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EP-4102199-B1 - STRAIN GAUGE LOAD CELL FOR MONITORING THE STRAIN IN PRESTRESSED ELEMENTS OR ELEMENTS SUBJECTED TO AXIAL STRAIN

EP4102199B1EP 4102199 B1EP4102199 B1EP 4102199B1EP-4102199-B1

Inventors

  • Gaute Alonso, Alvaro
  • Alonso Cobo, Carlos
  • García Sánchez, David
  • JIMENEZ, JOSE CARLOS
  • Ezquerro Andreu, Mikel

Dates

Publication Date
20260513
Application Date
20210610

Claims (13)

  1. A method for monitoring the strain in a prestressed element or element subjected to axial strain, the method comprising the steps of: - providing a strain gauge load cell (16) comprising a hollow cylindrical body (30), which comprises: an upper contact surface (1) configured to contact an anchor plate; a lower contact surface (6) configured to contact a bearing plate; a side wall (10) delimiting a hole (8) which extends from the upper contact surface (1) to the lower contact surface (6), an outer diameter Ø e and an inner diameter Øi defining the thickness of said side wall (10), the inner diameter Øi being the diameter of said hole (8), a height h such that h ≥ 2 5 ∅ e , the outer diameter Ø e fulfilling the following: ∅ e ≥ N fy 2 + π 4 ⋅ ∅ i 2 ⋅ 4 π wherein N is an axial strain for which the strain gauge load cell (16) has been dimensioned and fy is the elastic limit of a material with which the body (30) of the strain gauge load cell (16) has been manufactured; - arranging a plurality of strain gauges (12) on the side wall (10) of the body (30), wherein each strain gauge (12) is located at an equidistant distance between the upper contact surface (1) and the lower contact surface (6) of the body (30); - connecting the strain gauges (12) via an electronic assembly (40) in a full Wheatstone bridge configuration; - introducing the prestressed element or element subjected to axial strain in the hole (8); - applying loads through the upper (1) and lower (6) contact surfaces; - reading an output voltage from the electronic assembly (40); and - processing the output voltage to obtain the axial strain to which the prestressed element or element subjected to axial strain is subjected.
  2. The method of claim 1, wherein the height h of the body (30) is h ≥ ∅ e 2 .
  3. The method of any of claims 1-2, wherein the strain gauge load cell further comprises a protective cover (2) which surrounds and protects the plurality of strain gauges (12).
  4. The method of claim 3, wherein the side wall of the body (30) has at least one increased area (7) with a diameter larger than said outer diameter Ø e of the body (30), said increased area (7) being configured to fasten the protective cover (2) around said body (30).
  5. The method of claim 3, wherein the side wall of the body (30) has two increased areas (7) with a diameter larger than said outer diameter Ø e of the body (30), configured to fasten the protective cover (2) around said body (30).
  6. The method of any of claims 1-5, wherein the hollow cylindrical body (30) has been obtained by machining.
  7. The method of claims 1-6, wherein the body (30) is made of an isotropic material the elastic limit fy of which is greater than the average compression stress in the strain gauge load cell (16).
  8. The method of claim 7, wherein the body (30) is made of an isotropic material the elastic limit fy of which is greater than twice the average compression stress in the strain gauge load cell (16).
  9. The method of any of claims 1-8, wherein the arrangement of the plurality of strain gauges (12) on the side wall (10) of the body (30) at an equidistant distance between the upper contact surface (1) and the lower contact surface (6) of the body (30) enables the average axial deformation ε of the strain gauge load cell (16) to be obtained, the average axial deformation ε being linearly related to the axial prestressing strain N existing in the prestressing element or element subjected to axial strain to be characterised: N = ε ⋅ E ⋅ Ω wherein E is the modulus of elasticity E of the material with which the body (30) has been manufactured, N is the axial strain existing in the prestressing element or element subjected to axial strain to be characterised, and Ω is the average transverse cross section of the load cell (16).
  10. The method of any of the previous claims, wherein the output voltage provided by the electronic assembly in a full Wheatstone bridge is read by means of a data transmission cable (4).
  11. The method of any of the previous claims, wherein the output voltage is read by connecting the electronic assembly to a wireless transmitter device (42) configured to send the information to a wireless receiver device (43) through a wireless communication network.
  12. The method of claim 11, further comprising storing the information received by the wireless receiver device (43) in storage means (44) and processing said information in processing means connected to the wireless receiver device (43) to obtain the axial strain to which the prestressed element or element subjected to axial strain is subjected.
  13. The method of any of claims 11-12, further comprising adjusting the reading range of the output voltage provided by the electronic assembly (40) using a potentiometer (41).

Description

TECHNICAL FIELD The present invention belongs to the field of monitoring the strain existing in prestressed elements or elements subjected to axial strain. The invention has a special application in monitoring the strain existing in the active anchor of these elements. BACKGROUND OF THE INVENTION A structural element is an element belonging to a structure or which constitutes a structure in itself. A prestressed structural element is a structural element which is subjected to compressive axial strain prior to being put into operation. Prestressing units, also called prestressing elements, are used to prestress the structural element. The strain in the prestressed structural elements varies throughout the useful life thereof, influenced by phenomena such as the prestress losses or the strains to which the structural element is subjected during the service life thereof. The prestress losses can be instantaneous losses or delayed losses. Usually, the instantaneous prestress losses can be made up of three factors: due to penetration of wedges or tightening of a nut, due to elastic shortening of the structural element and due to friction with the prestressing duct or sheath. The delayed prestress losses can usually be made up of three other factors: due to shrinkage of the structural element, due to creep of the structural element and due to relaxation of the prestressing element. In order to determine the axial strain in prestressing units and in structural elements subjected to axial strain, so-called load cells are used. A prestressing unit has two anchoring areas: an active anchoring area (from which the prestressing force is exerted) and a passive anchoring area. Thus, in order to determine the axial strain in a prestressing unit, the load cell is placed in the active anchoring area of the prestressing unit. A particular type of prestressing unit are the so-called active ground anchors, wherein the delayed prestress losses are also influenced by the variations in the contour conditions of the passive anchor of the prestressing unit. The strain gauge load cells are devices, generally metallic, which enable the empirical characterisation of the structural strain existing in prestressed or compressed elements. The determination of this strain is possible thanks to the instrumentation of these devices by means of strain gauges which are connected to each other, normally by means of an assembly in a full Wheatstone bridge. For example, EP1486762B1 discloses a compression column load cell having a compression column machined with a square cross section, in the central plane of which a set of strain gauges is arranged in order to measure the strain to which the column is subjected. The load cells can also be used in weighing scales, as disclosed for example in US6596949B2. US3535923A discloses a load transducer capable of producing an indication of an applied load. FR1519874A discloses a measuring device for measuring stresses caused in a cable by a fluid collected in an enclosure. The correct measurement of the strain gauge load cells is influenced by the possible existence of significant flexion phenomena or concentration points of stresses in the walls of the load cell, generated by eccentricities in the application of the load and/or irregularities in the contour conditions of the device, such as irregularities in the bearing plates in the structural element. The aforementioned proposals have been designed as solid elements which do not enable the prestressing elements to pass therethrough, which makes it difficult or impossible to characterise the strain existing in the active anchors of prestressing elements. There are also load cells that do enable the prestressed elements to pass through an inner ring of the load cell. For example, EP1980712A1 discloses a nut-shaped strain gauge load cell which is introduced along a steel rod joined to the anchor the axial strain of which is to be characterised. Another example is disclosed in US4203318, which describes a strain gauge load cell for measuring the compression and axial elongation of load carrying members. This load cell is secured to the structural element the strain of which is to be measured by means of threaded ends. Another example can be seen in US2015075296A1, which discloses a strain gauge load cell for measuring the load of a rod. Finally, US7188535B1 discloses a strain gauge load cell designed to characterise forces and moments applied about multiple axes, for which various parameters related to the sensitivity of measurement in the aforementioned axes are detailed. JP2016186441-A discloses a device for detecting force applied on a constructed rotary body. The force is detected using a strain gauge tubular device. US2005/103123A1 discloses an apparatus for determining stress in a pipeline to which the apparatus is mechanically anchored. However, the load cells of these proposals do not enable the distortion caused by the phenomena of the concentrati