configuration at one end of the main section comprises a wooden post extending generally perpendicularly between the wooden top chord and the wooden bottom chord and is adjacent to the main section. A wooden diagonal web extends diagonally from the wooden top chord to the wooden bottom chord. A first set of metal plates is in alignment on opposed sides of the wooden joist to connect the wooden top chord to the diagonal web. A second set of metal plates is in alignment on opposed sides of the wooden joist to connect the diagonal web and the post to the bottom chord. A third set of metal plates is in alignment on opposed sides of the wooden joist to connect the wooden top chord to the post. An extension is defined by the wooden top chord extending beyond the diagonal web at an end of the wooden joist. The wooden joist is adapted to be in a top-chord bearing relation with a beam by the extension being on top of the beam.

While this joist is supported at the ends, the top and bottom chords and web between the end arrangements provide a section of the joist which is as a standard joist. Consequently, in the event of a fire, the central portion of the bottom chord of the section will burn through taking the web quickly with it. As a result, the tension is lost across the bottom chord and the remaining compression, now applied downwardly upon the section of the top chord, quickly causes the joist to fail and collapse. Thus this arrangement provides no better fire protection than a standard joist.

It is therefore an object of the present invention to provide a joist for use in timber floor construction with improved

fire properties over prior art joist arrangements.

According to a first aspect of the present invention there is provided a joist for use in Limber floor construction, the joist comprising top and bottom chord members, each of substantially uniform depth along their respective lengths, and means interconnecting said top and bottom chord members to form the joist, characterised in that the depth of the top chord is greater than the depth of the bottom chord.

In this way, the top chord can resist the applied compression which occurs in the event of a fire for a lcnger period so as to improve the fire properties of the timber floor construction.

Preferably, the top and bottom chord members are wooden chord members having a substantially rectangular shape and arranged in substantially parallel relationship with respect to one ancther. In this way, the bcttom chcrd member can be as is currently provided with the top chord member having the increased depth, thus reducing the spacing between the chords in a joist of standard depth. Additionally, the all wooden construction of the chords keeps the advantages of environmentally sustainable building materials.

Preferably, the top and bottom ohord members are of substantially the same width. In this way, the joist can be handled in oonstruotion in the same fashion as a prior art joist.

Preferably the depth of the top ohord is approximately twice the depth of the bottom chord. In this way, construction materials are used efficiently and those on site can easily distinguish the top chord.

The interconnecting means may be a panel. The panel may be routed in inner surfaces of the top and bottom chord. In this way, the joist is an I joist with a robust top chord.

Alternatively the interconnecting means may comprise a web.

The web may be formed from V shaped metal supports attached to sides of the top and bottom chords and arranged along the length of the chords. In this way, the joist is as open web joist with a robust top chord. Optionally, the web may be formed from a series of wooden supports arranged diagonally to provide alternating v-shaped supports between the chords. In this way, the joist is a wooden open web joist with a robust top chord.

Preferably each chord is formed of a single piece of timber.

Alternatively, one or more chords may be formed by sections joined together. The sections may be joined by finger jointing. Alternatively, the sections may be joined by applying nail plates. In his way, joists of significant span, say 4 meters, can be constructed.

According to a second aspect of the present invention there is provided a method of building construction to increase fire protection, comprising the steps: (a) providing a robust top chord joist according to the first aspect; and (b) usingthe robust top chcrd joist in place of a standard joist in the building construction.

By simply substituting the standard joists with the joists of the present invention, improved fire protection is provided to the timber frame construction.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figures 1(a) and 1(b) are end and plan views of a prior art I joist; Figures 2(a) and 2(b) are end and side views of a robust top ohord I joist according to an embodiment of the present invention; and Figures 3(a) and 3(b) are end and side views of a robust top chord open web joist according to an embodiment of the present invention.

Reference is initially made to Figures 2 (a) and (b) of the drawings which illustrates a joist, generally indicated by reference numeral 10, according to an embodiment of the present invention. Joist 10 has a top chord 12 being a substantially recuangular section of timber. On an inner face 16 there is a routed channel into which is located or glued, a panel 20. Panel 20 is typically OSB with a width substantially less than the width of the chord 12. There is also a bottom chord 14 which is also a substantially rectangular section of timber. On an inner face 18, facing inner face 16, is a routed channel into which the opposite side cf the panel 20 is lccated. The chords 12,14 and panel each have the same length as illustrated in Figure 2 (b) However, the depth of the top chord 12, labelled B' in the Figure, is twice the depth of the bottom chord, labelled A' in the Figure. Though the tcp chord 12 is shown with a depth being twice that of the bottom chord 14, the present invention is applicable with chord depths which reflect the top chord 12 having a greater depth than the bottom chord 14. The depth of each chord 12,14 is uniform along the length of the joist 10 and the chords are arranged in parallel.

It will also be appreciated by those skilled in the art that the joist 10 can easily be constructed using current methods to provide I joists. In particular, there are no additional supports or inserts reguired at either end of the joist.

Reference is now made to Figures 3 (a) and (b) of the drawings which illustrate a joist 110, according to a further embodiment of the present invention. Like parts to those of Figures 2 (a) and (b) have been given the same reference numeral but with the addition of one hundred. Joist 110 is provided with a top chord 112 being a length of timber with a rectangular cross-section. The length is selected to be the desired length of the joist 110. A second or bottom chord 114, also being a length of timber with a rectangular cross-section, is arranged in parallel with a fixed separation from the top chord 112. Each chord 112,114 has the same length so that the joist 110 has a fixed length. In this embodiment the chords 112,114 are separated and held together by a metal web.

On each side of the joist 110 there are arranged V shaped' metal plates or connectors 22. The connectors 22 are screwed into the sides of the top and bottom chords 112,114 at the side of the joist 110. These oonnectors are as known for open web joists.

The depth of the top chord 112 is greater than the depth of the bottom chord 114. Thus the top chord 112 has a greater volume of timber than the bottom chord 114 making it structurally of greater load bearing capacity. It is noted that the connectors 22 may be as used for standard open web joists as the depth of the joist 110 will be the same as for current open web joists and no additional pieces are required for attachment to the adapted top chord 112. It will be appreciated that additional screw/nail holes could be provided along the connector so that the screw/nails are attached across the depth of the top chord 112.

In each of the joists 10,110 it is apparent that the dimensions of the overall joist can remain as is known in the art. Thus joists of lengths of 4 metres are available. The depth of the top chord may be as great as 0.5 metres with an overall depth of joist being 1.25 metres. To obtain the overall length the chords 12,14,112,114 may be formed in sections which are connected together. A preferred method of joining the timber sections is by finger jointing as is known in the art. Alternatively plates may be applied across the joint and nailed or screwed in place to brace the join.

In use, a timber floor construction is built in the traditional mannerwith the load bearing joists being replaced with joists 10,110 of the present invention. If the construction is exposed to fire then, in the case of an I joist the panel will burn through first. As the fire travels upwards, it is the bottom chord which will suffer the quickest burn rate. As the timber in the bottom chord is burnt away, the chord will split and tension along the length of the bottom chord is lost. Without the tension on the bottom chord, the load on the joist, felt as compression on the top chord, will not be balanced. As a result, the top chord bears all the load. As the top chord in the present invention is muoh thicker than that of a standard or the bottom chord, it will bear the load more successfully. Additionally, as the fire takes hold of the top chord, the increased volume will take longer to burn and thus the joist of the present invention will support the structure for a greater length of time. Thus the joist of the present invention, which may be termed a robust top chord joist, maintains the structural integrity of a timber floor construction to prevent collapse.

The principle advantage of the present invention is that it provides a joist and a method of timber floor construction which mitigates collapse in the event of a fire for an increased period of time.

A further advantage of the present invention is that it provides a joist providing increased fire protection which can be manufactured, transported and handled by traditional methods.

Modifications may be made to the invention herein described without departing from the scope thereof. For example, though an I joist and an open web joist with metal connectors have been described, an all wooden open web joist may also be constructed. The timbers may be treated with a water repellent to improve their load bearing capacity. The timbers may also be treated with a fire retardant to decrease the burn rate.

Claims

CLAIMS1. A joist for use in timber floor construction, the joist comprising top and bottom chord members, each of substantially uniform depth along their respective lengths, and means interconnecting said top and bottom chord members to form the joist, characterised in that the depth of the top chord is greater than the depth of the bottom chord.
2. A joist according to claim 1 wherein the top and bottom chord members are wooden chord members having a substantially rectangular shape and arranged in substantially parallel relationship with respect to one another. C')
3. A joist according to claim 1 or claim 2 wherein the top and bottom chord members are of substantially the same width.
4. A joist according to any preceding claim wherein the depth of the top chord is approximately twice the depth of the bottom chord.
5. A joist according to any preceding claim wherein the interconnecting means is a panel.
6. A joist according to claim 5 wherein the panel is routed in inner surfaces of the top and bottom chord.
7. A joist according to any one of claims 1 to 4 wherein the interconnecting means comprises a web.
8. A joist according to claim 7 wherein the web is formed from V shaped metal supports attached to sides of the top and bottom chords and arranged along the length of the chords.
9. A joist according to claim 7 wherein the web is formed from a series of wcoden supports arranged diagonally to provide alternating v-shaped supports between the chords.
10. A joist according to any preceding claim wherein each chord is formed of a single piece of timber.
11. A joist according to any one of claims 1 to 9 wherein one or more chcrds are formed by sections of timber joined together. co
12. A joist according to claim 11 wherein the sections are joined by finger jointing.
13. A joist according to claim 11 wherein adjacent sections are joined by applying a nail plate.
14. A method of building construction to increase fire protection, comprising the steps: (a) providing a robust top chord joist according to any one of claims 1 to 13; and (b) using the robust top chord joist in place of a standard joist in the building construction.