JPH11303281A - Concrete floor-slab structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide concrete floor-slab structure, in which required strength characteristics can be ensured without particularly increasing slab thickness even when a floor lowering section is installed in an intermediate section between girders supporting the end sections of a floor slab respectively. SOLUTION: A floor lowering section Sw is mounted to an intermediate section between a pair of girders BY1, BY2 supporting the end sections of floor slabs F1-F4 and F5-F8 respectively in the concrete floor-slab structure. Beams Bs made of precast concrete are extended along the stepped section of the floor lowering section Sw, and the floor slabs F1-F4 and F5-F8 made of precast concrete are arranged on both sides of the beams Bs respectively at that time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a concrete floor slab structure, and more particularly to a concrete floor slab structure having a step on its upper surface.

[0002]

2. Description of the Related Art Conventionally, as a floor slab structure of a concrete building, a structure using a so-called precast concrete slab is generally well known. Widely adopted as. By adopting this type of slab for at least a part of the floor structure of the building, the workability at the site is much better and the safety of construction is higher than when concrete is cast at the construction site. Various advantages can be enjoyed, such as shortening the construction period and obtaining a high-quality concrete floor slab with little variation. In buildings with such precast concrete floor slabs,
Each floor slab is arranged so as to bridge between the girders of the building, and the girders support both ends.

[0003] By the way, as is well known, in a concrete building, it is sometimes required to provide a lowered floor portion on the floor surface (see, for example, JP-A-8-239934). In other words, taking the case of a multi-unit apartment as an example, it is necessary to form a descent by lowering the floor by a predetermined height from the other floor at the entrance, the water area or the Japanese-style room with tatami flooring of each dwelling unit. There is. Further, such a lowered floor portion may not be connected to the end of the slab on which the floor slab is supported, but may be provided in the middle of the girder supporting the floor slab, for example, near the center of the slab.

[0004]

The slab thickness is set to be thinner by the amount of step-down in the lower part of the floor than in the surrounding area. Therefore, when the lower part of the floor is provided in the middle between the girders, there is a problem that the design becomes difficult to secure the strength of the floor slab. That is,
As shown in FIG. 16, when both ends of the floor slab are supported by the girder, the magnitude of the moment acting on this floor slab F is supported by the girder as shown by a two-dot chain line curve in FIG. Although it becomes larger as it approaches the center of the slab from the end, when the floor descending portion S is provided in the middle of the girder, the slab thickness of this portion S is small, and especially the moment at the step portions D1, D2, etc. Load m1, m
2 may exceed the crack moment value of concrete.

In such a case, it is conceivable to secure the strength of the floor slab by setting the slab thickness to be large throughout. However, in this method, since the entire slab becomes thick, it is necessary to set a large height dimension for each floor in order to secure a required living room height (ceiling height). Under the restriction, there is a problem that the number of floors of the building has to be reduced in some cases. In particular, in recent years, in order to provide as large a living space as possible in a sunny southern direction, each dwelling unit in an apartment complex tends to be designed wider in the east-west direction than in the past. In the case of dwelling units, the slab thickness also needs to be set thicker because the spacing of the girders is wider. Therefore, in such a case, the problem of adverse effects on the room height becomes even more serious.

Accordingly, the present invention provides a concrete floor slab which can secure required strength characteristics without particularly increasing the slab thickness, even when a floor lowering portion is provided midway between the girders which respectively support the ends of the floor slab. Providing the structure was done as a fundamental purpose.

[0007]

For this reason, the invention according to claim 1 of the present application (hereinafter, referred to as a first invention) is characterized in that a floor lowering portion is provided halfway between a pair of girders respectively supporting ends of a floor slab. Is a concrete floor slab structure provided with a precast concrete small beam extending along the step of the floor descending portion, and a precast concrete floor slab is arranged on each side of the small beam. It is characterized by the following.

Further, the invention according to claim 2 of the present application (hereinafter referred to as the invention)
In the first invention, the reinforcing bar whose height is lower than the reinforcing bars of the other floor slabs by a predetermined dimension is provided on the upper surface of the slab portion corresponding to the lowered floor portion of the floor slab. Is provided.

Further, the invention according to claim 3 of the present application (hereinafter referred to as “the invention”)
A third invention) is characterized in that, in the second invention, the lower surface of each of the floor slabs and the lower surface of the small beam are substantially flush.

[0010] Further, the invention according to claim 4 of the present application.
(Hereinafter, referred to as a fourth invention) according to any one of the first to third inventions, wherein a side portion of each of the floor slabs and a side portion of the small beam are engaged with each other in combination. It is characterized in that the joints are provided respectively.

[0011] Further, the invention according to claim 5 of the present application.
(Hereinafter referred to as a fifth invention) is characterized in that, in the fourth invention, the engagement portion is formed with a tapered portion.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, taking as an example a case where the present invention is applied to a floor slab structure of an apartment house. FIG.
It is an explanatory plan view showing a layout (room arrangement) of one dwelling unit M of the apartment house according to the present embodiment. In addition, the above apartment house
For example, it is constructed as a middle-to-high-rise building of about 10 stories. As shown in FIG. 1, the dwelling unit M is arranged, for example, in a north-south direction, and has a door E on the north side (upper side in FIG. 1).
t and two Western-style rooms Ry1 and Ry2, a tatami-mat Japanese-style room Rj and a Western-style dining room LD on the south side (the lower side in FIG. 1), and a bathroom W1 and a toilet W2 in the middle part between north and south. A water area W, a kitchen K, and a Western-style room Ry3 in which the laundry room W3 is put together are arranged.

At the four corners of the dwelling unit M, pillars P that are erected in the height direction as part of the basic framework of the building are located, and these pillars P are located in the north-south direction (see FIG. 1). A pair of girders B _T 1 and B _T 2 extending in the up-down direction and a pair of girders B _Y 1 and B _Y 2 extending in the east-west direction (left-right direction in FIG. 1).
Are interconnected. By the way, the above dwelling unit M
Is designed to be wider in the east-west direction than before in order to provide as much room space as possible in the sunny south direction. In other words, the girders B _T 1,
Distance L _T of the center line of B _T 2 is a example about 9400Mm,
It is set longer than the unit area. On the other hand, the interval L _Y between the center lines of the girders _BY 1 and _BY 2 extending in the east-west direction is set to, for example, about 8700 mm.

FIG. 2 is an explanatory plan view showing the floor slab structure of the dwelling unit M. In the apartment house according to the present embodiment,
The floor slab of each dwelling unit including the dwelling unit M is a so-called precast concrete slab. As is well known, this precast concrete product is a concrete product that has been factory-produced with a formwork in advance based on required specifications with respect to main characteristics including, for example, shape, dimensions, and material. Compared to those obtained by casting, very high-precision and high-quality products can be stably obtained. Therefore, by adopting this type of slab for at least a part of the floor structure of the building, the workability at the site is much better and the safety of construction is higher than when concrete is cast at the construction site,
In addition, the construction period can be significantly reduced, and
Various advantages can be enjoyed, such as obtaining a high-quality concrete floor slab with little variation.

In the floor slab structure of the dwelling unit M, the entrance E
t, the Japanese-style room Rj with tatami mats and the water area W are lowered by predetermined heights from the other floors, respectively, and the floor is lowered S
_E, S _R and S _W are formed. These lowering parts S
_E, of S _R and S _W, the floor edge portion S _E corresponding to the entrance Et is concatenated NE pillar P and the two girders B _T 2, B _Y 1 (FIG. 1 and the upper right in FIG. 2) (That is,
Provided so as to be in contact with each other). In addition, the floor-down portion S _R corresponding to the Japanese-style room Rj is a column P and two large beams B _T 1 and B _{Y in the} southwest (lower left in FIGS. 1 and 2).
It is connected to 2. On the other hand, the floor edge portion S _W corresponding to the water around region W is on the relationship between the position in the vicinity of substantially the center of the floor, girders B _T 1 without connecting with any of the column or girder,
Are provided in (midway between them also girder B _Y 1, B _Y 2) halfway between B _T 2.

[0016] In this embodiment, among the steps of the floor edge portion S _W span is provided in the middle between the wider pair of girders B _T 1, B _T 2, more preferably, girders B _T 1, _BT 2
A small beam Bs made of precast concrete extends along a step D _W1 parallel to and near the center.
That is, the small beam Bs is a pair of large beams B _Y 1,
It extends in the north-south direction so as to bridge between _BY 2 and
And both ends thereof are coupled to each girder B _Y 1, B _Y 2. Also, the small beam Bs is preferably above girders B _T 1, B _T
The cross-sectional size is set smaller than 2, and the cast-in-place concrete is flush with the other floor.

On both sides of the small beam Bs, floor slabs F1 to F4 and F5 to F8 made of precast concrete are provided.
Are arranged respectively. Therefore, these floor slabs F
Floor slabs F1 to F4 arranged on the west side (left side in FIG. 2) of the small beam Bs among 1 to F4 and F5 to F8.
Has its left end portion is joined supported girder B _T 1, the right end is bonded supported the beams Bs. The floor slabs F5 to F8 arranged on the east side (the right side in FIG. 2) of the small beam Bs are:
The left end is joined and supported by the small beam Bs, and the right end is the large beam B.
It will be joined and supported by _T2 .

The above-mentioned floor slabs F1 to F4 and F5 to F8
As shown in FIGS. 3 to 8, as shown in FIGS. 3 to 8, a floor panel 10 made of precast concrete, and a large number of reinforcing bars 11 for reinforcing the floor panel 10 and cast-in-place concrete Cm (that is, concrete cast at a construction site). And, more preferably, a plurality of void forms 1 made of, for example, styrofoam.
5 is provided. By providing such a void form 15, floor slabs F1 to F4 can be maintained while maintaining the slab thickness.
And the weight reduction of F5 to F8 can be achieved.
Further, the small beam Bs includes a beam main body 20 made of precast concrete, and a number of reinforcing bars 21 whose lower ends are fixed to the beam main body 20 to reinforce the beam main body 20 and the cast-in-place concrete Cm. Note that, in FIG. 3, the illustration of the reinforcing bar 11 and the void form 15 is omitted for the floor slabs F1, F2, F5, and F7 in order to avoid excessive complexity of the drawing.

The reinforcing bars 11 of each of the floor slabs F1 to F4 and F5 to F8 are all made of steel, for example, the floor slabs F2 and F2.
6 will be described as an example, as shown in detail in FIG.
Numerous lower slab bars 12 buried in the floor panel 10 and extending vertically and horizontally, a number of diagonal members 13a whose lower ends are connected to the lower slab line 12 and extend obliquely upward, and the diagonal members 13a
Truss bars 13 having upper chord members 13b connecting the tops of the truss members, and a number of slab upper end bars 14 joined to the upper chord members 13b of the truss bars 13 and extending vertically and horizontally.

The reinforcing bar 21 of the small beam Bs is made of steel.
A plurality of substantially rectangular frame streaks 22 whose lower portions are buried in the beam main body 20, and small beams Bs which are arranged at inner corners and middle portions of the frame streaks 22 and are orthogonal to the frame streaks 22. It is provided with a bar-like bar 23 extending in the longitudinal direction, and is more preferably made of a so-called PC steel rod (steel rod for prestressed concrete) buried in the beam body 20 and extending in the longitudinal direction of the small beam Bs. It is provided with the PC muscle 24. That is, the small beam Bs is more preferably, by prestressed concrete construction method, girders B _T 1, B _T 2
Parallel thereto between, and are extended and looped between the girder B _Y 1, B _Y 2.

The lower part S _E , S _{R of the} floor slab structure
And of S _W, the floor edge portion S corresponding to the entrance Et
_E is, as can be clearly seen from FIG. 4, a state in which the cast-in-place concrete Cm is cast, and a predetermined height He from another floor surface.
(For example, He = 100 mm). In other words, the slab thickness is set to be smaller by the step-down amount He in the lower part S _E than in the surrounding area. The step-down amount He is appropriately set according to the step to be provided between the tile surface of the entrance and the floor surface of the corridor in the living room area.

As shown in FIGS. 6 and 7, in the floor-down portion S _R corresponding to the Japanese-style room Rj with tatami mats,
In a state where the cast-in-place concrete Cm has been cast, the step is lowered so as to be lower by a predetermined height Hr (for example, Hr = 25 mm) from another floor surface. In other words, the slab thickness is set to be smaller by the step-down amount Hr than the surrounding area in the floor-down portion S _R. The step-down amount Hr is appropriately set according to the height required for laying the tatami including the thickness of the tatami.

Furthermore, the floor edge portion S _W corresponding to the water around the area W, as best seen in FIGS. 5 and 8, the in-place concrete Cm while Da設, predetermined height from the other floor Hw (For example, Hw = 180 mm). In other words, in the floor edge portion S _W, indented amount H than the slab thickness around
It is set thin by w. The step-down amount Hw is appropriately set according to the set depth of the bathtub installation portion of the bathroom W1.

[0024] As best seen in FIG. 4 to FIG. 8 and 9, the floor edge portion S _E of the floor slab F1~F4 and F5～F8, the upper surface of the slab portion corresponding to S _R and S _W is height as compared with the reinforcing bar 11 of the other floor slab has a lower reinforcing bar 11 by a predetermined distance is disposed, extending the joists Bs manufactured precast concrete longer girders B _T 1, between B _T 2 to span after setting, the floor slab F1~F4 disposed between the joists Bs and girder B _T 1, on the floor slab F5~F8 is disposed between the girders B _T 2 joists Bs, floor By casting the cast-in-place concrete Cm as a whole, it is possible to change the thickness of the precast concrete floor panel 10 itself without lowering the floor descending portions S _E having the predetermined step-down amounts He, Hr and Hw. , it is possible to provide the S _R and S _W

In this case, the floor slabs F1 to F
4 and the lower surfaces of F5 to F8 and the lower surface of the small beam Bs are set to be substantially flush. As a result, the lower surface of the entire floor can be formed substantially flat, so that a clear space without protrusions can be secured below the entire floor, and there is no need to be restricted by the plan of the floor plan and the layout. Thus, each floor slab F1-F4 and F5-F8
The lower surface and also when set to be substantially flush to the underside of the joists Bs, bed down portion S _E of the floor slab F1~F4 and F5～F8, slab portion corresponding to S _R and S _W of Can be dealt with only by adjusting the height of the reinforcing bar 11 provided on the upper surface of the steel bar.

[0026] The floor edge portion S _E, of the S _R and S _W,
In the case of the lower part S _E corresponding to the entrance Et and the lower part S _R corresponding to the Japanese-style room Rj, a pillar P and two large beams _BT 2, _BY 1 or _BT 1, _BY 2, respectively. While being connected to the floor edge portion S _W corresponding to the water around region W is on the relationship between the position in the vicinity of substantially the center of the floor, girders B _T 1 without connecting with any of the column or girder, B _T is provided in the middle between the two, yet, its indented weight Hw also the floor edge portion S _E, the largest among the S _R and S _W (hence, also slab thickness thinnest) are set.

For this reason, as described above, in the case of a conventional floor slab structure in which each floor slab is bridged between girders and both ends are supported by these girders, this is shown by a two-dot chain line curve in FIG. As described above, there is a possibility that the moment loads m1 and m2 may exceed the cracking moment value of the concrete at the steps D1 and D2 (especially the steps D1 on the center side) of the floor descending portion S. In order to secure the slab thickness, it was necessary to set the slab thickness to be large throughout.

[0028] However, in the present embodiment, a wide girders B _T 1 of the span, the stepped portion of the floor edge portion S _W corresponding to the water around the area W which is provided in the middle between B _T 2 (step especially in the center-side Since the precast concrete beam Bs extends along the section D _W 1), the floor surface is
And precast concrete floor slabs F1 to F4 on both sides separated by the small beams Bs.
And F5 to F8 are arranged respectively. Therefore, the floor slab F1~F4 and F5~F8 becomes a to be supported by the much smaller span than when it is supported at both ends by the girder B _T 1, B _T 2, girders B _T 1, B
_Even when the floor lowering portion _SW is provided on the way between _T2 , the moment load can be significantly reduced as shown by the solid curve in FIG. As a result, required strength characteristics can be secured without particularly increasing the slab thickness, that is, without adversely affecting the ceiling height.

Further, since the floor is separated by the small beams Bs, the amplitude of the floor vibration due to the impact can be reduced with respect to the noise between the upper and lower floors due to the impact input to the floor and the like.
The noise reduction effect can be expected. Further, in this case,
Floor slabs F1 to F4 and F5 to F8 as well as small beams Bs
Since it is also made of precast concrete, the advantages of the precast concrete method can be enjoyed as a whole floor slab structure.

Next, each of the floor slabs F1 to F4 and F5
A portion where F8 and the small beam Bs are engaged will be described.
As can be clearly understood from FIG. 10, on both sides of the beam main body 20 of the small beam Bs, there are provided engagement portions 20a having tapered portions 20t which are inclined so as to fall down toward the side. On the other hand, the floor slabs F1 to F4 and F5 to F8 will be described with reference to the floor slab F6, for example, as shown in FIG.
As can be clearly understood from FIG. 1, an engaging portion 10a having a tapered portion 10t that is inclined so as to rise toward the side is provided at a side edge of the floor panel 10 on the side of the small beam Bs.

The tapered portion 20 of the beam main body 20 of the small beam Bs
t and the taper portion 10t of the floor panel 10 are set at the same angle (for example, 45 degrees), and as shown in FIG. 12, by combining the engagement portions 10a and 20a with each other, the taper portion 10t is formed. , 20t are engaged without any gap, and one side edge of the floor panel 10 is securely supported by the side edge of the small beam Bs. More preferably, in this support state, the lower surface of the small beam Bs and the lower surface of the floor panel 10 are set to be substantially flush.

Thus, the floor slabs F1 to F4 and F5
F8 and the side portion of the small beam Bs are provided with engaging portions 10a and 20a which are combined with each other and engaged with each other, so that the floor slabs F1 to F4 and F5 When arranging the floor slab, the floor slab can be easily and reliably held in a state where one side of each of the floor slabs F1 to F4 and F5 to F8 is engaged with the side of the small beam Bs. The workability when arranging F1 to F4 and F5 to F8 can be improved. Particularly, since the engaging portions 10a and 20a are formed with the tapered portions 10t and 20t, the side portions of the floor slabs F1 to F4 and F5 to F8 and the side portions of the small beams Bs are taperedly engaged. Can be combined with higher accuracy.

The side portions of the small beams Bs and the floor slabs F1 to F1
Various modifications are conceivable for the engaging portions provided on the side portions of F4 and F5 to F8. For example, as shown in FIG. 13, the engaging portion 40a of the beam main body 40 of the small beam Bs and the floor panel 3
The entire surface of the zero engaging portion 30a may be formed as a tapered portion. Further, as shown in FIG. 14, the beam main body 60 of the small beam Bs is formed.
And the engaging portion 50a of the floor panel 50 may be formed in a stepped shape so that the two stepped portions 60b and 50b are engaged with each other. In this case, the lower surface of the beam main body 60 of the small beam Bs and the lower surface of the floor panel 50 can be set to be more and more easily and surely substantially flush as compared with the case of the tapered engagement. Further, as shown in FIG.
On the side of the beam body 80 of the small beam Bs and the side of the floor panel 70,
In particular, without providing the engaging portion, the two members 80 and 70 may be arranged with the lower end surfaces thereof being supported, and the cast members 80 and 70 may be joined by casting concrete in place.

Although the above embodiment has been described with reference to, for example, a medium-to-high-rise apartment building, the present invention is not limited to such a case. It is needless to say that the present invention can be applied to concrete buildings without any problem. Although only one small beam of the floor slab structure is provided between a pair of large beams, a plurality of such small beams may be provided when the span between the large beams is longer. . As described above, the present invention is not limited to the above-described embodiment, and it goes without saying that various improvements or design changes can be made without departing from the gist of the present invention.

[0035]

According to the first aspect of the present invention, since the precast concrete small beams are extended along the steps of the lower part of the floor provided between the girders of the building, the floor surface is reduced. The beams will be separated. And floor slabs made of precast concrete are arranged on both sides separated by the small beams. Therefore, each floor slab is supported by a much smaller span than when both ends are supported by the girder, and even when a floor descent is provided in the middle between the girder, the slab thickness is particularly increased. The required strength characteristics can be secured without performing, that is, without adversely affecting the securing of the ceiling height. Further, since the floor is separated by small beams, the amplitude of floor vibration due to the impact can be reduced with respect to the noise between the upper and lower floors due to the impact input to the floor and the like, and a noise reduction effect can be expected. In this case, since not only the floor slab but also the small beams are made of precast concrete, the advantages of the precast concrete method can be enjoyed as the whole floor slab structure.

According to the second aspect of the present invention, basically, the same effects as those of the first aspect can be obtained. Particularly, on the upper surface of the slab portion corresponding to the descending portion of the floor slab, a reinforcing bar whose height is lower by a predetermined dimension than the reinforcing bars of the other floor slabs is arranged, so that the precast concrete floor is used. A floor down section can be provided without having to change the thickness of the slab itself.

Further, according to the third aspect of the present invention, basically the same effects as those of the second aspect can be obtained. In particular, since the lower surface of each floor slab and the lower surface of the small beam are substantially flush with each other, the lower surface of the entire floor can be formed substantially flat, and a clear space without protrusion below the entire floor. Can be secured, and there is no need to be restricted by the floor plan and the layout design.

Further, according to the fourth invention of the present application,
Basically, the same effect as any one of the first to third inventions can be obtained. In particular, since the side portions of each floor slab and the side portions of the small beams are provided with engaging portions that are combined with each other and engaged with each other, when arranging the floor slabs on both sides of the small beams, The floor slab can be easily and reliably held in a state in which the side portion at one end is engaged with the side portion of the small beam, and the workability when arranging the floor slab can be enhanced.

Further, according to the fifth invention of the present application,
Basically, the same effect as the fourth invention can be obtained. In particular, since the engaging portion is formed with a tapered portion, the side portion of each floor slab and the side portion of the small beam can be more accurately combined by the tapered engagement.

[Brief description of the drawings]

FIG. 1 is an explanatory plan view showing a layout of dwelling units of an apartment house according to an embodiment of the present invention.

FIG. 2 is an explanatory plan view showing a floor slab structure of the dwelling unit.

FIG. 3 is a perspective view showing a floor slab structure of the dwelling unit.

FIG. 4 is an explanatory sectional view taken along line Y4-Y4 in FIG. 2;

FIG. 5 is an explanatory sectional view taken along line Y5-Y5 in FIG. 2;

FIG. 6 is an explanatory sectional view taken along line Y6-Y6 of FIG. 2;

FIG. 7 is an explanatory sectional view taken along line Y7-Y7 in FIG. 2;

FIG. 8 is an explanatory sectional view taken along line Y8-Y8 in FIG. 2;

FIG. 9 is an explanatory sectional view showing an enlarged main part of FIG. 5;

FIG. 10 is an explanatory sectional view of a small beam according to the embodiment.

FIG. 11 is an explanatory cross-sectional view showing a side of the floor slab according to the embodiment, where the floor slab is engaged with a small beam.

FIG. 12 is an enlarged sectional explanatory view showing an engagement portion between the floor slab and the small beam.

FIG. 13 is an explanatory sectional view showing a modified example of an engaging portion between the floor slab and the small beam.

FIG. 14 is an explanatory sectional view showing another modified example of the engaging portion between the floor slab and the small beam.

FIG. 15 is a cross-sectional explanatory view showing another example of a portion where the floor slab and the small beam are joined.

FIG. 16 is an explanatory diagram schematically showing the magnitude of a moment acting on a floor slab.

[Explanation of symbols]

10, 30, 50, 60 ... floor panel 10a, 30a, 50a ... floor slab engaging portion 10t ... floor slab taper portion 11 ... floor slab reinforcing bar 13 ... truss bar 14 ... slab upper bar bar 20, 40, 60, 80 ... small beam beam body 20a, 40a, 60a ... joists engaging portion 20t ... joists of the tapered portion Bs ... joists B _T 1, B _T 2 ... girder D _W 1 ... floor edge portion of the step portion F1 ~F8 ... floor edge portion corresponding to the floor slab S _W ... water around the area

Claims (5)

[Claims] 1. A concrete floor slab structure in which a floor lowering portion is provided halfway between a pair of girders respectively supporting end portions of a floor slab, and a precast concrete small slab structure is provided along the step of the floor lowering portion. A concrete floor slab structure, wherein a beam is extended, and floor slabs made of precast concrete are respectively arranged on both sides of the small beam. 2. A reinforcing bar whose height is lower by a predetermined dimension than reinforcing bars of other floor slabs is provided on an upper surface of a slab portion of the floor slab corresponding to a lowered floor portion. The concrete floor slab structure according to claim 1, wherein 3. The concrete floor slab structure according to claim 2, wherein a lower surface of each of said floor slabs and a lower surface of said small beam are substantially flush with each other. 4. An engaging portion which is engaged with each other on a side portion of each of the floor slabs and a side portion of each of the small beams. The concrete floor slab structure according to any one of the above. 5. The concrete floor slab structure according to claim 4, wherein said engaging portion is formed with a tapered portion.