JP4276552B2 - Steel stairs - Google Patents

Description

  The present invention relates to a steel staircase that has been subjected to a silencing process.

  In recent years, steel stairs are lightweight, can be installed inexpensively and with a short construction period, and can be used for many buildings, both indoors and outdoors, because they can be made durable by applying an appropriate rust prevention treatment. Yes. However, the steel staircase has a simple structure in which thin steel members are bolted or welded together, and the tread surface is finished with a decorative board or tiles. There has been a noise problem in which falling sounds from objects tend to resonate.

  This noise is mainly caused by the impact sound caused by the impact on the tread and the vibration sound caused by the vibration of the steel staircase (step plate member, girder) subjected to the impact. Moreover, this vibration sound becomes loud when the rigidity of the steel step (step plate member, girder) is low.

  In view of this, various steel frame staircases that have been subjected to noise reduction have been proposed. As a steel frame staircase that has been subjected to a silencing process, for example, there is one provided with a foamed resin layer formed by laying a foam sheet on a tread (see, for example, Patent Document 1). With this steel-framed staircase, it is possible to suppress the impact sound caused by the impact on the tread. Further, the vibration of the tread can be attenuated, and the vibration sound generated from the tread can be suppressed.

Japanese Utility Model Publication No. 02-87838

  However, in the steel step described in Patent Document 1, the vibration of the tread cannot be sufficiently damped only by the foamed resin layer (buffer member) on the tread, and the vibration sound from the tread cannot be sufficiently suppressed. There was a problem.

  In addition, there is a problem that an impact is transmitted from the corrugated plate member to the girders, and the girders vibrate to generate vibration noise.

  SUMMARY OF THE INVENTION Accordingly, the present invention provides a steel staircase that can suppress an impact sound generated due to an impact on a tread, and can suppress a vibration sound generated by vibration of a steel frame stair (step plate member, girder) that has received the impact. Aim.

In order to achieve the object, the first structure of the steel staircase according to the present invention includes two girders arranged at an inclination, and is attached to the girders, and is formed by bending one steel plate into a substantially L-shape. a steel staircase having to a Danban member having a tread surface and riser plates were, and a first buffer member alleviating an impact applied to the tread surface of the step plate member, the impact transmitted to the digit from the stage plate member A second buffer member to be relaxed and a single steel plate having a length substantially the same as the width of the corrugated plate member, a substantially central portion of the bottom surface of the tread surface in the depth direction and the height direction of the back surface of the kick plate And a corrugated plate reinforcing member that is connected in a bridging manner by welding the substantially central portion thereof across the entire width direction .

  Further, a second structure of the steel staircase according to the present invention is the steel staircase of the first structure, wherein the girders are provided one at each end of the step plate member, and are provided on the inner side of the respective girders. It is a side beam which supports the said corrugated board group via a board receiving member, and adjacent corrugated board members are the above-mentioned corrugated board member in which the lower connection part provided in the lower end of the above-mentioned kick board of the above-mentioned corrugated board member is a lower stage The second buffer member is coupled between the step surface of the step plate member and the step plate receiving member, and is connected to the upper connection portion provided at the rear end of the step surface. It is provided between the lower connection part.

  A third structure of the steel staircase according to the present invention is the steel structure staircase of the first structure, wherein the girders are provided one on each side of the step plate member, and the flanges are provided at the upper ends of the respective girders. The step girder is a force girder that supports the step plate member from the lower side of the step plate member, and the step plate member is provided at the upper portion of the flange and at the lower end of the kick plate of the upper step plate member. The connecting portion is coupled so as to overlap the upper connecting portion provided at the rear end of the tread surface of the lower stepped plate member, and the second buffer member is connected to the lower connecting portion of the upper stepped plate member and the lower connected portion. It is provided between the upper connection part of the said corrugated board member, It is characterized by the above-mentioned.

  In the structure of the steel staircase according to the present invention, the first buffer member is provided on the top of the tread surface of the step plate member. Thereby, the impact sound by the impact to a tread can be suppressed. Moreover, the vibration sound of the step plate member can be suppressed.

  Moreover, the 2nd buffer member which relieve | moderates the impact transmitted from the said corrugated board member to the said girder was provided. Thereby, the impact transmitted from the corrugated plate member to the girders can be alleviated, and the vibration sound generated by vibrating the girders can be suppressed.

  Further, in the side-girder-type steel frame staircase, adjacent step plate members are connected to each other at the lower end of the kick plate of the upper step plate member and the rear end of the tread surface of the lower step plate member. It was combined so as to overlap with the upper connecting part provided in. Thereby, the impact received by the upper step plate member is transmitted to the lower step plate member, and this impact load is also transmitted from the lower step plate member to the side beam. For this reason, the impact load is dispersed and applied to the side beam, and the vibration noise of the beam can be further reduced. Further, the impact load received by each step plate member is also dispersed and reduced, and the vibration noise of the step plate member can be reduced. Furthermore, the vibration noise of the side beams can be further reduced by increasing the number of second buffer members that relieve the impact before the impact load is transmitted to the side beams.

  Moreover, the corrugated board reinforcement member which connects the lower surface of a tread surface and the back surface of a kick board in a bridge shape was provided. As a result, the rigidity of the step plate member can be increased, and the vibration sound generated by the vibration of the step plate member due to the impact received by the tread can be suppressed.

[First embodiment]
A first embodiment of a steel staircase according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a side girder type steel staircase according to the present embodiment, FIG. 2 is a side view of a side girder type steel staircase, and FIG. 3 is a diagram showing an experimental result of a silencing effect of the steel staircase.

(Structure of steel stairs)
As shown in FIGS. 1 and 2, the steel staircase A according to the present embodiment is a side staircase type steel staircase provided with a side girder 1, a step plate member 2, and a step plate receiving member 3.

  The side girders 1 are disposed at two ends, one on each side of the step plate member 2, and are inclined, and the step plate member 2 is interposed via a step plate receiving member 3 attached to the inside of each side beam 1. Support.

  The step plate member 2 is formed by bending a single steel plate into a substantially L shape to form a tread surface 2a and a kick plate 2b. The step plate member 2 has an upper connecting portion 2c bent obliquely upward at the rear end of the tread 2a, and a lower connecting portion 2d bent obliquely upward at the lower end of the kick plate 2b.

  On the entire surface of the tread 2a, an adhesive sheet 4, a buffer material 5 as a first buffer member, a sinking prevention steel plate 6, and a decorative plate 7 are attached in order. The tread surface 2a has two coupling holes 2e on each side. The upper connecting portion 2c and the lower connecting portion 2d are formed in parallel. The upper connecting portion 2c is formed with two coupling holes 2f on both sides. Similarly, the lower connecting portion 2d has a coupling hole 2g, one on each side, corresponding to the coupling hole 2f.

  The pressure-sensitive adhesive sheet 4 has a cushioning material 5 bonded on the tread surface 2a. The cushioning material 5 is formed of an elastic body having a high cushioning property and a high damping effect against vibration, that is, a so-called low density and a large amount of deformation, such as foamed rubber. For this reason, the cushioning material 5 attenuates the vibration transmitted to the tread 2a and attenuates the vibration of the tread 2a.

  The sinking prevention steel plate 6 is coated with an adhesive on both sides to bond the cushioning material 5 and the decorative plate 7 together. Further, the sinking prevention steel plate 6 is a rigid member that is difficult to be deformed, and prevents the sinking of the foot due to the cushioning material 5 and prevents the fall due to the sinking of the foot, thereby increasing the safety of the steel staircase A. The decorative board 7 is formed of a vinyl chloride sheet or the like, and covers the surface of the step board member 2 from the upper part of the kick board 2b to the entire surface of the tread 2a.

  The corrugated plate receiving member 3 is formed by bending a steel plate at a right angle, one surface is attached to the inside of the side beam 1, and the other support surface 3a is coupled at two positions corresponding to the coupling holes 2e of the tread surface 2a. The hole 3b is drilled. A buffer material 8 as a second buffer member is placed on the support surface 3a, and the tread surface 2a is overlaid so as to sandwich the buffer material 8. The cushioning material 8 is formed with the same dimensions as the support surface 3a, and two coupling holes 8a are formed at positions corresponding to the coupling holes 3b. The step plate member 2 is bolted to the inside of the side beam 1 via the step plate receiving member 3 by passing the bolts 11 through the coupling holes 2e, 3b, 8a.

  Here, the cushioning material 8 has a cushioning property and is formed of an elastic body that can withstand a relatively high load, for example, a high-density rubber plate or the like. The impact transmitted to the step board receiving member 3 (side girder 1) is reduced.

  A buffer material 9 as a second buffer member is placed on the upper connection portion 2c of the lower step plate member 2, and the lower connection portion 2d of the upper step plate member 2 is sandwiched between the buffer materials 9. It is superimposed. The buffer material 9 is formed with the same dimensions as the upper connecting portion 2c, and two coupling holes 9a are formed at positions corresponding to the coupling holes 9a. Then, by passing the bolts 12 through the coupling holes 2f, 2g, and 9a, the upper step plate member 2 is bolted on the lower step plate member 2, and this connection is performed over a plurality of steps from the bottom to the top of the steps. By doing so, a corrugated board group is formed.

  Here, the cushioning material 9 has a cushioning property and is formed of an elastic body that can withstand a relatively high load, for example, a high-density rubber plate or the like. 2 reduces the impact transmitted to the lower plate member 2.

  On the back side of the step plate member 2, a step plate reinforcement member 10 that reinforces the step plate member 2 is provided. The step board reinforcing member 10 has an upper side joined to the lower surface of the tread surface 2a, a lower side joined to the back surface of the kick plate 2b, and the lower surface of the tread surface 2a and the rear surface of the kick plate 2b are connected in a bridging manner. Further, the corrugated plate reinforcing member 10 is formed of a single steel plate having a length substantially the same as the width of the corrugated plate member 2, and is formed on the corrugated plate member 2 over the entire length (the entire width direction) of the corrugated plate member 2. Welded. At both upper ends of the corrugated plate reinforcing member 10, notches are formed in an L shape so as not to interfere with the corrugated plate receiving member 3.

(Silence effect of steel stairs)
Next, the silencing effect of the steel frame staircase A will be described. First, when a pedestrian puts his or her foot on the decorative plate 7 when moving up and down the steel staircase A, the impact load is transmitted to the anti-sinking steel plate 6 via the decorative plate 7 and is dispersed throughout the anti-sinking steel plate 6. It is transmitted to the buffer material 5. Here, the shock-absorbing material 5 is deformed by the impact load, the impact is relaxed, and the relaxed impact load is transmitted to the tread 2a.

  Thereby, it is possible to suppress an impact sound due to the impact on the tread surface 2a as compared with the steel staircase in which the impact load is directly applied from the decorative board 7 to the tread surface 2a.

  Next, the impact load transmitted to the tread surface 2a is transmitted vertically downward from the tread surface 2a to the side beam 1 via the cushioning material 8 and the step plate receiving member 3. Here, the cushioning material 8 is deformed when the impact load is transmitted, so as to mitigate the impact. Thereby, the impact load transmitted to the side beam 1 can be reduced, the vibration of the side beam 1 can be reduced, and the vibration sound generated from the side beam 1 can be suppressed.

  3 (a-1) is a cross-sectional view of a steel staircase a having a stepped plate reinforcing member 10 and no cushioning materials 8 and 9, and FIG. 3 (b-1) is a stepped plate reinforcing member 10 and a cushioning material 8 having a cushioning material. FIG. 3 (a-2) and FIG. 3 (b-2) are cross-sectional views of the steel step b without 9 and FIG. 3 (b-2) show the experimental results of the silencing effect when the ball is dropped on the tread 2a of the steel steps a and b. FIG.

  As shown in FIGS. 3 (a) and 3 (b), compared to the steel staircase a without the buffer material 8, the measured sound of the steel staircase b provided with the buffer material 8 is smaller, and the silencing effect is improved. I understand that it is expensive.

  A part of the impact load transmitted to the tread surface 2a is transmitted from the tread surface 2a to the kick plate 2b and the lower connecting portion 2d, and the upper connecting portion of the lower stepped plate member 2 from the lower connecting portion 2d via the cushioning material 9. 2c is transmitted downward in the vertical direction. The impact load is transmitted from the lower step plate member 2 to the side beam 1 via the step plate receiving member 3.

  Here, the cushioning material 9 is deformed when the impact load is transmitted, so as to mitigate the impact. As a result, the impact load transmitted from the upper step plate member 2 to the lower step plate member 2 is reduced, the impact load transmitted from the lower step plate member 2 to the side beam 1 is reduced, and the vibration of the side beam 1 is reduced. Thus, the vibration sound generated from the side beam 1 can be suppressed.

  3 (d-1) is a cross-sectional view of the steel staircase d with the corrugated plate reinforcing member 10 and the cushioning materials 8 and 9, and FIG. 3 (d-2) is a diagram in which a ball is dropped on the tread surface 2a of the steel staircase d. It is a figure which shows the experimental result of the silencing effect at the time of hitting.

  As shown in FIG. 3B and FIG. 3D, compared to the steel step b without the buffer material 9, the measured sound of the steel step d provided with the buffer material 9 is smaller, and the silencing effect is effective. I understand that it is expensive.

  Adjacent step plate members were joined so that the lower connection portion 2d of the upper step plate member 2 overlaps the upper connection portion 2c of the lower step plate member 2. Thereby, the impact received by the step plate member 2 is transmitted to the lower step plate member 2, and this impact load is also transmitted from the lower step plate member 2 to the side beam 1. On the contrary, the impact received by the step plate member 2 is also transmitted to the upper step plate member 2, and this impact load is also transmitted from the upper step plate member 2 to the side beam 1. As described above, the impact load is dispersed and applied to the side beam 1 so that the vibration noise of the side beam 1 can be further reduced. Further, the impact load received by each step plate member 2 is also dispersed and reduced, and the vibration noise of the step plate member 2 can be reduced. Furthermore, there is an effect that the number of shock absorbing materials 8 and 9 for reducing the impact is increased before the impact load is transmitted to the side beam 1 and the vibration noise of the side beam 1 is further reduced.

  Next, the impact load transmitted to the tread surface 2a vibrates the corrugated plate member 2 to generate a vibration sound. Such vibration noise is suppressed by the shock absorbing material 5 absorbing the vibration of the tread surface 2a. However, the step plate member is likely to vibrate when the rigidity is low, and the vibration noise is increased.

  Here, since the corrugated plate member 2 is formed by bending a single steel plate, the rigidity is high. Further, the step plate member 2 has a truss structure by connecting the lower surface of the tread surface 2 a and the back surface of the kick plate 2 b with a step plate reinforcing member 10 to increase the rigidity of the step plate member 2. Thereby, the vibration of the step plate member 2 can be suppressed, and the vibration sound generated by the vibration of the step plate member 2 can be suppressed. Further, since the corrugated plate reinforcing member 10 is connected over the entire length (the entire width direction) of the corrugated plate member 2, the vibration noise of the corrugated plate member 2 can be suppressed no matter where the impact load is applied on the tread surface 2a. .

  3 (c-1) is a cross-sectional view of the steel staircase c without the step plate reinforcing member 10 and the cushioning materials 8 and 9, and FIG. 3 (c-2) is a ball drop on the tread surface 2a of the steel staircase c. It is a figure which shows the experimental result of the silencing effect at the time of hitting.

  As shown in FIGS. 3 (c) and 3 (d), the measured sound is smaller in the steel step d provided with the step plate reinforcing member 10 than in the steel step c without the step plate reinforcing member 10. It can be seen that the silencing effect is high.

  Further, among the steel steps shown in FIG. 3, the steel step d provided with all the step plate reinforcing members 10 and the buffer materials 8 and 9 has the highest noise reduction effect, and the step plate reinforcing member 10 and the buffer materials 8 and 9 are combined. Therefore, a higher silencing effect can be obtained. In this experiment, in order to investigate the silencing effect by the corrugated plate reinforcing member 10 and the cushioning materials 8 and 9, a steel staircase without the cushioning material 5 was used. However, by providing the cushioning material 5, a higher silencing effect can be obtained. Can do.

  As described above, the cushioning material 5 is provided between the tread surface 2 a of the corrugated board member 2 and the decorative board 7. Thereby, the impact sound by the impact to the tread 2a can be suppressed. Moreover, the vibration sound of the step board member 2 can be suppressed.

  Further, the cushioning material 8 is provided at the lower part (between the tread surface 2a of the step plate member 2 and the step plate receiving member 3) of the portion transmitting the load downward in the vertical direction of the step plate member 2. Thereby, the impact load transmitted to the side beam 1 via the step plate receiving member 3 can be reduced, the vibration of the side beam 1 can be reduced, and the vibration sound generated from the side beam 1 can be suppressed.

  In addition, the cushioning material 9 is provided at a lower portion (between the upper connecting portion 2c and the lower connecting portion 2d) of the step plate member 2 that transmits the load downward in the vertical direction. As a result, the impact load transmitted from the upper step plate member 2 to the lower step plate member 2 is reduced, the impact load transmitted from the lower step plate member 2 to the side beam 1 is reduced, and the side beam 1 vibrates and is generated. You can suppress the vibration sound.

  Adjacent step plate members were joined so that the lower connection portion 2d of the upper step plate member 2 overlaps the upper connection portion 2c of the lower step plate member 2. Thereby, the impact received by the upper step plate member 2 is transmitted to the lower step plate member 2, and this impact load is also transmitted from the lower step plate member 2 to the side beam 1. For this reason, the impact load is dispersed and applied to the side beam 1, and the vibration noise of the side beam 1 can be further reduced. Further, the impact load received by each step plate member 2 is also dispersed and reduced, and the vibration noise of the step plate member 2 can be reduced. Furthermore, the vibration noise of the side beam 1 can be further reduced by increasing the number of cushioning materials 8 and 9 that reduce the shock before the impact load is transmitted to the side beam 1.

  In addition, by providing a step plate reinforcing member 10 that connects the lower surface of the tread surface 2a and the back surface of the kick plate 2b in a bridging manner, the rigidity of the step plate member 2 is increased, and the step plate member 2 is caused by the impact received by the tread surface 2a. Vibration sound generated by vibration can be suppressed.

[Second Embodiment]
Next, a second embodiment of a steel staircase according to the present invention will be described with reference to the drawings. FIG. 4 is a configuration diagram of a power girder type steel staircase according to the present embodiment, and FIG. 5 is a side view of the power girder type steel staircase. About the part which overlaps with said 1st embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted.

(Structure of steel stairs)
As shown in FIGS. 4 and 5, the steel staircase B according to the present embodiment is obtained by changing the side beam 1 of the first embodiment to a force beam 21 and directly attaching the step plate member 2 to the force beam 21. It is a steel girder of a power girder method in which the plate receiving member 3 is omitted.

  Since the step plate receiving member 3 is not provided, the steel staircase B is not provided with the cushioning material 8, and the tread surface 2a is not provided with the coupling hole 2e. Further, the corrugated plate reinforcing member 10 does not interfere with the corrugated plate receiving member 3 and therefore has no notch.

  The force girder 21 is disposed on both sides of the step plate member 2 in a slanted manner, one in total, and the step plate member 2 is connected to the step plate member 2 by a flange 21a provided at the upper end of each force girder 21. Support from below.

  On the flange 21a, an upper connecting portion 2c, a cushioning material 9, and a lower connecting portion 2d are superposed in order, and the flange 21a corresponds to the coupling holes 2f and 2g of the upper connecting portion 2c and the lower connecting portion 2d. The coupling hole 21b is drilled. Then, by passing the bolts 12 through the coupling holes 2f, 2g, 9a, 21b, the step plate member 2 is bolted onto the force beam 21, and this coupling is performed over a plurality of stages from the bottom to the top of the stairs. A corrugated board group is formed.

(Silence effect of steel stairs)
Next, the silencing effect of the steel frame staircase B will be described. In the steel staircase B of the present embodiment, the cushioning material 5 and the corrugated plate reinforcing member 10 can provide the same silencing effect (an effect of suppressing impact sound and vibration sound) as in the first embodiment.

  The steel staircase B directly connects the upper connecting portion 2 c to the force beam 21 without providing the step plate receiving member 3 and the buffer material 8. Here, the shock absorbing material 9 is deformed by receiving an impact load from the lower connecting portion 2d and relaxes the impact, and transmits the impact load to the flange 21a via the upper connecting portion 2c of the lower stepped plate member 2. For this reason, the shock absorbing material 9 reduces the impact load transmitted from the step plate member 2 to the force girder 21, reduces the vibration of the force girder 21, and suppresses the vibration sound generated from the force girder 21.

  In the present embodiment, no cushioning material is provided between the upper connecting portion 2c and the flange 21a. This is because an impact load is mainly applied from the vicinity of the center of the tread surface 2a to the tip when the stairs are moved up and down, and the impact load applied to the vicinity of the center of the tread surface since the step plate reinforcing member 10 is joined to the center portion of the tread surface 2a. However, it almost rests on the tip side (the kick plate 2b side) of the tread surface 2a. For this reason, a large impact load is not applied to the lower portion of the upper connecting portion 2c, and a cushioning material is unnecessary.

  In addition, in order to maintain accuracy over a long time on a force beam disposed obliquely, either one of the lower connecting portion 2d and the upper connecting portion 2c or the upper connecting portion 2c and the flange 21a is provided with a cushioning material. It is because it is preferable to fix without interposing.

  As described above, by providing the cushioning material 5 between the tread surface 2a of the step board member 2 and the decorative plate 7 as in the first embodiment, it is possible to suppress the impact sound due to the impact on the tread surface 2a.

  Further, the cushioning material 9 is provided at the lower part (between the lower connecting portion 2d and the flange 21a) of the portion that transmits the load downward in the vertical direction of the step plate member 2. Thereby, the impact transmitted from the step plate member 2 to the force beam 21 can be reduced, the vibration of the force beam 21 can be reduced, and the vibration sound generated by the vibration of the force beam 21 can be suppressed.

  Further, as in the first embodiment, by providing the step plate reinforcing member 10 that connects the lower surface of the tread surface 2a and the back surface of the kick plate 2b in a bridging manner, the rigidity of the step plate member 2 is increased, and the tread surface 2a The vibration sound generated by the vibration of the stepped plate member 2 due to the received impact can be suppressed.

Examples of applications of the present invention, it is possible to apply to the stairs of the various buildings that require noise countermeasures.

It is a block diagram of the steel frame staircase of the side beam type which concerns on 1st embodiment. It is a side view of the steel frame staircase of a side girder method. It is a figure which shows the experimental result of the silencing effect of a steel stairs. It is a block diagram of the steel beam staircase of the power girder system which concerns on 2nd embodiment. It is a side view of a steel girder of a power girder method.

Explanation of symbols

A, B, ad ... Steel staircase 1 ... Side girder 2 ... Step plate member 2a ... Tread surface 2b ... Kick plate 2c ... Upper connecting part 2d ... Lower connecting part 2e, 2f, 2g, 3b, 8a, 9a, 21b ... Bonding hole 3 ... Corrugated plate receiving member 3a ... Support surface 4 ... Adhesive sheet 5, 8, 9 ... Buffer material 6 ... Subduction prevention steel plate 7 ... Decorative plate
10… corrugated plate reinforcement
21… Power girder
21a ... Flange

Claims (3)

A steel staircase having two girders arranged at an inclination, and a step plate member having a tread surface and a kick plate attached to the girders and formed by bending one steel plate into a substantially L shape. ,
A first buffer member for relaxing the impact applied to the tread surface of the step plate member;
A second buffer member for reducing the impact transmitted from the step plate member to the beam;
It is formed of a single steel plate having a length substantially the same as the width of the step plate member, and a substantially central portion in the depth direction of the lower surface of the tread surface and a substantially central portion in the height direction of the back surface of the kick plate are disposed in the entire width direction. A steel frame staircase having a step plate reinforcing member connected in a bridging manner by crossing and welding . The girder is a side girder that is provided one by one at both ends of the corrugated plate member, and supports the corrugated plate group via a corrugated plate receiving member provided inside each of the girders.
Adjacent corrugated plate members are such that the lower connecting portion provided at the lower end of the kick plate of the upper stepped plate member overlaps the upper connecting portion provided at the rear end of the tread surface of the lower stepped plate member. Combined with
The second buffer member is provided between the tread surface of the step plate member and the step plate receiving member, and between the upper connection portion and the lower connection portion. Steel stairs as described in. The girder is a force girder that is provided one by one on both sides of the corrugated plate member, and supports the corrugated plate member from the lower side of the corrugated plate member with a flange provided at the upper end of each girder.
The step plate member has an upper connection provided at a rear end of the tread surface of the step plate member at the lower step, and a lower connection portion provided at the lower end of the kick plate of the upper step plate member at the upper portion of the flange. Combined to overlap the part,
2. The steel staircase according to claim 1, wherein the second buffer member is provided between a lower connecting portion of the upper step plate member and an upper connecting portion of the lower step plate member. .

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