Pile body internal force correction calculation method based on concrete damage model

Abstract

The invention discloses a pile body internal force correction calculation method based on a concrete damage model, and the method comprises the following steps: S1, installing stressometers at different sections in a pile body, and collecting pile body internal force test vibrating wire frequency data, temperature data and static load test data; s2, analyzing test data at the calibrated section of the pile body to obtain curve characteristics; s3, inverting concrete damage model parameters by using test data change characteristics, and constructing concrete damage models under different load effects; and S4, calculating the axial force of the pile body by using the parameters obtained in the step S2 and the step S3. According to the method, the concrete damage model and the multi-parameter inversion method are used for pile body internal force calculation, the problem that the on-site elastic modulus of the pile body concrete is difficult to obtain is solved, and the actual calculation precision and the theoretical interpretable degree are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of building monitoring, and particularly relates to a method for correcting and calculating the internal force of a pile shaft based on a concrete damage model. Background Art

[0002] The test of the internal force of a pile shaft refers to a test method for calculating the lateral resistance, end resistance or bending moment of a pile shaft under the action of a load by testing the strain and displacement of the pile shaft. It is an important way to understand the bearing capacity of a foundation pile during a static load test, and it is also a powerful measure to optimize the design of a pile foundation and ensure the safety of a building. The test results of the internal force of a pile shaft can provide reasonable parameters for design and construction, so as to reduce costs and save the construction period and achieve investment control on the premise of ensuring the safety of geotechnical engineering. It plays an extremely important role in the reasonable design and construction of buildings, bridges and port projects. Therefore, the internal force of a pile shaft is an important index in complex site areas and the optimization of pile foundation design.

[0003] Currently, the most conventional method for testing the internal force of a pile shaft is to embed sensors such as reinforcing bar stress gauges and strain gauges in the foundation pile, obtain the strain of the pile shaft cross-section by measuring the frequency change of the sensor, and then calculate the axial force of the pile shaft by combining the elastic modulus and cross-sectional area of the reinforcing bars and concrete of the foundation pile. However, the defects of this method are that the internal force of the pile shaft is relatively complex inside the pile shaft concrete and difficult to analyze, and it is affected by factors such as the elastic modulus of concrete, pouring quality, and the embedding quality and test accuracy of sensors. From the data of the calibration section, there is a large difference between the currently measured value of the internal force of the foundation pile and the measured value of the pile top load. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for correcting and calculating the internal force of a pile shaft based on a concrete damage model that can improve the test accuracy.

[0005] The purpose of the present invention is achieved by the following technical measures: A method for correcting and calculating the internal force of a pile shaft based on a concrete damage model, characterized by including the following steps:

[0006] S1. Install stress gauges at different cross-sections inside the pile shaft, and collect the vibrating wire frequency data, temperature data and static load test data of the internal force test of the pile shaft;

[0007] S2. Analyze the test data at the calibration section of the pile shaft to obtain the curve characteristics;

[0008] S3. Invert the parameters of the concrete damage model by using the change characteristics of the test data, and construct the concrete damage model under different loads;

[0009] S4. Calculate the axial force of the pile shaft by using the parameters obtained in step S2 and step S3.

[0010] The present invention applies a concrete damage model and a multi-parameter inversion method to the calculation of internal forces of pile shafts, solves the problem that it is difficult to obtain the elastic modulus of in-situ pile shaft concrete, and significantly improves the actual calculation accuracy and theoretical interpretability.

[0011] In step S1 of the present invention, 3 to 4 stress gauges are arranged at each cross-section, and 4 stress gauges are symmetrically arranged at the calibration cross-section.

[0012] In step S2 of the present invention:

[0013] ⑴ Calculate the coefficient of variation of micro-strain at the calibration cross-section of the pile shaft, and draw a curve of the coefficient of variation changing with the value of the static load test load;

[0014] ⑵ Draw a curve of the steel bar stress changing with the value of the static load test load;

[0015] ⑶ According to the change of the coefficient of variation, divide the micro-strain test data corresponding to the static load test load value into two sections of data: the rising section in the early stage of loading and the falling section in the later stage;

[0016] ⑷ Fit the data points between the average value of the steel bar stress and the pile top load in sections with a straight line to obtain the slopes K1 and K2 of the two straight lines.

[0017] In step S3 of the present invention:

[0018] ⑴ Calculate the elastic moduli E c1 、E c2 of the concrete in the early and later stages during the loading process of the pile shaft;

[0019] ⑵ Calculate the concrete compressive strength value f c,r or the tensile strength value f t,r ;

[0020] ⑶ Calculate the function of the concrete damage evolution parameter d t corresponding to different micro-strains in the concrete damage model.

[0021] In step S3⑶ of the present invention:

[0022]

[0023] Wherein:

[0024]

[0025]

[0026] In the formula: ε is the average value of the micro-strain of the data points included in the falling section of the coefficient of variation in the later stage, and ε t,r is the peak tensile strain of the concrete corresponding to the representative value f t,r of the uniaxial tensile strength of the concrete.

[0027] In step S4 of the present invention: the axial force F of the pile body i is calculated by the following formula:

[0028] F i = A c E c修 ε i + A s E s ε i

[0029] where: E c修 = (1 - d t )E c1 .

[0030] In the present invention, the calibrated section of the pile body is 1 to 2 times the pile diameter away from the pile top.

[0031] Compared with the prior art, the present invention has the following remarkable effects:

[0032] By adopting a concrete damage model to calculate the internal force of the pile body and using the actually obtained test data of the internal force of the pile body, the present invention gives a specific inversion calculation method for the parameters in the model. Each parameter has a clear physical meaning and a specific calculation formula, which greatly improves the accuracy and interpretability of the detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0034] Figure 1 is the flow chart of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be described in detail below with reference to the embodiments and their accompanying drawings to help those skilled in the art better understand the inventive concept of the present invention. However, the protection scope of the claims of the present invention is not limited to the following embodiments. For those skilled in the art, all other embodiments obtained without creative labor under the premise of not departing from the inventive concept of the present invention belong to the protection scope of the present invention.

[0036] As Figure 1 shown, a method for correcting and calculating the internal force of a pile body based on a concrete damage model according to the present invention includes the following steps:

[0037] S1. Install stress gauges at different sections inside the pile body, monitor the stress change and temperature change of the foundation pile through the stress gauges, and collect the vibrating wire frequency data, temperature data and static load test data of the internal force of the pile body;

[0038] Arrange 3 to 4 strain gauges at each cross-section, and it is advisable to symmetrically arrange 4 at the calibration cross-section. The number, position, and installation location of the cross-sections can be adjusted according to actual needs. In this example, there are a total of 9 cross-sections, and 4 strain gauges are symmetrically arranged at each cross-section.

[0039] S2. Analyze the test data at the calibration cross-section of the pile body to obtain the curve characteristics;

[0040] Only analyze the data of the 4 sensors at the calibration cross-section of the pile body (1 to 2 times the pile diameter away from the pile top), mainly referring to the steel bar stress of the pile body and the corresponding micro-strain calculated according to the calibration coefficient of the strain gauge after temperature calibration.

[0041] ⑴ Calculate the coefficient of variation of the micro-strain at the calibration cross-section of the pile body and draw the curve of the coefficient of variation changing with the static load test load value (generally 9 levels of load);

[0042] ⑵ Draw the curve of the steel bar stress changing with the static load test load value;

[0043] ⑶ According to the change of the coefficient of variation, divide the micro-strain test data corresponding to the static load test load value into two segments: the rising segment in the early stage of loading and the falling segment in the later stage;

[0044] ⑷ Fit the data points between the average value of the steel bar stress and the pile top load in segments to obtain the slopes K1 and K2 of the two straight lines.

[0045] S3. Use the change characteristics of the test data to invert the parameters of the concrete damage model and construct the concrete damage model under different loads; Apply the measured data in step S1 and the analysis results in step S2 to calculate the concrete elastic modulus E, which is one of the important parameters of the concrete damage model c1 、E c2 。

[0046] ⑴ Calculate the concrete elastic modulus E c1 、E c2 ;

[0047] The calculation formula is:

[0048] E c1 =(K1 - A s )·E s / ·A c Formula (1)

[0049] E c2 =(K2 - A s )·E s / ·A c Formula (2)

[0050] In the formula, E sis the elastic modulus of the steel bar, A s is the total area, A c is the area of the concrete.

[0051] ⑵ Calculate the concrete compressive strength value f c,r or the tensile strength value f t,r , that is, inversely calculate the concrete compressive strength value f c,r or the tensile strength value f t,r ;

[0052] ⑶ Calculate the concrete damage evolution parameter d corresponding to different micro-strains in the concrete damage model t as a function of, that is, apply the pile shaft concrete damage model constructed in step S3⑵ to obtain the concrete damage evolution parameter d corresponding to different micro-strains, which is the third important parameter of the concrete damage model t :

[0053] Taking the tensile strength value f t,r as an example:

[0054] The calculation formula is:

[0055]

[0056]

[0057] In the formula: ε is the average micro-strain of the data points included in the later descending section of the coefficient of variation, ε t,r is the peak tensile strain of the concrete corresponding to the representative value f t,r of the uniaxial tensile strength of the concrete. After calculating E c1 , E c2 , d t can be directly calculated using formula (8). The variable x at this time is the micro-strain value measured at different cross-sections of the pile shaft under different load conditions; since the calculation formula (8) of the concrete damage model parameter d t is relatively complex, but through formulas (3) to (8), it can finally be reduced to a function of f t,r , so the trial method can be used to obtain the accurate f t,r value.

[0058] This step is based on the confirmation of f t,r in step S3(2), and determines the constant terms in formula (8) according to formulas (3) to (6), including ε t,r , n, ρ t , α t , then substitute the variable x corresponding to the micro-strain value measured at different cross-sections of the pile shaft into formula (7), and finally construct the concrete damage model, which can be used for the axial force calculation of other pile shaft cross-sections.

[0059] S4. Calculate the axial force of the pile shaft at the corresponding micro-strain at different cross-sections using the parameters obtained in step S2 and step S3.

[0060] Axial force of the pile shaft F i It is calculated by the following formula:

[0061] F i = A c E c修 ε i + A s E s ε i

[0062] Where: E c修 =(1 - d t )E c1 .

[0063] In the formula, E c1 is the initial elastic modulus of concrete during the pile shaft loading process, d t is the concrete damage evolution parameter, ε is the stress of the pile shaft steel bar and the corresponding micro-strain calculated according to the calibration coefficient of the stress gauge after temperature calibration, and the subscript i represents different cross-sections of the pile shaft.

[0064] Regarding the correction calculation of the test results of the pile shaft internal force, the conventional formula for the pile shaft internal force is F i = A c E c ε i + A s E s ε i , the value of the concrete elastic modulus E c is a fixed value. For example, the standard value corresponding to the designed concrete grade C30 of the pile shaft is 30.0 GPa, which has no direct correlation with the on-site measured data. For precast piles with better curing conditions, the error is relatively small, but for cast-in-place concrete piles, there are significant differences. The elastic modulus of the pile shaft in the correction calculation method of the present invention is (1 - d t )E c1 , where E c1 is the calculation result of the measured value in step S3(1), d tThe parameters of the concrete damage model inversed after being tested by measured values are also equivalent to determining these two important parameters through in-situ field tests. Therefore, the correction algorithm of the present invention is reflected in two aspects. One is to make full use of the existing measured data; the other is the analytical calculation work carried out on the basis of the concrete damage model, and the relatively difficult-to-measure data of the concrete elastic modulus can be better explained. In addition, since the cast-in-place pile is placed in the rock and soil layer, the temperature change is small during the entire static load test and internal force test, and only the original temperature correction of the microstrain is required, and the influence of temperature can be ignored during the internal force calculation.

[0065] The concrete elastic modulus has a great influence on the test results of the pile body internal force. The accuracy of direct measurement using sensors is not high, and the situation where the measured result of the pile shaft force is greater than the pile top load often occurs. Especially when the load is large, the difference is greater. Therefore, based on the classical concrete damage model theory, the present invention uses measured data for parameter inversion and corrects the calculation result of the pile shaft force. When correcting, it is necessary to use measured data for piecewise fitting, which also verifies to a certain extent the situation that the concrete elastic modulus of the pile body changes with strain. This is different from the conventional calculation using a fixed elastic modulus.

[0066] In an example, for the internal force test of a certain cast-in-place pile, the pile diameter is 1m, and the cross-sectional area of the concrete in the pile body A c = 0.785m 2 , the steel bar area A s = 0.020m 2 . During the anti-pulling static load test, the load is divided into 12 levels. The first two levels are 1000 and 2000kN respectively, and the last 10 levels increase by 500kN step by step until the maximum loading amount of 7000kN. The relationship between the microstrain, inversion parameters measured by 4 steel bar stress gauges at the calibration section and the calculated pile body internal force value and the pile top load is shown in the following table:

[0067]

[0068] (Table 1).

Claims

1. A method for calculating pile internal force correction based on concrete damage model, characterized in that The following steps are involved: S1. Install stress gauges at different sections inside the pile body to collect vibration string frequency data, temperature data and static load test data for the internal force test of the pile body; S2, analyzing the test data at the calibrated section of the pile body to obtain curve characteristics; S3. Inverse the parameters of the concrete damage model using the variation characteristics of the test data to construct a concrete damage model under different loads; S4. Calculate the pile shaft axial force using the parameters obtained in step S2 and step S3.

2. The method for calculating pile body internal force correction based on concrete damage model according to claim 1 is characterized in that: In step S1, 3 to 4 stress gauges are arranged on each section. Four stress gauges are symmetrically arranged on the calibration section.

3. The method for calculating pile body internal force correction based on concrete damage model according to claim 2 is characterized in that: In step S2: ⑴ Calculate the microstrain variation coefficient at the calibrated section of the pile body and draw a curve showing the variation coefficient as the static load test load value changes; ⑵Draw a curve showing the change of steel bar stress with the static load test load value; ⑶ According to the variation of the coefficient of variation, the micro-strain test data corresponding to the load value of the static load test is divided into two sections: the early rising section and the late falling section of loading; (4) Perform straight line fitting on the data points between the average value of the steel bar stress and the pile top load in sections to obtain the slopes K1 and K2 of the two straight lines.

4. The method for calculating pile body internal force correction based on concrete damage model according to claim 3 is characterized in that: In step S3: ⑴ Calculate the concrete elastic modulus E in the early and late stages of pile loading c1 、E c2 ; ⑵Calculate the concrete compressive strength value f c,r Or tensile strength value f t,r ; ⑶ Calculate the concrete damage evolution parameter d corresponding to different microstrains in the concrete damage model t function.

5. The method for calculating pile body internal force correction based on concrete damage model according to claim 4 is characterized in that: In step S3(3): in: Where: ε is the average microstrain value of the data points included in the late descending section of the coefficient of variation, ε t,r is the representative value of the uniaxial tensile strength of concrete f t,r The corresponding peak tensile strain of concrete.

6. The method for calculating pile body internal force correction based on concrete damage model according to claim 5 is characterized in that: In step S4: the pile shaft axial force F i Calculated by the following formula: F i =A c E c修 e i +A s E s e i Where: E c修 =(1-d t )E c1 .

7. The method for calculating pile body internal force correction based on concrete damage model according to claim 6 is characterized in that: The marked section of the pile body is 1 to 2 times the pile diameter away from the pile top.

Images