DE69209472T2 - Cladding system for curtain walls - Google Patents

Description

The present invention relates to a polygonal panel for cladding buildings and a construction system comprising such a panel.

Building cladding panels are known in many forms. They essentially consist of first and second sheet metal trays, each of which has a central part with a polygonal circumference and inward-facing edge flanges, whereby the flanges of one tray fit between the flanges of the other tray and there is an insulating core inside the tray, which is usually attached to the tray by means of an adhesive or by directly injecting a molding compound. Profiles are attached to the inward-facing flange parts, which serve as edges for the cladding panels and allow the panels to be attached to one another and to a supporting structure.

EP-A-0361727 describes a polygonal panel for wall or ceiling cladding of the type in which certain longitudinal edges of the panels rest on or are clamped by support devices. The panel has a front panel, a back panel and a a core attached to it, e.g. by gluing, which preferably consists of a lightweight, non-combustible or fire-retardant and/or sound-absorbing material (e.g. a fiber material). The front sheet is in this case equipped with at least two lateral longitudinal flanges which are folded or curved towards the rear of the panel and at least partially enclose the core on at least these two long sides. These flanges in turn have freely protruding edge parts which are folded or curved outwards at an angle and are completely overlapped by edge parts of the rear sheet which extend beyond the core, the thickness of the core and the design and shape of the longitudinal edges and the edge parts being coordinated in such a way that the front and rear sheets do not directly touch each other.

Although the previously known panels have generally proven themselves in indoor areas, they are subject to significantly higher forces (mainly those caused by weather, particularly wind loads) when used outside the building, and the previously known systems have not achieved entirely satisfactory results in this application.

The present invention therefore provides a polygonal building cladding panel, which is equipped with, among other things,

- a first sheet metal tray with a first central part, which has a polygonal perimeter and on each side of this polygonal perimeter first inwardly directed edge flanges,

- a second sheet metal tray having a second central portion, which has a shape similar to the first tray but has a smaller area, and has on each side of its polygonal periphery second inwardly directed edge flanges, which are dimensioned so as to form a close fit with the first inwardly directed flanges of the first tray along these, and

- a structural core which essentially fills a cavity formed by said first and second central parts and their inwardly directed edge flanges,

wherein some of the first inwardly directed flanges have an edge part which projects transversely to the associated flange and substantially parallel to the central part of this trough, and said first and second inwardly directed edge flanges are fixed and positioned relative to one another by circumferential intermediate profiles, and wherein a laterally projecting stop body is in turn arranged on these intermediate profiles, which laterally overlaps said edge part in such a way that the first trough and the intermediate profile are held in the desired position relative to one another.

By equipping the edge flange with this stop body that overlaps the edge part, it is possible to prevent the edge part from coming loose as a result of wind-related tensile forces. The transversely projecting edge part is preferably provided on the edge of the outer tub.

The corresponding inward-facing flange is advantageously overlapped both inside and outside by a part of the intermediate profile.

In a preferred embodiment, this intermediate profile has a part into which a fastening element engages to fix the panel to a supporting structure. This part is preferably designed in such a way that when the fastening element is tightened, it moves the edge profile in a direction such that the stop body is pressed against the transversely projecting edge part. In this way, the fastening element can hold the two trays firmly pressed against the supporting structure and, in doing so, slightly press the sandwich core lying between these trays together.

The fastening element can be, for example, a screw element to which a rotatable body similar to a washer is assigned. The latter preferably has at least one groove into which an outwardly directed bead of the said profile part engages.

In many cases, the fastening element is assigned to the profiles of two adjacent panels that are mirror images of each other. The rotating body can have a partially circular shape, but with part of the circumference cut out, so that the rotating body can be rotated into a position in which it allows a second panel to be easily attached next to an already attached panel, after which it is then rotated (e.g. by 90º) in order to also grip the relevant part of this second panel.

In an alternative embodiment, the fastening element is designed as a body with a profiled head that can be brought into engagement with said part of the intermediate profile.

According to a further aspect of the invention, this provides a cladding panel system which

- a supporting structure,

- several cladding panels, each consisting of

* a first sheet metal tray with a central part which has a polygonal perimeter and first inward-facing edge flanges on at least one side of this polygonal perimeter,

* a second sheet metal panel having a second central portion having a shape similar to that of the first panel and having second inwardly directed edge flanges on at least one side of its polygonal periphery, and

* a structural core which essentially fills a cavity formed by the said second inwardly directed flanges,

- circumferential intermediate profiles which engage behind the first and second inwardly directed edge flanges and position them relative to one another, whereby this intermediate profile has a part in the form of a longitudinal groove with inwardly directed edges, and

- comprises a plurality of fastening elements, each of which has a fastening body and a head part engaging in the longitudinal groove, the longitudinal groove being displaceable relative to this head part and the fastening body having means mounted on the supporting structure and also fixing means for locking the fastening element relative to the longitudinal groove.

In these fastening elements, the head part has a hole into which a threaded fixing means is inserted, which can be tightened so that it presses against the intermediate profile part and fixes the latter relative to the fastening element.

The body part preferably has a groove which can be engaged with a supporting element of the supporting structure.

A filler strip is preferably arranged between two adjacent panels. This can be designed as a U-shaped sheet metal profile that has a seal on each of its flange sides. When pressed into the space between the two adjacent panels, this seal firstly creates a weatherproof seal between the opposing inward-facing flanges and secondly has an engagement part that engages behind corresponding edges on the two associated intermediate profiles.

While normally both the first and second inward facing flanges would have edge portions extending outward from the associated core are bent, it is assumed here that this can only be the case in the case of the first inward-facing flanges, for example, whereas the second inward-facing flanges can be equipped with edge parts pointing transversely inwards.

In a preferred design, these edge parts are provided on alternating flanges, but not on the flanges in between. In the most common case of a rectangular panel, for example, two opposing first flanges would be equipped with these edge parts, but the other two edge flanges would not. In this specific embodiment, the second flanges corresponding to these alternating flanges would each be designed without the edge parts, while the other flanges would have inward-facing edge parts. The polygonal shape can also be realized with a different number of sides, i.e. the polygonal building cladding panel can also be designed hexagonally, for example, whereby again preferably alternating first flanges would be equipped with the outward-facing edge parts, while the other three flanges would not.

For clarification, the present invention is described below by way of example with reference to the accompanying drawings, wherein

- Fig. 1 shows schematically an embodiment of a building cladding panel system according to the invention in external perspective;

- Fig. 2 shows an analogous view from the inside;

- Fig. 3 is a detailed view showing the rotating element associated with a screw fastening element;

- Fig. 4 shows an enlarged cross-section through two adjacent building cladding panels according to the invention with a filler strip in between;

- Figures 5a and 5b are similar views (but without the filler strip) showing the rotating body associated with the fastener in two different positions;

- Fig. 5c shows the filling bar on an enlarged scale;

- Fig. 6 is a view similar to Fig. 4 showing the other two sides of the corresponding panels with the filler strip inserted;

- Fig. 7 shows an exploded view of the panels shown in Figs. 1 to 6;

- Fig. 8 shows a view of the corner joint of two intermediate profiles;

- Figures 9a and 9b show schematically parts of two intermediate profiles according to the invention;

- Fig. 10 is an exploded view similar to Fig. 7 but with a hexagonal panel ;

- Fig. 11 shows an exploded view of a rectangular panel of modified construction;

- Fig. 12 shows a view of another modified panel similar to Fig. 4,

- Fig. 13 shows an enlarged cross-section along the line XIII-XIII of Fig. 12;

- Fig. 14 is a perspective view of another embodiment of a panel system according to the invention for cladding buildings;

- Fig. 15 shows a detail of the panel from Fig. 14 in a top view;

- Fig. 16 shows a pair of fastening elements from the arrangement shown in Figs. 14 and 15 in perspective view; and

- Fig. 17 shows a sectional view along the line XVII-XVII.

Figures 1 and 2 show a section of a curtain wall cladding system according to the invention. This section shows in particular part of a connection point between horizontal and vertical filling elements.

A supporting building framework or supporting structure 10 with a front flange or web 12 serves to suspend several individually removable and exchangeable panels 14. The supporting building framework can comprise any number of elongated profiles of L-, U- or I-shaped cross-section, each having a flange or web exposed in a front plane for fastening the panels. The individual panels are each fixed by means of a rotatable body 16 and a screw fastening element 18, which are brought into engagement with the upper and lower horizontal flanges 20 of stiffening profiles which form the edges of the panels.

The panels themselves consist of a first, outer sheet metal tray 22, a second, inner sheet metal tray 24, a light structural core element 26, and the horizontal and vertical intermediate stiffening profiles 28 and 30. The horizontal flanges 20 are formed in one piece with the horizontal profiles 28. Another inseparable part of the profiles 28 and 30 are the outwardly open outer grooves 32 in which the filler element profiles, generally identified by the reference numeral 34, can be brought into engagement. These filler element profiles 34 consist of an elastic seal 36 and a rigid outer strip or body element 38, which is designed as a U-shaped sheet metal profile.

As is clear from Figures 2 to 5b, the fastening of the panels 14 to the supporting structure 10 by means of the rotatable body 16 and the fastening element 18 is based on the fact that the rotatable body 16 has two parallel grooves 40 on its rear side, which overlap the protruding edges 42 of the flanges 20.

The system is designed in such a way that the lateral cross-sectional dimensions of the grooves 40 exceed the cross-sectional width of the protruding edges 42 in order to achieve certain Dimensional tolerances and differences due to thermal expansion or contraction must be taken into account. The substantially round rotatable bodies 16 have a flattened side 44 which can be rotated into a position corresponding to the flange 20 in which the profile can be passed through and at the same time another profile is held during assembly. This feature is shown in particular in Figures 5a and 5b in which the screw fastening element and the supporting structure have been omitted for reasons of clarity.

Fig. 4 shows a detail of the cross section through a horizontal filling element, with the panels 14 and the filling element profiles 34 already properly mounted. The screw fastening element 18 used in the illustrated embodiment has a drilling end 46 and a self-tapping thread 48 in order to carry out several steps at the same time. However, it should be clear that a screw fastening element of any type can be used here, provided that its thread 48 is long enough to allow the body 16 to be loosened and rotated over the protruding edge 42 without the body becoming detached from the structure.

Fig. 4 also shows how the filling element profile 34 engages in the outward-facing grooves of the intermediate profiles 28. This system can be illustrated in Fig. 6, which shows a cross-section through a vertical filling element, as well as in Fig. 5c, which shows the actual filling element profile. From the comparison between Fig. 6 and Fig. 5, it is immediately clear that the panels are not attached to the vertical filling element itself.

The filling element profile 34 is pressed in from the outside of the supporting structure. The seal 36 and the rigid body 38 are so elastic that minor unevenness in the building, which can result in slight differences in the distance between the grooves 32 of adjacent panels, can be compensated for. As can be seen in Fig. 5c, the seal 36 has outward-facing side lips 50 that seal the first inward-facing edge flanges 52 of the outer troughs in a weatherproof manner. The sealing profile also has inward-facing lips 54 that grip behind the ribs 56 on the outward-opening grooves 32 and thus hold the filling element profiles 34 in their position.

Fig. 5c shows the filling element profile in its initial form (unloaded state). Fig. 6 shows a cross section through a vertical filling element, while Fig. 4 shows a cross section through a horizontal filling element. From Figs. 4 and 6 it can be seen that the horizontal and vertical panel edges are designed differently. It is clear that the outer sheet metal trays 22 on the horizontal panel edges, as shown in Fig. 4, have inward-facing edge flanges 52 with laterally protruding edge parts 58.

The horizontal intermediate profiles 28 carry inward-facing stop bodies in the form of a rib 60 in the outward-opening grooves 32, which overlap the laterally protruding edge parts 58 of the outer troughs and can thus hold them back. In this way, the curtain wall system can withstand even excessive wind forces, since the outer troughs are held firmly to the supporting structure. The connection between the outer trough and the supporting structure is therefore not subject to chemical damage. which would occur, for example, if the outer sheet were only held in place by an adhesive bond.

The inward-facing edge flanges 52 of the vertical panel edges are, as can be seen from Fig. 6, designed without laterally protruding edges.

Fig. 7 shows the panel in a slightly simplified exploded view. The core element 26, which covers and adhesively connects substantially all of the major portion of the interior surfaces of the inner and outer tubs, is not shown in this Fig. 7, but forms part of the assembled arrangement.

However, the rigidity of the assembled panel and the flatness of the relatively thin-walled outer surface are particularly dependent on a uniform connection to the core and on its structural rigidity. Fig. 7 is intended to illustrate the assembly of the inner and outer troughs as well as the horizontal and vertical intermediate stiffening profiles.

The horizontal and vertical intermediate profiles 28 and have essentially the same cross-section, but differ in their length and in the shape of their ends.

As can be seen from Fig. 7, each intermediate profile has an outer major surface 62 which, during assembly, engages the inner surface of the inwardly directed flanges 52 of the outer pan 22. In addition, each intermediate profile has an inner major surface 64 which, during assembly, engages the outer surface of a second inwardly directed edge flange 66 of the inner pan 24. The inner sheet metal pan 24 has a central part 25 whose peripheral shape is similar to that of the central part 23 of the outer sheet metal pan 22, but has a slightly smaller surface area so that the inwardly directed flanges 66 of the inner pan fit between the inwardly directed flanges 52 of the outer pan and still leave space for the intermediate profiles 28, 30.

The intermediate profiles 28,30 are each equipped with an inner groove 68, which is open in the direction of the inward-facing flanges 66 of the inner trough. Since the free ends of the inward-facing flanges of the inner and outer troughs each sit in a groove (e.g. 68, 32), the loads and forces acting on the inner and outer troughs can be transferred to the intermediate profiles and thus introduced into the supporting structure.

For safety reasons, especially in systems such as building curtain walls, which must be relatively maintenance-free, the connections between the system parts should not be based exclusively on adhesive, since such a connection can easily be weakened by negative chemical influences. With the building cladding panel according to the invention, a force-fitting mechanical fixation of all components is achieved here, once the assembly on the supporting structure has taken place as described above.

The order in which the panels are assembled is particularly important. First, one horizontal Intermediate profile 28 is mounted on the corresponding horizontal flange 52 of the outer trough 22. For this purpose, the lateral edge part 58 is hooked behind the rib 60 of the groove 32 by rotating the horizontal profile 28 about its longitudinal axis.

As can be seen from Fig. 7, the inner grooves 68 of the horizontal profile 28 are shortened in an area 70 compared to the rest of the profile. This recess 70 allows the abutting end of the inner groove 68 of the adjacent vertical profile 30 to be accommodated.

Once the vertical profiles 30 have been inserted between opposite horizontal profiles 28, the latter can no longer be detached from the edges 58. Once the horizontal and vertical intermediate profiles 28, 30 have been mounted on the outer shell, preparations can be made for installing a structural core (preferably made of a lightweight material) in the interior of the outer shell 22 defined by the intermediate profiles. This structural core, which is not itself shown in Fig. 7, can be of any suitable shape (e.g. prefabricated as a block or plate made of polystyrene or polyurethane foam, mineral wool or another suitable material, or as a prefabricated core with a cellular structure such as self-supporting honeycomb material or similar).

To complete this "sandwich" structure, the inner tub is inserted into the inner grooves 68 of the intermediate profiles so that the free-standing ends of its edge flanges 66 are opposite the free-standing ends of the edge flanges 52 of the outer tub. The The central surface 25 of the inner tub is glued to the core and thereby also connected to the main surface 23 of the outer tub, thereby holding the entire assembly together. Such a sandwich construction is very advantageous because it allows the production of very large panels without the use of thick sheet metal.

With large-area panels, the most important thing is usually their flatness, as a defect in this regard would destroy the visual appearance of the wall cladding. The sandwich panels according to the invention ensure such flatness, while at the same time simplifying assembly due to their light weight and reducing the costs of the supporting structure.

Fig. 8 also shows a detailed view of a corner connection between a vertical and a horizontal intermediate profile. In Fig. 9a and 9b, the different ends of the horizontal intermediate profile 28 and the vertical intermediate profile 30 are shown again, as they appear on the special (more complex) profiles of the preferred embodiment according to Fig. 1 to 6.

Fig. 10 shows a non-rectangular (in this example hexagonal) variation of the panel. The assembly structure of the panel is similar to the embodiment shown in Fig. 7, but here three profiles of the "horizontal" type and three profiles of the "vertical" type are arranged alternately along the six peripheral flanges of the panel.

Another alternative setup is shown in Fig. 11. The difference between the system shown here and The embodiment shown in Fig. 7 consists in that two opposite inwardly directed edge flanges 66 of the inner tub each have an inwardly directed edge part 72 which can be pre-assembled on the intermediate profile 30 with an inner groove 74 (which is very similar in shape to the outer groove 32).

This last modification naturally requires a minor adjustment of the assembly method in that the "vertical" profile type 30 is now assembled with the inner tub 24.

Figures 12 and 13 show a further embodiment, in which the fastening structure differs from the system shown in Figures 1 to 4. Figures 12 and 13 show a vertical and a horizontal cross-section through a vertical filling element. In this further embodiment, the vertical supporting structure part 110 coincides with the vertical filling elements. The panels are therefore not fastened to their horizontal filling elements, as in the embodiments described at the beginning, but to their vertical filling elements.

In addition to the use of other intermediate profiles 120, the inner and outer troughs of the panels are also rotated by 90º, so that the horizontal edges become the vertical ones and vice versa.

Fig. 12 shows that the side edge parts 58 as well as the ribs 160 are now arranged in the vertical filling element.

From Fig. 13, which shows a cross section along the line XIII - XIII in Fig. 12, it can be seen that the panels 115 are lowered onto the screws 115 for fastening. These screws 115 are surrounded by hook-shaped elements 117, which are connected to the panels 114 via enlarged head parts that fit into undercut grooves 121 in the intermediate profiles 120.

The hook-shaped elements 117 can be fixed in the undercut grooves 121 by a fastening element 123, as shown in Fig. 13.

In order to prevent rattling noises from the connection and to give it a tight fit, the screw 115 can be additionally equipped with an elastic element which has additional means 127 for snapping onto the screw 115 and which also snaps into place behind a locking step 129 on the hook-shaped element 117.

The supporting structure part 110 preferably has a top-hat rail cross-section with lateral surfaces 112 for supporting the rear sides of the panels.

An elastic cushion in the form of the strips 113 can be provided between the surfaces 112 and the inner end faces of the panel.

The embodiment shown in Figures 12 and 13 would generally be preferred in applications where rectangular panels are to be mounted with their long side in the vertical direction, whereas the embodiment Figures 1 to 4 should be preferred for horizontally aligned rectangular panels.

Figures 14 - 17 show a further embodiment of the invention, in which the intermediate profiles are essentially similar to those of Figures 1 - 4 and the assembly arrangement is essentially similar to that of Figures 12 and 13, but which otherwise has some modifications.

In detail, the supporting structure is very similar to the design shown in Fig. 12 and includes a transverse screw 215.

Two panels 214, each comprising a first trough 222, a second trough 224 and a core 226, are shown side by side (Fig. 15). They are mounted on the supporting structure by means of fastening elements 216, each of which has a body part 217, an elongated head part 218 and a recess 219 extending downwards (Fig. 16) into which the screw 215 can be hooked.

As can be seen from Fig. 15, the head part 218 is inserted into a grooved area 227 of the associated intermediate profile 228, which has inwardly directed beaded edges 229 for engagement under this head part 218.

The head part 218 has at least one threaded hole into which a fixing element 252 is screwed in such a way that it presses against the intermediate profile 228 when screwed in and thus prevents a relative movement between the intermediate profile and the fastening element 216. The threaded holes 250 of the two As can be seen in Fig. 16, fastening elements are arranged offset from one another in the longitudinal direction of the head part. This makes it easier to place a wrench on the fixing element. In this way, the actual height and position of each panel 214 can be adapted to any special requirements. Otherwise, this construction is similar to the embodiment shown in Fig. 4, although it can be seen that a filling element profile 234 and a seal 236 are also used here. The latter are removable so that the fixing element 252 can be tightened or loosened using a suitable wrench.

It is obvious that this type of fastening is not only suitable for the structure shown here, but can also be used for other panels, provided they have an edge profile that is designed in such a way that the head of the fastening element can be engaged and fixed in place.

Claims (12)

1. Polygonal building cladding panel, equipped with - a first sheet metal tray (22,222) with a first central part (23) which has a polygonal periphery and on each side of this polygonal periphery first inwardly directed edge flanges (52), - a second sheet metal tray (24,224) having a second central portion (25) similar in shape to the first tray but with a smaller area, and having on each side of its polygonal periphery second inwardly directed edge flanges (66) sized to closely fit along the first inwardly directed flanges (52) of the first tray, and - a structural core (26,226) which essentially fills a cavity formed by said first and second central parts (23,25) and their inwardly directed edge flanges (66), some of said first inwardly directed flanges having an edge portion (58) extending transversely to the associated flange (52) and substantially parallel to the central portion of said tub, and said first and second inwardly directed edge flanges (52, 66) are fixed by circumferential intermediate profiles (28,30,120,228) and positioned relative to one another, and wherein a laterally projecting stop body (60, 160) is arranged on these intermediate profiles, which laterally overlaps the said edge part (58) in such a way that the first trough (22,222) and the intermediate profile (28,30,120,228) are held in the desired position relative to one another. 2. Panel according to claim 1, wherein the first and second inwardly directed flanges (66, 52) are engaged both internally and externally by a portion of the intermediate profile (28, 30, 120, 228). 3. Panel according to claim 1 or 2, wherein the intermediate profile (28,30,120,228) has a part (42,229) on which a fastening element (16,117,21) for fastening the panel to a supporting structure (10,110,210) engages. 4. Panel according to claim 3, wherein said part (42,229) is designed such that when the fastening element is tightened it presses on the intermediate profile (28,30,120,228) such that the stop body (60,160) is pressed against the transversely projecting edge part (58). 5. A panel according to any preceding claim, wherein the edge portion (58) is formed only on alternating inwardly directed flanges (52) of the first tray (22,222). 6. Panel system that - a supporting structure (10,110,210), - several cladding panels (14,114,214), each consisting of * a first sheet metal tray (22,222) with a central part (23) which has a polygonal periphery and on at least one side of this polygonal periphery first, inwardly directed edge flanges (52), * a second sheet metal panel (24) with a second central part (25) which has a shape similar to the first panel and which has second inwardly directed edge flanges (66) on at least one side of its polygonal periphery, and * a structural core (26) which substantially fills a cavity formed by said second inwardly directed flanges, - circumferential intermediate profiles (28,30,120,228) which engage behind the first and second inwardly directed edge flanges (52,66) and position them relative to one another, whereby this intermediate profile (28,30,120,228) has a molded flange (20,121,227) with inwardly directed edges (42,229), and - comprises a plurality of fastening elements (16,117,217), each of which comprises a fastening body and a head part engaging behind said flange (40,119, 218), wherein this flange is displaceable relative to this head part and the fastening element has means attached to the supporting structure (10,110,210) and also fixing means (18,123,252) for locking the fastening element relative to this flange. 7. Cladding panel system according to claim 6, additionally equipped with a fixing means in the form of a screw fastening element (18) to which a rotatable body (16) similar to a washer is assigned. 8. Cladding panel system according to claim 7, wherein the rotatable body (16) designed like a washer has at least one groove (40) into which an outwardly directed edge bead (42,229) engages, which is formed on the said part of the intermediate profile (28,30,120,228), and the rotatable body has a flattened part (44) in particular on its circumference, so that the rotatable body can be rotated into a position in which it allows the problem-free attachment of a second panel next to an already attached panel, after which it is then rotated (e.g. by 90º) in order to also engage behind the relevant part of this second panel. 9. Cladding panel system according to claim 6, additionally equipped with a fastening element which can be fastened to the supporting structure (110, 210) and has a body (117, 217) which has a profiled head (119, 218) for engagement in a groove (121, 227) of the intermediate profile (28, 30, 121, 228). 10. A trim panel system according to claim 9, wherein said head part (218) has a bore (250) into which a fixing means (252) with a thread is inserted, which can be tightened so that it presses against said part (227) of the intermediate profile (228) and thus locks the intermediate profile relative to the fastening element. 11. A panel system according to claim 8 or 9, wherein the threaded holes (250) of two fastening elements are arranged offset from one another in the longitudinal direction of the head in order to facilitate the application of a wrench to the fixing means, so that the height and position of each panel (214) can be adapted to specific requirements. 12. Cladding panel system according to one of claims 6 to 11, further equipped with a filler strip (34,234) which can be inserted between two adjacent panels and is designed as a U-shaped sheet metal profile with a seal (36,236) running on each side of this U-shaped filler strip, which, when the filler strip is pressed into the space between two adjacent panels, firstly produces a weather-resistant seal between the opposite inward-facing flanges (52) and secondly has engagement parts which engage behind corresponding edges (56) on the two intermediate profiles which are assigned to these opposite inward-facing flanges.