Static structures (e.g. – buildings) – Shaped or strengthened by fluid pressure – Comprising spaced – sheetlike members and fluid chamber...
Reexamination Certificate
2001-12-06
2002-12-24
Friedman, Carl D. (Department: 3635)
Static structures (e.g., buildings)
Shaped or strengthened by fluid pressure
Comprising spaced, sheetlike members and fluid chamber...
C052S002220, C135S090000, C135S097000
Reexamination Certificate
active
06497074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to building structures and, more particularly, to structural techniques utilizing lightweight materials to perform a load bearing function.
2. Description of the Prior Art
Let us first consider the fundamental difference between systems of rigid versus non-rigid building construction systems. Historically, buildings evolved from compression structures built from materials such as stone and clay bricks that are completely rigid. For these materials to fail under load, requires extreme compressive force that will cause the structural members to be crushed or to fracture. The problem with such constructions is the excessive weight of the materials.
Advancements in materials and construction technology created new components and structural members designed to work in both compression and tension. These structural components and members are designed with sufficient stiffness to prevent a member from buckling under compression loads. Wood is among the early materials having both good compression and tensile strength. Modern materials technology has focused on the use of metal structural members that have equal or greater tensile strength than compressive strength. These structural members can be used to fabricate engineered structural components such as Open Web Joists that can be used to construct rigid, free span structures using a minimum weight of material. For a given weight a non-rigid metal member or cable acting purely in tension can carry a greater load over a given span than any of the above mentioned structural members.
Conventional metal structures are designed with strict tolerance in regard to the stiffness of members in bending, because deflection of a member under compressive load will cause the member to buckle resulting in immediate structural failure. Engineered components such as open web joists are very light in weight, and work equally well in compression or tension, but perform poorly in deflection. When joists are used in the construction of conventional flat roofs the combined live and dead loads must be specified with careful reference to building codes so that the joists will be sufficiently rigid to prevent ponding. State-of-the-art design of flat roofs strictly limits deflection and gives careful consideration to the drainage of water off the roof, especially in the case where the formation of ice or collection of debris may cause drainage problems. Neglect in these matters can ultimately result in ponding on the roof and lead to structural failure.
More recently flexible architectural fabrics have been used as the cladding or “stressed skin” of the building envelope. These thin cladding materials act purely in tension. Stressed skin methods of construction differ from traditional tent-like structures in that tension forces are introduced into the sheet material after it has been installed. Referred to as “post-tension”, it is this force that is used to stabilize the thin cladding or skin. In traditional tents a skin is fitted to the frame but not stressed and therefore it is free to deflect in it's span between structural supports. Such traditional tents show noticeable movement and fluttering of the skin in the wind. Attempts to simply stretch the material tight are limited in their effectiveness, since a relatively weak force acting at a right angle to the skin will be able to significantly deflect the skin at the center span. Therefore, the modem approach is to use air pressure or structural tension members like cables to introduce a controlled amount of deflection and post-tension in the skin to create thereby a stressed skin structure.
High strength architectural fabrics have used air pressure and cable system post-tension methods in the construction of very large stressed skin roof systems such as Olympic Stadiums and airport terminal buildings. These large membrane roof systems have complex double curvature surface areas comprised of many individual membrane panels having irregular curved “sail shapes” which are joined together to form the shape of roof. In the opposite extreme, weak film materials have been used to construct large area greenhouse structures where, for example, polyethylene film is used as a stressed skin over arched frames to cover agricultural crops. The film material is placed over the arches and cables or cords are placed over the top of the film between the frames to draw down and tension the film. Alternatively, a double layer of polyethylene film is attached over the arched frames and then air pressure is used to create an air pillow type of stressed skin.
It is evident that the gradual evolution of building design toward lightweight, flat roof, construction is driven by the efficiency of such systems. In general these systems cost less to build. The problem is that state-of-the-art building design for flat and low-profile roofs do not make use of the full potential of flexible sheet material and non-rigid structural members, that work most efficiently in tension, because of the ponding problem mentioned above.
Another problem of the prior art systems resides in the fact that many of the sheet materials show elastic elongation under load which can exaggerate the deflection that may be anticipated. This is an especially serious problem in the case of the solar sheet materials like transparent films and translucent membranes, which can be more elastic. Many excellent solar sheet materials with good tensile strength exhibit significant elasticity under load, which, especially for economical flat roof construction, necessitates the specific methods provided herein in order to avoid the serious problem of ponding.
Previous stressed roof panels, such as those described in my U.S. Pat. No. 4,452,230 issued on Jun. 5, 1984, are typically built with significant slope across the panels span, as this is known to be a requirement to prevent the collapse or inversion of the flexible sheet material or panels due to live loads. These previous types of structures, even though built with strong architectural fabric would, if built with insufficient slope, suffer from ponding due to high live loads caused by snow and rain. Deflection of the stressed skin or panel would cause the pooling and the collecting of water, snow or ice, generally referred to as “ponding” that then produces even greater loads in the area of inverted skin. Such ponding and inversion of the fabric stressed skin roof systems can then lead to roof collapse and structural failure. Therefore, these previous roof construction systems are not suited to flat, low profile roof systems, which are the most economical to build.
SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to provide improved lightweight structures covered with flexible sheet materials.
It is also an aim of the present invention to provide an improved roof drainage system that prevents ponding on lightweight roofs covered with thin flexible sheet materials.
It is also an aim of the present invention to provide a new modular stressed-panel building envelope comprising a plurality of double-layer flexible panels.
The present invention also discloses improvements whereby the tensile strength of the building envelope materials are fully exploited while the structural members and/or the sheet materials are permitted to deflect in a pre-determined manner when under load.
Therefore, in accordance with the present invention, there is provided a modular stressed-panel building envelope, comprising a plurality of multi-layer flexible paneling modules adapted to be stretched between structural members, each of said multi-layer paneling modules having at least exterior and interior stressed layers defining a free space therebetween, said exterior and interior layers being operatively connected to work structurally in opposition to each other.
In accordance with a further general aspect of the present invention, there is provided a lightweight building construction system, comprising multiple similar stressed r
Friedman Carl D.
Mitchell Robert
Renault Ogilvy
Slack Naoko
Sunarc Structures Inc.
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