US3191727A

US3191727A - Framing structures

Info

Publication number: US3191727A
Application number: US235728A
Authority: US
Inventors: Andrew H Schmeltz; James J Davis
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1962-11-06
Filing date: 1962-11-06
Publication date: 1965-06-29
Anticipated expiration: 1982-06-29

Description

J 1965 A. H. SCHMELTZ ETAL 3,1 ,727

FRAMING STRUCTURES Filed Nov. 6, 1962 2 Sheets-Sheet 1 INVENTORS ANDREW H. SCHMELTZ JAMES J. DAVIS June 29, 1965 A. H. scHMELTZ ETAL 91,727

INVENTORS United States Patent 3,191,727 FRAMING QTRUQTURES Andrew H. Schrneitz, Ualimont, and .larnes J. Davis, Monroeville, Pa, assignors to Aluminum tlornpany of America, Pittsburgh, Pin, a corporation of Pennsylvania Filed Nov. 6, 1962, Ser. No. 235,728 6 Claims. (-Ci. res-4s This invention relates in general to framing members and structures made therefrom. It is more particularly addressed to unitary framing members in which spaced metallic elements are unitarily and structurally connected by a non-metallic thermal barrier disposed in secured relationship within the space therebetween.

'High heat conductivity of metals generally is probably the most undesirable property in their direct substitution for wood in building construction. This undesirable property can be best appreciated in window and similar panel structures, employing metallic framing instead of conventional wood window frames, in which case metallically framed windows are observed to be inferior in temperature insulating value to wood framed windows under differential temperature conditions, such as prevail in northern climates, between outdoor and indoor temperatures.

It is an object of the present invention to provide a unitary structural framing member incorporating inner and outer spaced metallic elements connected by an intermediate non-metallic thermal barrier insulating material preventing direct thermal conduction between the inner and outer metallic members.

It is another object of the invention to provide a unitary framing member having elongate metallic elements secured in interlocking structurally spaced parallel disposition by an intermediate non-metallic thermal barrier insulating strip.

Still another object of the invention is to provide metallically framed panels, such as windows, doors, dividers, window walls, and similar structures in which metallic framing is employed in marginal engagement with panels of glass or other non-metallic materials, the framing therefor comprising spaced metallic elements unitarily connected by means of a non-metallic strip of thermal insulation to provide an outer metallic mounting means and a thermally separated, but structurally connected, inner metallic supporting member.

The enunciated and other objects of the invention will become apparent to those skilled in the art to which the invention appertains on consideration of the following specification and illustrations, in which:

FIG. 1 represents a front or outside elevational view of a window or similar construction incorporating structural features of the invention;

FIG. 2 represents a partially assembled and exploded asymmetric view in partial section of a fragmentary length of a unitary structural framing member falling within the scope of the invention;

FIG. 3 represents a transverse partial sectional elevation taken on the plane 3-3 of FIG. 1;

FIG. 4 represents a transverse cross-sectional view of one element of the invention; and

FIGS. 5, 6, 7, 8 and 9 represent fragmentary transverse sectional views, to enlarged scale, illustrating progressive assembly of individual elements of the unitary structural framing members embraced by the invention.

Referring to the several above-identified illustrations, it will be observed that a unitary structural framing member has been provided in the form of two elongate

metallic elements

10 and 12 in separated or spaced parallel relationship and unitarily connected by a molded or otherwise fabricated non-metallic elongate element or strip 14 of a 3,l9.l,727 Patented June 29, 19 65 suitable thermal insulating material, such for example natural rubber or synthetic rubber compounds, generally classifiable as elastomeric materials. The metallic elements 1%) and 12, as well as the non-metallic element 14, lend themselves to fabrication by conventional extrusion practices and are preferably, although not necessarily obtained in that manner.

It is a further desirable feature of the unitary structural framing element thus far described, and illustrated to best advantage inunitary assembly in FIGS. 1 and 3, that the insulating element or thermal barrier strip 14 exhibits definite flexibility and/ or resiliency as an aid to its unitary assembly with the metallic elements it and 12, as hereinafter described in more detail, without forfeiture of that rigidity necessary to provide structural stability and load-carrying ability within and transversely across the unitary assembly of the separate elements 1t), 12 and 14-.

The thermal barrier connecting strip 14 may be molded or extruded to desired cross-sectional configuration from many well-known commercially available conventional materials, such as natural rubber, butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, polychloroprene suitable mixtures of these materials, as Well as polyvinyl chloride compounds, acrylic and methacrylate base plastics, and other synthetic materials exhibiting similar elastorneric characteristics. In this connection, a centrally located aperture 16 (FIG. 4) is often desirable to lend flexibility, reduce the weight and increase the insulating value of the thermal barrier connecting strip 14. It is also within the purview of the invention that additional rigidity and strength may be obtained in the assembled unitary framing structures of the invention by reinforcing the thermal barrier strip element 14, as by incorporating therewithin at fabrication strength-imparting structure in the form of reinforcing rods 18 and/ or flexible strips or expanded mesh 2% of glass, metal, or the like, as illustrated in FIG. 4.

The metallic elements it; and 12 have been primarily selected for purposes of illustration and simplicity in the general form of substantially similar T-sections. As such, and specific reference is herein made to FIGS. 1, 2 and 3, t e head or flange portions of

elements

10 and 12 are preferably offset slightly in opposite directions with the flange of one element (say element 16) serving as a mounting flange for securement of a framed structure in exposure to outside temperature (FIGS. 1 and 2) and the oppositely extending similar head or flange of element 12 providing a marginal flange for supporting or arresting a panel 22. It will thus be observed that the panel 22 and its metallic framing member 12 serving to support or arrest the same, are structurally separated or spaced from the outer metallic framing element 10 by the interposed thermal barrier strip 14, and that direct or bridging metallic thermal conduction between the

metallic elements

10 and 12 has been obviated.

Except for the unitary framing construction comprising the

metallic framing elements

10 and 12 and interposed connecting thermal barrier elastomeric strip 14, conventional practices can and have been followed in installing the panel 22, as best illustrated in FIGS. 1 and 3. For example, a conventional double wall insulated glazing panel 22 has been supported within a wall opening defined by a surrounding frame constructed from individual lengths of composite unitary framing members of the invention in spaced relationship thereto by means of suitable insulating blocks 24 of Wood or other suitable insulating material, the aforesaid wall opening being otherwise preferably provided with an angularly formed thin metallic fin strip surround 29 secured by nails or other conventional fasteners.

A conventional outer surface Weather strip insert 25 and an inner surface sealing strip insert 26, of suitable elastomeric material, are preferably secured within reentrant grooves or slots provided in the rear and outwardly facing surfaces of the

metallic framing elements

10 and 12, respectively (FIG. 3), and the glazing panel 22 is preferably held and pressed against the sealing strip 26 by means of a conventional resilient glazing molding strip 28, which may be a metallic spring type molding strip or fabricated from a non-metallic flexible plastic composition or natural rubber. Conventional caulking 2'7 is also preferably employed within any outwardly exposed spaces such as would normally occur between the lower front exposed edge of the bottom metallic framing element 10 and upper inclined surface of the exposed window sill (FIG. 3) adjacent thereto.

Itwill also be appreciated that the described structure and installation illustrated in FIGS. 1 and 3 provides a resilient, as well as a thermally insulated construction and supporting system for a panel 22, the latter being selectable in the form of glass, hard board, plywood, plaster board, or any other conventional non metallic building material, to thus serve as a substantially shock resisting construction.

Of cardinal significance in the design and construction of the unitary framing members of the present invention is the particular manner in which the metallic elements '10 and/or 12 are interconnected in unitary structural assembly by means of the elastomeric barrier strip 14. This interconnection and assembly, and manner of obtaining the same, is best illustrated in FIGS. 5 through 9 as now hereinafter described in specific detail.

Each metallic T-section 19 or 12 is provided with a 'web or stem preferably disposed normal to its flange or head, and for simplicity of description, the web or stem of each of the sections is illustrated as being of similar configuration and having a symmetrical transverse crosssection. In each instance, then, the web or stem is pref- 'erably configurated to provide lateral oppositely disposed spaced parallel exposed preferably flat root surfaces 30 normal to and integral with the underside of the head or flange of each of the T-section elements lit) and 12. In addition the webs or stems in each instance terminate at their free edges in lateral oppositely disposed and exposed flat parallel pilot surfaces 34 which are also parallel with the aforesaid surfaces 30, stepped slightly and uniformly below the same, and contiguously connected thereto by concave surfaces 32 which extend inwardly to provide an intermediate narrow or minimum thickness for each web or stem in the short transverse axis of each of the webs. I

Oppositely disposed marginal edge surfaces or faces of the thermal barrier strip 14 are preferably similarly inwardly recessed to receive and mate with the previously described and preferably identically configurated T-section webs or stems of the metallic elements and 12. In this regard, the recesses in the barrier strip 14 are purposely isolated from each other and the interior walls defining the same are substantially complementary to the exposed surfaces of each web or stem of the T-

sections

10 and 12, the transverse cross-sectional area of each recess preferably being slightly in excess of the transverse cross-sectional area of each T-section web (FIG. 9), the purpose of which will be hereinafter described. It is also to be observed that disposition of the oppositely disposed interior walls of the recesses in the barrier strip 14 is in reverse order from the entrances thereto to the disposition of the oppositely disposed exposed root, concave and pilot surfaces of the Webs of the

metallic elements

10 and 12 in the direction from the terminal free ends thereof to provide for complementary assembly of each configurated web within its recess.

As a result of the above-described exposed configuration of each web or stem of the T-

sections

10 and 12, and the substantially complementary mating surfaces defining the marginal edge recesses in the thermal barrier and connecting strip 14, snap assembly of the stems within the recesses can be accomplished with a minimum of flexure and absence or elimination of substantial reverse bending of the elastomeric barrier strip 14, reverse bending of elastomeric materials being recognized as detrimental to such materials and conducive to their early failure by fatigue.

Consideration and inspection of FIGS. 5 through 9, illustrating initiation and progressive snap assembly of one of the metallic framing elements 10 with its adjacent interconnecting non-metallic elastomeric strip 14, will clearly depict that the oppositely disposed parallel exposed terminal pilot surfaces 34 of the T-section web first engage with the inwardly projecting oppositely disposed convex surfaces 36 of the interior walls of the complementary recess in the strip 14 to angularly and flexurally spread the resilient bifurcated end forming the recess outwardly to a maximum contained angle, illustrated in FIG. 7, after which angular deflection of the strip 14 diminishes in response to continued relative contacting surface engagement between the web and recess, as illustrated in FIGS. 8 and 9, the latter depicting final installation and interlocking unitary assembly of the metallic element 10 and thermal barrier strip 14.

It will be observed that insertion of the Web within its complementary recess has been accomplished without progressively changing reverse bending of the legs 38 and it? of the thermal barrier strip forming its recessed or bifurcated end portion, the aforesaid progressive angular outward separation ofthe legs 38 and 40 (FIGS. 6 and 7) and subsequent progressive inward angular return thereof being confined to a substantially constant or fixed plane 'of angular bending or deflection of each of the legs 38 and '45 about a point for each leg located in a plane to the right of the entrance to the recess, which angular deflection does not involve deleterious fatigue-producing progressively changing bending movement as would occur if it were attempted to force a web or stem of greater localized free end terminal thickness into a recess in which the opposite walls forming the entrance thereto defined a width less than the localized terminal web thickness dimension.

The slight clearance (FIG. 9) between the mating surfaces of a web and recess forming the final connection between the metallic element It! and the elastomeric 'strip 14 is an important desideratum when it is intended to employ an adhesive to produce additional securement of the unitary assembly of the framing members of the invention. A wide variety of conventional adhesives are commercially available for this purpose and have been successfully employed in the form of liquids and heat-reactive pre-dried thermosetting and/or thermoplastic lacquer type cements. Regardless of the wide variety of commercial adhesives compatible with a particular selected elastomeric material for the barrier strip 114, a selected adhesive may be applied to the exposed surfaces of the webs or stems of the metallic members 10 .and 12, or on the interior walls of the recesses in the thermal barrier strip 14, or on both the said exposed web surfaces and recess interior walls.

It will be observed that snap insertion of the web or stem of either or both

metallic elements

10 and 12 within 'a substantially complementary recess in the thermal barrier strip 14, as above described (FIGS. 5 through 9), is accomplished with minimum rubbing contact between the exposed surfaces of a web of the

metallic elements

10 and 12 and the walls defining the complementary recesses in the barrier strip 14. In fact, consideration of FIGS. 6, 7 and 8 clearly establishes that the entire length of the root surfaces 3% along the long transverse axis of a web, measured normal to the underside of the flange of T-section iii, and substantially the entire arcuate length of the concave surfaces 32 contiguous therewith (FIG. 8), remain out of rubbing contact with any portion of the interior walls of the recess in the elastomeric strip 14 during unitary snap assembly of a composite framing member of the invention, which condition assures maximum surface retention of an adhesive, and particularly a liquid adhesive, earlier applied to the mating surfaces of the webs and/or recess walls.

The final assembled framing member illustrated in PEG. 9 provides interlocking engagement of the convex wall surfaces 3a: of a recess, within the thermal barrier strip 14, in registry within the concave mating surfaces 32 of the stem or web of a metallic element 19 assembled therewith, with and without the additional securement obtained through the use of an adhesive.

Having described and illustrated the invention in terms of specific unitary structural framing members and framed structures made therefrom, it is to be understood that other forms and embodiments may be adopted, as may fall within the scope or" the following claims.

What is claimed is:

l. A unitary structural framing assembly comprising spaced elongate metallic elements and an interlcckingly connecting elastomeric thermal barrier element therebetween, at least one of said metallic elements carrying a laterally extending web defined in transverse crosssection by parallel spaced exposed root surfaces, parallel spaced exposed terminal free end pilot surfaces, and an intermediate exposed concave surface in contiguous connection with a root and pilot surface on each side of the long transverse axis of the web, the thermal barrier element having a recess extending thereinto adjacent the web, said recess being defined in transverse cross-section by oppositely disposed parallel interior walls and an intermediate convex wall in contiguous connection with two parallel wall surfaces on each side of the long transverse of the recess, said interior walls of the recess eing substantially complementary in reverse disposition from the entrance thereto along the long transverse axis thereof to the disposition of the exposed root, concave and pilot surfaces of the web from the terminal free end thereof, and said interlocking assembly comprising said 7 ch snap inserted within the adjacent recess through resilient fiexure of the elastomeric thermal barrier element by contacting engagement of the parallel exposed terminal free end pilot suriaces of said web with the convex wall surfaces of the recess to provide interlocking registry of the oppositely disposed convex interior walls of the recess within the concave exposed surfaces of the ch.

2,. A unitary structural framing member comprising spaced elongate metallic elements in assembly with an interloclo'ngly connecting resilient elastomeric thermal barrier element thercbetween, said metallic elements each carrying an identical oppositely disposed laterally extending web defined in transverse cross-section by parallel spaced exposed root surfaces, parallel spaced exposed terminal free end pilot surfaces, and an intermediate exposed concave surface in contiguous connection with a root and pilot surface on each side of the long transverse axis of each web, the thermal barrier element having an identical isolated recess extending thereinto from oppositely disposed faces thereof adjacent each web, each said recess being defined in transverse cross-section by oppositely disposed para lel interior walls and an intermediate convex wall in contiguous connection with two parallel wall surfaces on each side of the long transverse axis of each recess, said interior walls of each recess being substantially complementary in reverse disposition from the entrance thereto along the long transverse axis thereof to the disposition of the exposed root, concave and pilot surfaces of each web from the terminal free end thereof, and said interlocking assembly comprising said webs each snap inserted within an adjacent recess to resiliently flex the body of the elastomeric thermal barrier element by contacting engagement of the parallel exposed terminal free end pilot surfaces of each web with the convex wall surfaces of each recess to provide interlocking registry of the oppositely disposed convex 2 7 interior walls of each recess within the concave exposed surfaces of each web.

3. A unitary structural framing member comprising spaced elongate metallic elements of generally T-shaped cross-section and an interlockingly connecting resilient elastomeric thermal barrier element therebetweeu, said metallic elements being disposed with their webs in spaced opposing relationship and each web being defined in transverse cross-section by parallel spaced exposed. root surfaces normal to the head flange, parallel spaced exposed te.minal free end pilot surfaces, and an intermediate exposed concave surface in contiguous connection with a root and pilot surface on each side of the long transverse axis of each Web, the thermal barrier element being of generally rectangular cross-section and having an identical isolated recess extending thereinto from oppositely disposed exposed faces t ereof adjacent each web, each said recess being defined in transverse cross-section by oppositely disposed parallel interior walls and an intermediate convex wall in contiguous connection with two parallel wall surfaces on each side of the long transverse axis of each recess, said interior walls of each recess being substantially complementary in reverse disposition from the entrance thereto along the long transverse axis thereof to the disposition of the exposed root, concave and pilot surfaces of each Web from the terminal free end thereof, and said interlocking connection comprising snap assembly of said webs each within an adjacent recess in the resilient elastomeric thermal barrier element through the entrance thereto to resiliently flex the body of the elastomeric thermal barrier element by contacting engagement of the parallel exposed terminal free end pilot surfaces of each v eb with the convex wall surfaces of each recess to provide interlocking registry of the oppositely disposed convex interior walls of each recess within the concave exposed surfaces of each web.

4. A unitary structural framing member comprising spaced elongate metallic elements and an interlockingly connecting resilient elastomeric thermal barrier element therebetween, said interlocking connection between each of the metallic elements and the thermal barrier element comprising resilient snap assembly of an extending web carried by each of the metallic elements within one each of two isolated recesses extending into the body of the thermal barrier element, each extending web being of elongate transverse cross-section and being defined symmetrically on both sides of the long transverse axis thereof by parallel exposed root surfaces, parallel exposed terminal free end pilot surfaces, and a contiguous connecting exposed concave surface, the short transverse thickness across the parallel exposed terminal surfaces being less than that across the parallel exposed root surfaces, each isolated recess in the thermal barrier element being of elongate transverse cross-section and symmetrical on both sides of the long transverse axis thereof, each recess being defined by oppositely disposed walls each comprising parallel surfaces contiguously connected by a convex wall surface, the exposed surfaces of each web and interior walls of each recess being substantially complementary in reverse disposition from the entrances to the recesses along the long transverse axis of each recess to the disposition of the exposed root, concave and free end pilot surfaces of the web from the terminal free end thereof receivable therewithin, and sa d parallel exposed terminal free end pilot surfaces of each web, on insertion of a web into its respective recess through the entrance thereto, resiliently flexing the body of the thermal barrier element by contacting engagement with the interior convex wall surfaces of each recess to provide interlocking registry of the interior convex wall surfaces within the exposed concave surfaces of each of the Webs.

5. A unitary structural framing member comprising spaced elongate metallic elements and an interlocliingly connecting resilient elastomeric thermal barrier element adhesively secured therebetween, said adhesively secured and interlocking connection between each of the metallic elements and the thermal barrier element comprising resilient snap assembly of an extending web carried by each of the metallic elements within one each of two isolated recesses extending into the body of the thermal barrier element, each extending web being of elongate transverse cross-section and being defined symmetrically on both sides of the long transverse axis thereof by parallel exposed root surfaces, parallel exposed terminal free end pilot surfaces, and a contiguous connecting exposed concave surface, the short transverse thickness across the parallel exposed terminal surfaces being less than that across the parallel exposed root surfaces, each isolated recess in the thermal barrier element being of elongate transverse crosssection and symmetrical on both sides of the long transverse axis thereof, each recess being defined by oppositely disposed walls each comprising parallel surfaces contiguously connected by a convex Wall surface, the exposed surfaces of each web and interior walls of each recess being substantially complementary and disposed in reverse coextensive relationship from the entrances to the recesses along the long transverse axis of each recess to the disposition of the exposed root, concave and free end pilot surfaces of the web from the terminal free end thereof receivable therewithin, and said parallel exposed terminal pilot surfaces of each web, on insertion of a web into its respective recess through the entrance thereto, and in the presence of an adhesive coating between the exposed surfaces of a web and substantially complementary coextensive interior walls of a recess, resiliently flexing the body of the thermal barrier element by contacting engagement with the interior convex wall surfaces of each recess to provide interlocking adhered registry of the interior convex wall surfaces in contact with the exposed concave surfaces of each of the webs.

6. A unitary structural framing member comprising spaced elongate metallic elements and an interlockingly connecting resilient elastomeric element adhesively secured therebetween, said adhesively secured and interlocking connection between each of the metallic elements and the elastomeric element comprising resilient snap assembly of an extending web carried by each of the metallic elements within one each of two isolated recesses extending into the body of the elastomeric element, each extending web being of elongate transverse cross-section and being defined symmetrically on both sides of the long transverse axis thereof by parallel exposed root surfaces, parallel exposed terminal free end pilot surfaces, and a contiguous connecting exposed concave surface, the short transverse thickness across the parallel exposed terminal surfaces being less than that across the parallel exposed root surfaces, each isolated recess in the clastomeric element being of elongate transverse cross-section and symmetrical on both sides of the long transverse axis thereof, each recess being defined by oppositely disposed wa-lls each comprising axially spaced parallel surfaces contiguously connected by a convex wall surface, the exposed surfaces of each web and interior walls of each recess being substantially complementary and disposed in reverse coextensive relationship from the entrances to the recesses along the long transverse axis of each recess to the disposition of the exposed root, concave and free end pilot surfaces of the web from the terminal free end thereof receivable therewithin, and said parallel exposed terminal free end pilot surfaces of each web, on insertion of a web into its respective recess through the entrance thereto, and in the presence of an adhesive coating between the exposed surfaces of a web and substantially complementary coextensive interior walls of a recess, resiliently and angularly flexing the body of the elastorneric element about a substantially constant plane of bending by contacting engagement with the interior convex wall surfaces of. each recess to provide interlocking adhered registry of the interior convex wall surfaces in contact with the exposed concave surfaces of each of the webs.

References Cited by the Examiner UNITED STATES PATENTS 2,835,360 5/58 Bernardoni et al 189-75 2,898,643 8/59 Bush et al 2()-11 3,093,217 6/63 Doede 18975 X 3,114,179 12/63 Briggs.

FOREIGN PATENTS 522,663 6/40 Great Britain. 1,138,912 6/57 France.

804,258 11/58 Great Britain.

HARRISON R. MGSELEY, Primary Examiner.

Claims

1. A UNITARY STRUCTURAL FRAMING ASSEMBLY COMPRISING SPACED ELONGATE METALLIC ELEMENTS AND AN INTERLOCKINGLY CONNECTING ELASTOMERIC THERMAL BARRIER ELEMENT THEREBETWEEN, AT LEAST ONE OF SAID METALLIC ELEMENTS CARRYING A LATERALLY EXTENDING WEB DEFINED IN TRANSVERSE CROSSSECTION BY PARALLEL SPACED EXPOSED ROOT SURFACES PARALLEL SPACED EXPOSED TERMINAL FREE END PILOT SURFACES, AND AN INTERMEDIATE EXPOSED CONCAVE SURFACE IN CONTIGUOUS CONNECTION WITH A ROOT AND PILOT SURFACES ON EACH SIDE OF THE LONG TRANSVERSE AXIS OF THE WEB, THE THERMAL BARRIER ELEMENT HAVING A RECESS EXTENDING THEREINTO ADJACENT THE WEB, SAID RECESS BEING DEFINED IN TRANSVERSE CROSS-SECTION BY OPPOSITELY DISPOSED PARALLEL INTERIOR WALLS AND AN INTERMEDIATE CONVEX WALL IN CONTIGUOUS CONNECTION WITH TWO PARALLEL WALL SURFACES ON EACH SIDE OF THE LONG TRANSVERSE AXIS OF THE RECESS, SAID INTERIOR WALLS OF THE RECESS BEING SUBSTANTIALLY COMPLMENTARY IN REVERSE DISPOSITION FROM THE ENTRANCE THERETO ALONG THE LONG TRANSVERSE AXIS THEREOF TO THE DISPOSITION OF THE EXPOSED ROOT, CONCAVE AND PILOT SURFACES OF THE WEB FROM THE TERMINAL FREE END THEREOF, AND SAID INTERLOCKING ASSEMBLY COMPRISING SAID WEB SNAP INSERTED WITHIN THE ADJACENT RECESS THROUGH RESILIENT FLEXURE OF THE ELASTOMERIC THERMAL BARRIER ELEMENT BY CONTACTING ENGAGEMENT OF THE PARALLEL EXPOSED TERMINAL FREE END PILOT SURFACES OF SAID WEB WITH THE CONVEX WALL SURFACES OF THE RECESS TO PROVIDE INTERLOCKING REGISTRY OF THE OPPOSITELY DISPOSED CONVEX INTERIOR WALLS OF THE RECESS WITHIN THE CONCAVE EXPOSED SURFACES OF THE WEB.