A method for determining the axial force and bending moment of an internal support in concrete based on a steel stress meter

Abstract

The application discloses a method for determining the axial force and bending moment of a concrete inner support based on a steel bar stress meter, and belongs to the technical field of inner force calculation of a steel support of underground engineering; the method comprises the following steps: embedding a vibrating string type steel bar stress meter in the outermost force-bearing steel bar of a reinforced concrete inner support, and obtaining the initial frequency and initial temperature of the upper and lower steel bars of a component section; obtaining the frequency value and temperature value in the process of foundation pit excavation; calculating the strain variation according to the frequency value of the upper and lower steel bars; obtaining the temperature-corrected strain variation according to the temperature value of the upper and lower steel bars and the calculated strain variation; and calculating the axial force and bending moment of the concrete inner support according to the temperature-corrected strain variation value. The method can accurately calculate the axial force and bending moment values of the concrete inner support, and can provide more accurate basis for analyzing the safety of the inner support according to the axial force and bending moment, and can provide more scientific theoretical guidance for similar engineering.

Description

Technical Field

[0001] This invention belongs to the field of internal force calculation technology for steel supports in underground engineering, specifically relating to a method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge. Background Technology

[0002] During the excavation of building foundations, internal support structures are often used to prevent the foundation pit from collapsing. Concrete supports are widely used in current projects due to their high rigidity. Concrete supports are the most important component of the foundation pit support structure, and the internal force of the supports is an important basis for evaluating the safety of the support structure and the entire foundation pit project. Currently, the internal force of concrete supports is mainly measured by embedding vibrating wire steel stress gauges within the supports.

[0003] During the excavation of the foundation pit, due to the settlement or bulging of the columns, the internal supports are not only subjected to axial forces but also to bending moments. However, most existing monitoring of internal support structures only evaluates their stress conditions by the axial force of the supports. The evaluation results have many errors compared with the actual situation and cannot truly reflect the real internal force state of the internal supports. When the axial force is small and the bending moment is large, there are greater safety hazards.

[0004] Therefore, there is an urgent need to design a method for calculating the internal forces of concrete supports that can simultaneously consider the axial force and bending moment of the supports within the concrete, so as to provide more accurate theoretical guidance for foundation pit engineering. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems existing in the prior art and provide a method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge. This method can accurately calculate the axial force and bending moment values ​​of the supports within the concrete based on the detection results of the pre-embedded vibrating wire steel bar stress gauge. It can also analyze the actual stress state of the internal supports based on the axial force and bending moment, providing more practical and accurate theoretical guidance for foundation pit engineering.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge, comprising the following steps:

[0007] S1. Before concrete pouring, vibrating wire rebar stress gauges are pre-embedded in the outer structural reinforcing bars of the reinforced concrete inner support. After the concrete inner support reaches its design strength and before excavation of the foundation pit, a vibrating wire reading instrument is used to read the rebar stress gauges and obtain the initial frequency value f of the rebar stress gauges arranged in the upper part of the component section. 0T and initial temperature T 0T The initial frequency value f of the stress gauge for the lower reinforcement arrangement. 0B and initial temperature T 0B ;

[0008] S2. During the foundation pit excavation process, the frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support are read respectively. T f B and temperature value T T T B ;

[0009] S3. Based on the frequency values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, calculate the strain change ε ​​of the upper and lower reinforcing bars of the member section respectively. T and ε B ;

[0010] The formula for calculating the strain change of the upper reinforcement of the member section is:

[0011] The formula for calculating the strain change of the lower reinforcement is: In the formula, K0 is the sensitivity coefficient of the sensor;

[0012] S4. Based on the temperature values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, and the strain change ε ​​calculated in step S3... T and ε B The strain change ε′ of the upper and lower reinforcement bars of the member section after temperature correction was calculated respectively. T and ε′ B ;

[0013] The formula for calculating the strain change of the upper reinforcement of the member section after temperature correction is: ε′ T =ε T -(T T -T 0T (F-F0),

[0014] The formula for calculating the strain change of the lower reinforcement due to temperature correction is: ε′ B =ε B -(T T -T 0B (F-F0),

[0015] In the formula, F is the linear expansion coefficient of the object being measured, and F0 is the temperature correction coefficient of the sensor.

[0016] S5. The strain change ε′ corrected for temperature, calculated in step S4 above. T and ε′ B Given the values ​​of the reinforced concrete internal support section, assuming the width of the section is B, the height is H, the average distance from the structural reinforcing bars to the upper and lower sections is a, and the average distance between two corresponding structural reinforcing bars on the upper and lower sections is h, and ignoring the zero tensile stress in the concrete, calculate the axial force and bending moment of the concrete internal support.

[0017] 1)ε′ T and ε′ B The forces are approximately equal and greater than zero. The internal force on this concrete support is only axial tension. The formula for calculating axial tension is E. S (ε′ T A ST +ε′ B A SB ), where E S For the elastic modulus of the reinforcing steel, A ST For the total area of ​​the upper reinforcement, A SB This represents the total area of ​​the bottom reinforcement bars, with zero bending moment.

[0018] 2)ε′ T and ε′ B The forces are approximately equal and less than zero. The internal force on this concrete support is only axial pressure, and the formula for calculating axial pressure is E. S (ε′ T A ST +ε′ B A SB )+E C (ε′ T +ε′ B A C / 2, where E C For the elastic modulus of concrete, A C The concrete cross-sectional area after deducting the area of ​​the reinforcing steel bars has a zero bending moment.

[0019] 3)ε′ T and ε′ B The forces are not equal and are both greater than zero. The internal forces acting on the concrete support are axial tension and bending moment. The formula for calculating the axial tension is E. S (ε′ T A ST +ε′ B A SB The formula for calculating the bending moment is E. S |ε′ T A ST -ε′ B A SB |×h / 2;

[0020] 4)ε′ T and ε′ B The forces are not equal and are both less than zero. The internal forces acting on this concrete support are axial pressure and bending moment. The formula for calculating axial pressure is E. S (ε′ T A ST +ε′ B A SB )+E C (ε′ T+ε′ B A C / 2, the formula for calculating the bending moment is E S |ε′ T A ST -ε′ B A SB |×h / 2+E C |ε T ′-ε′ B |A C / 2×h / 6;

[0021] 5)ε′ T and ε′ B The forces are not equal and are one positive and one negative. The internal forces acting on the concrete support are axial tensile force and bending moment, or axial compressive force and bending moment. Let ε′ be the internal forces. T The value is negative (the upper end of the cross-section is under pressure).

[0022] The formula for calculating axial force is: The formula for calculating bending moment is:

[0023] If ε′ B If it is negative, then ε′ in the above formula will be... T With ε′ B exchange.

[0024] In step S1, the arrangement of the vibrating wire rebar stress gauge is as follows: at the initial stage of foundation pit excavation, after the rebar in the reinforced concrete inner support is tied and before concrete pouring, the vibrating wire rebar stress gauge is pre-embedded in the structural stress rebar at the upper and lower middle positions on the outer side of the reinforced concrete inner support section. Then, the wires for measuring data are led to the outside of the component, the concrete inner support is poured, and it is cured.

[0025] The connection method between the vibrating wire rebar stress gauge and the structural reinforcing steel is as follows: the structural reinforcing steel is cut to the appropriate length, and then both ends of the vibrating wire rebar stress gauge are welded to the structural reinforcing steel. During welding, the sensor part in the middle section of the vibrating wire rebar stress gauge is cooled.

[0026] The beneficial effects of this invention are:

[0027] 1) Compared with the prior art, the method of the present invention takes into account the influence of temperature, which can more accurately reflect the true stress state of the internal support.

[0028] 2) In the method of the present invention, by pre-embedding vibrating wire rebar stress gauges in the outermost structural reinforcing bars of the inner support, the axial force and bending moment of the support in the concrete can be accurately calculated based on the detection results of the pre-embedded vibrating wire rebar stress gauges. By measuring and calculating the actual stress state of the inner support, not only can the bearing capacity of the inner support itself be evaluated, but also the settlement and heave state of the inner support can be analyzed based on the magnitude and direction of the bending moment, providing a theoretical basis for the safe excavation of the foundation pit. Attached Figure Description

[0029] Figure 1 This is a diagram showing the arrangement of the vibrating wire rebar stress gauge in this invention on the supporting rebar within reinforced concrete;

[0030] Figure 2 This is a schematic diagram of the structural connection between the vibrating wire type rebar stress gauge and the rebar in this invention;

[0031] Figure 3 This is a schematic diagram showing the dimensions of the reinforced concrete internal support section and the structural reinforcing steel bars in this invention;

[0032] Figure 4 This is a schematic diagram showing the dimensions of the reinforced concrete internal support section and the longitudinal reinforcing bars in an embodiment of the present invention.

[0033] In the figure, 1-reinforced concrete internal support, 2-structural reinforcing steel, 3-vibrating wire reinforcing steel stress gauge, 4-welded joint, 5-conductor, 6-stirrup, B-width of the reinforced concrete internal support section, H-height of the reinforced concrete internal support section, a-average distance from the structural reinforcing steel to the upper and lower sections, h-average distance between two corresponding structural reinforcing steels on the upper and lower sections. Detailed Implementation

[0034] The present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments.

[0035] Example 1: As Figure 1-4 As shown, the present invention provides a method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge, comprising the following steps:

[0036] S1. Before concrete pouring, a vibrating wire type rebar stress gauge is pre-embedded in the structural reinforcing steel bars 2 of the reinforced concrete support 1; such as Figure 1 As shown, at the beginning of the foundation pit excavation, after the steel bars in the reinforced concrete inner support 1 are tied, before the concrete is poured, a vibrating wire type steel stress gauge 3 is pre-embedded in the structural stress steel bars 2 at the upper and lower middle positions on the outer side of the reinforced concrete inner support 1 section. Then, the wire 5 of the measurement data is led to the outside of the component, the concrete inner support is poured, and it is cured.

[0037] like Figure 2As shown, the connection method between the vibrating wire type steel bar stress gauge 3 and the structural reinforcing steel bar 2 is as follows: the structural reinforcing steel bar 2 is cut to the corresponding length, and then both ends of the vibrating wire type steel bar stress gauge 3 are welded to the structural reinforcing steel bar 2. The structural connection point between the two is the weld 4. During welding, the sensor part of the middle section of the vibrating wire type steel bar stress gauge 3 is cooled down, and the wire 5 is led out of the component from the sensor.

[0038] After the concrete internal supports reach their design strength, and before excavation of the foundation pit, a vibrating wire reading instrument is used to read the rebar stress gauges to obtain the initial frequency value f of the rebar stress gauges at the top of the structural member cross-section. 0T =1400Hz and initial temperature T 0T =20℃, and the initial frequency value f of the stress gauge for the lower reinforcement arrangement. 0B =1400Hz and initial temperature T 0B =20℃.

[0039] S2. During the foundation pit excavation process, the frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support are read respectively. T =1600Hz, f B =1600Hz and temperature value T T =25℃, T B =25℃.

[0040] S3. Based on the frequency values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, calculate the strain change ε ​​of the upper and lower reinforcing bars of the member section respectively. T and ε B Where K0 = 1.73 × 10 -10 ;

[0041]

[0042] S4. Based on the temperature values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, and the strain change ε ​​calculated in step S3... T and ε B The strain change ε′ of the upper and lower reinforcement bars of the member section after temperature correction was calculated respectively. T and ε′ B , F=10με / ℃, F0=12.2με / ℃;

[0043] ε′ T =ε T -(T T -T 0T (F-F0)=114.8(με), ε′ B =ε B -(T T -T 0B)(F-F0)=114.8(με).

[0044] S5. The strain change ε′ corrected for temperature, calculated in step S4 above. T and ε′ B Based on the assumptions of plane section and zero tensile stress in concrete, the axial force and bending moment of the internal concrete support are calculated respectively.

[0045] like Figure 3-4 As shown, the outer perimeter of the structural reinforcing bars is tied with stirrups 6. Vibrating wire reinforcing bar stress gauges 3 are embedded on the upper and lower structural reinforcing bars 2 located in the middle. Ignoring the waist reinforcing bars, draw the structural distribution diagram of the structural reinforcing bars. The grade of concrete material is C35. The width B of the reinforced concrete inner support section is 1000mm, the height H is 800mm, the average distance a from the structural reinforcing bars to the upper and lower sections is 54mm, and the average distance h between the two corresponding structural reinforcing bars on the upper and lower sections is 692mm.

[0046] Due to ε′ T and ε′ B The forces are equal and greater than zero. The internal force on this concrete support is only axial tension, and the bending moment is zero. The axial tension is:

[0047] E S (ε′ T A ST +ε′ B A SB ) = 2.0 × 10 5 ×(114.8×10 -6 ×4917+114.8×10 -6 (×4917)=225.8kN.

[0048] Example 2: The difference from Example 1 above is:

[0049] The frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support T =1200Hz, f B =1200Hz.

[0050] The strain changes of the upper and lower reinforcing bars in the member section are respectively:

[0051]

[0052] The strain changes of the upper and lower reinforcement bars of the component section after temperature correction are as follows:

[0053] ε′ T =ε T -(T T -T 0T(F-F0)=-79(με), ε′ B =ε B -(T T -T 0B )(F-F0)=-79(με).

[0054] Due to ε′ T and ε′ B The forces are equal and less than zero. The internal force on this concrete support is only axial pressure, and the bending moment is zero. The axial pressure is:

[0055]

[0056] Example 3: The difference from Example 1 above is:

[0057] The frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support T =1500Hz, f B =1600Hz.

[0058] The strain changes of the upper and lower reinforcing bars in the member section are respectively:

[0059]

[0060] The strain changes of the upper and lower reinforcement bars of the component section after temperature correction are as follows:

[0061] ε′ T =ε T -(T T -T 0T )(F-F0)=61.2(με), ε′ B =ε B -(T T -T 0B )(F-F0)=114.8(με).

[0062] Due to ε′ T and ε′ B The forces are not equal and are both greater than zero. The internal forces acting on the concrete support are axial tension and bending moment. The axial tension is:

[0063] E S (ε′ T A ST +ε′ B A SB ) = 2.0 × 10 5 ×(114.8×10 -6 ×4917+61.2×10 -6 ×4917)=173.1kN,

[0064] The bending moment is:

[0065] E S |ε′ T A ST -ε′ B A SB |×h / 2=2.0×10 5 ×(114.8×10 -6 ×4917-61.2×10 -6 (×4917)×400=21.1kN.m.

[0066] Example 4: The difference from Example 1 above is:

[0067] The frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support T =1300Hz, f B =1200Hz.

[0068] The strain changes of the upper and lower reinforcing bars in the member section are respectively:

[0069]

[0070] The strain changes of the upper and lower reinforcement bars of the component section after temperature correction are as follows:

[0071] ε′ T =ε T -(T T -T 0T (F-F0)=-35.7(με), ε′ B =ε B -(T T -T 0B )(F-F0)=-79(με).

[0072] Due to ε′ T and ε′ B The forces are not equal and are both less than zero. The internal forces acting on the concrete support are axial pressure and bending moment. The axial pressure is:

[0073]

[0074] The bending moment is:

[0075] E S |ε′ T A ST -ε′ B A SB |×h / 2+E C |ε′ T -ε′ B |A C / 2×h / 6=2.0×105 ×(79×10 -6 ×4917-35.7×10 -6 (×4917)×400+3.15×10 4 ×(79×10 -6 -35.7×10 -6 )×800×1000 / 2×800 / 6=17+72.7=89.7kN.m.

[0076] Example 5: The difference from Example 1 above is:

[0077] The frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support T =1200Hz, f B =1600Hz.

[0078] The strain changes of the upper and lower reinforcing bars in the member section are respectively:

[0079]

[0080] The strain changes of the upper and lower reinforcement bars of the component section after temperature correction are as follows:

[0081] ε′ T =ε T -(T T -T 0T (F-F0)=-79(με), ε′ B =ε B -(T T -T 0B )(F-F0)=114.8(με).

[0082] Due to ε′ T and ε′ B The forces are not equal, and one is positive and the other is negative. The internal forces acting on the concrete support are axial tensile force and bending moment, or axial compressive force and bending moment. The axial force is:

[0083]

[0084] The bending moment is:

[0085]

[0086] The method of this invention can accurately calculate the axial force and bending moment of the support in the concrete based on the detection results of the pre-embedded vibrating wire rebar stress gauge. It can also analyze the actual stress state of the internal support based on the axial force and bending moment, providing more scientific theoretical guidance for engineering construction.

[0087] The above description is only used to illustrate the technical solution of the present invention and is not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention, as long as they do not depart from the spirit and scope of the technical solution of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge, characterized in that: Includes the following steps: S1. Before concrete pouring, vibrating wire rebar stress gauges are pre-embedded in the outer structural reinforcing bars of the reinforced concrete inner support. After the concrete inner support reaches its design strength and before excavation of the foundation pit, a vibrating wire reading instrument is used to read the rebar stress gauges and obtain the initial frequency value f of the rebar stress gauges arranged in the upper part of the component section. 0T and initial temperature T 0T The initial frequency value f of the stress gauge for the lower reinforcement arrangement. 0B and initial temperature T 0B ; S2. During the foundation pit excavation process, the frequency values ​​f of the vibrating wire rebar stress gauges at the upper and lower parts of the pre-embedded section in the concrete internal support are read respectively. T f B and temperature value T T T B ; S3. Based on the frequency values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, calculate the strain change ε ​​of the upper and lower reinforcing bars of the member section respectively. T and ε B ; The formula for calculating the strain change of the upper reinforcement of the member section is: The formula for calculating the strain change of the lower reinforcement is: In the formula, K0 is the sensitivity coefficient of the sensor; S4. Based on the temperature values ​​of the upper and lower reinforcing bars obtained in steps S1 and S2 above, and the strain change ε ​​calculated in step S3... T and ε B The strain change ε′ of the upper and lower reinforcement bars of the member section after temperature correction was calculated respectively. T and ε′ B ; The formula for calculating the strain change of the upper reinforcement of the member section after temperature correction is: ε′ T =ε T -(T T -T 0T (F-F0), The formula for calculating the strain change of the lower reinforcement due to temperature correction is: ε′ B =ε B -(T T -T 0B (F-F0), In the formula, F is the linear expansion coefficient of the object being measured, and F0 is the temperature correction coefficient of the sensor. S5. The strain change ε′ corrected for temperature, calculated in step S4 above. T and ε′ B Given the values ​​of the reinforced concrete internal support section, assuming the width of the section is B, the height is H, the average distance from the structural reinforcing bars to the upper and lower sections is a, and the average distance between two corresponding structural reinforcing bars on the upper and lower sections is h, and ignoring the zero tensile stress in the concrete, calculate the axial force and bending moment of the concrete internal support. 1)ε′ T and ε′ B The forces are approximately equal and greater than zero. The internal force on this concrete support is only axial tension. The formula for calculating axial tension is E. S (ε′ T A ST +ε′ B A SB ), where E S For the elastic modulus of the reinforcing steel, A ST For the total area of ​​the upper reinforcement, A SB This represents the total area of ​​the bottom reinforcement bars, with zero bending moment. 2)ε′ T and ε′ B The forces are approximately equal and less than zero. The internal force on this concrete support is only axial pressure, and the formula for calculating axial pressure is E. S (ε′ T A ST +ε′ B A SB )+E C (ε′ T +ε′ B A C / 2, where E C For the elastic modulus of concrete, A C The concrete cross-sectional area after deducting the area of ​​the reinforcing steel bars has a zero bending moment. 3)ε′ T and ε′ B The forces are not equal and are both greater than zero. The internal forces acting on the concrete support are axial tension and bending moment. The formula for calculating the axial tension is E. S (ε′ T A ST +ε′ B A SB The formula for calculating the bending moment is E. S |ε′ T A ST -ε′ B A SB |×h / 2; 4)ε′ T and ε′ B The forces are not equal and are both less than zero. The internal forces acting on this concrete support are axial pressure and bending moment. The formula for calculating axial pressure is E. S (ε′ T A ST +ε′ B A SB )+E C (ε′ T +ε′ B A C / 2, the formula for calculating the bending moment is E S |ε′ T A ST -ε′ B A SB |×h / 2+E C |ε T ′-ε′ B |A C / 2×h / 6; 5)ε′ T and ε′ B The forces are not equal, and one is positive and the other is negative. The internal forces acting on the concrete support are axial tensile force and bending moment, or axial compressive force and bending moment. Let ε′ T If it is negative, the formula for calculating the axial force is: The formula for calculating bending moment is: If ε′ B If it is negative, then ε′ in the above formula will be... T With ε′ B exchange.

2. The method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge according to claim 1, characterized in that: In step S1, the arrangement of the vibrating wire rebar stress gauge is as follows: at the initial stage of foundation pit excavation, after the rebar in the reinforced concrete inner support is tied and before concrete pouring, the vibrating wire rebar stress gauge is pre-embedded in the structural stress rebar at the upper and lower middle positions on the outer side of the reinforced concrete inner support section. Then, the wires for measuring data are led to the outside of the component, the concrete inner support is poured, and it is cured.

3. The method for determining the axial force and bending moment of concrete internal supports based on a steel bar stress gauge according to claim 2, characterized in that: The connection method between the vibrating wire rebar stress gauge and the structural reinforcing steel is as follows: the structural reinforcing steel is cut to the appropriate length, and then both ends of the vibrating wire rebar stress gauge are welded to the structural reinforcing steel. During welding, the sensor part in the middle section of the vibrating wire rebar stress gauge is cooled.

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