JP7223668B2 - large space structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a large-space structure provided in a sports stadium or the like.

A strung beam structure, a truss structure, and the like are often adopted for large-space structures that require a large internal space, such as sports facilities such as baseball fields, soccer fields, and skating rinks, convention halls, and warehouses. For example, a method for constructing a large-space structure using truss beams in which upper chord members and lower chord members are assembled with diagonal members has been proposed (Patent Document 1).

On the other hand, a large space structure using a folded plate structure of RC slabs has also been put into practical use (Non-Patent Document 1). A large space structure using such a folded plate structure has a novel appearance while realizing a rigid frame by joining planes that are not parallel to each other.

JP-A-7-109771

http://www. takasaki-foundation. or. jp/center/hall. html

However, in the folded plate structure of Non-Patent Document 1, the weight of the RC slab increases and the out-of-plane deformation of the plate increases. In addition, it is necessary to strictly manage the construction accuracy of formwork construction.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a large-space structure that is excellent in rigidity against vertical and horizontal loads and that can be lightened.

The present invention has been made to solve at least part of the above problems, and can be implemented as the following aspects or application examples.

[1] One aspect of the large-space structure according to the present invention is
A large space structure in which a plurality of units are integrally and continuously formed in the short direction of the unit,
Said unit is
a top connection;
two bottom connections that are below the top connection and are spaced apart from each other;
two first valley beams respectively connecting the top connecting portion and the two bottom connecting portions;
four intermediate connections located at a height between the top connection portion and the bottom connection portion and at positions corresponding to the vertices of an imaginary rectangle centered on the top connection portion in a plan view;
four first edge beams extending from the top connection to four of the intermediate connections;
four second edge beams extending from each of the intermediate connections to the bottom connection;
with
the adjacent units are united at the intermediate connection,
To each of the bottom connection portions, the lower end of one of the first valley beams and the lower ends of two of the second ridge beams arranged at equal angles across the first valley beam are fixed. is,
The first valley beam is located inside an imaginary line that connects the two intermediate connection portions arranged on both sides of the first valley beam.

According to one aspect of the large-space structure, by combining two types of frame structures, namely, a frame structure with two first valley beams and a frame structure with a first ridge beam and a second ridge beam, vertical load is and excellent rigidity against horizontal load. Further, according to one aspect of the large-space structure, by adopting the first valley beam, the first ridge beam, and the second ridge beam, the weight can be reduced compared to a folded plate structure using an RC slab. can be done.

[2] In one aspect of the large space structure,
A tie beam may be further provided that connects the two bottom connection portions that are arranged to face each other.

According to one aspect of the large space structure, the tie beam suppresses the deformation acting in the direction in which the two bottom connection portions separate, that is, in the direction in which the lower ends of the two first valley line beams separate, thereby increasing the vertical load. It is possible to increase the rigidity of the spatial structure.

[3] In one aspect of the large space structure,
The first edge beam and the second edge beam are steel pipes,
The first valley beam may be H-shaped steel.

According to one aspect of the large space structure, weight can be reduced by using steel pipes for the first ridge beam and the second ridge beam, and high rigidity can be achieved by using H-shaped steel for the first valley beam. be prepared.

[4] In one aspect of the large space structure,
The large space structure has five units that are continuously continuous,
The top connections of the units at both ends may be lower than the top connections of the central unit.

According to one aspect of the large space structure, by providing the units at both ends lower than the center, the force of each unit trying to expand in the horizontal direction due to the vertical load is balanced, so that the vertical direction of the central top connection portion The amount of deformation becomes smaller.

According to the large-space structure according to the present invention, the rigidity against vertical load and horizontal load is excellent, and the weight can be reduced.

1 is a perspective view of a large space structure according to one embodiment; FIG. 1 is a front view of a large space structure according to one embodiment; FIG. 1 is a plan view of a large space structure according to one embodiment; FIG. It is an AA cross-sectional view of a large space structure according to one embodiment. It is a front view explaining the amount of deformation of a structure with large space due to a vertical load. 1 is a perspective view of a dome structure; FIG. 1 is a front view of a dome structure; FIG. It is a BB sectional view of the dome structure.

Preferred embodiments of the present invention will be described in detail below with reference to the drawings. It should be noted that the embodiments described below do not unduly limit the scope of the invention described in the claims. Moreover, not all the configurations described below are essential constituent elements of the present invention.

A large-space structure according to an embodiment of the present invention is a large-space structure in which a plurality of units are integrally and continuously formed in the width direction of the unit, and the unit includes a top connecting portion, two bottom connection portions located below the top connection portion and opposed to each other with a gap therebetween; and two first valley line beams connecting the top connection portion and the two bottom connection portions, respectively; four intermediate joints located at a height between the top joint and the bottom joint and at vertices of an imaginary rectangle centered on the top joint in a plan view; and the top. comprising four first edge beams extending from a connection to four said intermediate connections and four second edge beams extending from each of said intermediate connections to said bottom connection, said adjacent units comprising: Each of the bottom connection portions is integrated at the intermediate connection portion, and each of the bottom connection portions is connected to the lower end of one of the first valley beams and the two of the second valley beams arranged at equal angles across the first valley beam. The lower end of the ridge line beam is fixed, and the first valley line beam is located inside an imaginary line connecting the two intermediate connection portions arranged on both sides of the first valley line beam.

1. Large Spatial Structure A large spatial structure 1 according to one embodiment will be described in detail with reference to FIGS. 1 to 5. FIG. FIG. 1 is a perspective view of a large spatial structure 1 according to one embodiment, FIG. 2 is a front view of the large spatial structure 1 according to one embodiment, and FIG. 3 is a large spatial structure according to one embodiment. 1, FIG. 4 is a cross-sectional view taken along the line AA in FIG. 1 of the large-scale structure 1 according to one embodiment, and FIG. It is a diagram. Here, FIG. 3 (a) shows the entire large-space structure 1, and (b) shows only the first unit 11. As shown in FIG. The large spatial structure 1 is constructed on the XY plane, the longitudinal direction of the large spatial structure 1 is the direction along the Y axis, and the short direction of the large spatial structure 1 is the direction along the X axis. direction, and the height direction of the large spatial structure 1 is the direction along the Z axis.

As shown in FIGS. 1 to 3, the large space structure 1 is constructed by integrally connecting a plurality of units (11 to 15) in the short direction of the units (11 to 15). Here, the "transverse direction" is the direction along the Y-axis in FIGS. The large spatial structure 1 according to the present embodiment includes five units, a first unit 11, a second unit 12, a third unit 13, a fourth unit 14, and a fifth unit 15, which are integrally continuous. The number of units is not limited to five, and can be set according to the use of the structure. For example, an odd number of units is preferable in order to suppress deformation under vertical load.

The large-space structure 1 is preferably composed of a plurality of types of units. The large-space structure 1 is composed of, for example, a combination of three types of units. The second unit 12 and the third unit 13 are units of the same type, and the fourth unit 14 and the fifth unit 15 are units of the same type that are symmetrical with respect to the plane along the lateral direction. The first unit 11 is a different type of unit from the second to fifth units 12 to 15. The length of the first edge beam 31 is longer than the second to fifth units 12 to 15, and the overall height of the unit is also higher. . The second unit 12 and the third unit 13 are units of a different type from the fourth unit 14 and the fifth unit 15, the length of the first ridgeline beam 31 is longer than the fourth unit 14 and the fifth unit 15, The total height of the unit is also high. By combining these three types of units in a well-balanced manner, excellent rigidity is provided against vertical and horizontal loads.

A large space structure 1 is constructed on a plurality of foundations. The large-space structure 1 includes two first foundations 51 and second foundations 52 spaced apart in the direction along the X-axis and extending in the direction along the Y-axis. The large-space structure 1 further comprises two third foundations 53 and may further comprise twelve fourth foundations 54, for example.

1-1. Unit As shown in FIGS. 1 to 3, the first unit 11, the second unit 12, the third unit 13, the fourth unit 14, and the fifth unit 15 have different beam lengths, etc., but have basic configurations. are the same, the first unit 11 located in the center of the large spatial structure 1 in the longitudinal direction will be mainly described here.

The first unit 11 is a unit of a framework formed between the bottom connection part 22 fixed to the first foundation 51 and the bottom connection part 22 fixed to the second foundation, with the top connection part 20 as the center. be. The first unit 11 includes a top connection part 20, two bottom connection parts 22 located below the top connection part 20 and opposed to each other with a space therebetween, and between the top connection part 20 and the bottom connection part 22. and four intermediate connections (first intermediate connection 24 to fourth intermediate connection 27) located at the vertices of an imaginary rectangle 23 centered on the top connection 20 in plan view And prepare. The imaginary rectangle 23 is a hatched area enclosed by a dashed line in FIG. 3(a).

Adjacent units (11-15) come together at intermediate connections (24-27). The first unit 11 is integrated with the second unit 12 at the second intermediate connection portion 25 and the fourth intermediate connection portion 27, and is integrated with the third unit 13 at the first intermediate connection portion 24 and the third intermediate connection portion 26. . Specifically, the first edge beam 31 and the second edge beam 32 are fixed at the first intermediate connection portion 24 of the first unit 11, and the first edge beam 31 and the second edge beam of the second unit 12 are fixed. It also functions as the second intermediate connection portion 25 of the second unit 12 . Other intermediate connecting portions in the first unit 11 also function as intermediate connecting portions for adjacent units, thereby being integrated with the adjacent units.

The first intermediate connection portion 24 , the second intermediate connection portion, the third intermediate connection portion, and the fourth intermediate connection portion 27 have the same height from the bottom connection portion 22 in the Z-axis direction. Each of the first intermediate connection portion 24 to the fourth intermediate connection portion 27 may be fixed to the upper end of the column 56 . When the pillars 56 are provided, the lower ends of the pillars 56 are fixed to four independent fourth foundations 54, and the pillars 56 extend along the Z-axis direction to support the intermediate connection portions (24 to 27). It is possible. Note that even without the pillar 56, the first intermediate connection portion 24 to the fourth intermediate connection portion 27 can be arranged at predetermined positions due to the balance of the beams.

The first unit 11 includes a ridge line frame 30 forming the ridge line of the mountain when the large spatial structure 1 is regarded as a mountain, and a valley line frame 40 similarly forming the trough line of the mountain. In FIG. 3B, the ridge frame 30 and the valley frame 40 are shown by omitting other configurations in order to facilitate understanding of the configurations of the ridge frame 30 and the valley frame 40 . The ridge line frame 30 and the valley line frame 40 are line-symmetrical with respect to the top connecting portion 20 and plane-symmetrical with respect to the YZ plane passing through the top connecting portion 20 .

The ridgeline frame 30 includes four first ridgeline beams 31 extending from the top connection portion 20 to four intermediate connection portions (first intermediate connection portion 24 to fourth intermediate connection portion 27), and each intermediate connection portion (first intermediate connection portion 24 to fourth intermediate connection portion 27). Four second ridgeline beams 32 extending from the connecting portion 24 to the fourth intermediate connecting portion 27) to the bottom connecting portion 22 are provided. The first ridgeline beam 31 and the second ridgeline beam 32 can be made of known structural materials for construction, such as steel pipes. Weight can be reduced by using steel pipes for the first edge beam 31 and the second edge beam 32 . Since the ridgeline frame 30 can disperse the load on the large-space structure 1 by the valleyline frame 40, it is possible to reduce the weight.

The valley line frame 40 includes two first valley line beams 41 connecting the top connecting portion 20 and the two bottom connecting portions 22 respectively. The first valley beam 41 is indicated by a dashed line. One of the first valley beams 41 is located inside an imaginary line that connects the first intermediate connection portion 24 and the second intermediate connection portion 25 that are arranged with the first valley beam 41 therebetween. The valley beam 41 is located inside an imaginary line that connects the third intermediate connection portion 26 and the fourth intermediate connection portion 27 arranged with the first valley beam 41 interposed therebetween. Moreover, as shown in FIG. 4, the first valley beam 41 bends upwards less than the first ridge beam 31 and the second ridge beam 32, and the imaginary straight line connecting the top connection portion 20 and the bottom connection portion 22 Because it is close to , it has excellent rigidity against vertical load. Therefore, it is preferable to use a structural material having higher rigidity than that of the first ridge beam 31 for the first valley beam 41, so that the load on the first ridge beam 31 can be transferred to the first valley beam 31 by a small beam 38, which will be described later, for example. The wire beam 41 can be made to bear. The first valley beam 41 can be made of a known architectural structural material, such as H-shaped steel. High rigidity can be provided by using H-shaped steel for the first valley beam 41 .

Each bottom connecting portion 22 has a lower end of one first valley beam 41 and lower ends of two second ridge beams 32 arranged at an equal angle α with the first valley beam 41 interposed therebetween. Fixed. To the top connection portion 20, the upper ends of four first ridge beams 31, the upper ends of two first valley beams 41, and the upper ends of two third ridge beams 33, which will be described later, are fixed. Fixing of the beams at each connecting portion may be performed by directly connecting the beams using a known connection method, or by using metal fittings.

The top connection portion 20, the bottom connection portion 22, and the first intermediate connection portion 24 to the fourth intermediate connection portion 27 may include joint members that connect the beams constituting the ridge line frame 30 and the valley line frame 40. FIG. The joining members are, for example, cast steel hardware or forged steel hardware for fixing beams of each shape by welding or bolts. The connecting member in the top connecting portion 20 includes connecting rods (not shown) that connect to the eight beams, and the connecting rods extend radially from the top to form a substantially "rice" shape in plan view.

According to the large-space structure 1, by combining two types of frame structures, namely, the frame structure of the two first valley beams 41 and the frame structure of the first edge beams 31 and the second edge beams 32, the vertical load is and excellent rigidity against horizontal load. Further, according to the large-space structure 1, by adopting the first valley beam 41, the first ridge beam 31, and the second ridge beam 32, the weight can be reduced compared to the folded plate structure using RC slabs. can be done.

As shown in FIG. 4 , the first valley beam 41 bends slightly upward from an imaginary straight line connecting the top connecting portion 20 and the bottom connecting portion 22 . The first valley beam 41 may be curved upward, for example. The first valley line beam 41 that bends or curves upward can exhibit high rigidity as the valley line frame 40 mainly against vertical loads. The bent portion of the first valley beam 41 and the first intermediate connection portion 24 and the second intermediate connection portion 25 are connected by two second valley beams 42 . The second valley beam 42 may be, for example, a steel pipe.

A third valley beam 43 connects between the first intermediate connection portion 24 and the third intermediate connection portion 26 and between the second intermediate connection portion 25 and the fourth intermediate connection portion 27, respectively. The third valley beam 43 is arranged one by one between each unit. The third valley beam 43 is indicated by a dashed line. The third valley beam 43 may be, for example, a steel pipe. One of the third valley beams 43 bends slightly upward from an imaginary straight line connecting the first intermediate connection portion 24 and the third intermediate connection portion 26 . The other third valley beam 43 bends slightly upward from the imaginary straight line connecting the second intermediate connection portion 25 and the fourth intermediate connection portion 27 . The third valley beam 43 may be curved upward, for example. By bending or curving the third valley beam 43 upward, when a roof is provided along each frame, it acts as a rain gutter along the third valley beam 43 to efficiently drain rainwater. . In addition, even when snow is accumulated, the snow can be efficiently dropped along the third valley beam 43. For example, if a heating wire is arranged along the third valley beam 43, the snow can be efficiently melted to the roof. of snow can be prevented.

The bent portion of the third valley beam 43 and the top connecting portion 20 are connected by the third ridge beam 33 . The third edge beam 33 is indicated by a dashed line. The third edge beam 33 may be, for example, a steel pipe.

Next, the 2nd unit 12 and the 3rd unit 13 are demonstrated. The second unit 12 has the same shape and structure as the third unit 13 . The second unit 12 and the third unit 13 are arranged so that the top connection part 20 is set at a position lower than the top connection part 20 than the first unit 11, the second edge line beam 31, the second edge line beam 32, and the first unit 11. It has basically the same configuration as the first unit 11 except that the length of the valley beam 41 is short.

Next, the 4th unit 14 and the 5th unit 15 are demonstrated. The fourth unit 14 is arranged next to the second unit 12 in the direction along the X axis, and the fifth unit 15 is arranged next to the third unit 13 in the same direction. The fourth unit 14 and the fifth unit 15 are located at both ends of the large spatial structure 1 in the longitudinal direction. The top connections 20 of the fourth unit 14 and the fifth unit 15 are at a lower position than the top connections 20 of the second unit 12 and the third unit 13 . The first edge beams 31 of the first unit 11 to the fifth unit 15 are connected via intermediate connection portions (24 to 27), and five quadrangular pyramids are connected in the direction along the Y axis.

Each of the fourth unit 14 and the fifth unit 15 includes a fourth edge beam 34 extending in the direction along the X-axis and two fifth edge beams extending from both ends of the fourth edge beam 34 toward the third base 53. 35. A lower end of the fifth edge beam 35 is fixed to the third foundation 53 .

Both ends of the fourth edge beam 34 of the fourth unit 14 are connected to the second intermediate connection portion 25 and the fourth intermediate connection portion 27 of the fourth unit 14 . Both ends of the fourth edge beam 34 of the fifth unit 15 are connected to the first intermediate connection portion 24 and the third intermediate connection portion 26 of the fifth unit 15 .

The first unit 11 to fifth unit 15 are provided with a large number of small beams 38 . Although beams 38 are shown only in the fourth unit 14 and part of the second unit 12 in FIG. 1, all other units have beams 38 as well. This is to clarify the illustration of symbols and the like. Small beams 38 span between each ridge line beam and each valley line beam to construct a roof structure. By transmitting the load from each ridge line beam to each valley line beam through the small beam 38, a substantially triangular frame structure composed of, for example, two first valley line beams 41 and tie beams 50, which will be described later, can receive a vertical load and a lateral load. directional horizontal loads can be efficiently resisted, and the frame structure of the first edge beam 31 and the second edge beam 32 can efficiently resist horizontal loads in the longitudinal direction.

As shown in FIG. 4, the large-space structure 1 includes tie beams 50 that connect two bottom connecting portions 22 that are opposed to each other in the direction along the X-axis. Note that the tie beam 50 is indicated by a thick dashed line in FIG. By suppressing the deformation acting in the direction in which the two bottom connection portions 22 are separated by the tie beams 50, that is, in the direction in which the lower ends of the two first valley line beams 41 are separated, the rigidity of the large-space structure 1 against vertical loads is increased. can be done. The tie beam 50 is made of a material that satisfies the required rigidity to withstand thrust, and for example, H-section steel can be used.

1-2. Vertical Load and Horizontal Load A case where a vertical load and a horizontal load act on the large-space structure 1 will be described with reference to FIGS. 4 and 5. FIG.

In FIGS. 4 and 5, arrows along the Z-axis indicate vertical loads. When a vertical load acts on the large space structure 1, mainly the two first valley beams 41 extending in an inverted V shape of each valley frame 40 resist deformation due to the vertical load. The tie beam 50 suppresses the deformation of the two first valley beams 41 that tends to expand in the direction along the X axis. The frame structure of the first ridge beam 31 and the second ridge beam 32 is bent so as to protrude outward from the large space structure 1 compared to the frame structure of the inverted V-shaped first valley beam 41. , relatively weak to deformation due to vertical load. Therefore, the frame structure of the first edge beam 31 and the second edge beam 32 alone does not provide sufficient rigidity to the vertical load of the large-space structure 1 . However, since the frame structure of the first valley beams 41 resists the deformation due to the vertical load, the rigidity of the large-space structure 1 as a whole due to the vertical load can be increased.

Furthermore, the top connection portions 20 of the fourth and fifth units 14 and 15 at both ends are lower than the top connection portion 20 of the first unit 11 in the center. By providing the fourth and fifth units 14 and 15 at both ends in the longitudinal direction that are lower than the first unit 11 in the center, the twenty second edge beams 32 and the four fifth edge beams 35 move along the Y-axis due to the vertical load. (small arrow in the direction along the Y-axis in FIG. 5) are balanced, so the amount of vertical deformation of the central top connection portion 20 is reduced.

Also, when a horizontal load (a large arrow in the direction along the Y-axis in FIG. 5) acts on the large-space structure 1, the second ridgeline beams 32 of the adjacent units are fixed at the intermediate connection portions so as to support each other. Therefore, the deformation of the large spatial structure 1 in the direction along the Y axis is suppressed.

2. Dome Structure The dome structure 2 will be described with reference to FIGS. 6 to 8. FIG. 6 is a perspective view of the dome structure 2, FIG. 7 is a front view of the dome structure 2, and FIG. 8 is a cross-sectional view of the dome structure 2 taken along line BB in FIG. In FIGS. 6 and 7, surfaces having the same shape are indicated by the same kind of hatching.

The dome structure 2 has a roof and walls of a folded plate structure in which a plurality of triangular faces are continuous by laying roofing materials between the ridge line beams and the valley line beams of the large space structure 1 described above. The dome structure 2 is composed of surfaces having five types of triangular outer shapes (hereinafter referred to as “triangular surfaces”).

The first triangular face 61 is formed between the first edge beam 31, the first valley beam 41 and the second valley beam 42 of the large spatial structure 1. As shown in FIG. The second triangular face 62 is formed between the first edge beam 31 , the third valley beam 43 and the third edge beam 33 of the large spatial structure 1 . The third triangular face 63 is formed between the second edge beam 32, the first valley beam 41 and the second valley beam 42 of the large spatial structure 1. As shown in FIG. A fourth triangular face 64 is formed between the second edge beam 32, the post 56 and the ground. Two fifth triangular faces 65 are formed side by side between the fourth edge beam 34 and the fifth edge beam 35 .

The dome structure 2 has five peaks when viewed from the front, and looks like a mountain range having multiple peaks and multiple valleys.

As shown in FIG. 8, in the dome structure 2, the internal space 70 formed by the first to fifth triangular surfaces 61 to 65 is a large space. The interior space 70 can include audience seats 72 arranged to surround an arena 74 . The dome structure 2 can be used for sports facilities, convention halls, warehouses, and the like.

The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same function, method, and result, or configurations that have the same purpose and effect). Moreover, the present invention includes configurations in which non-essential portions of the configurations described in the embodiments are replaced. In addition, the present invention includes a configuration that achieves the same effects or achieves the same purpose as the configurations described in the embodiments. In addition, the present invention includes configurations obtained by adding known techniques to the configurations described in the embodiments.

1 large space structure 2 dome structure 11 first unit 12 second unit 1
3... Third unit, 14... Fourth unit, 15... Fifth unit, 20... Top connection part, 22... Bottom connection part, 23... Virtual rectangle, 24... First intermediate connection part, 25... Second intermediate connection part , 26... Third intermediate connection portion, 27... Fourth intermediate connection portion, 30... Ridge frame, 31... First edge beam, 32... Second edge beam, 33... Third edge beam, 34... Fourth edge beam, 35... Fifth ridge line beam, 38... Small beam, 40... Valley line frame, 41... First valley line beam, 42... Second valley line beam, 43... Third valley line beam, 50... Tie beam, 51... First first valley line beam Foundation 52 Second foundation 53 Third foundation 54 Fourth foundation 56 Column 60 Dome unit 61 First triangular surface 62 Second triangular surface 63 Third triangular surface 64... Fourth triangular plane, 65... Fifth triangular plane, 70... Interior space, 72... Audience seats, 74... Arena, α... Angle

Claims (4)

A large space structure in which a plurality of units are integrally and continuously formed in the short direction of the unit,
Said unit is
a top connection;
two bottom connections that are below the top connection and are spaced apart from each other;
two first valley beams respectively connecting the top connecting portion and the two bottom connecting portions;
four intermediate connections located at a height between the top connection portion and the bottom connection portion and at positions corresponding to the vertices of an imaginary rectangle centered on the top connection portion in a plan view;
four first edge beams extending from the top connection to four of the intermediate connections;
four second edge beams extending from each of the intermediate connections to the bottom connection;
with
the adjacent units are united at the intermediate connection,
To each of the bottom connection portions, the lower end of one of the first valley beams and the lower ends of two of the second ridge beams arranged at equal angles across the first valley beam are fixed. is,
The large-space structure, wherein the first valley beam is located inside an imaginary line that connects two intermediate connection portions that are arranged on both sides of the first valley beam. In claim 1,
A large-space structure, further comprising a tie beam that connects the two bottom connecting portions arranged opposite to each other. In claim 1 or 2,
The first edge beam and the second edge beam are steel pipes,
A large-space structure, wherein the first valley beam is made of H-shaped steel. In any one of claims 1 to 3,
The large space structure has five units that are continuously continuous,
A large-space structure, wherein the top connection portions of the units at both ends are lower than the top connection portions of the central unit.

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