JP3348548B2 - Refractory beam - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refractory beam using an H-section steel frame such as an H-section steel, and more particularly to reducing the fire-resistant covering area of an H-section steel beam to improve work efficiency. Related to those that have enabled cost reduction.

[0002]

2. Description of the Related Art Conventionally, fireproof coatings of H-shaped steel beams have been
The work was performed uniformly over the entire circumference and the entire length regardless of the stress state. For example, when rock wool is used as a fire-resistant coating material, the rock wool is coated by spraying, but when the entire circumference is fire-resistant coated, for example, there is a flange edge portion as a site where the spraying operation is difficult. JP-A-63-223246 discloses a method in which rock wool is processed into a molded plate in advance to facilitate this spraying operation, and the molded plate is attached to a beam to cover the molded plate. This coating method employs a rock wool molding plate 3 having a plurality of notches 3a as shown in FIG.
In this method, the lower flange 2a of the H-shaped steel beam 2 attached to the beam 2 is wrapped, and the web portion 2b of the beam 2 is covered with another rock wool refractory material 4. As a method of omitting the spraying work itself at the construction site, there is also a pre-coating method in which a beam is provided with a fire-resistant coating in advance in a factory and then is carried into the construction site. As a coating material used in the precoating method, a mortar, a ceramic blanket, a refractory paint, or the like is used.

[0003]

The coating method described in the above-mentioned conventional Japanese Patent Application Laid-Open No. 63-223246 aims at improving the working efficiency, but the conventional on-site work remains. There is room for improvement in work efficiency. Normally, structural members are determined by large short-term loads such as earthquakes and winds.However, in the event of a fire, the only loads acting on the structure are the structure's own weight and the loading load, and these loads are equally applied to the entire beam. Instead, despite the fact that there is sufficient room for structural strength depending on the location in the beam, conventionally, this has been uniformly applied to fireproof coating. However, considering that the stress state is not uniform even in the structure,
There is a problem that uniform refractory coating is wasteful.

The present invention has been made to solve such a problem. In view of the fact that the stress state is not uniform even in the structure, the entire surface is not covered with fireproof coating, but is partially covered. It is an object of the present invention to obtain a refractory beam that can not only drastically improve the work efficiency but also reduce the cost by keeping the refractory coating to a minimum.

[0005]

According to the present invention, there is provided a refractory beam comprising:
Both ends of the steel beam having an H-shaped cross section in which both ends are supported in a simple supporting state and the flat surface of the structure is attached to the flange surface of one of the flanges are all fireproof except for the edge surfaces of both flanges of the structure. All parts other than the center part of the steel beam are fire-resistant coated with a coating material, except for the flange surface where the plane part of the structure is attached and at least the flange part where the plane part of the structure is not attached or the flange part which is attached. It is formed by coating with a fireproof material. Here, the simple support state refers to a support state in which when a vertical load is applied to the beam, the beam supports the shear force in the vertical direction at the fulcrum but does not support the bending moment.

Another refractory beam according to the present invention is:
The center and both ends of an H-shaped steel beam having both ends supported in a fixed support state and the flat surface of the structure attached to the flange surface of one of the flanges, leaving the edge surfaces of both flanges of the structure. All the parts other than the central part and both ends of the steel beam are covered with a fire-resistant coating material, and the flange surface to which the plane part of the structure is attached and at least the flange part or the flange part to which the plane part of the structure is not attached are attached. Except for the flange portion, all are formed by fireproof coating with a fireproof coating material. Here, the fixed support means a support state in which when a vertical load is applied to the beam, both the vertical shear force and the bending moment are supported at the fulcrum.

[0007]

FIG. 1 shows a refractory beam provided with a refractory coating material.
FIG. 2 is a sectional view showing various fireproof coating states, FIG. 2 is an explanatory view showing an effective cross section in the fireproof state of FIGS. 1A to 1D, and FIG. FIG. 4 is an explanatory view showing an operation state when a concentrated load is applied to the center of the span of the beam in the state, and bending moments and shear forces at respective parts. FIG. 4 shows a refractory covering material of a refractory beam of which both ends are simply supported according to the present invention. FIG. 5 is a sectional view showing the state of each part, FIG. 5 is an explanatory view showing the operating state when a concentrated load is applied to the center of the span of the beam with both ends fixed, and the bending moment and shear force at each part. FIG. FIG. 7 is a cross-sectional view showing the state of each part provided with a refractory coating material of a refractory beam in which both ends are fixedly supported, FIG. 7 is a perspective view showing a transport state of a refractory beam in which both ends are simply supported, and FIG. FIG. 9 is a sectional view taken along line AA of FIG. 7, and FIG. It is a sectional view showing a truck mounted state of the refractory beams.

In the drawing, 1 is an H-shaped steel which is a steel beam having an H-shaped cross section, 2 is a fireproof coating material of rock wool applied to the H-shaped steel 1, and 2a is applied to a web 1a of the H-shaped steel 1. The refractory coating material 2b is provided on the back surface of the upper flange 1b of the H-section steel 1 to which the flat part of the structure 3 is attached.
Is a fire-resistant coating material applied to the surface of the lower flange 1c of the H-section steel 1 to which the plane part of the structure 3 is not attached, and 2d is the lower flange 1 of the H-section steel 1 to which the plane part of the structure 3 is not attached.
c is a fire-resistant coating material applied to the back surface.

In the present invention, both ends are simply supported,
With regard to the H-beam 1 which is a steel beam having an H-shaped cross-section in which the plane portion of the structure 3 is attached to the flange surface of one flange 1b, the center of the H-beam 1 is located at both flanges 1 of the structure 3.
b, 1c, except for the edge surfaces, which are all refractory coated with the refractory coating material 2.
The entire surface of the structure 3 is formed with a fireproof coating material 2 except for an edge surface of the flange 1b to which the flat portion is attached and at least a flange portion 1c or a flange portion 1b to which the flat portion of the structure 3 does not attach. Therefore, the H-section steel 1 which becomes a refractory beam becomes a partial refractory coating, so that the places where the spraying work is difficult are reduced, and the cost can be reduced by reducing the refractory coating area. Further, since the flange surface of the flange 1b and the edge surfaces of the flanges 1b and 1c of the H-section steel 1 to which the plane portion of the structure 3 is attached are not fire-resistant coated, as shown in FIGS. A crane sling by a wire 5 hanging a refractory beam provided with a refractory coating material 2 on the crane 1 and handling when loading with a cushioning material 7 on a truck bed 6 as shown in FIG. 9 are facilitated. This makes it possible to perform the fireproof coating only at the factory, and eliminates the on-site spraying work of the fireproof coating material 2 using the shoring, and the working efficiency is drastically improved.

[0010] The H-section steel 1 which is a steel beam having an H-section with the both ends fixedly supported and the flat portion of the structure 3 attached to the flange surface of one of the flanges 1b. Part and both ends are both flanges 1b of the structure 3,
1c, the entire surface of the H-shaped steel 1 is covered with the fire-resistant coating material 2 except for the edge surface of the flange 1b. The entire structure 3 is formed by fire-resistant coating with a fire-resistant coating material 2 except for a flange 1c portion or a flange 1b portion to which no flat portion is attached. Therefore, similarly to the refractory beam whose one end is supported, the H-section steel 1 serving as the refractory beam has a partial refractory coating, so that the places where the spraying work is difficult are reduced, and the cost reduction due to the reduction of the refractory coating area is achieved. In addition, it is easier to handle fire-resistant beams when crane slinging and loading trucks, enabling fire-resistant coating only at factories, eliminating the need for on-site spraying of fire-resistant coating materials using shoring. Work efficiency can be drastically improved.

In this manner, the H-section steel, which is an H-shaped steel beam having an H-shaped cross section, in which both ends are simply supported or fixedly supported and the flat portion of the structure is attached to one of the flange surfaces, can be partially covered with fireproof coating. The reason was as follows. The reason for applying the refractory coating is to prevent a steel frame having a high temperature and reduced strength at the time of fire from being unable to support a load. However, since the residual proof stress of each part of the steel frame at the time of fire differs depending on the difference in stress applied to each part at the time of fire, it is desirable to change the fireproof covering state of each part according to the stress of each part of the steel frame at the time of fire. For example, a portion to which a large stress is applied may be provided with a refractory coating, but a portion to which only a relatively small stress is applied may be omitted. As a guideline for omitting the refractory coating, it is necessary to maintain the residual proof stress, and that the various stresses resulting from the load do not exceed the allowable stress. That is,
If the allowable stress ratio (load stress / allowable stress) is 1.0 or less, it is considered that the refractory coating on the portion can be omitted.

Therefore, it is necessary to first check the always allowable stress ratio in each part when a load is applied to the beam to determine which part the refractory coating can be omitted. Since there are the following three allowable stress degree ratios, it is necessary to check each of them. It is the shear stress, bending stress,
It is a combination stress degree. When the beam cross section is asymmetric with respect to the neutral axis, the section modulus differs between the compression side and the tension side, resulting in different bending stress levels. However, since the lateral buckling on the compression side must be taken into account, the allowable stress levels differ between the compression side and the tension side. For the above reasons, it is necessary to check the compression side and the tension side.

Next, the H-section steel shown in FIG.
(4) Check each allowable stress ratio at the time of fire when each of the fireproof coatings is applied in the manner shown in (d), and appropriately select the fireproof coating mode so that the allowable stress ratio at each part of the beam is 1.0 or less. Then, it is possible to omit the partial fireproof coating. That is, the specific selection is, for example, as shown in FIG.
In the embodiment shown in (b), only the web 1a of the H-section steel 1 is coated with the fire-resistant coating material 2a. In this case, in the case of a fire, since the upper and lower flanges 1b and 1c are not coated with the fireproof, the temperature becomes high, the predetermined strength cannot be maintained, and the predetermined strength can be maintained by hatching in FIG. Only the effective section D shown in FIG.

In consideration of the lateral buckling, the bending stress on the compression side is generated in the flange. Therefore, in FIG. 2B having no flange, the bending stress degree becomes zero, and it cannot withstand the bending load. In other words, it is not possible to use a refractory coating in a mode as shown in FIG. In addition, in the embodiments shown in FIGS. 1 (c) and 1 (d), the flange is provided with a fireproof coating on each of the portions where the bending stress on the compression side is generated, and (c) is a positive bending, D) will be used for negative bending. D in (a), (c), and (d) of FIG. 2 indicate effective cross sections corresponding to (a), (c), and (d) of FIG.

First, a case where a concentrated load P acts on the center of a span in a simple beam having a span of L and a support condition of a pin at one end and a roller at the other end will be described. FIG. 3A shows a case where a concentrated load P acts on the center of the span.
(b) shows the state of the bending moment at this time, and (c) shows the state of the shear force at this time. The bending moment when the concentrated load P acts on the center of the span is maximum at the center and decreases as approaching the end. Also,
The shear force is zero at the center and P / 2 at half the maximum from there to the end. Thus, the external force distribution is not uniform. Therefore, in the event of a fire, the fireproof coating is applied as shown in Fig. 1 (a) because the load capacity is the maximum in the central part. However, the load resistance may be small at the end, and the lower flange is used since it is bent over the entire length. Can be omitted, and the coating may be performed as shown in FIG. Therefore, the cross section of the beam at the time of refractory coating in the span direction is checked with the allowable stress ratio, so that the center of the beam is refractory coated as shown in FIG. The portion can be partially covered with a fireproof coating as shown in FIG. On the other hand, in the case of negative bending, the stress distribution is opposite to the state shown in FIG. 3, so that the central part of the beam is a refractory coating as shown in FIG. 1A and both ends of the beam are shown in FIG. A refractory coating like

Next, the supporting conditions are fixed at both ends, and the span is L
An example in which a concentrated load P acts on the center of the span in the beam of FIG. 5A shows a case where a concentrated load P acts on the center of the span, FIG. 5B shows a state of the bending moment at this time, and FIG. 5C shows a state of the shear force at this time. . The bending moment when the concentrated load P acts on the center of the span becomes smaller as it approaches the quadrant. The shear force is zero at the center and P / 2, which is half of the maximum value from there to the end.

The refractory coating in the event of a fire is performed as follows. First, since the withstand load is the largest at the center and both ends, the fireproof coating is performed as shown in FIG. Next, although the vicinity of the center of the beam is forward-bent, the portion where the bending moment is gradually reduced can omit the covering of the flange. Considering the forward bending, as shown in FIG. Cover. In addition, since the vicinity of the end of the beam is negatively bent and the bending moment is smaller than the maximum, the portion may be covered with a fireproof coating as shown in FIG. Therefore, the beam cross section at the time of fireproof coating in the span direction is checked with the allowable stress ratio,
As shown in FIG. 6, the center and both ends of the beam are coated with fireproof as shown in FIG. 1A, the center of the beam is coated with fireproof as shown in FIG. As described in (d) above, it is possible to perform partial coating by refractory coating. In addition, for the actual beam in the case of simple support and fixed support, it is problematic at which position the fireproof coating type is partially omitted.
This will be described in the embodiment section.

[0018]

Examples are shown below. (Embodiment 1) First, a description will be given of an H-section steel beam having a support condition of a pin at one end and a roller at the other end and a span of 5 m. When a concentrated load 2t acts on the center of the span of this beam, the bending moment is zero at the end of the span, + 2.5t at the center of the span, the shearing force is 1t at the end of the span, and 1t at the center of the span, as shown in FIG. 2t. The beam is assumed to be SS400 class and its cross section is assumed to be H-250 × 125 × 6 × 9 (H-section steel height × flange width × web thickness × flange thickness). Table 1 shows the effective shear area and the section modulus when each of them was coated with a fireproof coating as shown in (a) to (d) of FIG.

[0019]

[Table 1]

FIG. 11 shows the normal allowable stress ratios when the refractory coating shown in FIG. FIG. 12 shows the allowable stress ratios at the time of fire when the fireproof coatings shown in FIGS. 1 (a) and 1 (c) are applied. These allowable stress ratios are calculated based on the following mathematical expressions. 1) Allowable stress ratio of shear stress τs / fs ≦ 1.0, τ
s = (Q / A) τs: shear stress, fs: allowable shear stress, Q: shear force, A: effective shear area 2) Permissible stress ratio of compressive bending stress c σb / c fb
≦ 1.0, c σb = (M / Zc) where c σb: compression side bending stress, c fb: compression side allowable bending stress, M: bending moment, Zc: compression side section modulus, tensile side bending stress Allowable stress ratio t σb / tfb ≦
1.0, t σb = (M / Zt) where t σb: Tensile bending stress, t fb: Tensile allowable bending stress (= ft) Zt: Tensile section modulus 3) Combined stress: One load Combination of shear stress and bending stress when acting only in the direction (σb ²
+ 3τs ² ) / ft ² ≦ 1.0 where σb: the larger value of c σb and t σb ft: Allowable tensile stress (allowable bending stress on the tensile side)

The allowable stress at the time of a fire is the yield point strength of a steel frame at a temperature caused by the fire characteristics due to the shape of the building. Here, it is assumed that the allowable stress is a short-term allowable stress. The short-term allowable stress generally refers to an allowable tensile stress during an earthquake, a storm, a snowfall, or the like. Since the normal allowable stress ratios shown in FIG. 11 are not at the time of fire, the effective cross section is the entire cross section of FIG. Each allowable stress ratio was 1.0 or less, indicating that there was sufficient residual proof stress. Here, having the residual proof stress means that the stress is within the allowable stress when a certain external force is applied, that is, the allowable stress ratio is less than 1.0.

Referring to the allowable stress ratios in the event of a fire shown in FIG. 12, when the fireproof coating shown in FIG. 1A is used, the allowable stress ratios are 1.0 or less, and the load bearing capacity is sufficient. There is. When the fireproof coating shown in FIG. 1C is applied, the allowable stress ratio between the bending tension and the combined force exceeds 1.0 at the center.
As a result, when the entire span is 1 as shown in FIG. 13, 0.24 at the center is the fireproof coating shown in FIG.
That is, both ends may be the fire-resistant coatings shown in FIG. The coating thickness of the refractory coating material varies depending on the target refractory time and the type of the refractory material.
mm or more and 65 mm or more for 3 hours are appropriate. Therefore, in the H-shaped steel to be coated with the fireproof, the places where the spraying work is difficult are reduced, and the cost can be reduced by reducing the area of the fireproof coating. In addition, since the flange surface of the flange to which the flat portion of the H-section steel structure is attached and the edge surfaces of both flanges are not fire-resistant coated, it is easy to handle the fire-resistant beam after the fire-resistant coating during crane slinging and truck loading. As a result, it becomes possible to perform fireproof coating only at the factory, and it is possible to eliminate the on-site spraying work of the fireproof coating material using the shoring, and the working efficiency is drastically improved.

(Embodiment 2) Next, a beam having a span of 5 m with fixed support conditions at both ends will be described. When a concentrated load 4t acts on the center of the span of this simple beam, the bending moment is +2.5 tm at the center of the span, as shown in FIG.
-2.5 tm at the end, 1 t of shear force at the end of the span,
It is 2t at the center of the span. The beam is assumed to be SS400 class and its cross section is assumed to be H-250 × 125 × 6 × 9 (H-section steel height × flange width × web thickness × flange thickness). FIG.
The effective shear area and section modulus when the fireproof coating is applied as shown in (a) to (d) are the same as in Table 1. FIG. 15 shows the normal allowable stress ratios when the refractory coating shown in FIG. FIG. 16 shows each allowable stress ratio at the time of fire when the fireproof coating shown in FIG. FIG. 17 shows the respective allowable stress ratios at the time of fire when the fireproof coatings shown in (c) and (d) of FIG. 1 are used. These allowable stress ratios are calculated based on the mathematical formula shown in the first embodiment.

As shown in FIG. 15, each of the allowable stress ratios is always 1.0 or less, indicating that there is sufficient residual proof stress. Also, looking at each allowable stress degree ratio at the time of fire shown in FIG. 16, when the fireproof coating of FIG. 1A is applied, each allowable stress degree ratio is 1.0 or less, and a sufficient residual proof stress is obtained. is there. Also, looking at the respective allowable stress ratios at the time of fire shown in FIG. 17, when the refractory coating shown in FIG. 1C is located at the center, the allowable stress ratio of bending tension and combined force is 1.0. There is a place that exceeds. When the refractory coating shown in FIG. 1 (d) is at the end, the allowable stress ratio between the bending tension and the combined force is 1.0%.
There is a place that exceeds. As a result, as shown in FIG. 18, when the allowable stress ratio is 1 for all spans, 0.12
Is the fireproof coating of FIG. 1A, 0.03 at both ends is the fireproof coating of FIG. 1A, 0.19 near the center is the fireproof coating of FIG. The area around the part is (d) in Fig. 1.
And a partial fireproof coating.

Therefore, in this embodiment as well, the H-shaped steel coated with the fire-resistant coating is reduced in the places where the spraying operation is difficult, and the cost can be reduced by reducing the area of the fire-resistant coating. Moreover, since the flange surface of the flange to which the flat portion of the H-section steel structure is attached and the edge surfaces of both flanges are not coated with fireproof,
Easier handling of fire-resistant beams after crane slinging and truck loading after fire-resistant coating makes it possible to apply fire-resistant coating only at factories, eliminating the need for onsite spraying of fire-resistant coating materials using shoring. Work efficiency can be drastically improved.

[0026]

As described above, according to the present invention, a steel beam having an H-shaped cross section in which both ends are simply supported or fixedly supported, and the flat surface of the structure is attached to the flange surface of one flange, Since it is possible to perform fire-resistant coating with a typical fire-resistant coating material, the places where it is difficult to spray H-shaped steel beams can be reduced, the cost can be reduced by reducing the fire-resistant coating area, and the H-shaped steel frame Since the flange surface of the flange to which the flat part of the beam structure is attached and the edge surface of both flanges are not fire-resistant coated, it becomes easier to handle the fire-resistant beam after the fire-resistant coating during crane slinging and truck loading, This makes it possible to perform fireproof coating only at the factory, eliminating the need for spraying the fireproof coating material at the site using the shoring work, and has the effect that the working efficiency is drastically improved.

[Brief description of the drawings]

FIG. 1 is a refractory beam provided with a refractory coating material.
It is sectional drawing which shows the various refractory coating states of (d).

FIGS. 2A to 2D are explanatory views showing effective cross sections in the fireproof coating state of FIGS.

FIG. 3 is an explanatory diagram showing an operating state when a concentrated load is applied to the center of a span of a beam in which both ends are simply supported, and a bending moment and a shearing force in each part.

FIG. 4 is a cross-sectional view showing a state of each part provided with a fire-resistant covering material of a fire-resistant beam whose both ends are simply supported according to the present invention.

FIG. 5 is an explanatory diagram showing an operating state when a concentrated load is applied to a center of a span of a beam in which both ends are fixedly supported, and a bending moment and a shearing force in each part.

FIG. 6 is a cross-sectional view showing a state of each part provided with a fire-resistant covering material of a fire-resistant beam having both ends fixedly supported according to the present invention.

FIG. 7 is a perspective view showing a state in which a refractory beam whose both ends are simply supported according to the present invention is transported.

FIG. 8 is a sectional view taken along line AA of FIG. 7;

FIG. 9 is a cross-sectional view showing a state in which a refractory beam in which both ends are simply supported according to the present invention is loaded on a track.

FIG. 10 is an explanatory view showing an operation state when a concentrated load is applied to a center of a span of a beam whose both ends are simply supported according to the present invention, and a bending moment and a shear force at each portion.

FIG. 11 is an explanatory diagram showing each allowable stress degree ratio at all times when the refractory coating shown in FIG.

FIG. 12 is an explanatory diagram showing respective allowable stress ratios at the time of a fire when the fireproof coating shown in (a) and (c) of FIG. 1 is applied.

FIG. 13 is an explanatory diagram showing a state of a fireproof coating in each portion when the entire span of a beam whose both ends are simply supported is set to an allowable stress ratio of 1 according to the present invention.

FIG. 14 is an explanatory diagram showing an operation state when a concentrated load is applied to a center of a span of a beam having both ends simply supported according to the present invention, and a bending moment and a shear force at each portion.

FIG. 15 is an explanatory diagram showing each allowable stress degree ratio at all times when the refractory coating shown in FIG.

FIG. 16 is an explanatory diagram showing respective allowable stress intensity ratios at the time of fire when the refractory coating shown in FIG. 1A is applied.

FIG. 17 is an explanatory diagram showing respective allowable stress ratios at the time of fire when the fireproof coatings shown in (d) and (c) of FIG. 1 are applied.

FIG. 18 is an explanatory view showing a state of a fireproof coating in each part when the entire span of a beam having both ends fixedly supported is set to an allowable stress ratio of 1.

FIG. 19 is a process chart of a conventional composite fire-resistant coating method for a steel structure.

FIG. 20 is an enlarged sectional view of a rock wool molded plate.

[Explanation of symbols]

 Reference Signs List 1 H-section steel 1a Web 1b Upper flange 1c Lower flange 2 Fireproof coating 2a Fireproof coating applied to web 2b Fireproof coating applied to back surface of upper flange 2c Fireproof coating applied to surface of lower flange 2d Fireproof coating on the back of the lower flange 3 Structure

────────────────────────────────────────────────── (5) References JP-A-63-223246 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) E04B 1/94

Claims (4)

(57) [Claims] 1. A central portion of an H-shaped steel beam having both ends supported in a simple support state and a flat surface of a structure attached to a flange surface of one flange is an edge surface of both flanges of the structure. All are fire-resistant coated with fire-resistant coating material,
A part other than the central part of the steel beam is coated with a fire-resistant coating material except for an edge surface of a flange to which a plane part of the structure is attached and at least a flange part to which a plane part of the structure is not attached. Features refractory beams. 2. A steel beam beam having an H-shaped cross-section in which both ends are fixedly supported and a flat surface of a structure is mounted on a flange surface of one flange, and both ends of the steel beam have both flanges of the structure. The entire surface of the steel beam is covered with a fire-resistant coating material except for the edge surface, and the portion other than the central portion and both end portions of the steel beam is the edge surface of the flange to which the flat portion of the structure is attached and at least the flat portion of the structure is attached. A refractory beam, characterized in that it is entirely coated with a refractory material except for a flange portion. 3. A central portion of an H-shaped steel beam having both ends supported in a simple supporting state and a flat surface of a structure attached to a flange surface of one flange is an edge surface of both flanges of the structure. All are fire-resistant coated with fire-resistant coating material,
The portion other than the central portion of the steel beam is coated with a fire-resistant coating material except for the edge surface of the flange to which the plane portion of the structure is attached and at least the flange portion to which the plane portion of the structure is attached. And refractory beams. 4. A central portion and both ends of an H-shaped steel beam in which both ends are fixedly supported and a flat portion of a structure is attached to a flange surface of one of the flanges. All of the steel beams are covered with a fire-resistant coating material except for the edge surfaces, and the portions other than the central portion and both end portions of the steel beam are fitted with the edge surface of the flange to which the flat portion of the structure is attached and at least the flat portion of the structure. A refractory beam characterized in that all parts except for a flange portion are coated with a refractory coating material.