A method for determining the mid-span deflection of a fish-belly beam in unsaturated homogeneous strata

Abstract

The present invention relates to a method for determining the mid-span deflection of a fish beam in an unsaturated homogeneous stratum, belonging to the field of foundation pit support technology, and specifically comprising the following steps: Step 1: obtaining basic physical parameters, hydraulic properties and unsaturated characteristic parameters of the soil; Step 2: calculating the steel purlin support force of the unsaturated homogeneous stratum; Step 3: determining the foundation pit type and selecting the fish beam support structure parameters based on the foundation pit engineering characteristics; Step 4: simplifying the complex fish beam structure into a simply supported beam structure for mechanical analysis, calculating the mid-span deflection of the fish beam in the unsaturated homogeneous stratum through the steel purlin support force of the unsaturated homogeneous stratum, the fish beam support structure parameters and the parameters of the foundation pit, and judging the rationality of the deflection. The present invention fully considers the actual stress characteristics of the fish beam support structure and has the characteristics of simple calculation, wide application range and reliable settlement results.

Description

Technical Field

[0001] The invention belongs to the technical field of foundation pit support, and in particular relates to a method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum. Background Art

[0002] With the accelerating pace of urbanization, new foundation pits are becoming increasingly tight, deep, and large. Especially when adjacent to existing buildings or underground pipelines, excavation can easily lead to a series of safety issues, including ground subsidence, cracking of existing building walls, and deformation of underground pipelines, posing a serious threat to people's production and daily life. Among foundation pit support systems, fish beam structures have gradually attracted the attention of scholars and engineers due to their advantages, including high construction efficiency, small footprint, recyclability, low cost, and economic and environmental friendliness. However, the current design of fish beam support structures is primarily based on saturated soil theory, which is seriously inconsistent with the actual conditions of foundation pit soils. Furthermore, fish beam structures have a large number of web members, making the calculation of mid-span deformation a highly indeterminate problem. This calculation is extremely complex, making it impossible to quickly and accurately determine the mid-span deformation of the fish beam. This has become a key challenge in the design of fish beam structures and the root cause of engineering disasters such as building tilting, foundation pit collapse, and underground pipeline rupture. Therefore, in-situ and unsaturated characteristic tests were conducted on unsaturated homogeneous stratum soil, and the stress characteristics of the fish belly beam structure were analyzed to establish a unified equation for the mid-span deflection of the fish belly beam in unsaturated homogeneous stratum. This is of great significance to the safety and stability, cost reduction and efficiency improvement, energy conservation and emission reduction, and sustainable development in the field of foundation pit engineering.

[0003] However, existing methods for determining the mid-span deflection of fish-belly beam support have certain limitations, such as not considering the influence of the actual moisture content of the soil on the mid-span deflection, the specificity of the mid-span deflection equation, and the complexity of the calculation process. These shortcomings seriously restrict the application of fish-belly beam support structures in foundation pit support.

[0004] Specifically, the current method for determining the mid-span deflection of fish belly beam support has the following shortcomings:

[0005] Reference 1, "Study on the Stiffness of Prefabricated Prestressed Fish-Bellied Steel Bracing Systems" (China Civil Engineering Journal, 2021, No. 4), discloses a method for determining the stiffness of prefabricated fish-belly steel bracing systems. This method includes an equation for calculating mid-span deflection, which effectively calculates the mid-span deflection when the steel strands follow a specific ideal curve model. However, the actual steel strand model of fish-belly beams varies from project to project, and the mid-span deflection equation proposed in this paper has significant limitations in practical applications.

[0006] Reference 2, "Analytical Solution of the Deformation of Fish-Beam Structures and Determination of Reasonable Prestress Values" (Building Structures, 2020, Issue S02), discloses an analytical solution for calculating the deformation of fish-beam structures. This includes an equation for calculating the mid-span deformation of fish-beam beams. However, this equation has limitations for calculating mid-span deflection. When the prestress exceeds a certain value, the calculated results have large errors, adversely affecting the stability of the project. Furthermore, this equation does not consider the actual condition of unsaturated foundation pit soil, which limits the application of the mid-span deflection equation.

[0007] Reference 3, "Design Method for Large-Span Fish Beam Support Based on the Deformation Control Mechanism of Deep Foundation Pit" (China Harbor Construction, Issue 7, 2022), discloses a design method for large-span fish beam supports based on the deformation control mechanism of deep foundation pits. This method simplifies the equation for calculating the fish beam's mid-span deflection. However, the equation does not clearly provide a calculation formula for the redundant unknown forces, resulting in two unknown quantities in the equation, making it impossible to directly calculate the fish beam's deflection value. Furthermore, the deflection calculation in this document is based on saturated soil theory, which does not conform to the actual conditions of the foundation pit soil.

[0008] Currently, the mid-span deflection equations for fish-belly beam support structures, developed for specific strand curve models, significantly limit the flexibility of the fish-belly beam structure. Furthermore, none of these current research results consider the unsaturated state of the foundation pit soil, whose stress characteristics are fundamentally different from those of the saturated state.

[0009] Therefore, in order to solve the defects of the above-mentioned calculation method of the mid-span deflection of the fish belly beam support structure, it is necessary and feasible to establish a unified equation for calculating the mid-span deflection of the fish belly beam based on actual soil conditions and applicable to various support scenarios. Summary of the Invention

[0010] In response to the shortcomings of the existing technology, the present invention provides a method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum, which fully considers the actual stress characteristics of the fish belly beam support structure and has the characteristics of simple calculation, wide applicability and reliable settlement results.

[0011] A method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum is characterized by comprising the following steps:

[0012] Step 1: Obtain basic physical parameters, hydraulic properties and unsaturated characteristic parameters of the soil;

[0013] Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata:

[0014] Step 3: Determine the type of foundation pit and select the parameters of the fish belly beam support structure based on the characteristics of the foundation pit project;

[0015] Step 4: Simplify the complex fishbelt beam structure into a simply supported beam structure for mechanical analysis. Calculate the mid-span deflection of the fishbelt beam in the unsaturated homogeneous stratum based on the steel purlin support force, fishbelt beam support structure parameters, and foundation pit parameters, and determine the rationality of the deflection.

[0016] The step 1 is obtained by conducting field investigation, in-situ testing and indoor testing on a certain unsaturated homogeneous sand foundation pit.

[0017] The parameters obtained in step 1 include the soil unsaturated matrix suction , effective cohesion of unsaturated homogeneous sand , effective internal friction angle of unsaturated homogeneous sand ; Passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit ; Active earth pressure coefficient of unsaturated homogeneous sand ; Soil density of unsaturated homogeneous sandy soil .

[0018] The method for calculating the supporting force of the steel purlin in the unsaturated homogeneous stratum in step 2 specifically includes:

[0019] Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit using the basic physical parameters of the soil and the unsaturated characteristic parameters ;

[0020] Step 2.2: Based on the basic physical parameters of the soil and the unsaturated characteristic parameters and the height of the compressive stress zone below the bottom of the foundation pit , calculate the steel purlin support force in unsaturated homogeneous strata .

[0021] The calculation method for the height of the compressive stress zone below the bottom of the foundation pit in step 2.1 is as follows:

[0022] ;in: is the effective cohesion of unsaturated homogeneous sand; is the passive earth pressure coefficient of the unsaturated homogeneous sand below the bottom of the foundation pit; is the unsaturated matrix suction of soil; is the active earth pressure coefficient of unsaturated homogeneous sand; is the soil density of unsaturated homogeneous sand; 、 is the fitting parameter of the soil and water characteristic curve model; is the depth of foundation pit;

[0023] It should be noted that in step 2.1, the height of the compressive stress zone below the bottom of the foundation pit is The calculation results are as follows , then it is determined that ;like ,but Normal calculation.

[0024] In step 2.2, the calculation method of the steel purlin support force in the unsaturated homogeneous stratum is as follows:

[0025] ;in: is the height of the compressive stress zone above the bottom of the foundation pit, is the height of the tensile stress zone above the bottom of the foundation pit; is the fish beam support depth; is the total vertical stress of the unsaturated soil outside the foundation pit, is the total vertical stress of unsaturated homogeneous sand in the foundation pit.

[0026] The total vertical stress of the unsaturated soil outside the foundation pit The calculation method is: ; The total vertical stress of the unsaturated homogeneous sand in the foundation pit The calculation method is: ,in, is the soil density of unsaturated homogeneous sand.

[0027] The fish beam support structure parameters in step 3 include: fish beam span length , the longest straight belly rod length of the fish belly beam , 1 / 2 of the length of the steel strand ; Elastic modulus of steel purlin , elastic modulus of steel strand , steel purlin section moment of inertia , steel purlin width , cross-sectional area of ​​steel purlin , cross-sectional area of ​​steel strand , prestressed steel strands .

[0028] The calculation method of the mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum is:

[0029] ;in: The steel purlin is subjected to stress and the steel purlin supports The action and reaction forces are proportional to the supporting force of the steel purlin. equal; The span of the fish belly beam is long; The longest belly rod length of the fish belly beam; 1 / 2 of the length of the steel strand; is the cross-sectional area of ​​the steel purlin; is the width of the steel purlin; is the moment of inertia of the steel purlin section; is the cross-sectional area of ​​the steel strand; Prestressing of steel strands; is the elastic modulus of the steel purlin; is the elastic modulus of the steel strand;

[0030] in: The linear load of the unsaturated soil outside the foundation pit borne by the steel purlin is the linear load of the unsaturated soil outside the foundation pit borne by the steel purlin. The calculation method is: , is the width of the steel purlin.

[0031] If the result obtained from step 4 is unreasonable, it is necessary to repeat step 3, modify the parameters of the fish belly beam support structure, and recalculate step 4.

[0032] By means of the above technical solution, the present invention has at least the following beneficial effects:

[0033] (1) This invention fully considers the unsaturated state of the foundation pit soil and calculates the distribution of the unsaturated earth pressure in the foundation pit based on the unsaturated theory. This method can be applied to the actual stress state of the fish-belly beam support structure at any moisture content and is of great significance for the design of fish-belly beam support structures in unsaturated strata.

[0034] (2) This invention simplifies the fish beam support structure by using the principle of simply supported beams, transforming a complex high-order statically indeterminate problem into a first-order statically indeterminate problem. This method has a sufficient theoretical basis, a simple force analysis process, simple calculations, and reliable results. This invention establishes a unified equation for calculating the mid-span deflection of fish beams in unsaturated homogeneous strata, which is of great significance for quickly and accurately determining the mid-span deflection of fish beam support structures with multiple webs and various strand models.

[0035] (3) The present invention calculates the mid-span deflection of the fish belly beam support structure based on its actual stress conditions, which can significantly reduce the use of steel and engineering costs, improve economic benefits, and help achieve stability control of the foundation pit, carbon emission reduction and sustainable development.

[0036] (4) The present invention is applicable to various types of homogeneous unsaturated and saturated strata such as sand, silt, clay, and loess. It is a supplement and improvement to the design theory and method of fish belly beam support structure in existing foundation pit engineering and has a wide range of applications.

[0037] The present invention can reduce engineering investment and improve the efficiency and reliability of foundation pit support while ensuring the safety of the foundation pit and surrounding structures, and provides a more effective method for determining the mid-span deflection for the fish belly beam foundation pit support design. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1A flow chart of a method for determining the mid-span deflection of a fish-belly beam support in an unsaturated homogeneous stratum provided by the present invention;

[0039] Figure 2 A schematic diagram of a fish belly beam support structure provided by an embodiment of the present invention;

[0040] Figure 3 A schematic diagram of the supporting force of the fish belly beam support structure provided by an embodiment of the present invention;

[0041] Figure 4 A schematic diagram of a simplified fish beam support structure according to an embodiment of the present invention;

[0042] in:

[0043] 1-Angle brace, 2-Straight web member, 3-Steel perimeter purlin, 4-Steel strand of steel perimeter purlin before flexural deformation, 5-Diagonal web member, 6-Unsaturated soil outside foundation pit, 7-Foundation pit, 8-Ground-connected wall, 9-Bottom of foundation pit, 10-Unsaturated homogeneous sand below the bottom of foundation pit, 11-Web member of steel perimeter purlin before flexural deformation, 12-Web member of steel perimeter purlin after flexural deformation, 13-Steel strand of steel perimeter purlin after flexural deformation. DETAILED DESCRIPTION

[0044] In order to better explain the present invention and facilitate understanding, the technical solutions and effects of the present invention are described in detail below with reference to the accompanying drawings through specific implementation methods.

[0045] Example 1

[0046] The present invention provides a method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum, which does not consider the change of soil pressure caused by the deformation of the surrounding purlin structure. Figure 1 As shown, the specific steps include:

[0047] Step 1: Conduct field research, in-situ testing, and indoor tests on an unsaturated homogeneous sand foundation pit 7 to obtain the basic physical parameters, hydraulic properties, and unsaturated characteristic parameters of the soil.

[0048] The depth of the unsaturated homogeneous sand foundation pit 7 in this embodiment is The fish belly beam support depth of the fish belly beam support structure is 6m. 3m, the height of the compressive stress zone above the bottom of the foundation pit 4.25m, the height of the tensile stress zone above the bottom of the foundation pit is 1.75m.

[0049] The unsaturated soil matrix suction in this example The effective cohesion of unsaturated homogeneous sand is 4kPa. The effective internal friction angle of unsaturated homogeneous sand is 5kPa. is 30°; active earth pressure coefficient of unsaturated homogeneous sand is 0.33; the soil density of unsaturated homogeneous sand is 17kN / m 3 The passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit is 10 is 3.0.

[0050] The fitting parameters of the water-soil characteristic curve model for the unsaturated homogeneous sand foundation pit is 0.01, and the fitting parameters of the soil and water characteristic curve model are is 2.0.

[0051] Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata:

[0052] Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit :

[0053] (1)

[0054] in: is the effective cohesion of unsaturated homogeneous sand; is the passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit; is the unsaturated matrix suction of soil; is the active earth pressure coefficient of unsaturated homogeneous sand; is the soil density of unsaturated homogeneous sand; 、 is the fitting parameter of the soil and water characteristic curve model; is the foundation pit depth.

[0055] Substitute the parameter values ​​obtained in step 1 into the above formula (1) to obtain the height of the compressive stress zone below the bottom of the foundation pit of the unsaturated homogeneous sand. It is -0.0024m.

[0056] Step 2.2: Calculate the steel purlin support force in unsaturated homogeneous soil :

[0057] Since the height of the compressive stress zone below the bottom of the foundation pit calculated in step 2.1 is -0.0024m, that is ,Pick Therefore, the supporting force of the steel purlin in the unsaturated homogeneous stratum is The calculation equation is as follows:

[0058] (2)

[0059] in: is the height of the compressive stress zone above the bottom of the foundation pit, is the height of the tensile stress zone above the bottom of the foundation pit; The support depth of the fish belly beam.

[0060] is the total vertical stress of the unsaturated soil 6 outside the foundation pit, is the total vertical stress of unsaturated homogeneous sand in the foundation pit, which is calculated as follows:

[0061] (3)

[0062] (4)

[0063] The calculation shows that the supporting force of the steel purlin in the unsaturated homogeneous stratum in this embodiment is 25.27 kPa.

[0064] Step 3: According to the characteristics of the foundation pit engineering, determine that the foundation pit belongs to the third-level foundation pit.

[0065] The fish belly beam support structure includes straight web members 2, diagonal web members 5, angle braces 1, steel purlins 3, foundation pit, ground connection wall 8, foundation pit bottom 9. Figure 2-Figure 3 As shown, select the fish beam support structure parameters: fish beam span length The longest straight belly rod length of the fish belly beam is 50m 5m, 1 / 2 of the length of the steel strand 25.50m; elastic modulus of steel purlin The elastic modulus of the steel strand is 206GPa. is 206GPa, the moment of inertia of the steel purlin section is 0.00899m 4 , steel purlin width 0.4m, steel purlin cross-sectional area 0.02195m 2 , cross-sectional area of ​​steel strand 0.00363m 2 , prestressed steel strands 500kN.

[0066] Step 4: Analyze the mechanical properties of the fish beam support structure, calculate the mid-span deflection of the prestressed fish beam in the unsaturated homogeneous stratum, and determine the rationality of the deflection:

[0067] According to the load-bearing components and load-transmitting components of the fish belly beam, the complex fish belly beam structure is simplified into a simply supported beam structure for mechanical analysis. The steel strand 4 before the steel perimeter purlin is deflected, the steel strand 13 after the steel perimeter purlin is deflected, the web member 11 before the steel perimeter purlin is deflected, and the web member 12 after the steel perimeter purlin is deflected are shown in FIG. Figure 4As shown in the figure, the mid-span deflection of a prestressed fish belly beam in unsaturated homogeneous strata is calculated:

[0068] (5)

[0069] in: The steel purlin is subjected to stress and the steel purlin supports The action and reaction forces are proportional to the supporting force of the steel purlin. equal; The span of the fish belly beam is long; The longest belly rod length of the fish belly beam; 1 / 2 of the length of the steel strand; is the cross-sectional area of ​​the steel purlin; is the moment of inertia of the steel purlin section; is the cross-sectional area of ​​the steel strand; Prestressing of steel strands; is the elastic modulus of the steel purlin; is the elastic modulus of the steel strand.

[0070] in, is the linear load of the unsaturated soil 6 outside the foundation pit borne by the steel purlin. The calculation method is: , is the width of the steel purlin. Calculation shows that the linear load of the soil outside the pit on the steel purlin is 10.11 kN / m.

[0071] The mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum of this embodiment is obtained by the above formula (6): It is 54.91mm, that is, the maximum deformation of the fish belly beam support structure is 54.91mm.

[0072] Based on the above results, the rationality of the fish belly beam support structure is judged:

[0073] According to the Technical Standard for Monitoring of Construction Foundation Pit Engineering (GB 50497-2019) and the Code for Design of Steel Structures (GB50017-2017), the mid-span deflection of the fishbelly beam of 54.91 mm is less than the minimum warning value for the medium-level third-level foundation pit, that is, less than the allowable deflection of the steel purlin. The deformation also meets the design requirements for bending members, that is, the maximum deformation is less than the allowable deformation of the foundation pit. Therefore, the parameters of the fishbelly beam support structure meet the requirements of the code and can be used for foundation pit support design.

[0074] If the above results do not meet the minimum warning values ​​for the corresponding foundation pit specified in the "Technical Standard for Monitoring of Construction Foundation Pit Engineering" (GB 50497-2019) and the design requirements for flexural members in the "Code for Design of Steel Structures" (GB50017-2017), it is necessary to repeat step 3, modify the fish beam support structure parameters, including the steel purlin 3, web members, and steel strand parameters, and re-calculate in step 4.

[0075] Example 2

[0076] The present invention provides a method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum, which does not consider the change of soil pressure caused by the deformation of the surrounding purlin structure. Figure 1 As shown, the specific steps include:

[0077] Step 1: Conduct field research, in-situ testing, and indoor tests on an unsaturated homogeneous sand foundation pit 7 to obtain the basic physical parameters, hydraulic properties, and unsaturated characteristic parameters of the soil.

[0078] The depth of the unsaturated homogeneous sand foundation pit 7 in this embodiment is The fish belly beam support depth of the fish belly beam support structure is 6m. 3m, the height of the compressive stress zone above the bottom of the foundation pit The height of the tensile stress zone above the bottom of the foundation pit is 4.73m. It is 1.27m.

[0079] The unsaturated soil matrix suction in this example The effective cohesion of unsaturated homogeneous sand is 2kPa. The effective internal friction angle of unsaturated homogeneous sand is 5kPa. is 30°; active earth pressure coefficient of unsaturated homogeneous sand is 0.33; the soil density of unsaturated homogeneous sand is 17kN / m 3 The passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit is 10 is 3.0.

[0080] The fitting parameters of the water-soil characteristic curve model for the unsaturated homogeneous sand foundation pit is 0.01, and the fitting parameters of the soil and water characteristic curve model are is 2.0.

[0081] Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata:

[0082] Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit :

[0083] (1)

[0084] in: is the effective cohesion of unsaturated homogeneous sand; is the passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit; is the unsaturated matrix suction of soil; is the active earth pressure coefficient of unsaturated homogeneous sand; is the soil density of unsaturated homogeneous sand; 、 is the fitting parameter of the soil and water characteristic curve model; is the foundation pit depth.

[0085] Substitute the parameter values ​​obtained in step 1 into the above formula (1) to obtain the height of the compressive stress zone below the bottom of the foundation pit of the unsaturated homogeneous sand. It is 0.1151m.

[0086] Step 2.2: Calculate the steel purlin support force in unsaturated homogeneous soil :

[0087] Since the height of the compressive stress zone below the bottom of the foundation pit calculated in step 2.1 is 0.1151m, that is ,Pick Therefore, the supporting force of the steel purlin in the unsaturated homogeneous stratum is The calculation equation is as follows:

[0088] (2)

[0089] in: is the height of the compressive stress zone above the bottom of the foundation pit, is the height of the tensile stress zone above the bottom of the foundation pit; The support depth of the fish belly beam.

[0090] is the total vertical stress of the unsaturated soil 6 outside the foundation pit, is the total vertical stress of unsaturated homogeneous sand in the foundation pit, which is calculated as follows:

[0091] (3)

[0092] (4)

[0093] The calculation shows that the supporting force of the steel purlin in the unsaturated homogeneous stratum in this embodiment is 26.53 kPa.

[0094] Step 3: According to the characteristics of the foundation pit engineering, determine that the foundation pit belongs to the third-level foundation pit.

[0095] The fish belly beam support structure includes straight web members 2, diagonal web members 5, angle braces 1, steel purlins 3, foundation pit, ground connection wall 8, foundation pit bottom 9. Figure 2-Figure 3 As shown, select the fish beam support structure parameters: fish beam span length The longest straight belly rod length of the fish belly beam is 50m 5m, 1 / 2 of the length of the steel strand 25.50m; elastic modulus of steel purlin The elastic modulus of the steel strand is 206GPa. is 206GPa, the moment of inertia of the steel purlin section is 0.00899m 4 , steel purlin width 0.4m, steel purlin cross-sectional area 0.02195m 2 , cross-sectional area of ​​steel strand 0.00363m 2 , prestressed steel strands It is 300kN.

[0096] Step 4: Analyze the mechanical properties of the fish beam support structure, calculate the mid-span deflection of the prestressed fish beam in the unsaturated homogeneous stratum, and determine the rationality of the deflection:

[0097] According to the load-bearing components and load-transmitting components of the fish belly beam, the complex fish belly beam structure is simplified into a simply supported beam structure for mechanical analysis. The steel strand 4 before the steel perimeter purlin is deflected, the steel strand 13 after the steel perimeter purlin is deflected, the web member 11 before the steel perimeter purlin is deflected, and the web member 12 after the steel perimeter purlin is deflected are shown in FIG. Figure 4 As shown in the figure, the mid-span deflection of a prestressed fish belly beam in unsaturated homogeneous strata is calculated:

[0098] (5)

[0099] in: The steel purlin is subjected to stress and the steel purlin supports The action and reaction forces are proportional to the supporting force of the steel purlin. equal; The span of the fish belly beam is long; The longest belly rod length of the fish belly beam; 1 / 2 of the length of the steel strand; is the cross-sectional area of ​​the steel purlin; is the moment of inertia of the steel purlin section; is the cross-sectional area of ​​the steel strand; Prestressing of steel strands; is the elastic modulus of the steel purlin; is the elastic modulus of the steel strand.

[0100] in, is the linear load of the unsaturated soil 6 outside the foundation pit borne by the steel purlin. The calculation method is: , is the width of the steel purlin. It is calculated that the linear load of the soil outside the pit on the steel purlin is 10.61kN / m.

[0101] The mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum of this embodiment is obtained by the above formula (6): It is 86.30mm, that is, the maximum deformation of the fish belly beam supporting structure is 86.30mm.

[0102] Based on the above results, the rationality of the fish belly beam support structure is judged:

[0103] According to the Technical Standard for Monitoring Construction Pit Engineering (GB 50497-2019) and the Code for Design of Steel Structures (GB50017-2017), the mid-span deflection of the fish beam of 86.30 mm is greater than the minimum warning value for a Class III foundation pit, which is less than the allowable deflection of the steel purlin. This does not meet the minimum warning value for the corresponding foundation pit specified in the Technical Standard for Monitoring Construction Pit Engineering (GB 50497-2019). Therefore, return to step 3 to adjust the parameters of the fish beam support structure.

[0104] The adjusted fish belly beam structure parameters are: fish belly beam span length The longest straight belly rod length of the fish belly beam is 50m 5m, 1 / 2 of the length of the steel strand 25.50m; elastic modulus of steel purlin The elastic modulus of the steel strand is 206GPa. is 206GPa, the moment of inertia of the steel purlin section is 0.00899m 4 , steel purlin width 0.4m, steel purlin cross-sectional area 0.02195m 2 , cross-sectional area of ​​steel strand 0.00363m 2 , prestressed steel strands It is 550kN.

[0105] The mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum of this embodiment is obtained by the above formula (6): It is 54.45mm, that is, the maximum deformation of the fish belly beam supporting structure is 54.45mm.

[0106] Based on the above results, the rationality of the fish belly beam support structure is judged:

[0107] According to the Technical Standard for Monitoring of Construction Foundation Pit Engineering (GB 50497-2019) and the Code for Design of Steel Structures (GB50017-2017), the mid-span deflection of the above-mentioned fish belly beam is 54.45mm is less than the minimum warning value of the third-level foundation pit, that is, less than the allowable value of the deflection of the steel purlin, and the deformation also meets the design requirements for bending members, that is, the maximum deformation is less than the allowable deformation of the foundation pit. Therefore, the parameters of the fish belly beam support structure meet the requirements of the specification and can be used in foundation pit support design.

Claims

1. A method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum, characterized in that: The specific steps include: Step 1: Obtain basic physical parameters, hydraulic properties and unsaturated characteristic parameters of the soil; The parameters obtained in step 1 include the soil unsaturated matrix suction , effective cohesion of unsaturated homogeneous sand , effective internal friction angle of unsaturated homogeneous sand ; Passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit ; Active earth pressure coefficient of unsaturated homogeneous sand ; Soil density of unsaturated homogeneous sandy soil ; Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata: The method for calculating the supporting force of the steel purlin in the unsaturated homogeneous stratum in step 2 specifically includes: Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit using the basic physical parameters of the soil and the unsaturated characteristic parameters ; Step 2.2: Based on the basic physical parameters of the soil and the unsaturated characteristic parameters and the height of the compressive stress zone below the bottom of the foundation pit , calculate the steel purlin support force in unsaturated homogeneous strata ; The calculation method for the height of the compressive stress zone below the bottom of the foundation pit in step 2.1 is as follows: ;in: is the effective cohesion of unsaturated homogeneous sand; is the passive earth pressure coefficient of the unsaturated homogeneous sand below the bottom of the foundation pit; is the unsaturated matrix suction of soil; is the active earth pressure coefficient of unsaturated homogeneous sand; is the soil density of unsaturated homogeneous sand; 、 is the fitting parameter of the soil and water characteristic curve model; is the depth of foundation pit; It should be noted that in step 2.1, the height of the compressive stress zone below the bottom of the foundation pit is The calculation results are as follows , then it is determined that ;like ,but Normal calculation; In step 2.2, the calculation method of the steel purlin support force in the unsaturated homogeneous stratum is as follows: ;in: is the height of the compressive stress zone above the bottom of the foundation pit, is the height of the tensile stress zone above the bottom of the foundation pit; is the fish beam support depth; is the total vertical stress of the unsaturated soil outside the foundation pit, is the total vertical stress of unsaturated homogeneous sand in the foundation pit; Step 3: Determine the type of foundation pit and select the parameters of the fish belly beam support structure based on the characteristics of the foundation pit project; The fish beam support structure parameters in step 3 include: fish beam span length , the longest straight belly rod length of the fish belly beam , 1 / 2 of the length of the steel strand ; Elastic modulus of steel purlin , elastic modulus of steel strand , steel purlin section moment of inertia , steel purlin width , cross-sectional area of ​​steel purlin , cross-sectional area of ​​steel strand , prestressed steel strands ; Step 4: Simplify the complex fishbelt beam structure into a simply supported beam structure for mechanical analysis. Calculate the mid-span deflection of the fishbelt beam in the unsaturated homogeneous stratum based on the steel purlin support force, fishbelt beam support structure parameters, and foundation pit parameters, and determine the rationality of the deflection. The calculation method of the mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum is: ;in: The steel purlin is subjected to stress and the steel purlin supports The action and reaction forces are proportional to the supporting force of the steel purlin. equal; The span of the fish belly beam is long; The longest belly rod length of the fish belly beam; 1 / 2 of the length of the steel strand; is the cross-sectional area of ​​the steel purlin; is the width of the steel purlin; is the moment of inertia of the steel purlin section; is the cross-sectional area of ​​the steel strand; For prestressing of steel strands; is the elastic modulus of the steel purlin; is the elastic modulus of the steel strand; in: The linear load of the unsaturated soil outside the foundation pit borne by the steel purlin is the linear load of the unsaturated soil outside the foundation pit borne by the steel purlin. The calculation method is: , is the width of the steel purlin.

2. The method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum according to claim 1, characterized in that: The step 1 is obtained by conducting field investigation, in-situ testing and indoor testing on a certain unsaturated homogeneous sand foundation pit.

3. The method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum according to claim 1, characterized in that: The total vertical stress of the unsaturated soil outside the foundation pit The calculation method is: ; The total vertical stress of the unsaturated homogeneous sand in the foundation pit The calculation method is: ,in, is the soil density of unsaturated homogeneous sand.

4. The method for determining the mid-span deflection of a fish-belly beam in an unsaturated homogeneous stratum according to claim 1, wherein: If the result obtained from step 4 is unreasonable, it is necessary to repeat step 3, modify the parameters of the fish belly beam support structure, and recalculate step 4.

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