A method for detecting and estimating the bearing capacity of existing power transmission tower structure

Abstract

The application discloses a kind of existing power transmission tower structure bearing capacity detection estimation method, comprising: obtaining modal parameter of inherent mode shape and modal parameter of existing power transmission tower structure;Based on inherent mode shape and modal parameter and the modal parameter of existing power transmission tower structure, identify the local damage position of power transmission tower structure;Based on the local damage position of power transmission tower structure, obtain buckling modal analysis model;Based on buckling modal analysis model, carry out buckling modal analysis of power transmission tower under initial design condition and actual damage condition respectively, obtain buckling modal ultimate bearing capacity;Based on buckling modal ultimate bearing capacity, assess the bearing capacity of existing power transmission tower structure.The application reveals the attenuation influence of power transmission tower bearing capacity by the change of measured tower structure vibration mode, indirectly assesses the attenuation of bearing capacity by tower structure buckling modal strain energy, avoids the problems of stress and deformation and other local index limitations and low efficiency of conventional bearing capacity calculation method.

Description

Technical Field

[0001] This invention belongs to the field of performance testing and maintenance throughout the entire life cycle of civil engineering, and in particular relates to a method for testing and estimating the bearing capacity of existing power transmission tower structures. Background Technology

[0002] As a type of linear energy transmission project, power transmission towers inevitably traverse complex and varied terrains and geological environments. They are also affected by extreme weather conditions (hurricanes, heavy icing, etc.), which impact the long-term durability of the tower structure and the impact of geological disasters on the foundation, thus affecting the long-term load-bearing capacity of the tower structure. Therefore, it is crucial to detect the ultimate load-bearing capacity of power transmission towers, promptly grasp early changes in structural load-bearing capacity, predict the trend of performance degradation, and take timely reinforcement and repair measures to ensure the safe operation of the line. To this end, a method for detecting and estimating the load-bearing capacity of existing power transmission tower structures has been developed.

[0003] CN114021599A discloses a system and method for identifying safety hazards in power towers. This invention monitors the tilt status of power towers in real time through an edge computing terminal and a relay terminal. A pose monitoring device is installed on the tower, capable of detecting tilt and communicating with the BeiDou satellite and the relay terminal. The relay terminal collects pose signals and connects to the edge computing terminal, which processes all detection information and communicates with the monitoring station. The monitoring station analyzes data from each edge computing terminal, assesses the criticality coefficient of the power tower, and generates emergency commands. By measuring the angular changes between sensors, the degree of tilt of the power tower can be accurately determined, and a dedicated information transmission channel is established to avoid information congestion, enabling online detection of the power tower's tilt status.

[0004] CN115496726A discloses a method and system for assessing the safety of bridge structures. This invention analyzes the bridge structure to determine the ultimate bearing capacity and location of each load-bearing characteristic point. It then acquires images of bridge defects and analyzes their characteristics to generate defect information. Based on this information, the ultimate bearing capacity values ​​of the characteristic points are adjusted, and characteristic points that do not meet the threshold are marked as abnormal. This method effectively reduces reliance on manual experience, improves the accuracy of identifying and repairing bridge safety hazards, and solves the efficiency problem in existing technologies. Ultimately, it enables rapid and accurate identification of bridge load-bearing defects and their locations, helping construction personnel efficiently remove defects and quickly restore the bridge's traffic capacity.

[0005] The aforementioned patents still rely on traditional detection targets such as stress, strain, deformation, and displacement as reference quantities. These can only reflect the characteristics of local structural damage and cannot track the decay process of the overall bearing capacity of the transmission tower structure. It is necessary to propose physical quantities that characterize the overall bearing capacity of the structure and to construct methods for its detection and estimation. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for detecting and estimating the load-bearing capacity of existing power transmission tower structures, thereby resolving the issues present in the prior art.

[0007] To achieve the above objectives, the present invention provides a method for detecting and estimating the bearing capacity of existing transmission tower structures, comprising:

[0008] The structural modes of the transmission towers during the construction period were calibrated to obtain the modal parameters of the natural vibration modes;

[0009] Modal detection of existing transmission tower structures is performed based on modal parameters of natural vibration modes to obtain modal parameters of existing transmission tower structures;

[0010] The location of local damage to the transmission tower structure is identified based on the inherent vibration mode and modal parameters and the modal parameters of the existing transmission tower structure.

[0011] A buckling modal analysis model was obtained based on the local damage locations of the transmission tower structure.

[0012] Based on the buckling modal analysis model, buckling modal analysis of transmission towers was performed under initial design conditions and actual damage conditions to obtain the ultimate buckling modal bearing capacity.

[0013] Assessment of the bearing capacity of existing transmission tower structures based on buckling mode ultimate bearing capacity.

[0014] Optionally, the process of obtaining the modal parameters of the natural vibration modes includes:

[0015] Obtain engineering data of transmission towers after completion during the construction period, and establish a finite element geometric model of the transmission tower structure-foundation-ground based on the engineering data of transmission towers after completion during the construction period; perform vibration modal eigenvalue analysis on the finite element model to obtain the modal parameters of the natural vibration modes; wherein, the natural vibration modes refer to the first 10 natural vibration modes, and the modal parameters include but are not limited to the mode mode, modal curvature, and modal deformation.

[0016] Optionally, the process of obtaining the modal parameters of an existing transmission tower structure includes:

[0017] Based on the modal parameters of the inherent vibration mode, members whose modal displacements meet preset conditions are selected. The selected members are used as placement points, and inertial acceleration sensors are set at the placement points. The inertial acceleration sensors, charge amplifiers, and data acquisition instruments are connected by wires, and the wires are fixed to the transmission tower. Response data is obtained by moving hammer impact. Based on the response data, the vibration acceleration time history curve of the existing structure of the transmission tower is obtained. The vibration acceleration time history curve is analyzed using a random subspace algorithm and a stability graph method to obtain the modal parameters of the existing transmission tower structure.

[0018] Optionally, the process of identifying the location of localized damage to the transmission tower structure includes:

[0019] The structural stiffness at the local damage location is used to characterize the damage state, and the structural stiffness at the local damage location is gradually reduced from the initial design conditions. Operational modal analysis is used to obtain the structural stiffness parameters at the local damage location that match the measured modal parameters of the existing transmission tower. The modal confidence criterion of the mode shape is used, with the mode curvature mode change ratio as the modal confidence parameter. The modal parameters of the existing transmission tower structure are compared with the modal parameters of the natural mode shape to determine the local damage location of the existing transmission tower structure.

[0020] Optionally, the process of obtaining the buckling modal analysis model includes:

[0021] Under modal consistency conditions, the equivalent support constraint stiffness is adjusted until an equivalent support constraint stiffness matching the modal parameters of the natural vibration mode is obtained; the matching equivalent support constraint replaces the foundation constraint, and a buckling modal analysis model is established. The buckling modal analysis model uses stiffness parameters characterizing local damage to correct the numerical model of the superstructure, and uses the equivalent support constraint stiffness under corresponding conditions to characterize the substructure.

[0022] Optionally, the process of obtaining the buckling modal ultimate bearing capacity includes:

[0023] Based on the buckling modal analysis model, buckling modal analysis of transmission towers was performed under initial design conditions and actual damage conditions. The buckling modal strain energy trend diagram method of different modal orders was used to obtain the modal characteristic order corresponding to the first abrupt change in buckling modal strain energy. Buckling nonlinear analysis was performed under the modal characteristic order to obtain the buckling modal ultimate bearing capacity.

[0024] Optionally, the degree of bearing capacity attenuation of existing transmission tower structures can be assessed by comparing the ratio of buckling modal ultimate bearing capacity under actual damage conditions to that under initial design conditions.

[0025] The present invention also discloses a computer device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor executes the computer program to implement the steps of the above-described method for detecting and estimating the bearing capacity of an existing transmission tower structure.

[0026] The present invention also discloses a computer-readable storage medium storing a computer program thereon, characterized in that the computer program, when executed by a processor, implements the steps of the above-mentioned method for detecting and estimating the bearing capacity of an existing transmission tower structure.

[0027] Compared with the prior art, the present invention has the following advantages and technical effects:

[0028] This invention first performs modal calibration of the transmission tower structure during its construction phase to obtain the modal parameters of its natural vibration modes; then, based on the modal parameters of the natural vibration modes, it performs modal detection of the existing structure to obtain the modal parameters of the existing transmission tower structure; next, based on the natural vibration modes, modal parameters, and modal parameters of the existing transmission tower structure, it identifies the locations of local damage to the transmission tower structure; based on the locations of local damage to the transmission tower structure, it obtains a buckling modal analysis model; then, based on the buckling modal analysis model, it performs buckling modal analysis of the transmission tower under initial design conditions and actual damage conditions to obtain the ultimate buckling modal bearing capacity; finally, it evaluates the bearing capacity of the existing transmission tower structure based on the ultimate buckling modal bearing capacity.

[0029] This invention reveals the impact of transmission tower bearing capacity attenuation by measuring the vibration mode changes of the tower structure, and indirectly assesses the bearing capacity attenuation by using the buckling mode strain energy of the tower structure. This avoids the limitations of local indicators such as stress and deformation, as well as the low efficiency of conventional bearing capacity calculation methods. This invention solves the problem of the lack of quantitative detection and estimation methods for structural bearing capacity status in the operation, maintenance and reinforcement of transmission towers throughout their entire life cycle. Attached Figure Description

[0030] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0031] Figure 1 This is a flowchart of a method according to an embodiment of the present invention. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0034] Example 1

[0035] like Figure 1 As shown, this embodiment provides a method for detecting and estimating the bearing capacity of existing transmission tower structures, including:

[0036] The required equipment includes an inertial acceleration sensor, sensor-specific adhesive, data cables, a charge amplifier, a data acquisition unit, and a force hammer.

[0037] S1: Modal calibration of the structure during the construction period;

[0038] After obtaining the completed power pole tower project data, a finite element geometric model of the pole tower structure-foundation-subsoil was established based on parameters such as tower type, height, foundation type, and site strata. The pole tower structure was modeled using pole elements, the subsoil using solid elements, the soil using a Mohr-Coulomb constitutive model, and the concrete and steel using an elastoplastic DP constitutive model. Vibration modal eigenvalue analysis was conducted on the finite element model of the power pole tower structure-foundation-subsoil to obtain modal parameters such as mode shape, modal curvature, and modal deformation of the first 10 natural vibration modes.

[0039] S2: Modal analysis of the current structural condition;

[0040] Based on the modal deformations of the first 10 natural vibration modes calculated during the structural modal calibration at the construction stage, the members with larger modal displacements were recorded as placement points for inertial acceleration sensors. The inertial acceleration sensors were adhered to these placement points using sensor-specific adhesive. The inertial acceleration sensors, charge amplifiers, and data acquisition instruments were connected by wires, which were then secured to the main structure of the power transmission tower with tape. A moving hammer method was used, striking each measurement point three times to obtain the vibration acceleration time history curves of the existing structure of the power transmission tower. A random subspace algorithm and stability graph method were used to identify the modal parameters of the existing transmission tower structure, including natural frequencies, damping ratios, and mode shapes. The random state-space model is as follows:

[0041]

[0042] The Hankle matrix consists of 2i+j-1 discrete-time series response data from l measurement points of the system, divided into 2i blocks (each block has l rows and j columns), as follows:

[0043]

[0044] In the formula: y i ∈R l×1 Y represents the system's response data at l measurement points at time i; P and Y f These represent the output for the "past" and the output for the "future", respectively.

[0045] In random subspace recognition, the classic assumption is to obtain a long-running sequence (i→∞) that traverses all states. However, in practical engineering, j cannot be infinitely long. Therefore, to satisfy the assumption, the acquisition time is generally made as long as possible, resulting in a very large Hankel matrix. To reduce the amount of data, data-driven random subspace algorithms use QR decomposition to calculate the projection matrix.

[0046]

[0047] QT It is an orthogonal matrix, i.e., Q T Q = QQ T =1.

[0048] In the stochastic subspace algorithm, the relationship between the system output data and the system state vector is estimated by Kalman filtering. The Kalman filter state sequence is defined as follows:

[0049]

[0050] According to the subspace system recognition theory, the projection matrix Q i It can be decomposed into an extended observable matrix Γ i With Kalman filter state sequence X i The product of, i.e.:

[0051] For the projection matrix Q i Perform singular value decomposition:

[0052]

[0053] In the formula: U and V are orthogonal matrices, S = diag(σ1, σ2, ..., σ n ) represents the non-zero singular values ​​σ in descending order. i The diagonal matrix formed by these elements, where n is the order of the system.

[0054] Substituting the output matrix and the Kalman filter state sequence into the stochastic state-space model yields the least-squares solution for the system matrix A and the output matrix C:

[0055]

[0056] After obtaining the system matrix A and the output matrix C, the modal parameters of the system can be obtained by solving the eigenvalues ​​of the system matrix: A = ΨΛΨ -1 .

[0057] The stability diagram method is a way to determine the order of a system. It can distinguish between true and false modes from many modes and is suitable for mode identification in situations with strong noise. In determining whether a point on the stability diagram is a stable point, there are three screening criteria: frequency stability, damping ratio stability, and mode stability. Only points that fully satisfy all three criteria are considered completely stable.

[0058] Since the eigenvalues ​​obtained after eigenvalue decomposition of the system state matrix A are pairwise conjugates, the number of modal parameters obtained is only half that of the order of matrix A. Therefore, in the random subspace algorithm calculation, the order of matrix A must be even.

[0059] After obtaining the system matrix A and the output matrix C, the modal parameters of the system can be obtained by solving the eigenvalues ​​of the system matrix: A = ΨΛΨ -1 Ψ is the eigenvector matrix of the system, and its value is a complex number; Λ is the matrix derived from the system's eigenvalues ​​λ. i The diagonal matrix formed by these equations has pairwise conjugate complex eigenvalues. In the derivation of the discrete state-space equations of the system, considering the sampling limitations of practical instruments, it is necessary to discretize the continuous-time state-space equations. However, the actual system is continuous; therefore, when solving for the modal parameters, it is necessary to transform the discrete system state matrix A into a continuous system state matrix A₀. c The relationship between the two is as follows: Therefore, the relationship between the eigenvalues ​​of the two can be obtained as follows:

[0060]

[0061] The relationship between the system eigenvalues ​​and the original system frequency ω and damping ratio ξ is as follows:

[0062]

[0063] According to the above formula, we can obtain:

[0064] Modal frequencies:

[0065] Modal damping:

[0066] Modal shape: φ=CΨ

[0067] S3: Operational mode damage identification;

[0068] The damage state is characterized by the reduction in structural stiffness at the location of local damage. The structural stiffness at the local damage location is progressively reduced from the initial design conditions. Operational modal analysis is used to obtain structural stiffness parameters at the local damage location that match the measured modal parameters of the existing transmission tower. A modal confidence criterion based on mode shape (mode curvature) is employed, with the mode curvature change ratio used as the modal confidence parameter. The measured modal parameters are compared with the initial modal parameters to determine the location of local structural damage in the existing transmission tower.

[0069]

[0070] Modal curvature can exhibit abrupt changes at the location of damage. Furthermore, the curvature mode change ratio can be defined through modal curvature, which is a more obvious derivative of modal curvature than the results of modal curvature identification.

[0071]

[0072] In the formula, αj Indicates the curvature mode variation ratio. and These represent the curvature modes under lossless and lossy conditions, respectively.

[0073] The optimization search process adopts the Fibonacci search method, compares the modal parameter indices before and after correction, and sets the frequency difference rate between the corrected modal frequency and the target modal frequency at 5%, and the modal confidence criterion between the corrected modal strain energy and the target modal strain energy at 10% as the optimization objectives.

[0074] S4: Buckling mode model conversion;

[0075] First, the vibration modes need to be converted into buckling mode calculation models. The vibration mode analysis model is an integrated numerical model of the transmission tower structure and foundation. The buckling mode analysis model uses equivalent support constraints to replace the foundation constraints. The conversion is achieved by obtaining the equivalent support constraint stiffness under modal consistency conditions in both models. Operational modal analysis is used, and the equivalent support constraint stiffness is gradually reduced from the near-rigid fixed-end support constraint state to obtain the equivalent support constraint stiffness that matches the target modal parameters. Finally, the buckling calculation model of the existing transmission tower uses stiffness parameters characterizing local damage to modify the numerical model of the superstructure. Local damage refers to the damage location determined in the previous step, and the underlying foundation is characterized by the equivalent support constraint stiffness under the corresponding conditions. The buckling calculation model of the transmission tower under the initial design conditions only changes the underlying foundation to the equivalent support constraint stiffness under the corresponding conditions.

[0076] S5: Calculation of ultimate bearing capacity under buckling modes;

[0077] Buckling modal analysis of transmission towers was conducted under both initial design conditions and actual damage conditions. The buckling modal strain energy trend diagram method for different modal orders was used to identify the modal characteristic order corresponding to the first abrupt change in buckling modal strain energy. Buckling nonlinear analysis was then performed under this modal characteristic order to obtain the ultimate buckling modal bearing capacity. The initial design conditions refer to those under construction conditions, while the actual damage conditions refer to those under conditions of damage already in use.

[0078] For the superstructure, the "consistent defect method" was used to correct the nodal coordinates, and eigenvalue buckling analysis was performed. The displacements of each buckling mode were read, and an initial defect of L / 300 of the structural span was applied to each mode for nonlinear analysis to obtain the ultimate bearing capacity of the structure. The actual ultimate bearing capacity of the structure was calculated through wind load logic search, and compared with the ultimate bearing capacity obtained by the consistent defect method to determine the structural stability ultimate bearing capacity under the defect mode. For the substructure, combined with measured data, operational modal analysis (OMA) was used to correct the equivalent support three-dimensional connection stiffness, and the stability ultimate bearing capacity of the corrected model was calculated. Based on modal strain energy theory, the ultimate bearing capacity coefficient corresponding to the overall abrupt change in modal strain energy was found, which facilitates the calculation of the allowable bearing capacity of the transmission tower.

[0079] S6: Estimation of structural bearing capacity attenuation;

[0080] The degree of bearing capacity attenuation of existing transmission tower structures is evaluated by the ratio of the buckling modal ultimate bearing capacity under actual damage conditions to that under initial design conditions.

[0081] This embodiment also discloses a computer device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor executes the computer program to implement the steps of the above-described method for detecting and estimating the bearing capacity of an existing transmission tower structure.

[0082] This embodiment also discloses a computer-readable storage medium storing a computer program thereon, characterized in that the computer program, when executed by a processor, implements the steps of the above-described method for detecting and estimating the bearing capacity of an existing transmission tower structure.

[0083] Based on the detection of modal parameters of transmission tower structures under conditions of construction and current operation, this invention compares the differences in modal parameters between the two conditions and uses operational modal analysis to inversely search for the location and extent of local damage to existing transmission towers. The buckling modal ultimate bearing capacity at the characteristic order corresponding to the first abrupt change in buckling modal strain energy is used as an index characterizing the bearing capacity of the transmission tower. Buckling modal analysis of transmission towers is carried out under initial design conditions and actual damage conditions, and the ratio of the buckling modal ultimate bearing capacity under the two conditions is used to evaluate the degree of bearing capacity attenuation of the transmission tower structure.

[0084] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for detecting and estimating the bearing capacity of existing transmission tower structures, characterized in that, Includes the following steps: The structural modes of the transmission towers during the construction period were calibrated to obtain the modal parameters of the natural vibration modes; Modal detection of existing transmission tower structures is performed based on modal parameters of natural vibration modes to obtain modal parameters of existing transmission tower structures; The location of local damage to the transmission tower structure is identified based on the inherent vibration mode and modal parameters and the modal parameters of the existing transmission tower structure. A buckling modal analysis model was obtained based on the local damage locations of the transmission tower structure. Based on the buckling modal analysis model, buckling modal analysis of transmission towers was performed under initial design conditions and actual damage conditions to obtain the ultimate buckling modal bearing capacity. Assessment of the bearing capacity of existing transmission tower structures based on buckling mode ultimate bearing capacity.

2. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 1, characterized in that, The process of obtaining the modal parameters of the natural vibration mode includes: Obtain post-construction data of the transmission tower project, and establish a finite element geometric model of the transmission tower structure-foundation-soil based on the post-construction data. Perform vibration modal eigenvalue analysis on the finite element geometric model of the transmission tower structure-foundation-soil to obtain the modal parameters of the natural vibration modes. The natural vibration modes refer to the first 10 natural vibration modes, and the modal parameters include, but are not limited to, the mode shape, modal curvature, and modal deformation.

3. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 1, characterized in that, The process of obtaining the modal parameters of existing transmission tower structures includes: Based on the modal parameters of the inherent vibration mode, members whose modal displacements meet preset conditions are selected. The selected members are used as placement points, and inertial acceleration sensors are set at the placement points. The inertial acceleration sensors, charge amplifiers, and data acquisition instruments are connected by wires, and the wires are fixed to the transmission tower. Response data is obtained by moving hammer impact. Based on the response data, the vibration acceleration time history curve of the existing structure of the transmission tower is obtained. The vibration acceleration time history curve is analyzed using a random subspace algorithm and a stability graph method to obtain the modal parameters of the existing transmission tower structure.

4. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 1, characterized in that, The process of identifying the location of localized damage to a transmission tower structure includes: The structural stiffness at the local damage location is used to characterize the damage state, and the structural stiffness at the local damage location is gradually reduced from the initial design conditions. Operational modal analysis is used to obtain the structural stiffness parameters at the local damage location that match the measured modal parameters of the existing transmission tower. The modal confidence criterion of the mode shape is used, with the mode curvature mode change ratio as the modal confidence parameter. The modal parameters of the existing transmission tower structure are compared with the modal parameters of the natural mode shape to determine the local damage location of the existing transmission tower structure.

5. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 1, characterized in that, The process of obtaining the buckling modal analysis model includes: Under modal consistency conditions, the equivalent support constraint stiffness is adjusted until an equivalent support constraint stiffness matching the modal parameters of the natural vibration mode is obtained; the matching equivalent support constraint replaces the foundation constraint, and a buckling modal analysis model is established. The buckling modal analysis model uses stiffness parameters characterizing local damage to correct the numerical model of the superstructure, and uses the equivalent support constraint stiffness under corresponding conditions to characterize the substructure.

6. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 5, characterized in that, The process of obtaining the buckling modal ultimate bearing capacity includes: Based on the buckling modal analysis model, buckling modal analysis of transmission towers was performed under initial design conditions and actual damage conditions. The buckling modal strain energy trend diagram method of different modal orders was used to obtain the modal characteristic order corresponding to the first abrupt change in buckling modal strain energy. Buckling nonlinear analysis was performed under the modal characteristic order to obtain the buckling modal ultimate bearing capacity.

7. The method for detecting and estimating the bearing capacity of existing transmission tower structures according to claim 6, characterized in that, The degree of bearing capacity attenuation of existing transmission tower structures is assessed by comparing the ratio of buckling modal ultimate bearing capacity under actual damage conditions to that under initial design conditions.

8. A computer device comprising: The memory and processor contain a computer program stored in the memory and executable on the processor, characterized in that the processor executes the computer program to implement the steps of the method for detecting and estimating the bearing capacity of an existing transmission tower structure as described in any one of claims 1-7.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the steps of the method for detecting and estimating the bearing capacity of an existing transmission tower structure as described in any one of claims 1-7.

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