[0001] The invention relates to a tongue and groove joint without a fastening element for
wall or ceiling panels of a building, said panels comprising a metallic outer surface
plate, a metallic inner surface plate arranged at a thickness from the outer surface
plate, and heat insulation therebetween; in said tongue and groove joint, the groove
is placed at the first edge of the panel and comprises a fold that is bent inwardly
from one of said edges of the surface plate in a second direction, and, as an extension
thereof, a first bend pointing in the opposite first direction inside said thickness;
and the tongue is placed at the other edge of the panel and comprises a bead strip
extending outwards, in said second direction of a surface plate, and as an extension
thereof, a second bend pointing in said first direction inside said thickness. In
particular, the invention relates to a fire resistant panel of this type.
[0002] Currently used seams, i.e. tongue and groove joints provided between wall or ceiling
panels made of metallic surface plates and insulation provided therebetween, are not
the best possible joints. One such tongue and groove joint between light panels is
described in the publication WO-01/77457. The panel described in said publication
comprises an upper and a lower metal or plastic plate, and therebetween, insulation
made of elastic material, such as mineral wool. In particular, the publication relates
to a hidden joint provided between the panels, by which joint the panels are attached
to the load-bearing structure. The joining to the load-bearing structure is carried
out by screw fasteners at the tongue provided at the panel edge, and through a plate-like
additional load distribution element arranged in between the screw head and the metal
or plastic plate constituting the tongue and groove. The groove edge of the adjacent
panel covers said fasteners and load-distributing elements. By this fastening to the
load-bearing structure at the tongue and groove, it is also attempted to prevent the
breaking up of the tongue and groove and hence the access of additional air in a fire
site. Although said fastening method might prevent larger motions of the tongue and
groove area in the case of a fire, the solution does not, however, prevent combustion
gases from infiltrating too fast into the panels and further through the tongue and
groove to the opposite side of the panels, which means that the fire resistant capacity
is lost. In comparison with said publication, the publications GB-2,142,670 and EP-1,260,647
describe somewhat more complicated tongue and groove joints, which are provided with
reinforcing additional folds of the plate, when compared with the tongue and groove
of the above mentioned WO publication. The tongue and groove of the GB publication
is designed as a snap-on joint, which means that the making of the tongue and groove
joint requires the use of considerable strength. The high rigidity of the elements
of the tongue and groove joint makes the installation difficult in other ways, too,
especially if the elements of the tongue and groove joint have formal defects. In
both publications, the gap between the tongue and groove plates inside the panels
opens directly up to the border surface of the insulations of the top and bottom panel,
in the GB publication even in the clear gap left between the insulations, and as a
result, in these tongue and groove joints combustion gases have an easy and rapid
access to infiltrate into the panels and further through the tongue and groove to
the opposite side, which means that the fire resistant capacity is lost. In the case
of the structure described in the GB publication, a non-desirable infiltration of
exhaust gases is speeded up by a large tubular sealing, which bums rapidly down leaving
a large space for the penetration of the combustion gases. Moreover, in the case of
a fire, the tongue and groove joints described in said publications become slack or
loose owing to the heat, thus making the situation even worse.
[0003] The object of the invention is to realize a tongue and groove joint of the type described
in the beginning of this specification, which joint would have a distinctly better
fire resistance than the prior art tongue and groove structures, and which joint would
also otherwise have better airtightness and watertightness than known tongue and groove
joints. Another object of the invention is to achieve said main objects without using
any fire resistant paint or other thermally expanding mass between the insulations
of the connected panels, and also without locking the tongue and groove joint with
screws or similar means. A third object of the invention is to achieve, for a tongue
and groove of this type, also a shape that does not result in mutual matching problems
when installing the panels in place, as is sometimes the case with known seam solutions.
[0004] The above described drawbacks are eliminated, and the above defined objects achieved
by means of a tongue and groove according to the invention, characterized by what
is set forth in the characterizing part of claim 1.
[0005] It has surprisingly been found that by designing in the tongue and groove joints
of wall or ceiling panels both the tongue section and the groove section, so that
they include folds and first bends as extensions of the surface plates, and in addition
by arranging, as extensions of said bends, another set of inner flanges, at the same
time as the tongue bend is preferably arranged on the opposite side of the bead strip
than the surface plates, there is achieved a shape-locked joint between the tongue
and the groove. The shape-locked joint that is achieved owing to these several plate
bends or plate folds, designed according to the invention, drastically improves the
sealing of the tongue and groove, particularly in the case of a fire. The reason for
this improvement is that in the tongue and groove joint, the combustion gases infiltrating
through the plate bends are in the case of a fire conducted towards the heat insulation,
and not directly to the seam joint of the insulations provided between the panels,
in which case also the insulation capacity remains good, in addition to the improved
sealing. We can speak of a three-step locking, where the vertical parts of the tongue
and groove, i.e. the parts that are essentially parallel with the panel, stick against
each other, and the innermost bend of the panel, pointing towards the inside of the
panel, secures the locking, which in the case of a fire is tightened more owing to
the expansion of the panels caused by the combustion heat, and owing to the deformations
caused by said expansion. In the tongue and groove joint, the same conducting of the
combustion gases works in both directions. In addition, the groove part is set precisely
and reliably, without wedging, in the tongue part, because the tongue and groove parts
do not include inclined surfaces. Thus the wall or ceiling surface formed of the panels
is smooth. When being locked, the panel surface plates cannot slide or slip apart.
When necessary, a double sealing can be utilized in this tongue and groove joint.
As for a flow of water, caused by rain and wind, that attempts to penetrate into the
tongue and groove joint, it can be prevented by a capillary groove or water-preventing
trough according to the invention. The capillary groove is invisible, hidden in the
vertical part of the tongue; owing to the expanded transversal surface, it prevents
the water from rising deeper in the tongue and groove joint, and is left invisible
after the installation of the next panel. The capillary groove can also be used as
an extra sealing in the tongue and groove joint, by extruding for instance sealing
mass therein, either in the factory or during the installation step at the building
site.
[0006] The invention is described in more detail below with reference to the appended drawings.
[0007] Fig. 1 illustrates a first embodiment of the tongue and groove according to the invention,
seen in a cross-section in perpendicular to the panel plane and to the length of the
tongue and groove joint.
[0008] Fig. 2 illustrates a second embodiment of the tongue and groove according to the invention,
seen in the same view as in figure 1, in a cross-section in perpendicular to the panel
plane and the length of the tongue and groove joint.
[0009] The figures illustrate a panel construction to be used as wall or ceiling panels
1 in a building. This kind of light ceiling or wall panels comprise a metallic outer
surface plate 3a, a metallic inner surface plate 3b placed at a thickness W1 thereof
and heat insulation 6 in between said two surfaces. For fire safety, the heat insulation
is generally mineral wool, but in some cases other heat insulations can also be used,
such as foam polymers, for instance foam polyurethane or the like. Each of two superimposed
wall panels 1a, 1b and respectively each of two adjacent ceiling panels 1a, 1b are
interconnected at the edges by means of a mutual tongue and groove joint 2 without
a fastening element. The fact that a fastening element is not provided means that
in between the groove 7 and the tongue 8, there are not provided any mutually fastening
elements, but there may be provided fasteners, such as screws or bolts, that penetrate
the tongue alone and attach it to structures, not illustrated in the drawings. In
the preferred embodiments illustrated in the drawings, not even these are provided,
but the panels are fastened by other means. The absence of a mutual fastening element
between the groove and the tongue enables a self-sealing capacity to be described
below. Owing to said lack of fastening elements, in practice - the attached drawings
representing, on the other hand, theoretical measures - at least one of the first
bends 9a or 9b according to the invention is in contact with at least one of the second
bends 10a or respectively 10b, i.e. the bend 9a is in contact with the bend 10a and/or
the bend 9b is in contact with the bend 10b. At least between said bends 9a, 9b, 10a,
10b and the plate curves 13a, 13b, 14a, 14b, 15a, 15b, 16a, 16b to be described below,
there is not provided any other material than the sealings 21 and 22. In most cases,
in similar fashion, at least one of the folds 4a or 4b is in contact with at least
one of the second bead strips 5a or respectively 5b. The groove 7 of the tongue and
groove joint is provided at the first edge of each panel and comprises a fold 4a or
4b, folded from said edge of a surface plate inwards in said panel 1a, towards the
tongue 8, i.e. in the second direction D2, and as an extension thereof, a first bend
9a, 9b pointed in the opposite first direction D1, i.e. outwards towards the edge
1a of said panel, inside the panel thickness W1. The tongue 8 of the tongue and groove
joint is located at the second edge of each panel, said edge being opposite to the
first edge, and comprises a bead strip 5a or 5b, extending outwards from said panel
1b of a surface plate, towards said edge, i.e. when the groove and tongue are mutually
attached, in said second direction D2, and as an extension thereof, a second bend
10a, 10b pointing in said first direction, i.e. in this case towards the panel 1b,
inside said panel and in the direction D1 towards said groove 7, inside said second
thickness W1. It is pointed out that said panels 1, of which in the above specification,
for the sake of clarity there are used individual reference numbers 1a and 1b, are
typically identical, containing a groove at one edge and a tongue at the other edge,
i.e. in general, there is used a common reference number 1. For special purposes,
there can naturally be made panels that are mutually different. If said panel 1 is
used as a wall panel or a ceiling panel installed in an inclined position, the tongue
8 is provided at the top edge of the panel, and the groove 7 is provided at the bottom
edge of the panel. Thus the first direction D1 points downwards, and the second direction
D2 points upwards. In any case, the directions D1 and D2 are parallel with the center
line connecting the panel edge on the tongue side and the panel edge on the groove
side. If the surface plates are in parallel, the directions D1, D2 are also parallel
with them.
[0010] According to the invention, the groove 7 comprises a fold 4a, 4b and a first bend
9a, 9b as an extension of both surface plates 3a and 3b, and the tongue 8 is provided
with a bead strip 5a, 5b and a second bend 10a, 10b as an extension of both surface
plates 3a and 3b, which thus means that the first tongue and groove section 31 located
on the side of the first surface plate 3a is essentially of the same type as the second
tongue and groove section 32 located on the side of the second surface plate 3b. The
first and second tongue and groove sections 31, 32 are not necessarily mutually identical
or located at precisely the same height in the directions D1, D2, but according to
the invention, they are in principle corresponding in shape, and contain sections
or shapes that are functionally of the same type. Thus for example the second tongue
and groove section 32 can be located upper, i.e. shifted in the direction D2, or lower,
shifted in the direction D1, the curvatures to be dealt with below can be of different
orders etc. However, it is most advantageous that the first and second tongue and
groove section 31, 32 are identical in measures, in which case the tongue and groove
joint 1 is mirror symmetrical with respect to the center plane that is parallel with
the panel surface plates. The bead strips 5a, 5b of the tongue 8 according to the
invention, as well as the folds 4a, 4b of the groove 7 arranged on both sides thereof,
can be essentially parallel with the surface plates 3a, 3b, as is depicted by unbroken
lines in the drawings, but especially the tongue bead strips 5a, 5b can also in the
second direction D2 approach each other, so that the bead strips form, with respect
to the direction of the surface plates, an angle K3 that is smaller than 10°, generally
not larger than 7° and typically of the order 5°, as is depicted by dotted lines in
figure 1. The groove folds 4a, 4b are generally parallel with the surface plates,
but in any case the folds form a smaller angle, when compared with the direction of
the surface plates, than the bead strips. Likewise, advantageously the first groove
bends 9a, 9b according to the invention and the second bends 10a, 10b of the tongue
8, arranged on both sides thereof, are essentially parallel with the surface plates
3a, 3b. In addition, according to the invention the groove 7 comprises, as extensions
of both of said first bends 9a, 9b, first inner flanges 11a, 11b pointing essentially
in said second direction D2, and the tongue 8 also comprises, as extensions of both
of said second bends 10a, 10b, second inner flanges 12a, 12b pointing essentially
in said second direction D2. With respect to the second direction D2, the first inner
flanges 11a, 11b form a first angle K1, and with respect to the second direction D2,
the second inner flanges 12a, 12b form a second angle K2, said angles K1, K2 being
at maximum 60°, preferably not larger than 45° and typically within the range 30°
- 0°, in which case the gap 26 between the first inner flanges 11a, 11b and the second
inner flanges 12a, 12b is oriented, direction D
F, away from the border surface formed by the edges 19a, 19b of the heat insulations.
In size, the second angle K2 can be equal to the first angle K1, but advantageously
the second angle K2 is larger, for example by 5° - 10° larger than the first angle
K1. Said second bend 10a, 10b of the tongue is placed on the opposite side of the
bead strip 5a and respectively 5b, than the outer surface plate 3a or respectively
the inner surface plate 3b.
[0011] In the groove 7, the distance W2 between the folds 4a, 4b is smaller than the thickness
W1 of the wall elements, and in addition the distance W2 of these folds 4a, 4b is
larger than the distance W3 of the bead strips 5a, 5b of the tongue 8, but by no more
than for the length of the first clearances P1. In similar fashion, in the tongue
8 the distances W4 of the second bends 10a, 10b are larger than the distances W5 off
the first bends 9a, 9b of the groove 7, but by no more than for the length of the
second clearances P2. Typically these second clearances P2 are smaller than the first
clearances P1, and further the second clearances P2 are at least at some point of
the distance between the first and second plate curves 13a, 13b, 14a, 14b on one hand,
and the distance between the third and fourth plate curves 15a, 15b, 16a, 16b on the
other hand, parallel with the surface plates 3a, 3b, preferably along the length H
X, i.e. at this point or length, the first and second bends 9a, 9b, 10a, 10b are in
parallel. By utilizing this configuration, there is obtained a self-sealing quality
for the tongue and groove joint 2 by means of thermal expansion during a fire, which
is further enhanced by the existence of the first and second plate curves 13a, 13,
14a, 14b located on both sides of the length H
X of the second clearance P2, oriented in the opposite directions and shapewise accurately
matching, as well as the existence of the shapewise accurately matching third and
fourth plate curves 15a, 15b, 16a, 16b. The mutual matching of said plate curves is
achieved so that the distance from the first plate curves to the third plate curves
is essentially equal to the distance from the second plate curves to the fourth plate
curves, and in a ready-installed tongue and groove joint 2, the first and second plate
curves 13a, 13, 14a, 14b mutually and the third and fourth plate curves 15a, 15b,
16a, 16b mutually are simultaneously positioned in corresponding positions, i.e. they
could, without the sealing materials 21, 22, get into contact with each other. Thus
the above described self-sealing is achieved both in the direction of the surfaces
of the panels 1 and in the directions of the panel thicknesses W1. The size of the
second clearance P2, as well as both gaps between the first and second plate curves
and the third and fourth plate curves, where the first and second sealing materials
21, 22 are located, are at the most three times the thickness S of the surface plates,
and advantageously at the most two times the thickness S of the surface plates, in
which case it should be observed that the surface plates may have different thicknesses.
Typically the size of the second clearance is within the range 0.5 - 1.2 times the
thickness of the surface plates, in test pieces about 0.85 times the thickness of
the plate. As for the first clearance P1, it is at least 1.2 times but not more than
three times the second clearance P2, preferably within the range 1.5 - 2.5 times the
second clearance. By means of the design and measures of the tongue and groove sections
according to the invention, the tongue and groove joint 2 is when installing and as
a ready-made structure fairly loose in a normal situation, but in the case of a fire
at least one of the second clearances P2, or at a later stage both second clearances
approach zero, thus closing the route of combustion gases. The surface plates 3a,
3b are curved as groove folds 4a, 4b in the semi-curves 17, and as tongue bead strips
5a, 5b in the opposite curve sections 18a and 18b, as is shown in the drawings. When
necessary, for example when the panel is used as an exterior wall panel, the tongue
8 according to the invention also comprises, at least in the bead strip 5a on the
side of the outer surface plate 3a, pointing in the other direction, a capillary trough
20 parallel with the tongue and groove, which capillary trough decreases the wind-oriented
tendency of rain water to penetrate into the tongue and groove joint.
[0012] The groove folds 4a, 4b and the first bends 9a, 9b are interconnected by first plate
curves 13a, 13b, and the bead strips 5a, 5b and the second bends 10a, 10b of the tongues
are interconnected by second plate curves 14a, 14b. The first plate curves 13a, 13b
have a first curvature R1, and the second plate curves 14a, 14b have a second curvature
R2, and the first curvature R1 is larger than the second curvature R2. The first curvature
R1 and the second curvature R2 are convex in the same direction, and especially in
the second direction D2, i.e. the first plate curves 13a, 13b and the second plate
curves 14a, 14b are all convex in the second direction D2 and concave in the opposite
first direction D1. Further, the third plate curves and the fourth plate curves are
roughly semicircles, in which case there must naturally be observed the angles K3
possibly formed by the bead strips 5a, 5b. Moreover, the first bends 9a, 9b and the
first inner flanges 11a, 11b of the groove are interconnected by third plate curves
15a, 15b, and the second bends 10a, 10b and the second inner flanges 12a, 12b of the
tongue are interconnected by fourth plate curves 16a, 16b. The third plate curves
15a, 15b have a third curvature R3, the fourth plate curves 16a, 16b have a fourth
curvature R4, and the third curvature is smaller than the fourth curvature. The third
curvature R3 and the fourth curvature R4 are convex in the same direction, and especially
in the first direction D1, i.e. the third plate curves 15a, 15b and the fourth plate
curves 16a, 16b are all convex in the first direction D1 and concave in the opposite
second direction D2. Further, the third plate curves and the fourth plate curves are
roughly semicircles, in which case - when there are taken into account the above described
first and second angles K1, K2 and the fact that the first and second bends 9a, 9b,
10a, 10b are at least roughly parallel with the surface plates 3a, 3b - the third
plate curves 15a, 15b form an arch that is at least 30°, preferably at least 45° and
typically 60° - 90°, and the fourth plate curves 16a, 16b respectively form an arch
that is at least 20° or 25°, preferably at least 35° or 40° and typically 50° - 80°
or 55° - 85°. The tongue and groove joint is sealed at least by a first sealing 21
that is arranged in between the above described first plate curves 13a, 13b and the
second plate curves 14a, 14b. Advantageously the tongue and groove joint also comprises
second sealings 22, which are arranged in between the third plate curves 15a, 15b
and the fourth plate curves 16a, 16b. The sealings 21 and 22 are made of a suitable
elastic sealing material. The edges 19a, 19b of the heat insulation 6 located between
the surface plates are placed, when observed in the directions of the panels or their
planes D1, D2, in the area of the third plate curves 15a, 15b and the fourth plate
curves 16a, 16b, i.e. in the thickness direction of the panels, between the opposite
fourth plate curves 16a and 16b, set against each other, so that they form a mutual
border surface 29, in which case the edges 23, 24 of the first inner flanges 11a,
11b and the second inner flanges 12a, 12b are located at the heights H' and H" from
the border surface formed by the heat insulation edges 19a, 19b, and on the same side
of said border surface 29, i.e. in the second direction D2 when observed from the
border surface. It is possible to place the border surface 29 also in the direction
of the panel thickness in between the opposite second inner flanges 12a and 12b, but
by no means as far as the edges 24 of the second inner flanges; in between said edges
24 and the border surface 29, there must be left clearly said first height H', the
size of which is at least half of the third curvature R3, more precisely of the radius
of curvature and preferably at least half of the radius of curvature corresponding
to the third curvature. The edges 23 of the first inner flanges are located at least
at the second height H", which in a preferred embodiment of the figures is at least
as large as the first height, i.e. H" ≧ H', but it may also be somewhat smaller than
the first height, i.e. H" ≲ H'.
[0013] The folds 4a, 4b of the groove, folded in the second direction D2, have a first height
H, and the bends 5a, 5b of the tongue, pointing in the second direction D2, have a
second height H2, said second height being larger than the first height. Moreover,
the first bends 9a, 9b of the groove, pointing in the first direction D1, have a third
height H3, and the second bends 10a, 10b of the tongue, pointing in the first direction
D1, have a fourth height H4, and the third height is equal to the fourth height or
larger or smaller than the fourth height for the length of the third clearance P3.
1. A tongue and groove joint (2) without fastening elements for wall or ceiling panels
of a building, said panels comprising a metallic outer surface plate (3a), a metallic
inner surface plate (3b) placed at a thickness (W1) of said outer surface plate, and
heat insulation (6) therebetween, where:
- the groove (7) is located at a first edge of the panel and comprises inside said
thickness a fold (4a or 4b) folded inwards from said edge of the surface plate, in
a second direction (D2), and as an extension thereof, a first bend (9a or resp. 9b),
bent in the opposite first direction (D1);
- the tongue (8) is located at a second edge of the panel and comprises inside said
thickness a bead strip (5a or 5b) of the surface plate extending outwardly in said
second direction (D2), and as an extension thereof, a second bend (10a or resp. 10b)
pointing in said opposite first direction (D1); and
- the groove (7) is provided with said fold and first bend as an extension of both
surface plates (3a and 3b), and the tongue (8) is provided with said bead strip and
second bend as an extension of both surface plates (3a and 3b),
characterized in that the groove (7) further comprises, as extensions of both of said first bends (9a,
9b), first inner flanges (11a, 11b) pointing substantially in said second direction
(D2), and that the tongue (8) further comprises, as extensions of both of said second
bends (10a, 10b), second inner flanges (12a, 12b) pointing substantially in said second
direction (D2), so that a gap (26) left between the first inner flanges (11a, 11b)
and the second inner flanges (12a, 12b) is in a direction (D
F) directed away from a border surface (29) formed by edges (19a, 19b) of the heat
insulations.
2. A tongue and groove joint according to claim 1, characterized in that a distance (W2) between said folds (4a, 4b) is smaller than the thickness (W1) of
the wall elements; and that the distance (W2) between said folds (4a, 4b) is at the
maximum by the size of first clearances (P1) larger than a distance (W3) between said
bead strips (5a, 5b).
3. A tongue and groove joint according to claim 1 or 2, characterized in that distances (W4) between the second bends (10a, 10b) are at the maximum by the size
of second clearances (P2) larger than distances (W5) between the first bends (9a,
9b).
4. A tongue and groove joint according to claim 1, characterized in that said folds (4a, 4b) and first bends (9a, 9b) are interconnected by first plate curves
(13a, 13b); and that said bead strips (5a, 5b) and second bends (10a, 10b) are interconnected
by second plate curves (14a, 14b).
5. A tongue and groove joint according to claim 1, characterized in that the first bends (9a, 9b) and the first inner flanges (11a, 11b) are interconnected
by third plate curves (15a, 15b); and that the second bends (10a, 10b) and the second
inner flanges (12a, 12b) are interconnected by fourth plate curves (16a, 16b).
6. A tongue and groove joint according to claim 4 or 5, characterized in that the first plate curves (13a, 13b) have a first curvature (R1) and the second plate
curves (14a, 14b) have a second curvature (R2); and that the first curvature is larger
than the second curvature.
7. A tongue and groove joint according to claim 4 or 5, characterized in that the third plate curves (15a, 15b) have a third curvature (R3), and the fourth plate
curves (16a, 16b) have a fourth curvature (R4); and that the third curvature is smaller
than the fourth curvature.
8. A tongue and groove joint according to claim 1, characterized in that it further comprises first sealings (21) between the first plate curves (13a, 13b)
and the second plate curves (14a, 14b).
9. A tongue and groove joint according to claim 8, characterized in that it further comprises second sealings (22) between the third plate curves (15a, 15b)
and the fourth plate curves (16a, 16b).
10. A tongue and groove joint according to claim 1, characterized in that said folds (4a, 4b) folded in the second direction (D2) have a first height (H1),
and said sections (5a, 5b) pointing in the second direction (D2) have a second height
(H2); and that the second height is larger than the first height.
11. A tongue and groove joint according to claim 1 or 10, characterized in that said first bends (9a, 9b) pointing in the first direction (D1) have a third height
(H3), and said second bends (10a, 10b) pointing in the first direction (D1) have a
fourth height (H4); and that the third height is equal to the fourth height, or by
the size of a third clearance (P3) larger or smaller than the fourth height.
12. A tongue and groove joint according to any of the preceding claims, characterized in that it further comprises a capillary trough (20) in parallel with the tongue and groove,
provided at least in the bead strip (5a) placed on the side of the outer surface plate
(3a) and pointing in the second direction.
13. A tongue and groove joint according to any of the preceding claims, characterized in that the tongue and groove joint (1) is mirror symmetrical with respect to a plane parallel
with the panel surface plates.
14. A tongue and groove joint according to claim 5, characterized in that the edges (19a, 19b) of the heat insulation (6) are located, in said directions (D1,
D2) of the panels, in a region of the third plate curves (15a, 15b) and the fourth
plate curves (16a, 16b).
15. A tongue and groove joint according to any of the preceding claims, characterized in that said second bend (10a, 10b) of the tongue is located on the opposite side of the
bead strip (5a and resp. 5b) than the outer surface plate (3a) or respectively the
inner surface plate (3b).