N. Prasad Rao

Non-linear behaviour of lattice panel of angle towers

Abstract Lattice microwave towers and transmission towers are frequently made of angles bolted together directly or through gussets. Such towers are normally analysed to obtain design forces using the linear static methods, assuming the members to be subjected to only axial loads and the deformations to be small. The effects of the end restraints, eccentricity of connections and secondary bracings (redundants) on the strength of the compression members are usually accounted for in the codal recommendations by modifying the effective length of the members and thus the design compressive strength. Hence, forces in the redundants are not known from the analysis and their design is empirical. In this study, non-linear analysis of angle compression members and the single panel of angle planar as well as three-dimensional lattice frames, as in typical lattice towers, are carried out using MSC-NASTRAN software. Account is taken of member eccentricity, local deformation as well as rotational rigidity of joints, beam-column effects and material non-linearity. The analytical models are calibrated with test results. Using this calibrated model, parametric studies are carried out to evaluate the forces in the redundants. The results are compared with codal provisions and recommendations for the design of redundants are presented.

Failure Analysis of Transmission Line Towers

Transmission line towers, though designed per code provisions, may fail during mandatory testing required in many countries. Different types of premature failures that were observed during full-scale testing of transmission line towers at Tower Testing and Research Station, Structural Engineering Research Centre, Chennai (CSIR-SERC) are studied, and the results are discussed in detail. The failures are modeled using finite-element software, and the analytical results and the test results are compared with various code provisions. The nonlinear finite-element analysis program NE-Nastran was used to model the elastoplastic behavior of towers. Bracing members with slenderness ratios above 170 become ineffective, even though they have to carry insignificant forces. Importance of design assumptions and connection detailing in overall performance of towers were studied. Nonlinear finite-element analysis is useful in understanding the system behavior and for prediction of the failure pattern and ultimate load. B...

FLEXURAL AND LOCAL BUCKLING INTERACTION OF STEEL ANGLES

The column buckling equation and plate buckling equation are merged to obtain a graphical relation in the elastic and inelastic buckling ranges. This graphical relation has been used to classify all possible modes of hot rolled angle sections investigated by previous researchers in their experimental works. Forced plate buckling experiments have been carried out to verify the graphical relation for plate buckling. Design practices adopted by various standards for design of steel angles, have been compared with respect to this graphical relation and discussed. The importance of width to thickness ratio has been used to emphasize the buckling characteristics of hot rolled angle sections. The objective of this paper is to discuss the plate buckling failures of the angle sections recorded during full scale testing of transmission towers at Tower Testing and Research Station, SERC, Chennai, India. The plate buckling failures in tower testing for the past eighteen years have been analyzed and suggestions are given to avoid such failures in real structures and during testing.

Detection of damage through coupled axial–flexural wave interactions in a sagged rod using the spectral finite element method

This paper presents the results of a computational and experimental validation exercise performed towards damage identification of a sagged rod with known damage by using the coupled axial–flexural wave interaction mechanics. Towards simulating the damage scenario in a sagged conductor made of steel wire rope, a prismatic steel rod is taken up for study. An initial axial wave, tangential to the curve of the arc, manifests as both axial and flexural waves as it propagates alongside the length of the rod. This interaction effect between axial and flexure wave propagation is studied in this paper. Impedance mismatch is made in the rod by changing its cross-sectional area along its length. Numerical simulations are implemented using the spectral finite element method with a combined axial and flexure effect. The concept of obtaining the exact spectral element dynamic stiffness matrix for a wave propagation analysis sagged rod is discussed. Computation is implemented in the Fourier domain using Fast Fourier Transform (FFT). In the time domain, post processing of the response is done, which is applicable in structural diagnostics in addition to the wave propagation problem. The predominant single-frequency-based amplitude-modulated, narrow-banded, burst wave propagation is found to be better matched if the elemental rod theory is replaced with a modified rod theory called the Love theory. The differences in the propagating waves allow identification of the damage location in a very clear-cut way. The methodology of the moving correlation coefficient is also successfully employed to detect the damage precisely. This fact is very encouraging for future work on structural health monitoring.

Experimental and Numerical Investigation on Longitudinal Wave Propagation in ROD with Structural Discontinuity

KEywords: Wave propagation; SHM; spectral finite element method (SFEM); damage detection. wave propagation based automatic structural health monitoring techniques and the warning-alert systems are widely adopted in aerospace and nuclear industry. These techniques are however, not in a matured state of development for civil structures and are evolving. Hence research in this area is of paramount importance. This paper deals with axial wave propagation in steel rod with structural discontinuity. discontinuity (impedance mismatch) is induced in the rod by welding two steel rods which in turn indicated the variation of its cross section area along its length. This paper presents the elegant method of development of spectral elements for analysis of longitudinal waves in rod. Numerical simulations are performed using elementary rod theory and Love theory using spectral finite element method (SFEM) and compared with experimental studies conducted on steel rod using instrumentation for Pitch catch configuration method.

Studies on Strengthening Techniques for Existing Transmission Line and Communication Towers

The transmission and communication towers are subjected to increased loading due to its voltage upgradation and installation of new antennas in existing towers for new line of sight, respectively. Hence, cost-effective structural upgrading scheme has to be developed that can be easily adopted by power and communication industries. In the present study, component level analytical and experimental investigations are carried out to strengthen existing angle section of transmission tower. Experimental investigation is conducted to determine the buckling capacity of single angle section of size 65 × 65 × 5 mm with slenderness ratio 40 strengthened with different connection patterns such as (i) double cross-plate, (ii) single cross-plate, (iii) friction grip, (iv) double angle, and (v) double cleat angle connections. The compression strength of angle section is increased on an average by 20%. The analytical investigation is carried out using ABAQUS software using C3D8R elements. The numerical and experimental results are in good agreement, and the variation is within 16%.

Experiments on coupled axial–flexural wave propagation in a sagged rod with structural discontinuity using piezoelectric transducers

A damage evaluation methodology in a sagged prismatic rod through application of wave propagation mechanics is presented, and an experimental investigation an experimental investigation is illustrated for damage detection using lead zirconate titanate (PZT) piezoelectric transducers. Discontinuity in the form of variation of the cross-sectional area (impedance mismatch) is induced in the longitudinal direction of the experimental specimen by butt welding two steel rods, as described in the previous work. The proposed methodology outlined for a sagged prismatic rod can be adopted for damage detection in a transmission line conductor made of wire rope. The corresponding wave reflections from the damage and boundaries are highlighted. Experiments are conducted on a predominantly single frequency based amplitude modulated narrow-banded, burst wave propagation. The novelty of the work is the experimental demonstration of narrow-banded single frequency wave generation with PZT patches with facility for a variable frequency excitation and controlled by a function generator. This is different from the conventional resonating piezo-crystal at ultrasonic frequency range, where the excitation is strong but the frequency is not variable. Also, the methodology is proposed for bifurcating the initial portion of the nondispersive wave propagation from a mixed-up response. For locating the bifurcation zone for axial–flexural wave interaction, a moving correlation coefficient method is employed. Thus, the use of PZT transducers for online monitoring of damage identification in sagged rod is promising for the future work.

Schifflerised angle sections for triangular-based communication towers

This paper is concerned with the use of Schifflerised angle sections for triangular-based lattice communication towers. The torsional-flexural buckling properties of conventional 90°, 60° and schifflerised angles are compared. The experimental results obtained from the compression tests conducted on three different sizes of schifflerised angle sections are compared with the analytical results predicted from finite element analysis and with the capacities predicted from the procedure given in the literature. The capacity of schifflerised angles are underestimated by the methods suggested in the literature. The FE model non-linear analysis predicts the failure loads closer to the test results with a variation of −3 to 10%. The strength of schifflerised angles is 5 to 20% higher compared to conventional 90° angles. The behaviour of 60 m high three-legged triangular communication tower with schifflerised angles for leg members and conventional equal angles for all other members tested at Tower Testing and Research Station, CSIR-Structural Engineering Research Centre, Chennai is presented. NE-NASTRAN, a finite element software, is used to model the tower using beam column and plate shell elements for predicting the behaviour. The analytical and test results are compared with the codal provisions.

ANALYTICAL AND EXPERIMENTAL STUDIES ON 400 kV S/C PORTAL-TYPE GUYED TOWERS

The experimental and analytical behavior of 400 kV S/C portal-type guyed towers under different loading conditions is presented. The portal-type tower essentially consists of two masts extending outward in the transverse direction from the beam level to the ground. In addition, two sets of guys connected at the ground level project outward along the longitudinal axes and converge in the transverse axes. The experimental behavior of the guyed tower is compared with the results of finite element analysis. The 400 kV portal-type guyed towers with III and IVI type insulator strings are analyzed using finite element software. Full scale tower test results are verified through comparison with the results of the finite element analysis. The initial prestress in the guys is allowed to vary from 5% to 15% in the finite element modeling. The effect of prestress variation of the guys on the tower behavior is also studied.