Field of the Invention
This invention relates to joists, a term intended to cover
similar structural components such as beams, struts or ties.
Background to the Invention
Where a building floor incorporates a stairwell, certain
supporting joists of the floor must be of a shorter length than the remaining normal
length joists spanning spaced walls of the building. The ends of these shorter length
joists are attached to a transverse joist which in turn spans the pair of normal
length joists defining the ends of the stairwell space. As a consequence, this pair
of standard length joists are subjected to higher loads than the remaining joists
of normal length, and the invention was devised to provide joists having increased
stiffness (i.e. greater resistance to deflection) to serve as the pair of joists
defining the ends of a stairwell space (as subsequently explained with reference
to Figure 1 of the accompanying drawings) but the invention has application to any
joists where increased stiffness and load carrying capacity are required.
Summary of the Invention
According to the invention a joist has a metal member providing
a web and having an upper surface bonded by adhesive to an upper timber chord and
a lower surface bonded by adhesive to a lower timber chord. The inherent stiffness
of the metal web, and the location of the upper and lower chords at distances spaced
from the neutral axis of the joist, imparts a substantial resistance to bending.
The adhesive may be a polyurethane adhesive suitable for
bonding wood to steel. A moisture curing polyurethane adhesive with a coverage of
400g/m2 has been found to be particularly suitable.
Preferably, the metal member has an upper flange projecting
at 90° from an upper edge of the web, and a lower flange also projecting at
90° from the lower edge of the web, the upper surface then being provided by
the upper surface of the upper flange and the lower surface being provided by the
lower surface of the lower flange.
The upper surface may also comprise the vertical surface
of a continuous lip projecting upwardly from the outer extremity of the upper flange
and the lower surface may also comprise the vertical surface of a continuous lip
projecting downwardly from the outer extremity of the lower flange. It will be appreciated
that the upper and lower metal flanges, and the upper and lower lips when present,
also contribute to the enhanced stiffness of the joist.
In the preferred embodiment the upper chord is rectangular
in cross-section, having a horizontal dimension corresponding to the horizontal
dimension of the upper flange and the lower chord is similarly rectangular in cross-section
having a horizontal dimension corresponding to the horizontal dimension of the lower
The web, the upper flange, the lower flange, the upper
lip and the lower lip are preferably formed by bending a single piece of metal,
such as galvanised steel, into a substantially u-shaped cross-section, the flanges
defining the end limbs of the u-shape.
A portion of the length of the web may have a series of
holes for the passage of electrical cables, plumbing pipes or conduits, and these
holes may be formed with lips to prevent such cables pipes or ducts snagging on
the edges of the holes.
A portion of the length of the joist at one end thereof
is preferably devoid of holes so that this length can form an edge of a stairwell,
where holes in joists are not required.
A plurality of joists of differing lengths but each in
accordance with the invention may be provided so that the user can select joists
of appropriate length, trimming one or both ends as necessary.
Brief Description of the Drawings
The invention will now be further described, by way of
example, with reference to the accompanying drawings, in which:
Detailed Description of the Drawings
- Figure 1 is a plan view of a floor having a stairwell, the ends of the stairwell
space being defined by a pair of joists each in accordance with the invention;
- Figure 2 is a side elevation of one of the pair of joists;
- Figure 3 is a isometric view of one end of the joist of Figure 2, and
- Figures 4 and 5 are cross-sectional views respectively taken on the planes defined
by the lines IV-IV and V-V in Figure 2.
Referring to Figure 1, a domestic building (such as a house
of block of flats) has opposed side walls 1 and 2 spanned by conventional joists
3. The joist ends are attached to the walls 1 and 2 either by conventional joist
hangers or by the ends of the joists being built into the walls, in conventional
fashion. The building has a rectangular stairwell 4 to accommodate a staircase,
the stairwell 4 having a length parallel to the wall 1 of 2400mm and width perpendicular
to the wall 1 of 1000mm. It can be seen that over the length of the stairwell there
are four joists 5 shorter in length than the normal length joists 3 spanning the
walls 1 and 2. At their ends remote from the wall 2, the ends of these shorter joists
5 are connected (e.g. by conventional joist hangers) to a transverse joist 6 defining
the stairwell edge parallel to the wall 1. The ends of this transverse joist 6 are
in turn connected (e.g. by conventional joist hangers) to the pair of respective
joists 7 and 8 defining the ends of the stairwell space.
This pair of joists 7 and 8 must therefore be designed
to withstand greater bending moments than the remaining joists, because part of
the loading applied to the floor laid over the shorter joists 5 will be transmitted
to the pair of joists 7 and 8 through the intermediary of the transverse joist 6.
A pair of conventional joists (like the joists 3) have sometimes been placed side
by side and screwed together to form a composite joist to serve as each of the joists
7 and 8. The invention provides a joist of special design to serve as each of the
joists 7 and 8 and Figures 2 to 5 show one such joist 7.
Referring to Figures 2 to 5, the joist 7 comprises a galvanised
steel channel member folded from 1Smm sheet steel so as to have a central vertical
web 10 with a height dimension of 218mm and two horizontal flanges 11 and 12 with
a width dimension of 72mm from the extremities of which project two out-turned vertical
lips 13 and 14 with a height dimension of 15mm.
A strip of timber 15 of rectangular section (72mm wide
x 35mm deep) is attached by a polyurethane adhesive to the whole of the upper surface
of the upper flange and also to the adjacent surface of the upturned lip 13. The
strip of timber 15 has a width corresponding to the width of the flange 11 and forms
an upper chord of the joist. A second and identical timber strip 16 of rectangular
section (72mm wide x 35mm deep) is attached by the same adhesive to the whole of
the lower surface of the lower flange 12 and also to the adjacent surface of the
down-turned lip 14. The timber strip 16 has a width corresponding to the width of
the lower flange 12 and forms a lower chord of the joist. The adhesive is sufficiently
strong to bond the wood to the steel so that the shear forces on this interface
are resisted, imparting substantial stiffness to the composite joist.
A series of six circular holes 18 of 80mm diameter are
formed in the web 10, the holes 18 being at a regular spacing of 600mm, this distance
also being the distance between an end of the joist (the right hand end in Figure
2) and the nearest hole 18. The other end of the joist has a length 19 of 1200mm
devoid of holes.
Each hole 18 has a lip 20 which is curved in section as
best seen in Figure 5, the lip 20 projecting 10mm from the web 10 on the same side
thereof as the upper and lower flanges 11,12. The holes 18 allow for the passage
through the joist of cables, pipes or ducting which can be passed through the holes
18 without the risk of snagging on the edges of the holes as a result of the provision
of the lips 20. The holes 18 are centred on the neutral axis of the joist so as
to minimise loss of stiffness or resistance to bending.
In use, the joist 7 spans the walls 1 and 2 in a position
defining one end of the stairwell 4, as shown in Figure 1. The ends of the joist
7 are supported on the walls 1 and 2 either by joist hangers or by the ends of the
joists being built into the walls 1 and 2. In either event, the ends of the joist
7 can be trimmed on site, it being understood that the length 19 of the joist which
is devoid of holes is positioned so as to define the edge of the stairwell space
and that the flat side of the joist (the right-hand side in Figure 4) faces towards
the stairwell 4. Similar considerations apply to the joist 8 which is identical
to the joist 7 except that it is "handed" to ensure that the length devoid of holes
defines the other side of the stairwell space. The load on the shorter joists 5
is transmitted to the transverse joist 6 the ends of which are attached to the joists
7 and 8 by conventional joist hangers. The joists 7 and 8, being of increased stiffness,
are able to take this load without unwanted deflection.
It is envisaged that a plurality of joists, each in accordance
with the invention but of differing lengths, should be made available. For example,
joist lengths may vary from 3600mm to 5400mm in steps of 600mm, each joist having
a length at one end devoid of holes and being trimmable at each end to suit the
floor being constructed.