[0001] The present invention relates to a steel joist for use in a composite steel and concrete
construction, and to a composite steel and concrete construction system including
such a joist. The present invention constitutes an improvement upon the invention
patented in United States Patent No. 3,845,594.
[0002] The present invention discloses an improvement on the aforesaid patented structure
in which a composite steel and concrete structure comprises a horizontal concrete
slab containing reinforcing mesh and surmounting and partically embedding a plurality
of steel joists, by providing a joist possessed of greatly superior properties in
comparison with prior art joists. Among such properties is a significant improvement
in lateral stiffness which greatly improves the strength of the composite structure
during the construction stages and permits safer construction procedures particularly
where long spans are involved.
[0003] The present invention constitutes a significant advancement and improvement on United
States Patent No. 3,845,594, and in particular provides a steel joist which has a
top chord and a bottom chord which are connected by a web. The improved joist of the
invention is characterised in that the top chord comprises a pair of S or Z shaped
members adapted for embeddment in a concrete slab, said members being positioned in
mirror relation to one another thereby providing a joist having symmetry about a vertical
axis.
[0004] In such a joist, the web may be of the well-known zig-zag or continuous type of open
web, or the web may be composed of a plurality of structural shapes connected together
to form an open truss structure between the top and bottom chords.
[0005] In an alternative form of the invention, the web may be constructed of solid sheet
material, either integral with or separate from the top and bottom chords of the steel
joist.
[0006] Joists of the invention make it possible to erect long span constructions in excess
of the present limit of 13m, and may extend these spans to 20 or 25m without difficulty.
By utilizing angles, channels or tubes for web members, it is possible to increase
the radius of gyration of these sections over a solid round section and thus higher
unit stresses may be tolerated in the web members and a saving in steel weight results
in a more efficient joist construction.
[0007] The joists of the present invention are constructed from high strength steel, and
a plurality of the steel joists may be incorporated into a composite steel and concrete
structure which possesses a two hour fire classification rating. The top chord of
each joist comprising a pair of symmetrical, oppositely positioned S or Z shaped flanges
extending the length of the joist, is embedded in a horizontal concrete slab of the
structure. This is a matter of great significance in the advancement of composite
construction, as safety considerations are of ultimate importance in any building
intended for use for residential or office purposes.
[0008] The steel joist of the present invention, when compared with the prior structure,
is even more stable laterally and torsionally during the non-composite stage, and
accordingly longer spans may be more safely constructed than were possible in accordance
with the prior art.
[0009] Some embodiments of the invention will now be described, by way of example, with
reference to the accompanying drawings, in which:-
Figure 1 is a perspective view from below of a composite steel and concrete construction
utilizing steel joists of the present invention,
Figures 2A and 2B are vertical sections through joists constructed in accordance with
the present invention illustrating alternative arrangements of the top chord members,
Figures 3A, 3B, 3C and 3D illustrate features of joists in which the web is a continuous
sheet,
Figures 4A, 4B, 4C, 4D, 4E and 4F illustrate further details of top chord to web connections,
and joist fabrication techniques,
Figure 5 is a perspective of an in-fill framing system using joists the cross-section
of which is shown in Figure 4E, and
Figure 6 is a section through a composite steel and concrete construction in accordance
with another aspect of the present invention.
[0010] With reference to Figure 1 of the drawings, there is shown a composite steel and
concrete floor system 10 consisting of a plurality of open web steel joists 11 connected
together with roll or spanner bars 12, which serve to support form work 13, on which
a concrete slab 14 is poured, which slab includes reinforcing mesh 15. Each of the
open web joists 11 consists of a bottom chord 16, which as shown in Figure 1 may consist
of a pair of right angled members 16, a series of web members 17 and dual top chords
18. The top chords are provided with appropriate slots though which the roll or spanner
bars may be inserted to support form work, the roll or spanner bars being either permanent
roll bars intended to remain in the structure when complete, or may be removable roll
bars as described for example in United States Patent No. 3,945,168.
[0011] Referring to Figure 2A there is shown a vertical section, an open web steel joist
11 having a pair of bottom angles 16, forming the bottom chord of the joist, a pair
of top chord members 18, and a web which may be for example formed of hollow rectangular
cross-section tubular members, or of channel members 17. As illustrated in Figure
2A the two top chord members are welded together at 19 and to the web members 17 by
welds 20. The welds 20 as illustrated may be spot or seam welds, and are positioned
to provide the maximum strength, and to enable a welding electrode to be inserted
inside the member 17, to make a satisfactory weld. Each top chord member 18 is formed
of an identical cross-section profile shape having an upper S or Z portion 21 and
a downwardly depending leg 22. It is intended that the S or Z shaped portions 21 be
embedded in the concrete slab of the composite construction, the S or Z shapes providing
a superior shear connection between the concrete slab and the joists to provide a
true composite action between the joists and the slab. It should be noted that the
two top chord members are positioned in mirror image relationship to one another,
thus providing a perfectly symmetrical configuration of joist about a vertical axis
of the joist, which symmetry provides structural advantages during the non-composite
or installation stage, where the unpropped joist is required to carry the weight of
wet concrete, form work, its own weight and other construction live loads that may
be imposed, such as the weight of workmen, or possible excess concrete due to localised
thickness or impact of concrete pouring buckets. The double top chord profile provides
a greater cross-sectional area in this critical component of a long span joist, which
improves its lateral slenderness properties thereby making it stiffer. This increased
stiffness increases the capacity of the joist to resist compressive stresses. Correspondingly
this reduces the degree of lateral restraint that need be provided to the top chord
or top flange during the construction stage, which lateral restraint is normally provided
by a combination of roll bars and plywood form work securely attached to lateral supporting
wall beams and the like. There are clear economic advantages to this improved performance
of the joist, resulting in cost savings during the erection of composite steel and
concrete floor systems, which savings may be translated into lower cost per square
metre of floor space which may be passed on to owners and occupants in the form of
reduced capital costs, reduced rents and the like. Also, the joist of the present
invention being symmetrical provides significant improvements in the fabrication stage,
since distortion caused by heating during welding procedures is minimized.
[0012] Sweep is a phenomenon encountered when constructing a welded joist, where the welds
all occur on one side of the web. This creates a stress in the joist which tends to
cause a curvature to occur in the completed joist. This curvature has been overcome
in present practice by pre-curving the top chord in a direction opposite to the direction
in which the sweep will occur, so that the completed joist when welded together returns
to a substantially straight longitudinal configuration. With the symmetrical properties
of the joist of the present invention, the entire problem of sweep due to welding
stresses is avoided, and a straight joist is obtained without the necessity for complicated
precompensation techniques during fabrication.
[0013] Figure 2B illustrates a similar joist constructed in ascccordance with the present
invention, also possessed of vertical symmetry, but in which the top chord members
are arranged facing oppositely to the members in Figure 2A. As before the top chord
members are secured to the web 17 by suitable welds 20, and an additional filler plate
23 is secured between the top chord members 18, and serves to seal the space between
the two top chord members to prevent the loss of concrete through the open web of
the joist, during construction. The function of the two embodiments illustrated in
Figures 2A and 2B is virtually identical.
[0014] Figure 2B also illustrates an optional form of top chord member 18, which may be
provided with an optional lip 24, which is useful for increasing the compressive strength
of the joist in the non-composite mode, that is before the top chord has been embedded
in concrete.
[0015] Figure 3A illustrates in perspective an alternative form of joist in accordance with
the invention in which the web and bottom chord are rolled from a single strip of
steel. The web 37 of Figure 3A is formed unitarily with the bottom chord 36, for example
by the cold rolling of a suitable strip of sheet steel. As before, top chord members
18 are conected to the web 37 by welding, and opening 38 in the top chord and the
web may be formed either before or after welding by a suitable punching operation.
It will be appreciated that if the slots 38 are formed before the top chord members
18 are welded to the web 37, it will be necessary to provide means for aligning the
openings 38 which extend entirely through both top chord members and the web 37 prior
to welding. This alignment may create problems in fabrication in certain
'circumstances, and accordingly Figure 3B illustrates an alternative to the structure
illustrated in Figure 3A in which the top chord members 39 are provided with only
a very short downwardly depending leg 40 on the cross-sectional shape which leg is,
as before, welded to the web 37. In this case, the web may readily be punched for
the openings 38 prior to affixing the top chord members 39, and there is no necessity
to align openings in the top chord members with corresponding openings in the web.
[0016] Figure 3C illustrates in exploded perspective an alternative form of joist construction
in accordance with the invention. In Figure 3C the joist is formed of a strip or plate
41, angles 42 as bottom chords and top chord members 43. The joist of Figure 3C is
fabricated by welding, and appropriate slots are formed in the top chord members 43
and in the web 41 prior to or after welding, with the necessary alignment being made
so that the openings 44, 45 and 46 in the top chord members 43 and the web 41 would
be appropriately aligned prior to running the welds.
[0017] Figure 3D illustrates an alternative form of bottom chord 47, which can be used in
place of the angles 42 of Figure 3C. In this case a cold rolled steel bottom chord
shape as shown in Figure 3D would be attached to the web 41 as by welding.
[0018] Figure 4A illustrates a form of top chord member 50 provided with a longitudinal
rib 51 on the vertical leg 52 of the top chord section, which rib 51 would assist
in electric resistance welding of the top chord member 50 to an appropriate web structure.
[0019] Figure 4B illustrates an alternative form of top chord member 53 provided with a
plurality of slots 54 in the sloping face of the top chord section to enhance the
shear connection between the top chord and the concrete slab by permitting concrete
to fill the slots 54 when the slab is being poured.
[0020] The top chord member 53 is also provided with a right angle flange 55 which may be
used to support appropriate form-work, as an alternative, or ancillary to the use
of conventional spanner or roll bars.
[0021] Figures 4C and 4D are perspective views illustrating the use of channel. shapes as
web members 60, which web members may be positioned centrally of the joist as in Figure
4D, wherein the channel member 60 is positioned between the vertical legs 61 and 62
of the open web joist. In Figure 4D where heavier construction loads are to be encountered,
the vertical legs 61 and 62 of the top chord members are positioned tightly together,
and channel web members 60 are positioned on either side of the vertical legs 61 and
62. Similarly, bottom chord members 63 shown in Figure 4D as angle members may be
positioned between the web members 60.
[0022] A further and highly desirable fabrication practice is to use channels for all compression
members of the truss or open web joist, which are positioned inside or between the
top chord members, and to use angles for tension members, which are positioned outside
the top chord flanges.
[0023] Figures 4E and 4F illustrate two additional fabrication techniques. In Figure 4E
two identically shaped members 70 and 71 are welded back-to-back to provide a complete
joist. Each member 70 and 71 is one half of the completed joist, the joint between
the two members coinciding with the vertical axis of the completed joist. In Figure
4F a first member 72 includes an S or Z shaped top chord 73, a bottom chord portion
74, and a web 75. A second top chord portion 76 is welded to the web 75 to form the
completed double top chord joist.
[0024] The fabrication technique illustrated in Figure 4E provides a particularly advantageous
technique in practicing the present invention. The symmetrical sections may be rolled,
welded and punched to provide an economical and versatile joist for use in composite
construction.
[0025] When fabricated as a shallow depth joist an efficient infill technique for steel
beam structures is obtained. If a deeper joist is formed, an efficient regular span
joist is obtained.
[0026] The present application also provides an improved composite steel and concrete floor
system, utilizing a novel form of steel joist having a pair of symmetrically opposed
top chord members connected to a suitable web which in turn is connected to a suitable
bottom chord structure. The top chord now consists of two S shaped members with the
downward vertical leg modified in that it may be lengthened to provide the required
additional welding surface for web connections. The lip in the present top chord construction
may be either deleted or rolled in the opposite direction if necessary so as not to
interfere with web members which are placed between or outside the top chord elements.
[0027] In the alternative, some web members may be located between the top chord elements
and some outside the top chord elements. This option has definite advantages so far
as welded connections are concerned in order to more easily align the web and chord
members. The top chord member of the present invention is stronger than the prior
top chord since the joist is symmetrical about its vertical axis. The downstanding
legs of the top chord elements may of course be extended to increase welding surface
area as required.
[0028] Insofar as slots are concerned, which are provided so that roll or spanner bars may
be inserted therein to support form work, the function of these slots is known. However
the long ends of the roll bars should be cut back so that they do not foul the opposite
top chord element when being inserted during erection of a composite floor system.
Alternatively further saw cuts may be made in the roll bar to accommodate the vertical
flanges of the double top chord structure.
[0029] The web system of the present invention may be constructed of individual members
which can be any shape conventionally used for such members. Commonly angles, flats,
channels and rectangular sections may be used, although round rod pieces or serpentine
webs could also be used if required for any particular application. The bottom chord
of the joist may be of any conventional shape although generally a pair of angles
is the most commonly encountered configuration.
[0030] The cover of filler plate used to fill the top of the joist between the top chord
members may be made of very light gauge material and its purpose is simply to prevent
concrete from spilling through between the top chord elements. The uppermost portion
of the individual web members is positioned so that it does not protrude above this
cover plate. It would be logical to make the cover plate of light gauge steel and
simply tack weld it into place. It would be considered a non-structural element and
thus not included in the design calculations for the joist. Alternatively, however,
a heavier cover plate could be utilized and welded into position to provide a more
positive lateral connection between the top chord elements than that which would normally
occur as a result of the top chord to web member welded connections. This heavier
cover plate might then be included in the design calculations for the load bearing
strength of the joist.
[0031] In general, the advantage of the applicant's improved joist structure is a great
increase in lateral stability as a result of the double top chord elements which are
connected together by either web connections or a heavier gauge filler plate. The
increased lateral stability or rigidity reduces the slenderness ratio of the top chord
element and provides additional compression capacity during the non-composite structural
stage of construction. Furthermore, the addition of a second top chord element provides
increased cross-sectional area further enhancing the compression capacity of the joist.
Thirdly the configuration now allows individual web members to be utilized more readily
and provides for a more efficient web system which is lighter in weight especially
in the longer spans of 10.5m and over, enabling the double top chord joist of the
present application to be utilized in spans of 18m or more.
[0032] Referring to Figure 5, there is shown a portion of a building floor system, including
steel beams 80 and 81 supporting a pair of double top chord joists 82 and 83. Spanner
bars 84 and 85 as described in United States Patent No. 3,845,594 connect the joists
82 and 83 and would support suitable sheeting (not shown) on which a concrete deck
slab may be poured. Thus a smooth concrete slab may be poured, forming the floor of
a building with a steel beam subframe.
[0033] In Figure 6, a pair of steel beams 90 and 91 support ledger angles 92 and 93 on which
a joist 94 is shown, embedded in and supporting a concrete slab 95. By the use of
the ledger angles 92 and 93, a thickened slab may be obtained compared to the slab
of Figure 5, which may for example be used for an in-floor electrical distribution
system (not shown).
1. A steel joist for use in composite steel and concrete construction comprising a
top chord (18), a bottom chord (16) and a web (17) joining the top chord (18) and
the bottom chord (16), characterised in that the top chord (18) comprises a pair of
S or Z shaped members (21) adapted for embeddment in a concrete slab, said members
(21) being positioned in mirror relation to one another thereby providing a joist
having symmetry about a vertical axis.
2. A joist as claimed in Claim 1, wherein the web (17) is comprised of a continuous
serpentine bar.
3. A joist as claimed in Claim 1, wherein the web (17) comprises a plurality of channel
shaped compression members and angle shaped tension members.
4. A joist as claimed in Claim 1, wherein the web (17) comprises a sheet or plate
(41).
5. A joist as claimed in any one of Claims 1 to 4, wherein the bottom chord (16) comprises
a pair of angle members connected to the web (17) by welding.
6. A joist as claimed in Claim 5, wherein the angle members (16) are connected to
the web (17) by welded connections.
7. A joist as claimed in Claim 1, wherein the bottom chord (36) is integral with and
rolled from a single piece of steel together with the web (37).
8. A joist in accordance with any one of Claims 1 to 7, wherein openings (38) are
provided adapted to receive spanner bars (12) to support form work during erection
of a composite steel and concrete structure.
9. A joist as claimed in Claim 1, wherein the joist is formed of two identical cross-section
members (70, 71) joined at said vertical axis.
10. A steel joist for use in composite steel and concrete construction, said joist
having a top chord (18) adapted to be embedded in a concrete slab to form a shear
connector to said slab, a web (17) and a bottom chord (16), characterised in that
the joist is formed by two identically shaped members (70, 71) joined back-to-back,
with each member having one half of the top chord, web and bottom chord formed therein,
said members being formed of rolled sections of sheet steel.
11. A joist as claimed in Claim 10, wherein the top chord consists of a pair of S
or Z cross-section shapes.
12. A joist as claimed in Claim 1, wherein the top chord comprises two substantially
identical elongated rolled steel members (18) arranged in mirror image relation to
each other, each member (18) being of S or Z cross-section with a downwardly-depending
leg (22), the members (18) being attached to the web (17) by welding the downwardly-depending
leg (22) of each top chord member to the web.
13. A joist as claimed in Claim 12, wherein the downwardly-depending legs of the top
chord members are provided with a plurality of longitudinally spaced openings (44,
46) therein adapted to receive the ends of spanner bars to support concrete retaining
forms on said spanner bars.
14. A plurality of steel joists in a composite steel and concrete structure comprising
a horizontal concrete slab (14) containing reinforcing mesh (15) and surmounting and
partially embedding a plurality of steel joists (11) each having a top chord (18)
and a bottom chord (16) which are connected by a web (17), characterised in that the
top chord (18) comprises a pair of symmetrical, oppositely positioned S or Z shaped
flanges (21) extending the length of the joist and being embedded in said concrete
slab, the web (17) and bottom chord portions (16) of the joist not being embedded
in said concrete.
15. A structure as claimed in Claim 14, wherein each of the S or Z shaped flanges
(21) has a downwardly-depending leg (22) and is connected to the web (17) by welding
the leg (22) and the web (17) together.
16. A structure as claimed in Claim 14 or Claim 15, wherein the joists are supported
by steel beams (80, 81) and together with said slab (14) form an in-fill panel.
17. A structure as claimed in Claim 16, wherein the beams (90, 91) are provided with
ledgers (92, 93) fixed to the webs thereof whereby a thickened slab is obtained.