EP3744915A1

EP3744915A1 - Seismic constraint system, particularly for prefabricated panels and connection device concerning this system

Info

Publication number: EP3744915A1
Application number: EP20177230.8A
Authority: EP
Inventors: Luigia CECCARELLI
Original assignee: Ceccarelli Energia SNC Di Luigia Ceccarelli
Current assignee: Ceccarelli Energia SNC Di Luigia Ceccarelli
Priority date: 2019-05-31
Filing date: 2020-05-28
Publication date: 2020-12-02
Anticipated expiration: 2040-05-28
Also published as: EP3744915B1; EP3744915C0; IT201900007728A1

Abstract

Described is an earthquake protection system, particularly for prefabricated panels which comprises a device (1, 101, 201, 301) for connecting between two walls (21, 22) of a building: a slab (21) and a panel (22). The connecting device (1, 101, 201, 301) is designed to be moved at least along a direction of translation (10, 20, 110, 220, 320) by guide means (7, 8, 108, 207, 307) associated with at least one of the two walls (21, 22). The direction of translation (10, 20, 110, 220, 320) is substantially parallel to the plane in which at least one of the two walls (21, 22) lies, which has at least one degree of freedom.

Description

This invention relates to an earthquake protection system, particularly for prefabricated panels and to a connecting device between a slab and a panel of a building for the earthquake protection system.
It is known that the earthquake action can be broken down into two components substantially normal to each other: a first component substantially parallel to the surface of the walls consisting of infill panels for prefabricated buildings, and a second component, substantially perpendicular to the surface of the panels.
With regard to the earthquake action parallel to the surface of the panels, the walls, discharging their weight to the ground, have a considerable rigidity, so much so that it is essential to use connections which allow the structure to move independently of the panels which, on the other hand, remain fixed, without their mass moving.
On the other hand, the panels do not offer any rigidity to the earthquake action perpendicular to their surface and their connection to the structure must therefore be designed on the basis of their mass, with constraints which must be in relation to the design acceleration.
According to the prior art, the anchors tested and marketed comprise the making of an earthquake node which can translate both along the direction of propagation of the earthquake action parallel to the panel and along the normal direction by means of a double slider which engages, respectively, on a horizontal guide integrally associated with the slab, for example with screws and bolts and/or embedded in the concrete, and on a vertical guide similarly associated with the wall.
However, these devices have serious problems.
This type of panel is prepared in the factory, perforated according to the design, and then mounted on site. However, during installation, just a few millimetres of misalignment between the guides are sufficient, which occurs frequently for many reasons (even independent of the skills of the operators, such as, for example, the ground, the type of concrete used, the humidity of the air, etc.), to misalign the device and to causing jerking of the panel during the earthquake phase.
The articulated coupling, in fact, represents a fixed constraint and earthquakes - even relatively weak ones - can cause an oscillation even of tens of centimetres with disastrous consequences for the infill walls and consequent risks for the person in these buildings.
Moreover, as already mentioned, this type of earthquake fixing is difficult to position and to mount, but even more delicate and complex to replace or to restore, precisely because they require a complete integration (by embedding) inside the walls on which they are mounted.
According to an earthquake fixing device for prefabricated objects, which is currently on the market, a slider is simultaneously guided along the normal and along the direction parallel to the ground, by guides associated respectively with a wall and a slab of a same building.
In this case, the guides protrude from the slab.
When the earthquake force breaks down into the first and second components, respectively along the direction normal and parallel to the ground, these components are thus absorbed by translation of the slider on the guides.
This device, however, requires a direct and destructive intervention on the building, creating an "entry window" for the track on which the slider, by means of a runner, can slide along the normal to the slab; it is therefore necessary to form a housing space in the wall for positioning the track, with all the consequences in terms of costs and inconvenience.
The aim of the invention is to overcome the above-mentioned drawbacks of known types of earthquake protection system, particularly for prefabricated panels which allow a quick and easy installation, without the need to make an entry window for the slider.
In the context of the above-mentioned purpose, an aim of the invention is to provide an earthquake protection system, particularly for prefabricated panels, which - despite its simplified execution - guarantees excellent operation avoiding the risk of misalignments.
Another aim of the invention is that the system, according to the invention, offers excellent resistance also in the case of strong earthquake events.
Yet another aim of the invention is to provide an earthquake protection system, particularly for prefabricated panels, with means which are easily available on the market and using materials of common use, in such a way that the device is economically competitive.
This purpose, as well as these and other aims which will become clearer as this description continues, are achieved by an earthquake protection system, particularly for prefabricated panels, according to the invention, comprising:

a connecting device between two walls of a building: a slab and a panel;
said connecting device being designed to be moved at least along a direction of translation using guide means associated with said at least one wall;
the direction of translation being substantially parallel to the plane in which at least one of said two walls lies, which have at least one degree of freedom.

Advantageously, the connecting device is designed to be moved, by suitable guide means, both along a first direction of translation substantially parallel to the plane in which the slab lies (horizontal guide) and along a second direction of translation substantially parallel to the plane in which the panel lies (vertical guide).
In this case, the connecting device preferably comprises at least two sheet-like elements mutually coupled and which extend in a plane substantially parallel to the plane in which the slab lies. The two sheet-like elements are designed to be connected to at least one pair of plates, which can also be coupled to each other and which, on the other hand, extend in a plane substantially normal relative to the plane in which the panel lies. The sheet-like elements and the pair of plates have means for inserting inside the respective guide means.
In a second variant of the system, the connecting device is designed to be moved, again by guide means, along a first direction of translation substantially parallel to the plane in which the slab lies.
In this second scenario, the connecting device preferably comprises two sheet-like elements mutually coupled which extend in a plane substantially parallel to the plane in which the slab lies. The two sheet-like elements are also designed to be connected to a fixed point of the panel and have means for inserting inside the relative guide means.
In a third embodiment, the connecting device is designed to be moved, by suitable guide means, along a second direction of translation substantially parallel to the plane in which the panel lies.
In this system, the connecting device has at least one pair of plates mutually coupled, which extend in a plane substantially normal to the plane in which the panel lies and which are designed to be connected to a fixed point of the slab. The pair of plates also has means for inserting inside the guide means.
For this reason, the connecting device between a slab and a panel of a building for an earthquake protection system, particularly for prefabricated panels, which is also an object of the invention, comprises at least one between:

the at least two sheet-like elements: a lower element and an upper element mutually coupled;
the pair of plates: a right plate and a left plate which are also mutually coupled. The sheet-like elements and the pair of plates extend, respectively, in a plane substantially parallel to the plane in which the slab lies and in a plane substantially normal to the plane in which the panel lies. Moreover, the sheet-like elements and the pair of plates have means for inserting inside the respective guide means.

In this invention, the terms "upper" and "lower" refer to the position of the sheet-like elements relative to the slab (and therefore the sheet-like element will be considered as "lower" which on average has a smaller distance from the slab relative to the "upper" element).
Similarly, the terms "right" and "left" referred to the plates only indicate their position in space relative to the axis of symmetry of the vertical guide, so that the right plate will be the one shown to the right of the axis of symmetry and the left plate will be the one shown to the left of the same axis.
The Applicant has perceived that in order to avoid making an entry window for the connecting device, once the guides have been fixed to the slab and to the panel, where required, it would have been sufficient to subsequently insert, on site, the various components of the connecting device, inserting them in a snap-on fashion in the respective guides.
Advantageously, the insertion means comprise a hook formed in the end portion of the lower element and a protrusion shaped like the letter "S" on the end portion of the upper element, in such a way that, when the two sheet-like elements are moved towards each other during their engagement with the horizontal guide, the curved parts of the hook and of the protrusion are substantially coupled, that is to say, with the concavity oriented on the same side.
This configuration allows the simultaneous sliding of the upper and lower sheet-like element by engaging the first hook and the protrusion with the horizontal guide defined in the slab. The contextual nature of the movement is actuated by two plates which advantageously rise from the edges of the lower element and which define the lateral support of the upper element, in such a way that during the translation of the lower element, the upper element is dragged by the contact with the plates.
Similarly, the insertion means also advantageously consist of a second hook formed in the end portion of each of the plates of the pair of plates, in such a way that, during the movement of the plates towards each other in their engagement with the relative vertical guide, the curved parts of the second hooks are facing each other, with the concavities facing in the opposite direction, symmetrically along the direction of extension S of the plates.
In this case, too, similarly to what is stated above, the second hooks of the two right and left plates engage with the vertical guide defined in the panel. Advantageously, in the making of a system with two degrees of freedom, where the connecting device must necessarily move along the line parallel to the slab and to the panel, the two plates which rise from the lower plate are interposed in two pairs of plates each engaging in a respective vertical guide, by means of removable connections, such as bolts and nuts.
If, on the other hand, the system has a single degree of freedom, then the translation allowed to the connecting device is only one: along the line parallel to the slab or to the panel.
To perform the translation along the parallel to the slab, a fixed plate integral with a fixed point of the panel is, advantageously, interposed and fixed between the two sheet-like elements, upper and lower, by means of removable connections. The first hook and the protrusion of the sheet-like elements are, on the other hand, free to slide along the horizontal guide which engage.
Similarly, in order to perform the movement along the parallel to the panel, a fixed plate integral with a fixed point of the slab is advantageously interposed between the right and left plate and fixed by removable connections. Preferably, for reasons of symmetry, two pairs of fixing plates are used for two vertical plates; the two pairs of plates therefore allow, by means of the two hooks free to slide along the horizontal guide, the desired translation in the case of an earthquake event.
Further features and advantages of the invention are more apparent in the detailed description below, with reference to a preferred, non-limiting embodiment of the earthquake protection system, particularly for prefabricated panels, illustrated by way of example and without limiting the scope of the invention, with the aid of the accompanying drawings, in which:

Figure 1 shows a first embodiment of the protection system, according to the invention, wherein the connecting device 1 is shown in the cross section substantially normal to the plane in which the slab 21 lies and passing through the axis of symmetry M of the device 1;
Figure 2 shows the device 1 of Figure 1 raised, with a half in cross section according to section II-II and an enlarged detail of the cross section;
Figure 3 is a cross section substantially normal relative to the plane in which the slab 21 of a second variant of the protection system lies, with the connecting device 101;
Figure 4 is a cross section substantially parallel to the plane in which the panel 22 of a third alternative of the protection system lies, with the connecting device 201;
Figure 5 shows the system of Figure 4, again in a cross section substantially normal relative to the plane in which the slab 21 lies, wherein the connecting device 301 has a fold 305'b which allows the retaining and correct centring during assembly of the other left plate 305" and of the fixed plate 321;
Figure 6 shows a perspective view of the sheet-like element 2 equipped with a first hook 2a;
Figure 7 is a perspective view of the plate 5";
Figure 8 is a perspective view of the sheet-like element 3, with the protrusion 3a;
Figure 9 is a perspective view of the sheet-like element 102, with the protrusion 102a;
Figure 10 illustrates the insertion in the guide 7 and 107 of the sheet- like elements 2, 102 and 3, 103;
Figure 11 illustrates the insertion in the guide 8, 208 and 308 of the pairs of plates 5'-5, 205'-205" and 305'-305".

The above-mentioned drawings show a preferred embodiment of an earthquake protection system, particularly for prefabricated panels, according to the invention, which comprises a connecting device, defined in its entirety with the reference numerals 1, 101, 201 and 301, between two walls 21 and 22 of a building: a slab 21 and a panel 22.
To do this, the connecting device 1, 101, 201 or 301 is designed to be moved at least along a direction of translation 10, 20, 110, 220 or 320, substantially parallel to the plane in which the slab 21 and/or the panel 22 lies, by guide means 8, 7, 107 or 208: the horizontal guide 7 or 107 and/or the vertical guide 8 or 208 associated respectively with the slab 21 and the panel 22. The translation along the direction of translation 10, 20, 110, 220 or 320 depends on the degree of freedom of the walls 21 and 22; there are, therefore, three cases:

i) the connecting device 1 is designed to translate along the direction 10 and 20, engaging the horizontal guide 7 and the vertical guide 8 with double track, since both the slab 21 and the panel 22 have a degree of freedom (Figures 1 and 2);
ii) the connecting device 101 can only move along the direction 110 substantially parallel to the plane in which the slab 21 lies, engaging only the horizontal guide 107, due to the fact that only the panel 22 has a degree of freedom (Figure 3);
iii) the connecting device 201 (and 301) translates along the direction 220 (and 320) substantially parallel to the plane in which the panel 22 lies, engaging only the horizontal guide 208, because only the slab 21 has a degree of freedom (Figures 4 and 5).

In the first alternative embodiment, the connecting device 1 comprises at least two sheet- like elements 2 and 3, a lower element 2 and an upper element 3, which can be coupled to each other. The two sheet- like elements 2 and 3 both extend in a plane substantially parallel to the plane in which the slab 21 lies and can be connected to two pairs of plates 5' and 5" which are substantially identical, positioned symmetrically relative to the centre axis M of the device 1 (for obvious reasons of equilibrium).
The plates 5' and 5" are also mutually coupled and extend in a plane substantially normal to the plane in which the panel 22 lies.
With reference to Figure 2, the coupling of the two plates 5' and 5" to the sheet- like elements 2 and 3 occurs due to the fact that the element 2 has two plates 4 (Figure 6) substantially identical to each other and positioned symmetrically relative to the central axis M (again for reasons of equilibrium of the device 1). The plates 4 rise from the edges of the body 2b of the lower element 2 to define the lateral support of the upper element 3, in such a way that - during the translation of the lower element 2 -, the body 3b of the upper element 3 is dragged by the contact with the plates 4 (Figure 7). Each of the plates 4 is also fixed to the pair of plates 5' and 5" by means of a connection with a bolt-locknut 6 which remains on a double slot 7', 8' and 8" (defined respectively on the plate 4 and on the right plate 5' and left plate 5") to facilitate the adjustment in situ, eliminating the problem of misalignments.
Both the sheet- like elements 2 and 3 and the pairs of plates 5' and 5" have means for inserting inside the respective guide means 7 and 8.
The insertion means consist, as regards the lower element 2, of a first hook 2a (Figure 6) and for the plates 5' and 5" of second hooks 5'a and 5"a (Figure 7), all formed in the end portion of the lower element 2, the right plate 5' and the left plate 5".
The upper element 3, on the other hand, has an "S" protrusion 3a on its end portion (Figure 8), in such a way that, during the reciprocal movement of the two sheet- like elements 2 and 3 towards each other, when they engage the guide 7, the curved parts of the hook 2a and of the protrusion 3a are substantially coupled and have the concavity facing in the same direction, as shown in Figure 1.
With regard to the two plates 5' and 5", on the other hand, during their moving towards each other and engagement with the guide 8, the curved parts of these second hooks 5'a and 5"a face each other, symmetrically along the direction of extension S of the right 5' and left 5" plates and have the concavities facing in the opposite direction (see Figure 4).
An alternative is represented by the connecting device 101, which is designed to be moved, by guide means 107, along the direction of translation 110 substantially parallel to the plane in which the slab 21 lies. In this case, the device 101 has two sheet-like elements 102 (lower) and 103 (upper), mutually coupled and which extend in a plane substantially parallel to the plane in which the slab 21 lies. The two elements 102 and 103 can only move along the direction 110 because they are connected to a fixed point integral with the panel 22, made, for example, by a fixed plate 122 associated with the panel 22 (Figure 3).
Similarly to what has been seen for the elements 2 and 3, the lower element 102 and the upper element 103 are also equipped with a first hook 102a and a protrusion 103a for inserting inside the horizontal guide 107; the lower element 102 is also illustrated in Figure 9 where the slot 108' (defined in the body 102b) is also visible, whilst the upper element 103 is identical to that shown in Figure 8 and also provided with a hole or an upper slot for the passage of the bolt-locknut 106 (Fig. 3) for blocking the "upper element 103-fixed plate 122-lower element 102".
Certainly, to guarantee the correct passage of the shank of the bolt 106, a hole or a slot will also be present in the fixed plate 122.
In exactly the same way as in the first embodiment of Figures 1 and 2, when the hook 102a and the protrusion 103a engage the guide 7, the elements 102 and 103 have their curved parts substantially coupled and with the concavities facing in the same direction.
In the third variant embodiment, the connecting device 201 (or 301) is designed to be moved, by means of the vertical guide 208 along the second direction of translation 210 (310) substantially parallel to the plane in which the panel 22 lies.
The device 201 (301) has two pairs of plates 205' and 205" (305' and 305"), even though Figures 4 and 5 show only one pair for reasons of symmetry, which extend on a plane substantially normal to the plane in which the panel 22 lies.
The plates 205' and 205" (305' and 305") are coupled to each other, connecting to a fixed point integral with the slab 21.
As shown in Figure 4, the fixing to the slab 21 is guaranteed by a fixed plate 221 interposed between the plates 205' and 205" and fixed to it by a bolt-locknut 206 which acts on slots or holes defined in the right plate 205', in the left plate 205" and, obviously, also at the fixed plate 221.
The fixed plate 321, with reference to the following drawing, is similarly interposed with the plates 305' and 305", where the right plate 305' has a fold 305'b in the end portion opposite the one where the hook for engaging with the respective guide is formed; the fold 305'b allows the retaining and correct centring of the other left plate 305" as well as of the fixed plate 321 during assembly (Figure 5).
In this third embodiment, too, a bolt-locknut system 306 allows the slab 21 to be integrally connected to the device 301.
In line with what has been described up to now, the plates 205' and 205" (and also the plates 305' and 305", provided they are not illustrated) have means for inserting in the guide 208, which comprise a second hook 205'a and 205"a respectively formed in the end portion of each plate 205' and 205". In this way, therefore, during their engagement with the guide 208, the curved parts of the second hooks 205'a and 205"a face each other, in a symmetrical manner, along the direction of extension S of the plates 205' and 205" (Figure 4).
The insertion of the sheet- like elements 2, 3, 102 and 103 in the respective guides 7 and 107 is schematically illustrated in Figure 10, where the various steps of inserting the upper element 3 and 103 are shown with different lines: in the first step, the head element 3 (103) is inserted, that is to say, with the protrusion 3a (103a) facing towards the bottom of the guide 8 (108) and it then rotates in the direction of the arrow (anticlockwise) until the protrusion 3a (103a) remains imprisoned inside the guide 8 (108), between the walls of the guide 8 (108) and the lower element 2 (102).
The element 2 (102), even if not shown, is inserted before the element 3 (103), again at the head, introducing the first hook 2a (102a) inside the guide 8 (108) and rotating it in a clockwise direction.
Figure 11, on the other hand, shows the insertion of the plates 5', 5", 205', 205", 305' and 305": each plate 5', 5", 205', 205", 305' and 305" is always introduced at the head, that is to say, on the side of the second hook 5'a, 5'a, 5"a, 205'a, 205"a, 305'a and 305" and then rotated towards the outside of the guide 7, 207, 307 relative to the axis of symmetry "S" in such as to imprison the hooks 5'a, 5'a, 5"a, 205'a, 205"a, 305'a and 305"a between a wall of the guide 7, 207, 307 and the head of the other plate 5', 5", 205', 205", 305' and 305".
From the above description it may be seen how the invention achieves the preset purpose and aims and in particular the fact that an earthquake protection system is made, particularly for prefabricated panels, to be installed quickly and easily, without ruining walls to make an entry window for the slider.
In particular, the insertion in situ of sheet-like elements and plates allows, on the one hand, an extremely simplified assembly and, on the other hand, the guarantee of excellent quality at low cost, since the components are made in a workshop and then installed on site.
Another advantage of the invention is due to the fact that the adjustment means by means of the slot or the double slot allow the adjustment of the coplanarity between the guides and the walls on site, eliminating any risk of misalignment.
Another advantage of the connecting device, according to the invention, is due to the fact that the sheet-like elements and the plates are obtained by means of simple machining operations, thus guaranteeing a considerable economic advantage.
Another advantage of the connecting device between a slab and a panel of a building for an earthquake protection system, particularly for prefabricated panels, is that, due to the fact that, providing the guide with a engagement dimension calibrated as necessary, theoretically as desired, the device offers excellent resistance even in the case of strong earthquake events. In effect, the portion of plate or sheet-like element to be engaged with the vertical or horizontal guide is at the discretion of the construction design and will be advantageously sized during the design step taking into account this feature.
Lastly, the use of means which are easily available on the market and the use of common materials makes the device economically competitive. The invention can be modified and adapted in several ways without thereby departing from the scope of the inventive concept.
Moreover, all the details of the invention may be substituted by other technically equivalent elements.
In practice, the materials used, as well as the dimensions, may be of any type, depending on requirements, provided that they are consistent with their production purposes.

Claims

An earthquake protection system, particularly for prefabricated panels, comprising:
a connecting device (1, 101, 201, 301) between two walls (21, 22) of a building: a slab (21) and a panel (22);

said connecting device (1, 101, 201, 301) being designed to be moved at least along a direction of translation (10, 20, 110, 220, 320) by guide means (7, 8, 108, 207, 307) associated with said at least one wall (21, 22);

said direction of translation (10, 20, 110, 220, 320) being substantially parallel to the plane in which at least one of said two walls (21, 22) lies, which have at least one degree of freedom.
The system according to claim 1, wherein said connecting device (1) is designed to be moved, by guide means (7, 8), both along a first direction of translation (10) substantially parallel to the plane in which said slab (21) lies and along a second direction of translation (20) substantially parallel to the plane in which said panel (22) lies.
The system according to claim 2, wherein said connecting device (1) comprises at least two sheet-like elements (2, 3) mutually coupled and extending in a plane substantially parallel to the plane in which said slab (21) lies, designed to be connected to at least one pair of plates (5', 5"), which can also be coupled to each other and extending in a plane substantially normal to the plane in which said panel (22) lies;
said sheet-like elements (2, 3) and said pair of plates (5', 5") having means (2a, 3a) for inserting inside the respective guide means (7, 8).
The system according to claim 2, wherein said connection device (101) is designed to be moved, by guide means (108) along a first direction of translation (110) substantially parallel to the plane in which the slab (21) lies.
The system according to claim 3, wherein the connecting device (101) comprises two sheet-like elements (102, 103) mutually coupled and extending in a plane substantially parallel to the plane in which said slab (21) lies, designed to be connected to a fixed point integral with said panel (22);
said sheet-like elements (102, 103) having means (102a, 103a) for inserting inside said guide means (108).
The system according to claim 2, wherein said connecting device (201, 301) is designed to be moved, by guide means (207, 307) along a second direction of translation (210, 310) substantially parallel to the plane in which said panel (22) lies.
The system according to claim 6, wherein said connecting device (201, 301) comprises at least one pair of plates (205', 205", 305', 305") mutually coupled, extending in a plane substantially normal relative to the plane in which said panel (22) lies and designed to be connected to a fixed point integral with said slab (21);
said at least one pair of plates (205', 205", 305', 305") having insertion means (205'a, 205"a, 305'a, 305"a) inside said guide means (207, 307).
A connecting device (1, 101, 201, 301) between a slab (21) and a panel (22) of a building for an earthquake protection system, particularly for prefabricated panels, comprising at least one between:
at least two sheet-like elements (2, 3, 102, 103): a lower element (2, 102) and an upper element (3, 103) mutually coupled;

at least one pair of plates (5', 5", 205', 205", 305', 305"): a right plate (5', 205', 305') and a left plate (5", 205", 305") which can also be mutually coupled;

said at least two sheet-like elements (2, 3) and said at least one pair of plates (5', 5", 205', 205", 305', 305") extending, respectively, in a plane substantially parallel to the plane in which said slab (21) lies and in a plane substantially normal to the plane in which said panel (22) lies;

and said at least two sheet-like elements (2, 3, 102, 103) and said at least one pair of plates (5', 5", 205', 205", 305', 305") having insertion means (2a, 3a, 102a, 103a, 5'a, 5'a, 205'a, 205"a, 305'a, 305"a) inside respective guide means (7, 8, 108, 207, 307).
The device (1, 101) according to claim 8, wherein said insertion means (2a, 3a, 102a, 103a) consist of a hook (2a, 102a) formed in the end portion of said lower element (2, 102) and a protrusion (3a, 103a) shaped in the form of an "S" on the end portion of said upper element (3, 103), in such a way that, when said at least two sheet-like elements (2, 3, 102, 103) are moved towards each other during their engagement with said guide means (8, 108), the curved parts of said hook (2a, 102a) and of said protrusion (3a, 103a) are substantially coupled.
The device (1, 201, 301) according to claim 8, wherein said inserting means (5'a, 5"a, 205'a, 205"a, 305'a, 305"a) comprise a second hook (5'a, 5'a, 205'a, 205"a, 305'a, 305"a) formed in the end portion of each of the plates of said at least one pair of plates (5', 5", 205', 205", 305', 305"), in such a way that, when the plates (5', 5", 205', 205", 305', 305") are moved towards each other during their engagement with said guide means (7, 207, 307), the curved parts of said second hooks (5'a, 5"a, 205'a, 205"a, 305'a, 305"a) face each other symmetrically along the direction of exension (S) of said plates (5', 5", 205', 205", 305', 305").