Static structures (e.g. – buildings) – Cast in situ composite slab
Reexamination Certificate
2000-11-07
2002-11-26
Johnson, Blair M. (Department: 3634)
Static structures (e.g., buildings)
Cast in situ composite slab
C052S745030, C249S211000
Reexamination Certificate
active
06484464
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to floor and roof reinforcing support structures for buildings, and more particularly to such a floor or roof structure utilizing a plurality of joists as integral parts of roof or floor sections such as a poured concrete slab composite construction.
BACKGROUND OF THE INVENTION
Over the past several years the need for stronger, lighter, less costly, and more durable roof and floor structures along with the need for more uniform materials has led to an ever increasing interest in steel joists and reinforcing members for floors and roofs. While various built-up, sheet metal and open truss shapes have been tried with various levels of success, few have met the criteria of manufacturing simplicity, flexibility for piping and electrical access, as well as ease in installation.
The flooring and roofing systems of buildings are complex integrated systems of components that must act together in a reliable and cost-effective manner during transportation, installation, and in service, where modifications are sometimes common.
One common approach taken by the building products industry to address these diverse needs is that of welded-member truss sections. These trusses are usually welded combinations of steel L-angle and round bar components. While these steel trusses can provide fairly good access for electrical and tubing routing needs, they are labor-intensive and require often-complex quality control measures associated with the weldment that are an integral part of their manufacture. As a result, they can be costly for a builder to specify. In addition, the stock must often be ordered to “exact length,” since any required modifications at the job site may be difficult and involved. This has led to quite restricted use of these trusses, especially in the residential building marketplace for truss lengths generally under 20 feet.
Still other members have consisted of thin sheet metal webs reinforced by angles as top and bottom chord or flange members. However, these have not gained wide acceptance for various reasons including the following. First, the top and bottom angle members are usually thicker than the web member, making welding without excessive imperfections in the thin sheet a difficult process. In addition, the welded portions are located in relatively high stress regions, and may be weakened by corrosion, since welding usually removes any pre-existing corrosion protection coatings. Furthermore, the nesting required for efficient stacking and transportation is an especially difficult problem, since these sections are easily damaged during transport and installation.
Still other approaches have included various wood I-beam built-up trusses where the top and bottom chords are glued or mechanically fastened to a web member. While these trusses are quite flexible and simple to install for intermediate applications, they are of limited utility for longer spans. Furthermore, they do not lend themselves for use in composite flooring systems because they lack the strength and rigidity to be integrated adequately with concrete aggregates.
While thin sheet metal hat-shaped Z-shaped, and C-channel cross-sections have been considered, these sections have some inherent disadvantages. One of these disadvantages is that these truss members or joists have a “blade edge.” This edge is very susceptible to imperfections in the sheet metal along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the truss or joist can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional sheet metal truss member or joist will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads. Thus, initiation of an edge “bulge” or “crimp” on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or “propagate” easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the truss member material itself
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the wind load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional truss member has the effect of enhancing an already established process of failure initiation.
Second, these truss members or joists, when manufactured out of relatively thin sheet metal are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the truss member associated with local compressive or shear stresses. Buckling can precipitate section failure of the truss member. For example, in a Z-section truss member with edge lips on the flange edges, when the top and bottom flanges are non-uniformly stressed, the result can be a kinking of the edge in the form of a crimp or buckle. This crimping can lead to complete failure of the section.
Finally, some thinner conventional truss members can experience “rolling” when placed under load. Rolling is when the shear stresses within the truss member results in a net torque about the centroid. of the thin walled cross-section thus causing the cross-section to twist possibly making the truss member unstable. Some manufacturers have increased the cross-sectional length of the flanges of the conventional C-channel stiffener or joist member trying to solve the rolling problem but were met with only marginal improvement. This is because the increased flange length had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross-sectional flange length caused difficulty in accessing the fasteners used in mounting the C-channel to the rest of the integrated structure.
Because of diverse market requirements, the need for a simple, scalable, and reliable truss member, and the problem of joining relatively thick sections to sections relatively less thick, there is a need within the industry today for a versatile new lightweight/lower cost truss or joist configuration that can address all of the above-mentioned drawbacks and short comings of the present state of the art, is suitable for use with substantially all standardized building methods, and can be made on a cost-effective basis.
SUMMARY OF THE INVENTION
The present invention alleviates and overcomes the above-mentioned problems and shortcomings of the present state of the art through a novel lightweight/lower cost joist member. The novelty and uniqueness of this invention is that it: 1) is made of thinner material to reduce the in-plane stresses found in the fastener or joint area when it is integrated with other structures, 2) resists deflection adequately to meet stringent building code requirements, 3) is resistant to buckling and rolling, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress, and 5) can be manufactured cost effectively by using conventional manufacturing methods such as roll forming.
This novel invention may be described as a substantially reconfigured or stabilized J-section sheet metal truss having a mounting or integrating flange. It should be noted here that due to their extreme susceptibility to rolling, conventional J-section sheet metal joist members are seldom used in buildings. The unexpectedly strong synergisms of the unique characteristics found in the present stabilized J-section truss not only addr
Browning & Bushman P.C.
ICOM Engineering Corporation
Johnson Blair M.
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