This invention relates to cold-formed steel joists and to assemblies of such joists to provide structural support for floors and roofs in the building construction industry.
For industrial and commercial applications the preferred roof and floor joist is a top chord bearing joist. In North America the top chord bearing joist market is predominantly serviced by the open web steel joist (OWSJ) market that is regulated by the Steel Joist Institute (SJI). The top chord joist provides an excellent method for erection when a crane is used, as top chord bearing joists passively stay in place by gravity. A problem with the present art is that the designs require an abundance of parts and considerable man hours to produce. The OWSJ is difficult to customize for the many alternative conditions that arise on construction projects today. Present top chord bearing joists as described in the SJI specifications are typically built using hot rolled steel shapes; however, some OWSJ designs have elements that are cold rolled shapes. There are top chord bearing joists using special cold formed shapes that are arranged in a manner similar to the OWSJ, so these proprietary OWSJ projects also have abundant parts and require abundant man hours to produce. The top chord bearing joists specified in SJI and the special shaped joists both require approximately 6 to 10 man hours per ton to manufacture and require additional man hours to customize.
Typical standard joists as identified in the Steel Joist Institute (SJI) specifications have top and bottom chords that are angle sections and the webs are round bars or cold formed U shapes or crimped angles; shoes for end bearing are fixed to the top chords at the ends, typically by welding. These joists are customized to suit the conditions of each project. When OWSJ's are used for sloping conditions, the shoes are typically drawn by a draftsman, arranged to fit the desired angle by a fitter in the shop and then welded. Installing sloped shoes can be very expensive. Concentrated loads often require the need of engineering to satisfy special loading conditions. On a 40 ft long joist there will be approximately 44 pieces. Many pieces are different sizes and weights; the pieces are custom cut on a saw line for each project. In today's industry joists are produced with a high factor of labour cost, from 6 to 10 man hours a ton depending on location, support infrastructure, plant capabilities and product mix.
Some cold formed joist systems are available on the market that have very similar assembly methods to that of the OWSJ. The available systems often have special shaped chords and web members. A cold formed top chord bearing joist system maintains a high quantity of parts utilization and associated man hours to assemble. Most of the available cold formed joist systems are difficult to customize similar to the OWSJ products described in SJI.
In the past many innovative steel joist designs have been developed and introduced to the market. The market is demanding in terms of performance requirements to suit alternative building design types, therefore these products require significant customization for each project. The present art of OWSJ designs, such as those shown in
Therefore, a new and improved way to provide top chord bearing joists would be to provide a joist that reduces labour hours, reduces material use and is easy to customize for the many alternative project conditions.
Embodiments of the present invention have been developed to facilitate the customization of top chord bearing joists to suit the many conditions that exist in the top chord joist market, using a minimal number of parts and person hours. The cold-formed steel joist as described herein satisfies all of the given alternatives for the application of top chord bearing joists and provides enhanced use of materials and facilitates superior advanced manufacturing methods. The end result is superior structural top chord bearing joist components at a lower cost.
The top chord bearing joist as described herein improves material use, reduces waste, reduces man hours to manufacture and increases daily output of product. Construction of the joist makes use of cold formed shapes that are not necessarily limited to single functions, thereby satisfying shifting needs of the market.
An embodiment of the present invention relates to an upper chord bearing joist comprising: a top chord member and a bottom chord member, each having a flange portion and a web receiving portion including two web receiving tabs, each made from a unitary piece of metal; a generally planar steel web, a portion of the web being attached to the the top chord member and to the bottom chord member, wherein a top portion of the web is between the two web receiving tabs of the top chord member and a bottom portion of the web is between the two web receiving tabs of the bottom chord member; and a first and second pair of support members, each support member including a shoe portion, a web attaching portion, and an angled portion, the web attaching portion portion being attached to the web receiving tabs and the angled portion being in contact with the web.
Another embodiment of the present invention relates to an upper chord bearing joist comprising: a top chord member being cold-formed from a unitary piece of sheet steel and having: a flange portion, a web receiving portion including two web receiving tabs, and a pair of integral inner flange portions, each inner flange portion extending substantially perpendicularly from one of the web receiving tabs so as to be in a spaced relationship to the flange portion, each top chord member being cold-formed from a unitary piece of sheet metal; a bottom chord member being cold-formed from a unitary piece of sheet steel and having a flange portion and a web receiving portion including two web receiving tabs; and a generally planar steel web, a portion of the web being attached to the the top chord member and to the bottom chord member, wherein a top portion of the web is between the two web receiving tabs of the top chord member and a bottom portion of the web is between the two web receiving tabs of the bottom chord member, said top portion defining a top surface area of contact, and said bottom portion defining a bottom surface area of contact; wherein each chord member is cambered about the web such that the top and bottom surface area of contact varies along a length of the joist.
Another embodiment of the present invention relates to an upper chord bearing joist comprising: a top chord member and a bottom chord member, each having a flange portion and a web receiving portion including two web receiving tabs, each made from a unitary piece of metal; a generally planar steel web, a portion of the web being attached to the the top chord member and to the bottom chord member, wherein a top portion of the web is between the two web receiving tabs of the top chord member and a bottom portion of the web is between the two web receiving tabs of the bottom chord member; and a first and second pair of support members, each support member including a shoe portion, a web attaching portion, and an angled portion, the web attaching portion portion being attached to the web receiving tabs and the angled portion being in contact with the web; wherein each chord member is cambered about the web such that the surface area of contact varies along a length of the joist.
Another embodiment of the present invention relates to an upper chord bearing joist comprising: a top chord member and a bottom chord member, each having a flange portion and a web receiving portion including two web receiving tabs, each made from a unitary piece of metal; a generally planar steel web, a portion of the web being attached to the the top chord member and to the bottom chord member, wherein a top portion of the web is between the two web receiving tabs of the top chord member and a bottom portion of the web is between the two web receiving tabs of the bottom chord member; and wherein the generally planar steel web includes a plurality of web segments which in combination define a generally planar steel web.
A further understanding of the functional and advantageous aspects of the present invention can be realized by reference to the following detailed description and drawings.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:
Without limitation, the majority of the systems described herein are directed to cold-formed steel joists. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms.
The figures are not to scale and some features may be exaggerated or minimized to show details of particular elements while related elements may have been eliminated to prevent obscuring novel aspects. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. For purposes of teaching and not limitation, the illustrated embodiments are directed to cold-formed steel joists.
The top chord bearing joist 10 has a diagonal member 30 and diagonal member 23 at each ends which may simultaneously function as joist bearing shoes 34 and 36 in conjunction with the top chord of the joist. The planar steel web 16 provides the ability to customize the joist 10 with stiffener components to suit special design loads. As discussed below, additional members may be affixed to the web or chords to increase the load capacity of the joist 10.
Referring to
Generally speaking, for a structural joist member 100 to be composite, it must have means to mechanically interlock with the concrete 102 to provide sheer bonding. Accordingly, a steel deck 108 is adapted to rest on a top surface of the inner flange portions 62 as shown in
The shear bond between the concrete engaging portion and the concrete may be increased by using rivets spot clinches or the like to increase the surface area of contact between the concrete and the top chord. Despite the asymmetry provided by the flange portion 64, this embodiment of the joist is substantially concentric since the concrete engaging portion is bonded to the concrete and the steel-concrete composite effectively distributes the applied load to each joist through its centre of gravity.
Interlock between the slab and the top chord of the joist provides the required shear bond capacity to allow composite action. The composite joist section acts as a ‘T’ beam, with the joist providing the required tensile resistance and the concrete substantially providing the compressive resistance.
Referring to
As shown in
a) illustrates a cross-sectional view of the upper chord 20 in which the web receiving portion 60 extends from the flange portion 64 and is in contact with the web 16. Inner flange portions 62 extend from the web receiving portion 60. In a preferred embodiment of the present invention, web receiving portion 60 comprises two web receiving tabs, and flange portion 64 comprises a lower bearing portion 68 and an upper bearing extension 66.
In operation, chord members and the web typically experience varying loads along their lengths. Accordingly, it is advantageous to provide a means of stiffening the chords and web by affixing segments thereto.
a) shows a moment diagram illustrating the magnitude of bending stress on the joist.
While in situ, the top chord 20 resists compressive forces while the bottom chord 22 resists tensile forces resulting from bending of the joist member 10. Often, there are one or more regions within the length of the joist that experiences larger bending stresses, the compressive and tensile stresses in the chord member are the greatest within these regions. To increase the efficiency of material use, a continuous top or bottom chord may be provided for the entire length of the joist such that the flexural resistance of the joist 10 is sufficiently larger then the regions of lowest bending stresses. A chord segment may then be fastened to the regions of higher bending stresses such that the reinforcement functions compositely with the joist, resulting in an increased flexural resistance. Accordingly, the quantity of material used is roughly proportional to the stresses experienced while in situ. As a result of local buckling of thin elements under compression, which reduces material efficiencies, chord reinforcement segmenting is often only required on the chord that resists compressive stresses (top chord 20). However, chord segmenting may also be applied to the tensile resisting chord (bottom chord 22) in order to increase flexural resistance of the section.
Using prior art methods, an extensive procedure must be followed when a sloped joist is required. Coordination between the building parties must be conducted and drawings and specifications must be created in order to accommodate a sloped joist condition. Shoes are generally installed in the shop, where workers perform the necessary layout and welding. Present methods of installing sloped shoes do not allow for any tolerance, and therefore extensive field repairs may be required when the designed slope, as governed by the distance between the supports and difference in elevation of the supports, varies from the as-built support conditions.
Accordingly, it is an aspect of the present invention to provide one standardized sloping shoe that can accommodate a wide range of angles via rotatable pin joint. Referring to
Compared to prior art methods of supporting an angled joist, the preferred embodiment of the present invention requires very little coordination, and the sloping shoe does not require any project specific drawings and requires no layout in the shop to determine joist angle. The sloping shoe may rotate about the pin and therefore may accommodate any slope that is expected in the field, regardless of as-built variances in slope requirements. The holes in the sloping shoe are aligned with the holes in the joists shoe; therefore installation may be performed by typical bolting procedures. The rotational degree of freedom introduced by the pin may be eliminated, if so desired, by field welding of the components connected by the pin.
In a further aspect of the present invention, one may effectively extend the bottom chord 22 by fastening it to an adjacent column. Referring to
Some non-limiting examples of motivations for fastening the bottom chord 22 include: (1) provision of lateral restraint of the bottom chord of the joist, (2) stabilization at the top of a column in order to satisfy column design assumptions, (3) torsion stabilization of a girder, and (4) provisions of a moment connection between joist and column.
In use, joists typically face special load conditions, such as concentrated loads P from mechanical units. The preferred embodiment of the present invention accommodates concentrated loads via supplementary stiffeners affixed to the web. Referring to
In production, standardized stiffeners may be fabricated that have standardized design values, allowing for expedited accommodation of concentrated loads where the strength of the web 16 alone cannot support the bearing stress. With current top chord bearing joists, once fabricated, a concentrated load can only be installed at a specified location. This invention allows for greater flexibility in accommodating design changes after the joists have already been fabricated and erected. Provided that the flexural capacity and the shear capacity are sufficient, a concentrated load may be repositioned or added anywhere along the length a joist member and reinforced using stiffeners 92.
While the preferred embodiment the present invention is shown in
For instance, a larger flange 64 provides increased flexural capacity and strucual efficiency (i.e. higher strength to mass ratio) as a result of distributing the material to the outermost regions of the cross section thereby increasing the second moment of area and increasing resistance to bending.
A larger web receiving portion 60 accommodates the camber requirements by providing an increased fastening surface area for the chord to web connection. As per the camber illustrated in
a) illustrates an embodiment with larger inner flange portions 62, (b) has a larger web receiving portion 60, (c) has a smaller reinforcement lip 62, (d) has a boxed top flange 64 with side portions 54, (e) has turned down edges 50, (f) has partially turned down edges 52, and (g) has a top flange 64 with a plurality of bends.
Regarding
Regarding
In markets where designers are concerned with seismic conditions and wish to use plywood decking to provide a horizontal diaphragm, it is common for the market to desire roof joists that have a continuous block of wood fastened to its top chord.
To strengthen the web 16 of joist 10, stiffeners may be added as in
Referring to
As used herein, the terms “comprises”, “comprising”, “includes” and “including” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms “comprises”, “comprising”, “includes” and “including” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.
The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.
This patent application relates to U.S. Provisional Patent Application Ser. No. 60/514,622 filed on Oct. 28, 2003 entitled SINGLE WEB COLD FORMED JOIST; U.S. patent application Ser. No. 10/721,610 filed on Nov. 25, 2003 entitled SEGMENTED COLD FORMED JOIST; and U.S. patent application Ser. No. 10/974,964 filed on Oct. 28, 2004 entitled COLD-FORMED STEEL JOIST each of which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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60514622 | Oct 2003 | US |
Number | Date | Country | |
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Parent | 10974964 | Oct 2004 | US |
Child | 12585402 | US | |
Parent | 10721610 | Nov 2003 | US |
Child | 10974964 | US |