Design method of fire protection layer of rectangular concrete-filled steel tubular column based on bearing capacity method

Abstract

The application discloses a design method of a fireproof protection layer of a rectangular steel pipe concrete column based on a bearing capacity method. The method comprises the following steps: designing the thickness and material of the fireproof protection layer of the steel pipe concrete column, determining the steel temperature and the concrete temperature of the steel pipe concrete column, obtaining the strength and the elastic modulus of the material under the fire condition by using the material temperature, determining the most unfavorable load effect combination design value of the steel pipe concrete column, processing to obtain the axial compression stability bearing capacity of the steel pipe concrete column under the fire condition, obtaining the residual bearing capacity of the steel pipe concrete column, judging whether the steel pipe concrete column meets the fireproof design requirement by using the residual bearing capacity, and designing the steel pipe concrete column by using the thickness of the fireproof protection layer meeting the fireproof design requirement. The application provides a bearing capacity judgment method for the fireproof design and the fire resistance checking of the rectangular steel pipe concrete column, the method is in good agreement with the numerical analysis result, and the method has strong applicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of structural design, and particularly relates to a design method of a fire protection layer of a building engineering structural member. BACKGROUND

[0002] A concrete-filled steel tubular column is formed by filling concrete in a hollow steel tube, and can be divided into a circular concrete-filled steel tubular column, a square concrete-filled steel tubular column and a rectangular concrete-filled steel tubular column according to different cross-sectional forms. In a stress process, the constraint of the steel tube on the internal concrete makes the concrete in a three-way compression state, thereby improving the compressive strength and deformation resistance of the concrete. The internal concrete of the steel tube can effectively prevent the steel tube from locally buckling, so that the concrete-filled steel tubular column has excellent mechanical properties, specifically high bearing capacity and high ductility, and is widely applied in multi-story and high-rise steel structure buildings.

[0003] Fire protection of steel structures has always been a concern, and there are still many deficiencies in the current fire design method for concrete-filled steel tubular columns. Two fire calculation methods, namely critical temperature method and bearing capacity method, are given in the Technical Code for Fire Protection of Steel Structures GB 51249-2017 for pure steel columns, but these two methods are not applicable to concrete-filled steel tubular columns. Because these two methods both assume that the cross section of the member under fire is in a uniform heating state, which is basically true for pure steel columns, but for concrete-filled steel tubular columns, the internal concrete can absorb heat, and existing research shows that the cross section of the concrete-filled steel tubular column is not uniformly heated, specifically the temperature of the outer steel tube is high, and the temperature of the internal concrete is low, so the cross section does not meet the assumption of uniform heating, and therefore the critical temperature method and the bearing capacity method given in GB 51249-2017 cannot be used for fire design and fire resistance calculation of concrete-filled steel tubular columns.

[0004] In the "Technical Code for Fire Protection of Steel Structures" GB 51249-2017 Chapter 8, in addition to the formula for calculating the thickness of the fire protection layer of the concrete-filled steel tube column, but the formula has many problems in use. First, the specification gives a fitting formula obtained by a large number of examples, the application scope of this formula is limited. Taking the commonly used rectangular concrete-filled steel tube column as an example, the specification 8.1.1 clearly points out that the premise of using the formula is that the column section width is 200mm~1400mm, the aspect ratio is 1~2, the steel ratio is 0.04~0.2, and the slenderness ratio is 10~60. The above parameter values cannot cover all the situations encountered in the current engineering. For example, in order to save indoor space and avoid convex beams and columns, the column section form with relatively wide and narrow is often used in actual engineering, that is, the column section width is generally 160mm~200mm, and the aspect ratio is 1.0~4.0. The calculation shows that the existing method is not applicable to this case. Secondly, the existing specification is under the premise of ISO834 standard temperature rising curve, and the formula for calculating the thickness of the fire protection layer of the concrete-filled steel tube column is proposed. If the actual fire temperature rising curve is used for fire protection layer thickness design, due to the difference in calculation premise, the specification formula is no longer applicable to this case. Finally, the specification only gives the calculation formula of the protection layer thickness when the non-expanding fireproof coating and cement mortar are used as the fireproof material, and other materials such as concrete can also be used for fire protection according to the specific situation in engineering. Since the specification gives the fitting formula for two specific materials, when other fireproof materials are used as the protection layer, it is beyond the scope of the specification, so it is difficult to quantitatively calculate the thickness of the protection layer of other fireproof materials in actual engineering. Generally, only experimental research or numerical simulation can be used, but these two methods are difficult for ordinary technical personnel to master, and bring great cost and time cost. In addition, there is no more general design method for the fire protection of the concrete-filled steel tube column from other materials. SUMMARY

[0005] In order to solve the technical problems encountered in the fire protection design of the concrete-filled steel tube column, the present application provides a design method for the fire protection layer of the rectangular concrete-filled steel tube column based on the bearing capacity method, which has a wider application scope and is more convenient for designers to operate, and solves the various problems encountered in the fire protection design of the rectangular concrete-filled steel tube column.

[0006] The present application scheme is as follows, including the following steps, as shown in Figure 1

[0007] Step (1): According to the design fire resistance rating of the concrete-filled steel tube column to be designed for fire protection, the thickness and material of the initial fire protection layer are determined;

[0008] Step (2): Obtain the design fire resistance limit t of the concrete-filled steel tube column under the initial fire protection layer thickness and material;Step (3): If t is less than the design fire resistance limit t of the concrete-filled steel tube column, the thickness and material of the initial fire protection layer are adjusted, and the design fire resistance limit t of the concrete-filled steel tube column under the adjusted thickness and material of the fire protection layer is obtained; Step (4): If t is greater than or equal to the design fire resistance limit t of the concrete-filled steel tube column, the thickness and material of the fire protection layer are determined.d Steel temperature T at time s and concrete temperature T c ;

[0009] Step (3): according to the concrete temperature T c get the strength and elastic modulus of the concrete under fire conditions, and according to the steel temperature T s processing to get the elastic modulus of the steel under fire conditions;

[0010] Step (4): according to the specification processing to get the most unfavorable load effect combination design value of the concrete filled steel tubular column, and the most unfavorable load effect combination design value is taken as the maximum axial force N max ;

[0011] The specification is specifically the provisions of article 3.2.2 of the Technical Code for Fire Protection of Building Steel Structures GB51249.

[0012] Step (5): obtain the axial compression stability bearing capacity N maxT of the concrete filled steel tubular column under fire conditions;

[0013] Step (6): first, the residual bearing capacity n of the concrete filled steel tubular column is processed, and then whether the fire protection layer of the concrete filled steel tubular column meets the fire design requirements is judged according to the residual bearing capacity n, if the fire protection design requirements are met, the fire protection layer design of the concrete filled steel tubular column is carried out by using the fire protection layer.

[0014] If the fire protection design requirements are not met, the thickness of the fire protection layer is increased, and the steps (2) and (6) are returned in order to rejudge whether the fire protection layer meets the fire design requirements, until the fire protection layer meets the fire design requirements, and the fire protection layer design of the concrete filled steel tubular column is completed.

[0015] The method of the application obtains the final thickness of the fire protection layer by repeating the steps (2) to (6), stops iteration when the fire protection layer of the concrete filled steel tubular column meets the fire design requirements, and takes the last obtained thickness of the fire protection layer as the final thickness of the fire protection layer, and constructs the concrete filled steel tubular column according to the final thickness of the fire protection layer.

[0016] The step (2) is specifically:

[0017] Step 2.1: if the concrete filled steel tubular column is not provided with a fire protection layer around, that is, the thickness of the fire protection layer is 0, the highest steel temperature T s of the steel plate in the concrete filled steel tubular column is iteratively processed according to the following formula:

[0018]

[0019]

[0020]

[0021]

[0022] In the formula:

[0023] ΔT s represents the temperature rise of the steel plate in time (t, t+Δt), unit (℃);

[0024] t represents the fire time, unit (h);

[0025] Δt represents the time step, unit (s), which should not be greater than 5s;

[0026] α represents the comprehensive heat transfer coefficient, unit [W / (m 2 ·℃)];

[0027] ρ s ,c s respectively represent the density of steel and the specific heat of steel;

[0028] F represents the fire surface area per unit length of the steel plate, unit (m 2 );

[0029] V represents the volume per unit length of the steel plate, unit (m 3 );

[0030] T s ,T g respectively represent the average temperature of the steel at t and the average temperature of the hot flue gas, unit (℃);

[0031] α c represents the heat convection coefficient, which is 25 W / (m 2 ·℃);

[0032] α r represents the heat radiation coefficient, unit [W / (m 2 ·℃)];

[0033] μ represents the heat capacity ratio of concrete to steel;

[0034] b represents the cross-sectional width of the concrete-filled steel tube column, unit (m);

[0035] ε r represents the comprehensive emissivity, which is 0.7;

[0036] σ represents the Stefan-Boltzmann constant, which is 5.67×10 -8 W / (m 2 ·℃)4 );

[0037] A c A s These represent the cross-sectional area of ​​concrete and the cross-sectional area of ​​steel, respectively, in mm². 2 );

[0038] ρ c ,c c These represent the density of concrete and the specific heat of steel, respectively.

[0039] The iterative process is specifically as follows:

[0040] Step 2.1.1: Using the initial steel temperature T s The temperature rise ΔT of the steel plate within a time step Δt is obtained. s ;

[0041] Step 2.1.2: Set the steel temperature T s Updated to the initial steel temperature T s With temperature rise ΔT s sum;

[0042] Step 2.1.3: Repeat the above steps to adjust the steel temperature T. s Iterative calculations were performed to obtain the steel temperature T at different times. s And will design fire resistance limit t d The temperature of the steel at time T s As the final steel temperature T s And used in the following steps of processing.

[0043] Step 2.2: If the steel-concrete composite column is not surrounded by a fireproof protective layer (i.e., the thickness of the fireproof protective layer is 0), the highest concrete temperature T in the steel-concrete composite column is obtained using the following formula. c :

[0044] T c =T0+θ c (T s -T0)

[0045]

[0046]

[0047] ζ = 11 + 46e -1.3t

[0048] In the formula:

[0049] T c The average temperature of the concrete at time t is expressed in degrees Celsius (°C).

[0050] T0 represents the initial ambient temperature, unit (℃);

[0051] θ c represents the temperature rise ratio;

[0052] φ represents the cross-sectional shape coefficient of concrete;

[0053] ζ is the temperature rise influence coefficient of concrete;

[0054] h represents the column cross-sectional height, unit (m);

[0055] t c represents the thickness of the steel pipe, unit (m);

[0056] e represents the natural constant;

[0057] Step 2.3: If the steel pipe concrete column is provided with a fire protection layer, the highest steel temperature T s in the steel pipe concrete column is obtained by iterative processing according to the following formula:

[0058]

[0059] If

[0060] If

[0061]

[0062] In the formula:

[0063] F i represents the fire surface area per unit length of the steel member, unit (m 2 );

[0064] η represents the heat capacity ratio of the fire protection layer to the cross-sectional steel pipe;

[0065] λ i represents the thermal conductivity of the fire protection layer, unit [W(m·℃)];

[0066] d i represents the thickness of the fire protection layer, unit (m);

[0067] c i represents the specific heat capacity of the fireproof material, unit [J / (kg·℃)];

[0068] ρ i represents the density of the fireproof material, unit (kg / m 3 );

[0069] The iterative processing in step 2.3 is synchronized with step 2.1;

[0070] Step 2.4: If the steel tube concrete column is provided with fire protection layer, the highest concrete temperature T of the concrete in the steel tube concrete column is obtained by processing according to the following formula c :

[0071] T c = T0+ θ c (T s -T0)

[0072]

[0073]

[0074] The step (3) is specifically:

[0075] Step 3.1 elastic modulus E of steel under fire condition sT The following formula is used to obtain:

[0076] E sT = χ sT E s

[0077]

[0078] In the formula:

[0079] E sT represents the elastic modulus of steel under fire condition, unit (N / mm 2 );

[0080] χ sT represents the elastic modulus reduction coefficient of steel under fire condition;

[0081] E s represents the elastic modulus of steel at normal temperature, unit (N / mm 2 ), which should be taken according to the provisions of the current national standard "Code for Design of Steel Structures" GB 50017;

[0082] T s represents the steel temperature of the component at the design fire resistance limit t d ;

[0083] The above step is specifically to determine the elastic modulus of steel under fire condition by using the formula in clause 5.1.3 of "Technical Code for Fire Protection of Steel Structures" GB 51249.

[0084] Step 3.2 strength f of concrete under fire condition ck T The following formula is used to obtain:

[0085]

[0086] wherein:

[0087] f ck represents the standard value of the strength of concrete at normal temperature, unit (N / mm 2 ) ;

[0088] T c represents the temperature of concrete at the time of the design fire resistance limit t d ;

[0089] The standard value of the compressive strength of concrete under fire condition is taken from the Technical Code for Concrete Filled Steel Tubular Structures GB50936 Appendix E.0.2.

[0090] Step 3.3 The elastic modulus E c T of concrete under fire condition is processed by using the following formula:

[0091]

[0092] wherein:

[0093] E c represents the elastic modulus of concrete at normal temperature, unit (MPa) ;

[0094] T c represents the temperature of concrete at the time of the design fire resistance limit t d ;

[0095] The value of the elastic modulus of concrete under fire condition is taken from the Technical Code for Concrete Filled Steel Tubular Structures GB50936 Appendix E.0.2.

[0096]

[0097] The step (5) is specifically: the axial compression stability bearing capacity N maxT of the concrete filled steel tubular column under fire condition is processed by using the following formula:

[0098]

[0099]

[0100] Φ = 0.5 [1 + 0.65 (λ n - 0.2) + λ n 2 ]

[0101] N uT = A s f yT + A c f cT

[0102] ​

[0103]

[0104] f yT = η sT f y

[0105]

[0106] wherein:

[0107] Kf represents the stability coefficient of the axial compression member under fire conditions;

[0108] N uT Kf represents the compression bearing capacity of the column section under fire conditions, in units of (N);

[0109] Φ represents the regularized slenderness ratio influence coefficient;

[0110] λ n represents the regularized slenderness ratio of the concrete-filled steel tube column;

[0111] f y represents the yield strength of the steel at room temperature, in units of (N / mm 2 );

[0112] f yT represents the yield strength of the steel under fire conditions, in units of (N / mm 2 );

[0113] f cT represents the compressive strength of the concrete under fire conditions, in units of (N / mm 2 );

[0114] N ET represents the Euler critical force of the column under fire conditions, in units of (N);

[0115] E sT represents the elastic modulus of the steel under fire conditions, in units of (N / mm 2 );

[0116] E cT represents the elastic modulus of the concrete under fire conditions, in units of (N / mm 2 );

[0117] I s represents the moment of inertia of the steel tube section, in units of (mm 4 );

[0118] I c represents the moment of inertia of the concrete section, in units of (mm 4 );

[0119] lo represents the calculated length of the axial compression member, which is the height of the member (i.e. column) in mm;

[0120] η sT represents the strength reduction factor of the steel under fire conditions.

[0121] The step (6) is specifically:

[0122] The residual bearing capacity n and the fire load ratio R of the concrete-filled steel tubular column are determined:

[0123] The residual bearing capacity n is the axial compression stability bearing capacity N of the column under fire conditions maxT The bearing capacity N of the composite column at normal temperature u The ratio of the maximum axial force N of the composite column under fire conditions max The bearing capacity of the composite column at normal temperature

[0124]

[0125]

[0126] N u =A s f y +A c f ck

[0127] In the formula:

[0128] N u represents the bearing capacity of the column section at normal temperature, in N;

[0129] N max represents the maximum axial force of the concrete-filled steel tubular column under fire conditions;

[0130] f ck represents the standard value of the strength of the concrete at normal temperature, in N / mm 2 ;

[0131] If the residual bearing capacity n is greater than or equal to the fire load ratio R, it indicates that the fire resistance bearing capacity of the concrete-filled steel tubular column meets the requirements, and the fire protection layer of the concrete-filled steel tubular column meets the design requirements, and the fire protection layer design of the concrete-filled steel tubular column is performed using the fire protection layer;

[0132] If the residual bearing capacity n is less than the fire load ratio R, it indicates that the fire resistance bearing capacity of the concrete-filled steel tubular column does not meet the requirements, and the fire protection layer of the concrete-filled steel tubular column does not meet the design requirements, and the thickness of the fire protection layer is increased, and the fire protection layer is re-determined whether it meets the fire protection design requirements in the order of steps (2) to (6).

[0133] The steel pipe concrete column is mainly composed of concrete poured in a rectangular steel pipe, and the ratio of the long side to the short side of the rectangular steel pipe is 1-4.

[0134] The fireproof protective layer adopts non-intumescent fire retardant coating, cement mortar, concrete, intumescent fire retardant coating, aerated concrete block, fireproof plate, gypsum-based fireproof slurry or flexible felt-like thermal insulation material, etc.

[0135] The method of the present application establishes a simplified algorithm for the average temperature of the cross section of the rectangular steel pipe concrete column and a formula for the axial compression stability bearing capacity of the rectangular steel pipe concrete column under fire, and proposes a bearing capacity judgment method for the fireproof design and fire resistance calculation of the rectangular steel pipe concrete column.

[0136] The beneficial effects of the present application are as follows:

[0137] 1. A method for calculating the axial compression stability bearing capacity of a component subjected to fire on four sides of a rectangular steel pipe concrete column is provided, while the prior art can only use numerical simulation method with high calculation cost to solve the problem;

[0138] 2. The problem that the current specification for the fireproof design of the rectangular steel pipe concrete column is only applicable to non-intumescent fire retardant coating and cement mortar as fireproof materials is solved, and the method proposed by the present application is applicable to the cases without fireproof protection and with various fireproof materials, and has wide application range;

[0139] 3. The thickness calculation formula of the fireproof protective layer of the rectangular steel pipe concrete column in the current specification is only applicable to the fire temperature rise according to the ISO834 standard curve, and the bearing capacity method proposed by the present application can be applicable to the calculation of various fire temperature rises, such as high and large space air temperature rise, etc.

[0140] 4. The bearing capacity method for the fireproof design of the rectangular steel pipe concrete column proposed by the present application fills the gap of the existing steel structure fireproof design technology, and the axial compression stability bearing capacity calculation formula of the rectangular steel pipe concrete column under fire is in good agreement with the numerical results, without the need for complex numerical simulation work. The bearing capacity method proposed by the present application can consider the temperature rise of both steel and concrete materials for fire resistance calculation, while the bearing capacity method used in the existing steel structure fireproof design can only be used for pure steel components. The method proposed by the present application greatly improves the simplicity, reliability and safety of the fireproof design of the rectangular steel pipe concrete column, and is easy for designers to master and apply, which is conducive to the further popularization of steel structure buildings. BRIEF DESCRIPTION OF DRAWINGS

[0141] Figure 1 The step flow chart of the present application;

[0142] Figure 2 Fireproof structure schematic diagram of rectangular steel pipe concrete column with non-intumescent fire retardant coating on the outside;

[0143] Figure 3 Fireproof structure schematic diagram of rectangular steel pipe concrete column with cement mortar rendering on the outside;

[0144] Figure 4 Fireproof structure schematic diagram of rectangular steel pipe concrete column with cast fine stone concrete on the outside. DETAILED DESCRIPTION

[0145] The application will be described in further detail below with reference to the drawings and embodiments.

[0146] The embodiments of the application and the specific steps are as follows, as shown in Figure 1 :

[0147] Embodiment 1

[0148] Step (1): The steel plate in the steel pipe concrete column is Q355, the concrete adopts C40 ordinary concrete, the load ratio is 0.3, the column height is 3000 mm, the column section width is 900 mm, the column section height is 900 mm, and the wall thickness is 35 mm. According to the Building Design Fireproofing Code GB50016-2014, the fire resistance grade of the column is determined to be four, and the fire resistance limit is 0.5 h; Step (2): according to the column section temperature field formula of the rectangular steel pipe concrete column when the four sides are on fire, the steel temperature T s and the concrete temperature T c of the rectangular steel pipe concrete column at 0.5 h are obtained:

[0149] Steel temperature formula of steel plate:

[0150]

[0151]

[0152]

[0153]

[0154] Column concrete temperature formula:

[0155] T c = T0+ θ c (T s -T0)

[0156]

[0157]

[0158] ζ = 11 + 46e-1.3t

[0159] According to the above formula, the steel plate temperature T s = 460°C, and the concrete temperature T c = 75°C.

[0160] Step (3): According to the "Technical Code for Fire Protection of Steel Structures" GB51249, the strength and elastic modulus of steel and concrete at steel temperature T s and concrete temperature T c are respectively E s T = 1.61 x 10 5 N / mm 2 ; f ck T = 26.738 N / mm 2 , E c T = 2.71 x 10 4 N / mm 2 .

[0161] Step (4): According to the specification, the most unfavorable load effect combination design value of the concrete-filled steel tube column is obtained;

[0162] Step (5): According to the following formula, the compressive stability bearing capacity, residual bearing capacity n and fire load ratio R of the column under fire conditions are obtained:

[0163]

[0164] f yT = η sT f y = 0.3465 x 355 = 123 kN / mm 2

[0165] N uT = A s f yT +A c f cT = 3.33 x 10 7 N

[0166]

[0167]

[0168] Φ = 0.5 [1 + 0.65 (λ n - 0.2) + λ n 2 ] = 0.4696

[0169]

[0170]

[0171] N u =A s f y +A c f ck =6.15×10 7 N

[0172]

[0173] The remaining bearing capacity n is greater than the fire load ratio R, indicating that the fire resistance bearing capacity of the steel-concrete composite column meets the requirements, and the steel-concrete composite column should be constructed according to this fireproof protective layer.

[0174] Example 2

[0175] Step (1): As Figure 2 As shown, the steel plate grade is Q355, the concrete is C40 ordinary concrete, the load ratio is 0.3, the column height is 3000mm, the column section width is 200mm, the height is 700mm, the wall thickness is 12mm, and the exterior is covered with 10mm non-intumescent fireproof coating. According to the "Code for Fire Protection Design of Buildings" GB50016-2014, the fire resistance rating of the column is determined to be Class I, and the fire resistance limit is 3h.

[0176] Step (2): Based on the formula for the temperature field of the column section when the rectangular steel tube concrete column is exposed to fire on all four sides, the steel temperature T of the column at the fire resistance limit of 3 hours is obtained. s and concrete temperature T c :

[0177] The steel temperature of the steel plate:

[0178]

[0179]

[0180]

[0181]

[0182]

[0183] Concrete temperature inside the column:

[0184] T c =T0+θ c (T s -T0)

[0185]

[0186]

[0187]

[0188] According to the above formula, the steel plate temperature T s = 444℃, the concrete temperature T c = 307℃.

[0189] Step (3): According to the "Technical Code for Fire Protection of Steel Structures" GB51249, the strength and elastic modulus of steel and concrete at steel temperature T s and concrete temperature T c are respectively E s T = 1.64 x 10 5 N / mm 2 ; f ck T = 23.19 N / mm 2 , E c T = 1.25 x 10 4 N / mm 2 .

[0190] Step (4): According to the specification, the most unfavorable load effect combination design value of the concrete-filled steel tube column is obtained;

[0191] Step (5): According to the following formula, the compressive stability bearing capacity, residual bearing capacity n and fire load ratio R of the column under fire conditions are obtained:

[0192]

[0193] f yT = η sT f y = 0.3702 x 355 = 131.4 kN / mm 2

[0194] N uT = A s f yT +A c f cT = 5.52 x 10 6 N

[0195]

[0196]

[0197] Φ = 0.5 [1 + 0.65 (λ n - 0.2) + λ n 2 ] = 0.6518

[0198]

[0199]

[0200] N u =A s f y +A c f ck =1.06×10 7 N

[0201]

[0202] The remaining bearing capacity n is greater than the fire load ratio R, indicating that the fire resistance bearing capacity of the steel-concrete composite column meets the requirements, and the steel-concrete composite column should be constructed according to this fireproof protective layer.

[0203] Example 3

[0204] Step (1): As Figure 3 As shown, the steel plate grade is Q355, the concrete is C40 ordinary concrete, the load ratio is 0.5, the column height is 3300mm, the column section width is 160mm, the height is 400mm, the wall thickness is 16mm, and the exterior is covered with 50mm cement mortar. According to the "Code for Fire Protection Design of Buildings" GB50016-2014, the fire resistance rating of the column is determined to be Class I, and the fire resistance limit is 3h.

[0205] Step (2): Based on the formula for the temperature field of the column section when the rectangular steel tube concrete column is exposed to fire on all four sides, the steel temperature T of the column at the fire resistance limit of 3 hours is obtained. s and concrete temperature T c :

[0206] The steel temperature of the steel plate:

[0207]

[0208]

[0209]

[0210]

[0211]

[0212] Formula for concrete temperature inside column:

[0213] T c =T0+θ c (T s -T0)

[0214]

[0215]

[0216]

[0217] According to the above formula, the steel plate temperature T s = 620℃, the concrete temperature T c = 522℃.

[0218] Step (3): According to the "Technical Code for Fire Protection of Steel Structures" GB51249, the strength and elastic modulus of steel and concrete at steel temperature T s and concrete temperature T c are respectively E s T = 8.48 x 10 4 N / mm 2 ; f ck T = 14.92 N / mm 2 , E c T = 6.09 x 10 3 N / mm 2 .

[0219] Step (4): According to the specification, the most unfavorable load effect combination design value of the concrete-filled steel tube column is obtained;

[0220] Step (5): According to the following formula, the compressive stability bearing capacity, residual bearing capacity n and fire load ratio R of the column under fire condition are obtained:

[0221]

[0222] f yT = η sT f y = 0.1573 x 355 = 55.9 kN / mm 2

[0223] N uT = A s f yT + A c f cT = 1.65 x 10 6 N

[0224]

[0225]

[0226] Φ = 0.5 [1 + 0.65 (λ n - 0.2) + λn 2 ] = 0.7460

[0227]

[0228]

[0229] N u =A s f y +A c f ck =7.21×10 6 N

[0230]

[0231] If the remaining bearing capacity n is less than the fire load ratio R, it means that the fire resistance bearing capacity of the column does not meet the requirements. It is necessary to increase the thickness of the fire protection layer and repeat the iterative steps (2)-(6) until the fire protection layer meets the fire protection design requirements.

[0232] Example 4

[0233] Step (1): As Figure 4 As shown, the steel plate grade is Q355, the concrete is C40 ordinary concrete, the load ratio is 0.3, the column height is 2900mm, the column section width is 200mm, the height is 400mm, the wall thickness is 16mm, and the exterior is covered with 80mm concrete. According to the "Code for Fire Protection Design of Buildings" GB50016-2014, the fire resistance rating of the column is determined to be Class I, and the fire resistance limit is 3h.

[0234] Step (2): Based on the formula for the temperature field of the column section when the rectangular steel tube concrete column is exposed to fire on all four sides, the steel temperature T of the column at the fire resistance limit of 3 hours is obtained. s and concrete temperature T c :

[0235] Steel plate temperature formula:

[0236]

[0237]

[0238]

[0239]

[0240]

[0241] Formula for concrete temperature inside column:

[0242] T c =T0+θ c(T s -T0)

[0243]

[0244]

[0245]

[0246] According to the above formula, the steel plate temperature T s = 500℃, the concrete temperature T c = 385℃.

[0247] Step (3): According to the "Technical Code for Fire Protection of Steel Structures" GB51249, the strength and elastic modulus of steel and concrete at steel temperature T s and concrete temperature T c are E s T = 1.50 x 10 5 N / mm 2 ; f ck T = 20.59 N / mm 2 , E c T = 9.63 x 10 3 N / mm 2 .

[0248] Step (4): According to the specification, the most unfavorable load effect combination design value of the concrete-filled steel tube column is obtained;

[0249] Step (5): According to the following formula, the compressive stability bearing capacity, residual bearing capacity n and fire load ratio R of the column under fire condition are obtained:

[0250]

[0251] f yT = η sT f y = 0.3702 x 355 = 131.4 kN / mm 2

[0252] N uT = A s f yT +A c f cT = 5.52 x 10 6 N

[0253]

[0254]

[0255] Φ = 0.5 [1 + 0.65 (λ n - 0.2) + λ n 2 ] = 0.6518

[0256]

[0257]

[0258] N u = A s f y + A c f ck = 1.06 x 10 7 N

[0259]

[0260] The residual bearing capacity n is greater than the fire load ratio R, which indicates that the fire resistance bearing capacity of the column meets the requirements, and the construction of the steel pipe concrete column is performed according to the fire protection layer.

[0261] The content described in the embodiments of the specification is only a list of implementation forms of the inventive concept, and the protection scope of the present application should not be regarded as being limited to the specific forms stated in the embodiments, and the protection scope of the present application also extends to equivalent technical means that can be thought of by those skilled in the art according to the inventive concept.

Claims

1. A design method for fireproof protective layers of rectangular steel-concrete composite columns based on the bearing capacity method, characterized in that, Includes the following steps: Step (1): Determine the initial thickness and material of the fire protection layer based on the design fire resistance rating of the steel-concrete composite column that requires fire protection design; Step (2): Obtain the design fire resistance limit t of the steel-concrete composite column. d The temperature of the steel at time T s and concrete temperature T c ; Step (3): Based on the concrete temperature T c To obtain the strength and elastic modulus of concrete under fire conditions, and simultaneously based on the steel temperature T s The elastic modulus of the steel under fire conditions was obtained through processing; Step (4): According to the specifications, obtain the design value of the most unfavorable load effect combination for the concrete-filled steel tube column, and use the design value of the most unfavorable load effect combination as the maximum axial force N of the concrete-filled steel tube column. max ; Step (5): Obtain the axial compressive stability bearing capacity N of the concrete-filled steel tube column under fire conditions based on the strength and elastic modulus of the concrete and the elastic modulus of the steel. maxT ; Step (6): First, process and obtain the remaining bearing capacity n of the steel-concrete composite column. Then, determine whether the fire protection layer of the steel-concrete composite column meets the fire protection design requirements based on the remaining bearing capacity n. If it meets the fire protection design requirements, then use the fire protection layer to design the fire protection layer of the steel-concrete composite column. If the fire protection design requirements are not met, the thickness of the fire protection layer is increased, and the process returns to step (2) and the fire protection layer is re-evaluated in the order of steps (2) to (6) to determine whether the fire protection layer meets the fire protection design requirements. Step (2) specifically refers to: Step 2.1: If there is no fireproof protective layer around the concrete-filled steel tube column, the highest steel temperature T in the steel plate of the concrete-filled steel tube column is obtained by iterative processing according to the following formula. s : In the formula: ΔT s This represents the temperature rise of the steel plate over time (t, t+Δt). t represents the time spent exposed to fire; Δt represents the time step; α represents the overall heat transfer coefficient; ρ s ,c s These represent the density and specific heat of steel, respectively. F represents the fire-exposed surface area per unit length of steel plate; V represents the volume of a steel plate per unit length; T s ,T g These are the average temperature of the steel and the average temperature of the hot flue gas at time t, respectively. α c Indicates the heat transfer coefficient by heat convection; α r Indicates the coefficient of thermal radiation heat transfer; μ represents the heat capacity ratio of concrete to steel; b represents the cross-sectional width of the concrete-filled steel tube column; ε r Indicates the overall emissivity; σ represents the Stefan-Boltzmann constant; A c A s These represent the cross-sectional area of ​​the concrete and the cross-sectional area of ​​the steel, respectively. ρ c ,c c These represent the density of concrete and the specific heat of steel, respectively. Step 2.2: If there is no fireproof protective layer around the concrete-filled steel tube column, the highest concrete temperature T in the concrete-filled steel tube column is obtained according to the following formula. c : T c =T0+θ c (T s -T0) g=11+46e -1.3t In the formula: T c This represents the average temperature of the concrete at time t; T0 represents the initial ambient temperature; θ c Indicates the temperature rise ratio; φ represents the cross-sectional shape factor of the concrete; ζ is the coefficient of influence of concrete temperature rise; h represents the height of the column section; t c Indicates the thickness of the steel pipe; e represents the natural constant; Step 2.3: If a fireproof protective layer is provided around the concrete-filled steel tube column, the highest steel temperature T in the steel plate of the concrete-filled steel tube column is obtained by iterative processing according to the following formula. s : like like In the formula: F i This represents the fire-exposed surface area per unit length of steel structural member; η represents the heat capacity ratio between the fireproof protective layer and the cross-section steel pipe; λ i This indicates the thermal conductivity of the fireproof protective layer; d i Indicates the thickness of the fire-resistant protective layer; c i This indicates the specific heat capacity of the fire-resistant material; ρ i Indicates the density of fire-resistant materials; Step 2.4: If a fireproof protective layer is provided around the concrete-filled steel tube column, the highest concrete temperature T in the concrete-filled steel tube column is obtained according to the following formula. c : T c =T0+θ c (T s -T0) 2. The design method for fireproof protective layer of rectangular steel tube concrete column based on bearing capacity method according to claim 1, characterized in that: The specific steps (3) are as follows: Step 3.1 Elastic modulus E of steel under fire conditions sT The following formula is used for setting: AND sT =χ sT AND s In the formula: E sT This indicates the elastic modulus of steel under fire conditions. χ sT This represents the reduction factor for the elastic modulus of steel under fire conditions. E s This represents the elastic modulus of steel at room temperature. T s Take the component at the design fire resistance limit t d The temperature of the steel at any given moment; Step 3.2 Strength f of concrete under fire conditions ck T The following formula is used for setting: 20℃≤T c ≤1200℃ In the formula: f ck This indicates the standard strength value of concrete at room temperature. T c Take the component at the design fire resistance limit t d The concrete temperature at any given moment; Step 3.3 Elastic modulus E of concrete under fire conditions c T The following formula is used to obtain it: 20℃≤T c ≤1200℃ In the formula: E c This represents the elastic modulus of concrete at room temperature. T c Take the component at the design fire resistance limit t d The temperature of the concrete at any given moment.

3. The design method for fireproof protective layer of rectangular steel tube concrete column based on bearing capacity method according to claim 1, characterized in that: The specific step (5) involves using the following formula to obtain the axial compressive stability bearing capacity N of the concrete-filled steel tube column under fire conditions. maxT ; Φ=0.5[1+0.65(λ n -0.2)+λ n 2 ] N uT =A s f yT +A c f cT f yT =the sT f y In the formula: This represents the stability coefficient of an axially compressed member under fire conditions. N uT This indicates the compressive bearing capacity of the column section under fire conditions; Φ represents the influence coefficient of regularized slenderness ratio; λ n This represents the regularized slenderness ratio of a concrete-filled steel tube column. f y This indicates the yield strength of steel at room temperature; f yT This indicates the yield strength of steel under fire conditions; f cT This indicates the compressive strength of concrete under fire conditions; N ET This represents the Euler critical force of the column under fire conditions. E sT This indicates the elastic modulus of steel under fire conditions. E cT This indicates the elastic modulus of concrete under fire conditions. I s The moment of inertia represents the cross-section of the steel pipe; I c The moment of inertia of the concrete cross section; l0 represents the calculated length of the axially compressed member; η sT This represents the strength reduction factor of steel under fire conditions.

4. The design method for fireproof protective layer of rectangular steel tube concrete column based on bearing capacity method according to claim 1, characterized in that: The specific steps (6) are as follows: The following formulas are used to determine the remaining bearing capacity n and the fire load ratio R of a concrete-filled steel tube column: N u =A s f y +A c f ck In the formula: N u This indicates the compressive bearing capacity of the column cross-section at room temperature. N max This indicates the maximum axial force in a concrete-filled steel tube column under fire conditions. f ck This indicates the standard strength value of concrete at room temperature. f y This indicates the yield strength of steel at room temperature; If the remaining bearing capacity n is greater than or equal to the fire load ratio R, it indicates that the fire resistance bearing capacity of the steel-concrete composite column meets the requirements, and the fire protection layer of the steel-concrete composite column meets the design requirements. The fire protection layer of the steel-concrete composite column is then used for fire protection layer design. If the remaining bearing capacity n is less than the fire load ratio R, it indicates that the fire resistance bearing capacity of the steel-concrete composite column does not meet the requirements, and the fire protection layer of the steel-concrete composite column does not meet the design requirements. Increase the thickness of the fire protection layer, return to step (2), and re-judge whether the fire protection layer meets the fire protection design requirements in the order of step (2) to step (6).

5. The design method for fireproof protective layer of rectangular steel tube concrete column based on bearing capacity method according to claim 1, characterized in that: The steel-concrete composite column is mainly formed by pouring concrete inside a rectangular steel tube, wherein the ratio of the long side to the short side of the rectangular steel tube is 1 to 4.

6. The design method for fireproof protective layer of rectangular steel tube concrete column based on bearing capacity method according to claim 1, characterized in that: The fireproof protective layer is made of non-intumescent fireproof coating, cement mortar, concrete, intumescent fireproof coating, aerated concrete blocks, fireproof board, gypsum-based fireproof slurry, or flexible felt-like heat insulation material.

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