Determination method for midspan deflection of unsaturated homogeneous stratum fish belly sill

Abstract

The invention relates to a method for determining the midspan deflection of an unsaturated homogeneous stratum fish-bellied sill, which belongs to the technical field of foundation pit supporting and specifically comprises the following steps: step 1, obtaining basic physical parameters, hydrological properties and unsaturated characteristic parameters of soil; 2, calculating the supporting force of the steel enclosing purlin of the unsaturated homogeneous stratum; 3, determining the type of the foundation pit according to the engineering characteristics of the foundation pit and selecting the parameters of a fish-belly sill supporting structure; 4, the complex fish-bellied sill structure is simplified into a simply supported beam structure for mechanical analysis, the midspan deflection of the fish-bellied sill in the unsaturated homogeneous stratum is calculated according to the steel enclosing purlin supporting force of the unsaturated homogeneous stratum, the parameters of the fish-bellied sill supporting structure and the parameters of the foundation pit, and the reasonability of the deflection is judged. The method fully considers the actual stress characteristics of the fish-bellied sill supporting structure, and has the characteristics of simple calculation, wide application range and reliable settlement result.

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 continuous acceleration of urbanization, new foundation pits are developing in the direction of "tight, deep and large". Especially when they are close to existing buildings or underground pipelines, foundation pit excavation is prone to cause a series of safety problems such as ground settlement, cracking of existing building walls, deformation of underground pipelines, etc., which pose a serious threat to people's production and life. In the foundation pit support system, the fish belly beam structure has gradually attracted the attention of scholars and engineers due to its advantages such as high construction efficiency, small space occupation, recyclability, low cost, economy and environmental protection. However, the current fish belly beam support structure design is mainly based on saturated soil theory, which is seriously inconsistent with the actual situation of the foundation pit soil. In addition, the fish belly beam structure has a large number of webs, and the calculation of mid-span deformation belongs to a high-order hyperstatic problem. The calculation is very complicated, and it is impossible to quickly and accurately obtain the mid-span deformation of the fish belly beam, which has become a key problem in the field of fish belly beam structure design and the root cause of engineering disasters such as building tilting, foundation pit collapse and underground pipeline rupture. Therefore, in-situ and unsaturated property tests were carried out on unsaturated homogeneous strata 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 strata, which 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, the 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 complex calculation process. These shortcomings seriously restrict the application of fish belly beam support structure 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-belly Steel Support System" (China Civil Engineering Journal, 2021, No. 4) discloses a method for determining the stiffness of a prefabricated prestressed fish-belly steel support system, including a mid-span deflection calculation equation, which can well calculate the mid-span deflection when the steel strand is a specific ideal curve model. However, the actual fish-belly beam steel strand model varies with different projects, and the mid-span deflection equation proposed in this paper has great limitations in practical applications.

[0006] Document 2, "Analytical Solution of Force-Deformation of Fish-Beam Structure and Determination of Reasonable Prestress Value" (Building Structure, 2020, No. S02), discloses an analytical solution calculation method for the force-deformation of fish-beam structure, which includes the mid-span deformation calculation equation of fish-beam. This equation has certain limitations for the calculation of mid-span deflection. When the prestress exceeds a certain value, the calculation result error is large, which has an adverse effect on the stability of the project. In addition, this calculation equation does not take into account the actual condition that the foundation pit soil is unsaturated, thereby limiting the scope of application of the mid-span deflection equation.

[0007] Document 3, "Design Method for Large-Span Fish Beam Support Based on Deep Foundation Pit Deformation Control Mechanism" (China Harbour Construction, Issue 7, 2022), discloses a design method for large-span fish beam support based on deep foundation pit deformation control mechanism. This method simplifies the calculation equation for the mid-span deflection of the fish beam. However, the equation does not clearly give the calculation formula for the redundant unknown forces, resulting in two unknown quantities in the equation, and the deflection value of the fish beam cannot be directly obtained. At the same time, the document calculates the deflection based on the saturated soil theory, which is inconsistent with the actual conditions of the foundation pit soil.

[0008] At present, the mid-span deflection equation of the fish beam support structure proposed for a specific steel strand curve model greatly limits the flexibility of the fish beam structure. At the same time, the current research results do not consider the unsaturated state of the foundation pit soil, and its stress characteristics are essentially different from 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 a variety of support scenarios. Summary of the invention

[0010] In view of the shortcomings of the prior art, 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 application range and reliable settlement results.

[0011] A method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum, characterized in that it specifically comprises the following steps: Step 1: Obtain basic physical parameters, hydraulic properties and unsaturated characteristic parameters of the soil; Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata: Step 3: Determine the type of foundation pit and select the parameters of the fish belly beam support structure according to the characteristics of the foundation pit project; 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 through the steel purlin support force, fishbelt beam support structure parameters and foundation pit parameters, and determine the rationality of the deflection.

[0012] The step 1 is obtained by conducting field investigation, in-situ testing and indoor testing on a certain unsaturated homogeneous sand foundation pit.

[0013] The parameters obtained in step 1 include the soil unsaturated matrix suction , effective cohesion of unsaturated homogeneous sand , the 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 .

[0014] 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 .

[0015] The calculation method of 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 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; 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 ;like ,but Calculate normally.

[0016] 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.

[0017] 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.

[0018] The fish beam support structure parameters in step 3 include: , the longest straight belly rod length of the fish belly beam , 1 / 2 of the length of the 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 strand .

[0019] The calculation method of the mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum is: ;in: The force of the steel purlin and the supporting force of the steel purlin is the action force and reaction force, the size of which is related to the steel purlin support force equal; The span of the fish belly beam is long; The longest belly bar 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; in: 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.

[0020] If the result obtained in step 4 is unreasonable, it is necessary to repeat step 3, modify the fish belly beam support structure parameters, and recalculate step 4.

[0021] By means of the above technical solution, the present invention has at least the following beneficial effects:

[0022] (1) The present invention fully considers the unsaturated state of the foundation pit soil and calculates the distribution of the unsaturated soil pressure of 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 under any water content, which is of great significance to the design of the fish belly beam support structure in unsaturated strata.

[0023] (2) The present invention uses the principle of simply supported beams to simplify the fish beam support structure, transforming the complex high-order statically indeterminate problem into a first-order statically indeterminate problem. The theoretical basis is sufficient, the force analysis process is simple, the calculation is simple, and the results are reliable. The present invention establishes a unified equation for calculating the mid-span deflection of the fish beam in unsaturated homogeneous strata, which is of great significance for quickly and accurately grasping the mid-span deflection of the fish beam support structure with multiple webs and multiple steel strand models.

[0024] (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.

[0025] (4) The present invention is applicable to various homogeneous unsaturated and saturated strata such as sand, silt, clay, loess, etc. It supplements and improves the design theory and method of fish belly beam support structure in existing foundation pit engineering and has a wide range of applications.

[0026] 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

[0027] Figure 1 A 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;

[0028] Figure 2 A schematic diagram of a fish belly beam support structure provided by an embodiment of the present invention;

[0029] Figure 3 A schematic diagram of the supporting force of the fish belly beam support structure provided by an embodiment of the present invention;

[0030] Figure 4 A simplified deformation diagram of the fish belly beam support structure provided by an embodiment of the present invention;

[0031] in:

[0032] 1-angle brace, 2-straight web member, 3-steel perimeter purlin, 4-steel strand before the steel perimeter purlin undergoes flexural deformation, 5-diagonal web member, 6-unsaturated soil outside the foundation pit, 7-foundation pit, 8-ground-connected wall, 9-foundation pit bottom, 10-unsaturated homogeneous sand below the bottom of the foundation pit, 11-web member before the steel perimeter purlin undergoes flexural deformation, 12-web member after the steel perimeter purlin undergoes flexural deformation, 13-steel strand after the steel perimeter purlin undergoes flexural deformation. DETAILED DESCRIPTION

[0033] 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 through specific implementation modes in conjunction with the accompanying drawings.

[0034] Example 1

[0035] 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:

[0036] Step 1: Conduct field investigation, 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.

[0037] The depth of the foundation pit 7 of the unsaturated homogeneous sand foundation pit in this embodiment is The fish beam support depth of the fish 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.

[0038] The unsaturated soil matrix suction in this example The effective cohesion of unsaturated homogeneous sand is 4 kPa. The effective internal friction angle of unsaturated homogeneous sand is 5 kPa. is 30°; active earth pressure coefficient of unsaturated homogeneous sand is 0.33; the soil weight of unsaturated homogeneous sand is 17kN / m 3The passive earth pressure coefficient of unsaturated homogeneous sand below the bottom of the foundation pit is 10 is 3.0.

[0039] 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.

[0040] Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata:

[0041] Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit :

[0042] (1)

[0043] 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.

[0044] 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.

[0045] Step 2.2: Calculate the steel purlin support force in unsaturated homogeneous soil :

[0046] 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 unsaturated homogeneous strata is The calculation equation is as follows:

[0047] (2)

[0048] 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; It is the support depth of fish belly beam.

[0049] 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:

[0050] (3)

[0051] (4)

[0052] The calculation shows that the supporting force of the steel purlin in the unsaturated homogeneous stratum in this embodiment is 25.27 kPa.

[0053] Step 3: According to the characteristics of the foundation pit engineering, determine that the foundation pit belongs to the third-level foundation pit.

[0054] The fish belly beam support structure includes a straight web member 2, an inclined web member 5, a corner brace 1, a steel purlin 3, a foundation pit, a ground connecting wall 8, and a 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 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 strand It is 500kN.

[0055] Step 4: Analyze the mechanical properties of the fish belly beam support structure, calculate the mid-span deflection of the prestressed fish belly beam in the unsaturated homogeneous stratum and determine the rationality of the deflection:

[0056] According to the load-bearing components and force-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. Figure 4 As shown in the figure, the mid-span deflection of a prestressed fish belly beam in unsaturated homogeneous strata is calculated:

[0057] (5)

[0058] in: The force of the steel purlin and the supporting force of the steel purlin is the action force and reaction force, the size of which is related to the steel purlin support force equal; The span of the fish belly beam is long; The longest belly bar 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.

[0059] 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 10.11 kN / m.

[0060] 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 supporting structure is 54.91mm.

[0061] According to the above results, the rationality of the fish belly beam support structure is judged:

[0062] 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 of 54.91 mm is less than the minimum warning value of the medium-level 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 for foundation pit support design.

[0063] If the above results do not meet the minimum warning value of the corresponding foundation pit specified in the "Technical Standard for Monitoring of Construction Foundation Pit Engineering" (GB 50497-2019) and the design requirements for bending members in the "Code for Design of Steel Structures" (GB50017-2017), it is necessary to repeat step 3, modify the parameters of the fish belly beam support structure, including the parameters of steel purlin 3, web members and steel strands, and re-calculate step 4.

[0064] Example 2

[0065] 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:

[0066] Step 1: Conduct field investigation, 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.

[0067] The depth of the foundation pit 7 of the unsaturated homogeneous sand foundation pit in this embodiment is The fish beam support depth of the fish 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.

[0068] The unsaturated soil matrix suction in this example The effective cohesion of unsaturated homogeneous sand is 2 kPa. The effective internal friction angle of unsaturated homogeneous sand is 5 kPa. is 30°; active earth pressure coefficient of unsaturated homogeneous sand is 0.33; the soil weight 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.

[0069] 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.

[0070] Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata:

[0071] Step 2.1: Calculate the height of the compressive stress zone below the bottom of the foundation pit :

[0072] (1)

[0073] 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.

[0074] 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.

[0075] Step 2.2: Calculate the steel purlin support force in unsaturated homogeneous soil :

[0076] 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 unsaturated homogeneous strata is The calculation equation is as follows:

[0077] (2)

[0078] 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; It is the support depth of fish belly beam.

[0079] 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:

[0080] (3)

[0081] (4)

[0082] The calculation shows that the supporting force of the steel purlin in the unsaturated homogeneous stratum in this embodiment is 26.53 kPa.

[0083] Step 3: According to the characteristics of the foundation pit engineering, determine that the foundation pit belongs to the third-level foundation pit.

[0084] The fish belly beam support structure includes a straight web member 2, an inclined web member 5, a corner brace 1, a steel purlin 3, a foundation pit, a ground connecting wall 8, and a 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 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 strand It is 300kN.

[0085] Step 4: Analyze the mechanical properties of the fish belly beam support structure, calculate the mid-span deflection of the prestressed fish belly beam in the unsaturated homogeneous stratum and determine the rationality of the deflection:

[0086] According to the load-bearing components and force-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. Figure 4 As shown in the figure, the mid-span deflection of a prestressed fish belly beam in unsaturated homogeneous strata is calculated:

[0087] (5)

[0088] in: The force of the steel purlin and the supporting force of the steel purlin is the action force and reaction force, the size of which is related to the steel purlin support force equal; The span of the fish belly beam is long; The longest belly bar 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.

[0089] 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 10.61 kN / m.

[0090] 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.

[0091] According to the above results, the rationality of the fish belly beam support structure is judged:

[0092] According to the Technical Standard for Monitoring of Construction Pit Engineering (GB 50497-2019) and the Code for Design of Steel Structures (GB50017-2017), the mid-span deflection of the above-mentioned fish beam of 86.30 mm is greater 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. It does not meet the minimum warning value of the corresponding foundation pit specified in the Technical Standard for Monitoring of Construction Pit Engineering (GB 50497-2019), and it is necessary to return to step 3 to adjust the parameters of the fish beam support structure.

[0093] 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 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 strand It is 550kN.

[0094] 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.

[0095] According to the above results, the rationality of the fish belly beam support structure is judged:

[0096] According to the Technical Standard for Monitoring of Construction 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 the 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 for 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; Step 2: Calculation of steel purlin support capacity in unsaturated homogeneous strata: Step 3: Determine the type of foundation pit and select the parameters of the fish belly beam support structure according to the characteristics of the foundation pit project; 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 through the steel purlin support force, fishbelt beam support structure parameters and foundation pit parameters, and determine the rationality of the deflection.

2. The method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum according to claim 1 is 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 2, characterized in that: The parameters obtained in step 1 include the soil unsaturated matrix suction , effective cohesion of unsaturated homogeneous sand , the 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 .

4. 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 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 .

5. The method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum according to claim 4, characterized in that: The calculation method of 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 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; 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 ;like ,but Calculate normally.

6. A method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum according to claim 4, characterized in that: 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.

7. The method for determining the mid-span deflection of a fish belly beam in an unsaturated homogeneous stratum according to claim 6, 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.

8. 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 fish beam support structure parameters in step 3 include: , the longest straight belly rod length of the fish belly beam , 1 / 2 of the length of the 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 strand .

9. 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 calculation method of the mid-span deflection of the fish belly beam in the unsaturated homogeneous stratum is: ;in: The force of the steel purlin and the supporting force of the steel purlin is the action force and reaction force, the size of which is related to the steel purlin support force equal; The span of the fish belly beam is long; The longest belly bar 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; in: 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.

10. 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: If the result obtained in step 4 is unreasonable, it is necessary to repeat step 3, modify the fish belly beam support structure parameters, and recalculate step 4.

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