US3927496A

US3927496A - Method for constructing a tensile-stress structure and resultant structures

Info

Publication number: US3927496A
Application number: US452363A
Authority: US
Inventors: Joseph A Kersavage
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-03-18
Filing date: 1974-03-18
Publication date: 1975-12-23
Anticipated expiration: 1992-12-23

Abstract

A tensile-stress structure having a surface of double opposed curvature including a terminal support member defining the perimeter of the curved surface and a plurality of elongated screen members spanning the space defined by the support member, each of the screen members including a plurality of longitudinal and transverse strands with the longitudinal strands of each screen member extending in a substantially straight line between spaced apart locations on the support member. The openings between the strands of the screen members are closed by semirigid means such as a plastic film. A method for manufacturing the structure is disclosed.

Description

[ 51 Dec. 23, 1975 United States Patent 1 Kersavage 3.394,720 7/1968 52/63 3 619 432 11/1971 Harrington............. 264/32 METHODFORCONSTRUCTINGA TENSlLE-STRESS STRUCTURE AND Pryor FOREIGN PATENTS OR APPLICATIONS Inventor: Joseph A. Kersavage, 4022 .400 4/1966 France....................... ..52/581 Sequoyah Ave., S. 1., Knoxville, Tenn. 37401 a Primary Examiner-Frank L. Abbott [22] Filed 1974 Assistant Examiner-Henry Raduazo [21] Appl. No.: 452,363

Attorney, Agent, or Firm-Fitch, Even, Tabin & Luedeka ABSTRACT A tensile-stress structure having a surface of double {52] US. 52/80; 52/748; 52/223 R; 52/309', 29/460 F; 264/32; 264/258; 52/245 [58] Field of 264/32, 35, 228, 263, 309, opposed curvature including a terminal support mem- 264/258, 231, 229; 52/80, 81, 581, 748; ber defining the perimeter of the curved surface and a plurality of elongated screen members spanning the space defined by the support member, each of the screen members including a plurality of longitudinal [56] References Cited f f n m 0 mm sa sfi d 0 m mm m rm f tSSt U m fi.m .m i a u m d a c n m n u b e a a u n n e m m m i ww F 0 m m .m 8 .m3 .m e W m mqrwbm m mm m m m D m e m m 7 m e mm em 1 u mm fi m m S rm II C m me .m m nvt fi 0 em 1 1mm mw n r. .h.. CEU t h w h c wmw m memsmwm 80000 BBBN 5H2222B5 22555502 55 .65

m A P m m m m m m Tm u nnnu nmn md M N r o y mbma ue w A 8 m S T a o o e a S OCPMRWLV D 172%6667 005 6666 99999999 11111111 Q MUVUWU 39522637 076090 33335 522 626% 7496237 5 .2 .2 223333 U.S. Patent Dec. 23, 1975 Sheet 1 of4 3,927,496

U.S. Patent Dec. 23, 1975 Sheet 2 of4 3,927,496

Fig. 3.

U.S. Patent Dec. 23, 1975 Sheet 3 of4 3,927,496

Fig. 4.

Fig. 5.

U.S. Patent Dec. 23, 1975 Sheet 4 of4 3,927,496

METHOD FOR CONSTRUCTING A TENSILE-STRESS STRUCTURE AND RESULTANT STRUCTURES This invention relates to tensile-stress structures and methods for their construction. Particularly, this invention relates to a method for constructing a tensile-stress structure which includes a surface of double opposed curvature.

Tensile-stress structures are part of the large group of suspended and stretched structures. In particular, this invention deals with tensile-stress structural forms such as membranes suspended between a perimetral structural support member, the membranes assuming a double opposed curvature geometry. The membranes commonly form a wall or roof section of a structure, e.g. a pavilion.

Membrane structural forms are of interest because of their light weight and strength. In addition, certain aesthetic values have been assigned to structures having curved surfaces as distinguished from planar surfaces. The membranes of prior art tensile-stress structures exhibit maximum structural integrity or rigidity when the forces acting upon the structure and tending to disrupt, dislocate or destroy it are applied in a direction which causes the disruptive forces to be resisted by the tensile strength of the membranes. A loose analogy is drawn to a large circus tent wherein wind forces directed inwardly against the loosely hung tent wall push the wall inwardly until the tent fabric assumes that curved geometry which brings the tensile strength of the fabric into play to resist the wind. As the wind direction changes, the pressure outside the tent becomes negative relative to the pressure inside the tent so that the tent wall is pushed outwardly until the tent fabric assumes that curved geometry which again brings into play the tensile strength of the fabric to resist the outward pressure. With changing wind currents, the tent wall appears to flap" because the tensile strength of the fabric is not immediately available for resisting the wind forces, but rather the tent fabric must first assume that curved geometry wherein the tensile strength of the fabric can successfully resist further movement of the fabric by the wind.

In the prior art, it has been common practice heretofore to use curved structural surfaces for bringing into play the tensile strength of a membrane for resisting forces acting upon the structure. Such curved surfaces have been formed by constructing a complex grid of support members and spanning the spaces between elements of the grid with a membrane. Various membranes have been employed in these structures, including coated woven fabrics. In these prior art structures large numbers of support members with numerous crossings between supports have been required. Particularly when employing cables to establish the support grid, the large number of crossings of cables has required a similar large number of crossing connectors for maintaining the cables in position relative to one another. These connectors at the cable crossings are both expensive and time consuming to install. In the absence of an extensive support grid, cablesupported structural surfaces are forced to assume curvatures corresponding to the curvature of the sag of the supporting cables. This is due, of course, to the extremely large forces exerted upon a cable when it is suspended in a substantially straight line and provided with a load intermediate'of its anchored ends.

The present inventor has discovered a method for constructing a structural surface of double opposed curvature wherein the space defined by a perimetral support is spanned by a plurality of elongated screen members aligned in a succession of substantially straight lines which in their aggregate define the surface. The longitudinal side edges of adjacent screen members are overlapped and a semirigid coating is applied which closes the openings between the strands of the screen and which, in one embodiment, bonds the overlapped portions of the screens to one another. The resultant structural surface is uniquely stable and resistant to those forces commonly acting upon structural surfaces.

It is therefore an object of this invention to provide a method forconstructing a tensile-stress structure of double opposed curvature. It is another object to provide a tensile-stress structure having a structural surface of double-opposed curvature that is defined by a plurality of screen elements aligned along a succession of substantially straight lines. Other objects and advantages of theinvention will be apparent from the following description including the drawings in which:

FIG. his a representation of a tensile-stress structure depicting various features of the invention;

FIG. 2 is aplan 'view of the structure shown in FIG. 1;

FIG. 3 is a fragmentary sectional view taken generally along line 3-3 of FIG. 1;

FIG. dis a representation of one embodiment for anchoring the ends of' a screen member;

FIG. 5 is a fragmentary representation of overlapping screen members employed in constructing the structure shown in FIG. I; and

FIGS. .6 and 7. are representations of general types of surfaces-of double-opposed curvature and which are suitable-for use in tensile-stress structures of the kind disclosed herein.

When speaking of a surface having a double opposed curvature-T, it ismeant that the surface is curved about two nonparallel axes such that the surface assumes a type of "saddle" appearance on each of its opposite sides. The radius of curvature of the surface curved about one axis may be equal to the radius of curvature of the surface that curves about the other axis, but in many instances these radii will be different. The perimeter of the surface may be defined by straight lines 'as seen in the hyperbolic parabloid depicted in FIG. 6 or defined by curved lines such as the elliptical ends of the geometrical structure depicted in FIG. 7. Similarly, the perimeter of the surface may be defined by a combination of straight and curved lines such as would occur by combination of variations of the geometrical structures of FIGS. 6 and 7. Other surfaces of double-opposed curvature will be apparent to one skilled in the art. Any geometrical configuration of double-opposed curvature capable of being formed by an open soap film spanning a space defined by one or perimetrical supports comprising a closed line is reproducible em loying the concepts disclosed herein. The geometrica configurations depicted in FIGS. 6 and 7 are intended asex-amples of two structural surfaces, each of double-opposed curvature. that may be formed by thedisclosed method. The resultant structural surface may itself define a housing. Commonly, however. a plurality of surfaces arecombined to define the ultimate structure.

One embodiment of a tensile-stress structure depicting various features of the invention is shown in FIG. I. This structure is built in situ directly on the ground 12 without added foundation. It comprises two outer structural surfaces 14 and [6, each of double-opposed curvature, with the surfaces being mutually cooperatively positioned so that they enclose a space [8 therebetween which is useful to house any of a variety ot things or even persons. The

surface

14 and 16 may be made transparent or transluscent so that the structure 10 suitably serves as a greenhouse.

In the depicted structure the perimeter of each of the

surfaces

14 and 16 is defined by supports. Specifically, straight upright supports 20 and 22 define two edges of the generally hyperbolic parabloid surface 14. These supports extend upwardly from the ground surface to a common point 24. It is noted that neither of the depicted supports 20 and 22 is vertical and need not be as will more appear more fully hereinafter. The remainder of the perimeter of the quadric surface 14 is defined by

straight supports

26 and 28 disposed on the ground. These latter two supports are connected to each other at one of their respective ends 30 and 32. Their other ends 34 and 36 are connected to the

ends

38 and 40 of the upright supports 20 and. 22, respectively, to fully enclose the perimeter of the surface 14. As necessary, the

supports

26 and 28 are anchored to the ground employing suitable anchoring means well known in the art.

The cooperating surface 16 similarly includes two

upright supports

44 and 46 connected to

further supports

48 and 50 laid on the ground, the ends of these supports being connected to define the perimeter of the quadric surface 16.

In the depicted embodiment the perimetral support for each of the structural surfaces 14 and I6 is defined by four straight support members, specifically wood 4 X 6 boards each about l6 feet long, connected at their ends. The present invention, however, lends itself to the construction of surfaces having perimeters that are of irregular curvature. For example, the elliptical paraboloid shown in FIG. 6 has two perimeters, each of an elliptical geometry. In such curved perimeters, the perimetral support 47 may comprise a single member. Alternatively, the support may comprise a plurality of individual curved members connected together. In any event, the function of the perimetral support is to define the perimeter of the desired surface and to serve as anchoring means for a plurality of screen members that span the space defined by the support.

In accordance with the present invention, the space between the

supports

20, 22, 26 and 28 is spanned by a plurality of elongated screen members 52, each of which includes a plurality of substantially parallel and laterally spaced apart strands or filaments 54 extending substantially parallel to the longitudinal dimension of the screen member and a plurality of substantially parallel spaced apart strands or filaments 56 extending substantially parallel to the transverse dimension of the screen member as seen in FIG. 5. The screen employed in the present invention is of an open weave", meaning that there is substantial open space 84 between adjacent strands. Such screens are typlified by the common insect screen available from many building supply or hardware sales outlets. These screens have strands of 55 the order of 0.0l inch in diameter that are woven in a 16 X [6 pattern (that is, there are 16 strands per lineal inch of width of the screen and 16 strands per lineal inch of screen length). Other strand sizes and other strand densities are acceptable, however, so long as the openness of the screen is sufficient to permit a bonding agent to readily permeate the screen, but not so open as to adversely affect the closing of each opening by the bonding agent, as by the formation of a film across the opening 84, for example. Plastic or glass fiber screens may be used, however, metal screens are preferred because of their greater tensile strengths which generally permit the use of a smaller diameter strands for obtaining equivalent strength in the membrane product. In general, metal screens also are easier to handle during construction of the structure.

Referring to the several FIGURES, the screen members 52 are laid in side by side relation with their longitudinal side edges overlapping the longitudinal side edges and 82 of adjacent screen members 52 and 52', respectively, and in sufficient number to cover the space outlined by the

supports

20, 22, 26 and 28. Each screen member is pulled taut and its opposite ends 58 and 60, hence the ends of the longitudinal strands 54, of the screen are anchored to the perimetral support at spaced apart locations. In the depicted embodiment, the ends of the screens that form the surface 14 are anchored to the

supports

26 and 22. The overlapped edges are bonded to each other to join the adjacent screens to one another and form a continuous screen spanning the space defined by the perimetral support.

The extent of overlap of the longitudinal side edges of the adjacent screen members is important in developing the maximum tensile strength in the membrane product. As noted, the ends of the longitudinal strands of each screen member are anchored to the perimetral support after having been pulled taut. This positions these strands for maximum realization of their individual and collective tensile strengths. By overlapping the longitudinal side edges of adjacent screens and bonding the overlapped portions together, the transverse strands of the screen members are connected together. The outside longitudinal edge of each screen adjacent the perimetral support is anchored to the support so that ultimately there continuity of the transverse strands across the space spanned by the screens.

The extent of overlap of the screen edges is chosen such that in combination with the bonding agent employed, the overlapping ends of the transverse strands of the screen members are held together with a force that is substantially equal to or greater than the tensile strength of the transverse strands. This bonding system ensures that in the resultant structure, the full tensile strength of the transverse strands is realized. Development of the strength in the overlapped and bonded areas is accomplished in one embodiment by adjusting the extent of the overlap such that the area is sufficiently large to permit the particular bonding agent employed to establish that bond strength between the ends of overlapping transverse strands which will at least substantially equal the tensile strength of the transverse strands. Thus, stronger bonding agents require less overlap and vice versa.

The openings 84 between the strands 54 and 56 of the screen members are closed, preferably after the screens are in place, as by filling the openings with a plastic 88 or other material that forms a fiim across each opening. This filler material bonds the strands S4 and 56 to one another thereby enhancing the stability of the resultant structure against shear forces. Importantly, the filler closes the screen openings, enabling the resultant surface to exclude the natural elements from the interior of the structure. The preferred filler is semi-rigid when is in its ultimate state of closing the screen openings. By By this means, the surface 14 is allowed to exhibit its integrity, but is caused to remain sufficiently flexible for absorbing impact forces that might otherwise penetrate the structural surface. Hail stones and thrown rocks or balls are examples of missiles that pose potential penetration hazards to a tensile stress structure employing membranes. Penetration by these and other like hazards is resisted by the disclosed structural surfaces. The preferred filler, therefore, resists hardening to a brittle state byreason of its reaction with the natural elements, particularly sunlight. Acrylic plastics provide suitable fillers as does neoprene rubber.

The preferred filler also serves to bond the overlapped screen portions to each other, thereby making a one-step operation of the bonding and filling steps. This requires, however, that the screen openings be suffrciently large as permits the filler material to permeate the screens to form an effective bond. Screens of the general type referred to hereinabove have satisfactorily large openings to receive many commercially available filler materials, for example, acrylic plastics, such as those manufactured by Rhome & Haas, Philadelphia, Pennsylvania, under the Rhoplex trade name.

One embodiment for anchoring the end of a screen to a support is depicted in FIGS. 3 and 4. In the depicted embodiment, the end 60 of the screen 52 is overlaid on the outer surface of the support 26. A wooden strip 64 is overlaid on the screen and thereafter fastened to the support 26 as by nails 66. In addition to the frictional engagement between the screen that is sandwiched between the support 26 and the wooden strip 64, it is advantageous in certain circumstances to apply a bonding agent 68, such as an epoxy resin, between the support and the overlying wooden strip. This epoxy resin is squeezed between the support and strip such that it permeates that portion of the screen disposed between the support and strip and effects a substantial bond between these three components.

It will be recognized that the screen 52 is subjected to substantial working movement at the location where the screen enters the space between the support 26 and the strip 64. In the depicted anchorage means, a wooden strip 70 having a rounded surface 72, such as the common quarter round" wood strip, is attached to that surface 74 of the support 26 which faces inwardly of the structural surface formed by the screen 52. The rounded surface 72 of the strip 70 is disposed in supporting relationship to the screen 52 on that side of the screen which forms its inside curvature so that the screen is prevented from forming sharp bends at its ends where it is attached to a support. This support for the screen has been found to be particularly important in reducing the severity of localized working of the screen when it is subjected to forces whose direction of application to the screen repeatedly changes. The screen thus is protected against premature rupture at its anchored ends.

Each strand spanning the space between spaced apart locations on the perimetral support constitutes a tensile member in the present structure. This is accomplished by orienting each elongated screen member between the supports such that its longitudinal strands 54 are aligned along substantially straight lines. Each strand, being straight, has no substantial catenary,

6 hence functions in substantially pure tension. In their aggregate, these strands define the surface 14 of double-opposed curvature. These strands and their straight line alignment is seen in FIGS. 1, 6 and 7. In symetrical double-opposed curvatures, the transverse strands 56 of the screen members also extend in substantially straight lines, crossing the longitudinal strands at generally right angles. In many embodiments, however, the

transverse strands will either only approximate straight lines or will follow curved paths, under which conditions the strands of the screen will not necessarily cross at right angles. This relationship of the transverse and longitudinal screen strands will arise in certain instances by reason of the geometry of the surface. It will also arise by reason of the use of the screen members which have a constant width, for in the alignment of these constant width screens along generally straight lines to span certain geometrical surfaces, it necessarily follows that the overlap will not be constant from end to end to a given pair of adjacent screen members. Consequently, the transverse strands of these overlapped screens will not lie in a straight line. They, however, when joined to each other, function in tension along their composite length. In this respect, it is noted that the overlap shown in FIG. 5 is idealized for purposes of illustration.

The longitudinal strands extending in substantially straight lines between points of attachment to the supports and crossed at intervals along their respective lengths by transverse strands that extend between other spaced apart locations on the support, define the surface of double opposed curvature. With reference to FIG. 7, the surface defined by the screen members includes an outside face 102 and an opposite inside face 104. The face 102 of the surface 100 resists forces directed against the surface 100 as represented by the arrows A in FIG. 7, while the face 104 resists forces directed against the surface 100 from a direction opp0 site to that indicated by the arrows A. [t is to be recognized that the direction of the forces exerted against the surface 100 (either face) will vary in direction and the arrows A are illustrative only of the forces involved. In any event, these forces constitute a load applied to the surface 100. In the present structure and referring to the loading imposed by the arrows A, the load is resisted by the effective curvature" of the face 102 (shaded area in FIG. 7) which transfers the load to the points A and D on the perimetral support. Consequently, radius of curvature of the surface is of importance in that such determines the loading that can be resisted by the surface 100 without rupture. Employing the concepts disclosed herein and using ordinary woven aluminum insect screen of a 16 X 18 weave pattern and 0.01 inch strand diameter, a double layer of screen (minimum overlap of about 1 inches), filled and bonded with an acrylic plastic will resist a loading of about 20 pounds per square foot of surface area fora span of about 100 feet if the curvature of the surface 100 is such that the center point of the curved surface is about 25 feet from a straight line connecting the opposite ends of the curved (points A and D of FIG. 7, for example). This loading is substantial for a membrane structural form and is particularly significant when consideration is taken of the weight of the membrane and the cost of its construction which are both relatively low compared to prior art membranes.

The maximum loading which can be resisted by the surface I00, assuming a given curvature, is increased by employing multiple layers of screen members. Multiple layered screen membranes are constructed preferably by laying up a first layer as described hereinbefore, but without application of a bonding or filler material to the screen members, and thereafter laying up a second layer of screens on the first layer, followed by application of the filler material. The screen members of the second layer preferably are slightly misaligned with the screen members of the second layer to develop nonregistration of the openings in the overlying screen members. In addition, the screen members of the second layer have been found to bear against the screen members of the first layer in such manner as to hold the first screens in their edge'overlapping positions without the use of ties. Because of this holding" feature, the lay-up construction time for dual layered surfaces is substantially less than twice the lay-up construction time for two single layers so that additional layers of screen members do not represent substantial increase in construction costs. Further, in certain geometrical configurations of the surface of doubleopposed curvature, a second layer of elongated screen members may be laid up with its longitudinal strands aligned in substantially straight lines that are oriented at angle to the straight lines followed by the longitudinal strands of the first layer screens. This type structure is depicted generally in FIG. 6 in which the strands 97 of the first layer screens are disposed at an angle to the strands 99 of the second layer screens. In this and like multi-layered structures, the cross-directed strands develope unusually large strength in the structure at relatively low cost in material and fabrication. importantly, the developed strength is multidirectional so that the resultant structure is resistant to potential destructive forces directed against the structure from diverse directions.

Forces developing within the interior of the structure, as by the action of wind currents outside the structure, which exert an outward thrust against the surface 100 are resisted by the effective curvature of the face 104 of the surface 100 in like manner as described above in the connection with forces acting against the face 102. Under either condition, the resistance to destruction, disruption or dislocation of the surface 100 is through the tensile strength of the screen strands as enhanced by the stabilizing influence of the filler applied to close the screen openings and which also interconnects the longitudinal and transverse strands of the screen members.

In one specific method for constructing a structural surface in accordance with the present invention, a plurality of supports, such as the

supports

20, 22, 26 and 28, were connected at their ends in the configuration shown in FIG. 1 to define the perimeter of a structural surface 14 having a generally hyperbolic parabloid geometry. A second group of

supports

44, 46, 48 and 50 were similarly connected at their ends in the configuration shown in FIG. 1 to define a second structural surface also of a generally hyperbolic parabloid geometry. Each of the supports comprised a common wood 4 X 6, approximately 16 feet long. These two groups of supports were then joined to one another at the corners thereof formed by the

supports

20 and 22, and supports 44 and 46. This corner joint 24 was disposed uppermost of the structure with the

supports

26, 28, 48 and 50 resting on the ground. These two groups of supports were further connected to each other by

cross members

90 and 92 to rigidify the structure and to define

door openings

94 and 96. In the present example, the cooperating

structural surfaces

14 and 16 defined an open space therebetween which served to house growing plants. A cap 91 was placed on top of the corner joint 24 to protect against the natural elements.

Elongated screen members 52, each comprising a length of approximately 16 feet and a constant width of 18 inches were disposed between the

support members

26 and 22 with the longitudinal strands of each of the screen members being disposed in approximately straight lines between the

supports

26 and 22. The screen employed in the present example was common aluminum window screen comprising 0.0l 1 inch diameter filaments woven in a 16 X 18 weave pattern.

Each end of each of the screens was anchored to respective support by means of a wooden strip placed over the screen and nailed to the support. An epoxy cement was placed between the wooden strip and the support to assist in bonding the support, screen, and wooden strip together. Each support was provided with a strip of wood quarter-round disposed on the inner face of the support and extending along the length of the support. The rounded surface of the quarter-round was positioned adjacent the screen at the location of the exit of the screen from between the support and the clamping wood strip. Each longitudinal strand of the screen was substantially taut at the time its ends were anchored.

The longitudinal side edges of the screens were overlapped a minimum of approximately 1 h inches inwardly from the side edge of each screen. During construction, ties were inserted in the overlapped portions of the screens at spaced intervals to temporarily hold the screens in place prior to the application of a bonding agent to the overlapped regions.

After the space defined by the

supports

20, 22, 26 and 28 are covered by screen members, an acrylic plastic identified as Rhoplex acrylic (Rohm & Haas Company), thickened as desired, was applied to the entire surface defined by the screens using a squeegee. Approximately one gallon of plastic, in liquid form, was applied to each square feet of screen area. This plastic formed a film across each of the openings between the screen strands and bonded the overlapped ends of the transverse strands of the adjacent screen members. The bond strength developed in the overlapped areas was greater than the tensile strength of the individual transverse strands of the screen members.

The resultant surface 14 assumed a double-opposed curvature as shown in FIG. 1. Each of the strands of the screen members, being aligned in a substantially straight line acted in substantially pure tension so that the surface 14 resisted disruptive forces acting upon the surface from diverse directions, including forces directed inwardly of the structure and outwardly of the structure. The space defined by the

supports

44, 46, 48 and 50 was covered by screen members in a like manner. As described above in connection with the structural surface 14, this second structural surface 16 exhibited like properties as did surface 14.

The plastic applied to the screens to fill the openings therein was transluscent, permitting sunlight to be admitted to the interior of the structure. The

surfaces

14 and 16, however, were solid in the sense that they effectively excluded all natural elements other than light.

Moreover, this plastic, when cured was semirigid. Consequently, the surface 14 and I6 successively resisted penetration upon impact by such objects as rocks thrown against the

surfaces

14 and 16.

Tensile-stress structures fabricated in accordance with the present disclosure exhibit good structural properties which are obtainable at low cost in materials and low construction expense. The framework of the structure is minimal and the screens are readily available from commercial sources at relatively low cost. Plastic fillers for closing the screen openings also are relatively inexpensive and easily applied. In addition, there is possible a large variety of geometrical configurations of the surface, giving the designer a storehouse of possible structures. Employing the disclosed method, the structures are erected in situ thereby providing a means for erecting an economical, but strong, useful structure in a remote location.

Whereas specific materials of construction have been referred to herein, it is not intended to limit the invention to such. For example, whereas aluminum screen of a stated strand size and spacing between strands has been disclosed, it is noted that other screens are suitable. Such other screens include other metal screens and plastic screens, either woven or nonwoven. As noted, multiple layers of screens may be employed as desired. The method chosen to anchor the screen ends clearly may be varied so long as provision is made for protecting the screen against excessively sharp bends at its ends.

While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure, but rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A method for fabricating a tensile-stress structure having a surface of double-opposed curvature comprising the steps of erecting a substantially rigid open frame means defining the perimeter of said surface and comprising a terminal support for the respective ends of a plurality of elongated screen members spanning the space within said perimeter,

positioning an elongated screen member comprising a plurality of longitudinal strands, across said space defined by said terminal support, aligning said screen member across said space with individual ones of said longitudinal strands thereof disposed along a substantially straight line,

tensioning said screen member in the direction of the length of said longitudinal strands,

anchoring the opposite ends of said screen member to said frame means,

applying successive elongated screen members across said space defined by said frame means with their respective adjacent side edges overlapping until said screen members collectively define said surface of double opposed curvature and,

closing the openings between the strands of said screen members with a semi-rigid material.

2. The method of claim 1 wherein the step of closing said openings includes bonding said overlapped margins to one another.

3. The method of claim 1 and and including the step of combining said surface of double-opposed curvature with a further like surface.

4. The method of claim 1 and including the step of overlaying a further plurality of elongated screen members across said space defined by said frame means prior to closing the openings between said strands with said semirigid material.

5. A tensile-stress structure having a surface of double-opposed curvature and comprising a substantially rigid open frame member defining the perimeter of said curved surface,

a plurality of elongated screen members disposed across said space defined by said frame means in side by side relation with their adjacent side edges disposed in overlapping relation with each other and collectively defining said surface of doubleopposed curvature, each of said screen members including a plurality of longitudinal and transverse strands, each of said screen members being tensioned in the direction of said longitudinal strands whereby each of said longitudinal strands extends in a substantially straight line between spaced apart locations on said frame member,

means anchoring the ends of each of said screen members to said frame member, and

semi-rigid means closing the openings between said strands of each of said screen members.

6. The tensile-stress structure of claim 5 wherein said frame member comprises a plurality of connected rigid elongated elements.

7. The tensile stress structure of claim 5 wherein the adjacent longitudinal side margins of said screen members are bonded to one another in the overlapped portions.

8. The tensile-stress structure of claim 7 wherein the bond strength in said overlapped portions is at least as great as the tensile strength of individual ones of the transverse strands of said screen members.

9. The tensile-stress structure of claim 5 and including means defining a transitional surface adjacent the anchored end of each screen member adapted to reduce the severity of bending of said screen member at each of its anchored ends.

10. The tensile-stress structure of claim 5 and including a further plurality of elongated screen members defining a plurality of further screen members overlaying the first plurality of screen members and spanning said space within said perimeter, the longitudinal strands of each of said further screen members being aligned along substantially straight lines that are nonparallel to the lines along which the strands of the screen members of said first screen members are aligned.

Ill i i

Claims

1. A method for fabricating a tensile-stress structure having a sUrface of double-opposed curvature comprising the steps of erecting a substantially rigid open frame means defining the perimeter of said surface and comprising a terminal support for the respective ends of a plurality of elongated screen members spanning the space within said perimeter, positioning an elongated screen member comprising a plurality of longitudinal strands, across said space defined by said terminal support, aligning said screen member across said space with individual ones of said longitudinal strands thereof disposed along a substantially straight line, tensioning said screen member in the direction of the length of said longitudinal strands, anchoring the opposite ends of said screen member to said frame means, applying successive elongated screen members across said space defined by said frame means with their respective adjacent side edges overlapping until said screen members collectively define said surface of double opposed curvature and, closing the openings between the strands of said screen members with a semi-rigid material.

5. A tensile-stress structure having a surface of doubleopposed curvature and comprising a substantially rigid open frame member defining the perimeter of said curved surface, a plurality of elongated screen members disposed across said space defined by said frame means in side by side relation with their adjacent side edges disposed in overlapping relation with each other and collectively defining said surface of doubleopposed curvature, each of said screen members including a plurality of longitudinal and transverse strands, each of said screen members being tensioned in the direction of said longitudinal strands whereby each of said longitudinal strands extends in a substantially straight line between spaced apart locations on said frame member, means anchoring the ends of each of said screen members to said frame member, and semi-rigid means closing the openings between said strands of each of said screen members.