A method for calculating critical temperature of compressed concrete-filled steel tube columns based on multiple temperature characteristic points

Abstract

A critical temperature calculation method for concrete - filled steel tubular columns under compression based on multiple temperature characteristic points belongs to the technical field of failure warning under fire. According to the conditions, the column load ratio n, the outer section (steel pipe) diameter or side length D / B, the load eccentricity e, the steel ratio α, and the slenderness ratio λ are obtained; the positions of the characteristic temperature point sets are determined and the simplified calculation methods for the critical temperatures of each point in the characteristic temperature point sets are determined, T Rcr，EQ is the critical temperature: For circular interior columns under fire: T Rcr,EQ = - 1241.89n - 2.19λ + 1094.74 (point 1). For circular corner columns under fire: #imgabs0#. For square interior columns under fire: #imgabs1#. For square corner columns under fire: #imgabs2#. The obtained critical temperature of the concrete - filled steel tubular column under compression is more accurate and more in line with the actual situation, which can provide a reliable method for the failure warning or fire - resistance design of structures under fire and has important practical value.

Description

Technical Field

[0001] The invention relates to a fire damage early warning method and a practical fire resistance design method for a building structure, and in particular to an improved critical temperature method for a compressed steel tube concrete column based on multiple temperature characteristic points. Background Art

[0002] In recent years, with the rise in the number of high-rise buildings, large-scale buildings and complex uses, the factors causing building fires have increased, and the scale and hazards of building fires have also tended to expand. When a fire occurs in a building structure, firefighters will inevitably have to enter the building fire scene for rescue. In order to protect the lives of firefighters, early warning technology for building damage under fire is very important. When a building structure reaches its fire resistance limit and is destroyed under fire, there are many indicators that can characterize its limit state, such as: (1) residual bearing capacity under fire. When the residual bearing capacity under fire is lower than the external load it bears, the structure is considered to have reached the limit state; (2) deformation or deformation rate. my country's national standard "Fire Resistance Test Method for Building Components" (GB / T9978.1-2008) stipulates that when the deformation or deformation rate exceeds the specified limit, the structure can be considered to have reached the limit state; (3) average cross-sectional temperature. my country's national standard "Technical Code for Fire Protection of Building Steel Structures" (GB51249-2017) provides the critical temperature method for steel structures. When the average cross-sectional temperature of the steel structure reaches the critical temperature, the structure is considered to have reached the limit state. When warning of damage to building structures under fire, if the structural bearing capacity, deformation and temperature of the building structure under fire can be obtained in time, theoretically, it is possible to predict the damage of the building structure and issue a warning signal. However, there are defects in using the above three indicators for warning in actual projects: (1) The residual bearing capacity of a component under fire in an actual fire scene cannot be measured in real time by equipment, and due to the complex influencing factors at the fire scene, the residual bearing capacity under fire obtained by numerical simulation is often quite different from the bearing capacity of the structure in the actual fire scene; (2) Although the deformation index can be measured in real time, the deformation or deformation rate limit values ​​given in the existing specifications are obtained based on the standard fire curve. Most actual fires do not develop according to the standard fire curve. Therefore, the deformation index has limited reference significance for predicting the limit state of the actual building structure; (3) The temperature index can be measured in real time at the fire scene through a pre-set sensor, and a large number of tests have also shown that the material properties of building materials will significantly deteriorate at high temperatures and lose their normal working ability. Therefore, from the perspective of warning of structural damage under fire, the temperature index has better operability and reliability than the other two indicators. However, the critical temperature method given in the existing national standards has certain defects. The critical temperature method given in the national standards can only be applied to steel structures, but not to concrete and steel-concrete structures. The critical temperature given in the national standards refers to the average temperature of the cross section, but under the actual fire, it may not necessarily make the temperature of the structure evenly distributed in the cross section and in the length direction. This critical temperature or temperature characteristic point selection method cannot fully reflect the impact of uneven temperature distribution caused by various factors when the actual building structure is affected by fire, and the results obtained cannot reflect the actual situation.Therefore, the present invention proposes a refined critical temperature method which can reflect the actual situation and takes the temperatures of multiple temperature characteristic points on the structure as the critical temperature, thereby providing theoretical support for the damage warning technology of building structures under fire and practical fire-resistant design methods. Summary of the invention

[0003] The purpose of the present invention is to overcome the shortcomings of the existing critical temperature method and establish an improved critical temperature method for compressed steel tube concrete columns based on multiple temperature characteristic points.

[0004] A method for calculating critical temperature of a compressed steel tube concrete column based on multiple temperature characteristic points, characterized by comprising the following steps:

[0005] (1) Determine the calculation conditions

[0006] According to the conditions, the column load ratio n (the ratio of the load acting on the steel tube concrete column under fire to the ultimate bearing capacity of the steel tube concrete column at room temperature), the outer section (steel tube) diameter or side length D / B, the load eccentricity e, the steel content α (the ratio of the cross-sectional area of ​​the steel tube to the cross-sectional area of ​​the concrete inside the steel tube) and the slenderness ratio λ (circle: λ = 4L / D, square: Where L is the calculated length of the steel tube concrete column);

[0007] (2) Determine the location of the characteristic temperature point set

[0008] For the middle column, the four sides are exposed to fire, that is, the fire-exposed surfaces are all around, and the temperature distribution is uniform; for the circular steel tube concrete column, any point of the outer side steel tube section, that is, point 1, is used as the feature point; for the square steel tube concrete column, any corner point of the four corners of the outer side steel tube section, that is, point 4, and any midpoint of the four sides of the outer side steel tube section, that is, point 5, are used as the feature point set; the above sections are all located in the middle section of the column.

[0009] For corner columns, the temperature of the fire-exposed surface is higher; for circular concrete-filled steel tube columns, the midpoint of the fire-exposed edge of the outer side steel tube section, i.e. point 2, and the endpoint of the fire-exposed edge, i.e. point 3, are used as the feature point set; for square concrete-filled steel tube columns, the endpoints of the continuous fire-exposed edges of the outer side steel tube section, i.e. point 6, and the midpoint of the continuous fire-exposed edges, i.e. point 7, are used as the feature point set; the above sections are all located in the middle section of the column.

[0010] Preferably, for corner columns, the fire-exposed surface of a circular concrete-filled steel tube column is the column side surface corresponding to the 90° central angle; for a square concrete-filled steel tube column, one of the four corners of the square concrete-filled steel tube column belongs to the fire-exposed surface range, the endpoints corresponding to the 6 points are the endpoints corresponding to the continuous length of the continuous fire-exposed edge of the outer side steel tube section, and the midpoint corresponding to the 7 points is the position corresponding to 1 / 2 of the continuous length of the fire-exposed edge.

[0011] (3) Simplified calculation method for determining the critical temperature of each point in the characteristic temperature point set

[0012] After determining the geometric positions of each point in the characteristic temperature point set, the critical temperature can be obtained by substituting it into the formula; where T Rcr，EQ is the critical temperature;

[0013] For round center columns subject to fire:

[0014] T Rcr,EQ =-1241.89n-2.19λ+1094.74(1 point)

[0015] For round corner columns subjected to fire:

[0016]

[0017] For square center columns subject to fire:

[0018]

[0019] For square corner columns subjected to fire:

[0020]

[0021] The applicable range of the invention formula is column load ratio n = 0.2 ~ 0.8, outer section diameter (side length) (D / B) is 300 ~ 1200mm, slenderness ratio λ = 10 ~ 60, steel content α = 0.05 ~ 0.2, load eccentricity e / r c =0~0.6(where r c is the core concrete radius), concrete strength f cu =30~80MPa, steel strength f y =235~420MPa.

[0022] The critical temperature of the compressed steel tube concrete column obtained by the present invention is more accurate and more in line with the actual situation, can provide a reliable method for damage warning or fire resistance design under structural fire, and has important practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Technical route for the embodiment

[0024] Figure 2 Compare the calculated value of the reference model with the calculated value of the formula of the present invention;

[0025] Figure 3 For fire conditions.

[0026] Figure 4 This is the correspondence diagram between the refined critical temperature and the characteristic points. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with examples, but the present invention is not limited to the following examples.

[0028] Example 1

[0029] The technical solution of the present invention is simulated and compared, and specifically comprises the following steps:

[0030] (1) Fire resistance test of concrete-filled steel tube columns

[0031] In order to study the critical temperature of steel tube concrete columns under fire, 12 steel tube concrete column specimens were designed and manufactured, and fire resistance tests were carried out to obtain test data such as the temperature variation law of characteristic temperature points, fire resistance limit and refined critical temperature based on test data.

[0032] According to the known conditions, we can obtain the column load ratio n (the ratio of the load acting on the steel tube concrete column under fire to the ultimate bearing capacity of the steel tube concrete column at room temperature), the outer section (steel tube) diameter or side length D / B, the load eccentricity e, the steel content α (the ratio of the cross-sectional area of ​​the steel tube to the cross-sectional area of ​​the concrete inside the steel tube) and the slenderness ratio λ (circle: λ = 4L / D, square: Where L is the calculated length of the steel tube concrete column);

[0033] (2) Establish a refined finite element calculation model of concrete-filled steel tube columns under fire

[0034] On the basis of the fire resistance test research, in order to further study the refined critical temperature of steel tube concrete columns under fire theoretically, the finite element software ABAQUS was used to establish the critical temperature theoretical analysis model of steel tube concrete columns, and the model was verified and corrected with the existing test data and the data obtained in this experiment, so as to establish the refined finite element calculation model of steel tube concrete columns under fire. On the basis of the verified finite element model, the refined critical temperature of steel tube concrete columns was further analyzed, and the refined critical temperature based on finite element analysis was obtained.

[0035] (3) Analysis of the working mechanism of concrete-filled steel tube columns under fire

[0036] On the basis of the established refined finite element analysis model of steel tube concrete columns under fire with a certain calculation accuracy, it is necessary to determine the basic calculation conditions that are more in line with the longitudinal and transverse fire conditions of the actual project, establish a finite element calculation model that is closer to the actual project size, and conduct in-depth discussion and analysis on the failure mode and deformation development, temperature development, stress development, load changes borne by steel tubes and concrete, and refined critical temperature of steel tube concrete columns under fire. A refined critical temperature determination method suitable for steel tube concrete columns in actual construction projects is proposed.

[0037] (4) Propose a refined critical temperature method for concrete-filled steel tube columns

[0038] Based on the analysis of the working mechanism of steel tube concrete columns under fire, the influence of column load ratio, outer section diameter (side length), slenderness ratio, steel content, load eccentricity, material strength and other parameters on the fire resistance limit and refined critical temperature of steel tube concrete columns are discussed. A simplified calculation formula for the refined critical temperature is given through multivariate linear regression analysis. Combined with the refined critical temperature determination method of steel tube concrete columns, a refined critical temperature calculation method suitable for steel tube concrete columns is proposed in the present invention. Using the proposed refined critical temperature calculation method for steel tube concrete columns and the high temperature early warning sensor device, the damage early warning method of steel tube concrete columns under fire is discussed.

[0039] The relationship between the refined critical temperature calculation method of the above-mentioned steel tube concrete column and the corresponding characteristic point correspondence diagram is shown in Figure 4 .

[0040] Figure 2 The figure shows the comparison between the simplified formula calculation value of the refined critical temperature of the steel tube concrete column and the parameter model calculation value under different parameters, where T Rcr,FEA is the value calculated by theoretical analysis model of critical temperature of steel tube concrete column, T Rcr,EQ The calculated value is the simplified formula of the present invention. The specific calculation conditions are as follows: D = 600 mm, t s =20mm, L=6000mm, slenderness ratio λ=40, cross-section steel content α=0.15, f cu =60MPa, f y =355MPa, column load ratio n=0.6; basic calculation conditions for square steel tube concrete column: B=600mm, t s =20mm, L=6928mm, slenderness ratio λ=40, cross-section steel content α=0.15, f cu =60MPa, f y =355MPa, column load ratio n=0.6. Boundary conditions are hinged at both ends, load eccentricity e=L / 1000, end plate size: 1000×1000×50mm. The calculation model fire curve adopts ISO-834 temperature rise curve. When one of the parameters is changed, the other parameters remain unchanged.

[0041] Refined critical temperature of circular center column under fire conditions: 1 point T Rcr,FEA / T Rcr,EQ The average value is 1.10 and the variance is 0.68; the critical temperature of the circular corner column under fire conditions is refined: 1 point T Rcr,FEA / T Rcr,EQ The mean is 1.02, the variance is 0.05, and the 2-point T Rcr,FEA / T Rcr,EQ The average value is 1.02 and the variance is 0.23; the critical temperature of the square column under fire conditions is refined: 4 points TRcr,FEA / T Rcr,EQ The mean is 1.00, the variance is 0.15, and the 5-point T Rcr,FEA / T Rcr,EQ The average value is 1.04 and the variance is 0.41; the critical temperature of the square corner column under fire conditions is refined: 6 points T Rcr,FEA / T Rcr,EQ The mean value is 1.02, the variance is 0.05, and the 7-point T Rcr,FEA / T Rcr,EQ The average value is 1.01 and the variance is 0.06. It can be seen that the simplified calculation formula is consistent with the calculated value of the parameter model, which can reflect the variation law of the refined critical temperature of the steel tube concrete column.

Claims

1. A method for calculating the critical temperature of a compressed concrete-filled steel tube column based on multiple temperature characteristic points, characterized in that: The following steps are involved: (1) Determine the calculation conditions According to the conditions, the column load ratio n, the outer section diameter D or side length B of the steel tube, the load eccentricity e, the steel content ratio α and the slenderness ratio λ are obtained; the column load ratio n is the ratio of the load acting on the steel tube concrete column under fire to the ultimate bearing capacity of the steel tube concrete column at room temperature, the steel content ratio α is the ratio of the cross-sectional area of ​​the steel tube to the cross-sectional area of ​​the concrete inside the steel tube, the slenderness ratio λ of the circle is: λ=4L / D, the slenderness ratio λ of the square is: Where L is the calculated length of the steel tube concrete column; (2) Determine the location of the characteristic temperature point set For the middle column, the four sides are exposed to fire, that is, the exposed surface is all around, and the temperature distribution is uniform; for the circular concrete-filled steel tube column, any point of the outer side steel tube section, i.e. point 1, is used as the feature point; for the square concrete-filled steel tube column, any corner point of the four corners of the outer side steel tube section, i.e. point 2, and any midpoint of the four sides of the outer side steel tube section, i.e. point 3, is used as the feature point set; For corner columns, the temperature of the fire-exposed surface is high; for circular concrete-filled steel tube columns, the midpoint of the fire-exposed edge of the outer side steel tube section, i.e. point 4, and the end point of the fire-exposed edge, i.e. point 5, are used as feature point sets; for square concrete-filled steel tube columns, the end point of the continuous fire-exposed edge of the outer side steel tube section, i.e. point 6, and the midpoint of the continuous fire-exposed edge, i.e. point 7, are used as feature point sets; the above sections are all located in the middle section of the column; (3) Simplified calculation method for determining the critical temperature of each point in the characteristic temperature point set After determining the geometric positions of each point in the characteristic temperature point set, the critical temperature can be obtained by substituting it into the formula; where T Rcr，EQ is the critical temperature; For round center columns subject to fire: T Rcr,EQ =-1241.89n-2.19λ+1094.74(1 point) For round corner columns subjected to fire: For square center columns subject to fire: For square corner columns subjected to fire:

2. The critical temperature calculation method of compressed concrete-filled steel tube columns based on multiple temperature characteristic points according to claim 1 is characterized in that: For corner columns, the fire-bearing surface of the circular steel tube concrete column is the column side surface corresponding to the 90° central angle; for the square steel tube concrete column, one of the four corners of the square steel tube concrete column belongs to the fire-bearing surface range, the endpoints corresponding to the 6 points are the endpoints corresponding to the continuous length of the continuous fire-bearing edge of the outer side steel tube section, and the midpoint corresponding to the 7 points is the position corresponding to 1 / 2 of the continuous length of the fire-bearing edge.

3. The critical temperature calculation method of compressed concrete-filled steel tube columns based on multiple temperature characteristic points according to claim 1 is characterized in that: The formula is applicable to column load ratio n = 0.2 ~ 0.8, outer section diameter D / side length B is 300 ~ 1200mm, slenderness ratio λ = 10 ~ 60, steel content α = 0.05 ~ 0.2, load eccentricity e / r c =0~0.6, where r c is the core concrete radius, concrete strength f cu =30~80MPa, steel strength f y =235~420MPa.

Images