The disclosure relates to the type of joist having an upper chord in the form of an inverted top-hat channel into which an insert is forced. The inner faces of the side walls of the channel converge towards one another at the top of the channel and the width of the bottom of the channel is greater than the width of the bottom of the insert.

This invention relates to a joist for use in a concrete forming structure 
for shoring concrete slabs employed in the building industry and, in 
particular, for use in construction of poured concrete floors. One such 
joist forms the subject of our Canadian Pat. No. 941,123 which issued on 
Feb. 5, 1974 and which will be hereinafter termed "the patented joist." 
Basically, the patented joist includes a cold rolled substantially U-shaped 
channel or top chord which has elastically deformable side walls capable 
of being sprung outwardly to admit a wooden nailable insert into the 
channel; a lower chord spaced from the top chord by primary and secondary 
chord connecting means; and a pair of slotted nailer-plates each located 
at an opposite end of the joist. 
A further type of known joist is formed from a pair of cold-rolled channel 
members welded together in back-to-back relation. The joist has an upper 
chord of U-shaped cross-section to receive a nailable wooden insert, a 
lower chord, intermediate chord connecting means having a constant depth, 
and returning side flanges inhibiting damage to the edges of said joist. 
These joists, although admirable for their respective purposes have proved 
to be heavy and somewhat costly to manufacture. 
A still further known type of joist is manufactured from extruded aluminum. 
This type of joist also has a U-shaped channel or upper chord into which a 
wooden joist member or insert is forced with the inner surfaces of said 
channel being provided with serrations to inhibit movement of the insert 
upwardly in a vertical plane, and, hence, to separate from the channel 
member. 
All of these joists are for the purpose of supporting plywood panels (which 
are nailed to the wooden inserts) during the pouring of concrete thereon. 
The inserts must be capable of removal from the channels because over long 
use, the inserts may become splintered due to the nailing of the plywood 
panels to the inserts. 
The object of the present invention is, therefore, to provide a lightweight 
joist which is cheaper to manufacture and handle and from which a damaged 
insert can be quickly removed and replaced.

Referring to the drawings, the joist shown therein is formed from extruded 
aluminum and consists of an elongated channel member presenting an upper 
chord, indicated generally at A, a lower chord indicated generally at B 
and chord connecting means (indicated generally at C) intermediate said 
chords A and B. 
The upper chord defines a channel (indicated generally at D) which is 
substantially U-shaped in cross-section and which is formed by side walls 
1, having inner 2 and outer faces 3, and a bottom wall 4. 
As will be seen from FIG. 1 the inner faces 2 of the side walls 1 converge 
towards one another in the direction of the upper portion of the channel D 
and the thickness of each said side wall 1 increases in transverse section 
from the bottom to the top thereof, with the outer faces 3 being 
substantially normal to said bottom wall 4. 
Each of the side walls 1, at the top thereof, merges into an upper, 
outwardly extending flange 5 each of which, in themselves, terminates in a 
downwardly extending stub-flange 6 parallel with and spaced from its 
associated said side wall 1. 
The lower chord B is defined by a pair of lower horizontal flanges 7 each 
parallel with and spaced from the upper flanges 5 and having an overall 
transverse dimension substantially equal to the overall transverse 
dimension of the upper chord. Each of the lower horizontal flanges 7 
terminates in an upwardly extending stub-flange 8. The interior of the 
lower chord B also includes a generally T-shaped slot indicated generally 
at 9 which is adapted to receive a bolt-head 20, the roof 10 of the side 
walls 11 terminating at its flanges 7 and being connected to a pair of 
spaced vertical side walls 11, each of said side walls 11 terminating, at 
its lower end, in a ledge 12 on which the head of the bolt is capable of 
sitting. 
The chord connecting means C comprises a central web 13 extending between 
the upper A and lower B chords with the stub-flanges 8 being substantially 
parallel with the vertical axis of said web C. 
Employing the basic form of joist, an insert 14 (FIG. 2) which can be of 
wood, is forced into the channel D the sides of the latter being, to a 
certain extent elastically deformable, although permitting entry of said 
insert squeezing the sides thereof towards the top. 
At this stage, it would be as well to mention the disadvantage which has 
always existed with known prior art joists employing an upper U-shaped 
chord or what is known in the trade as an "inverted top hat." In 
previously known joists, both the inner and outer faces of the side walls 
of the channel have always (a) extended parallel to one another and (b) 
extended parallel with the central vertical axis of the joist. As is also 
known, the inserts normally employed are of low-grade lumber and it is 
thus relatively impossible to cut them with exact right-angled corners. 
Hence, often as not, the inserts have had rounded corners so that when 
they have been driven into a channel member with bottom right-angled 
corners, said inserts have only sat on their corners because said 
right-angled corners would not accommodate the insert. In the majority of 
cases, therefore, the inserts have sat "proud" of the channel or, in other 
words, the upper horizontal surface of the insert has been higher than the 
neighboring and upper horizontal flanges of the joist. Thus when plywood 
panels have been placed on the joists (to enable the pouring of concrete 
floors on said panels), only the latter have supported the panels and 
concrete instead of the inserts and the upper horizontal flanges. The load 
transfer has therefore been uneven, notwithstanding that the weight has 
eventually crushed the corners of the insert until it has "sat" on the 
bottom of the channel. 
Thus, by arranging for the diverging walls at the bottom of the channel, 
this disadvantage has been obviated so that the channel will accommodate 
rounded corners on the insert without forcing the latter upwardly therein 
and thereby ensuring that the upper horizontal surface of the insert and 
the upper horizontal flanges, together, provide an even load bearing 
surface and a more even load transfer throughout the entire joist. This 
condition is clearly shown in FIG. 2 where the rounded bottom corners of 
the insert 14 do not touch the bottom of the channel and whereby the 
insert 14 sits quite naturally within the channel D with the side walls 1 
squeezing or gripping the upper surface of which together with the upper 
horizontal flanges 5, form an even load-bearing surface for plywood panels 
15 upon which the concrete is poured. 
If desired, of course, the inserts can be detachably secured within the 
channels, as is shown in FIG. 3, by means of nails 16 which are driven 
through the side walls 1 and into the insert. Alternatively portions of 
the side walls 1 can be punched inwardly when the insert is in situ within 
the channel, to form tangs 17 (see FIG. 4), either rectangular, square, or 
triangular and which bite into the side surfaces of said insert. 
The purpose of the T-shaped slot 9 will be apparent from reference to FIG. 
5 where it will be seen that the joist is capable of being clamped to 
another member 18 (such as an I-beam) by means of a slidable clamp 19 
bolted to the joist by means of a bolt 20 the head of which is slidably 
accommodated in said T-shaped slot, both said clamp and bolt being more 
clearly shown in FIG. 2. 
The reason that the outer face 3 of each side wall 1 is normal to the 
bottom wall 4 of the channel as opposed to being parallel with each inner 
face 2 which is angulated with respect to the bottom (thus increasing the 
thickness of each side wall 1 in transverse section from the bottom to the 
top thereof), is that it is desired to transmit the load transfer 
vertically downwardly to the bottom of the channel in the most efficient 
manner. 
The returning stub flanges 6 and 8 have the advantage that damage to the 
side edges of the horizontal flanges 5 and 7 is inhibited during frequent 
re-use of the joist. 
Although the use of nails 16 has been described above and illustrated in 
FIG. 3, it will be obvious to those skilled in the art that screws, 
self-tapping screws or power-actuated studs can be employed to secure the 
insert within the channel.