Mechanical behavior of steel transmission tower legs reinforced with innovative clamp under eccentric compression

Abstract

Most steel lattice towers have been in service for over 20 years in China. Many existing towers are now required to carry higher loads than those for which they were originally designed because of new heavier devices and revised wind design codes. An effective way of increasing the carrying capacity of a tower is to reinforce its core leg members by attaching additional elements with bolted connections. However, the reinforcing methods proposed in previous studies require drilling into the core leg members, which results in considerable construction inconvenience. Therefore, an innovative clamp-type (IC) method is proposed herein. To evaluate the effectiveness of the proposed IC method, this paper presents results from 90 new cases comprising 10 eccentric compression tests and 80 finite element (FE) analyses of the load capacity of starred compound members, by varying the interconnector arrangement, form, spacing and bolt row, and slenderness ratio of the compound members. The experimental results show that the compound members with the IC interconnector mainly exhibit buckling failure in the core member. However, in the case of instability about the imaginary axis, the bearing capacity of the compound members with the IC interconnector is approximately 41%–62% higher than that of the single angle member. A nonlinear FE model is developed, which considers the material, contact non-linearity, and geometric imperfections. The FE model is validated against the experimental test results, which show good agreement, both in terms of failure mode and load–displacement curve. The validated FE model is used to conduct a parametric analysis to investigate the effect of slenderness ratio on the bearing capacity of the compound member. Four width–thickness ratios of connected angles and five IC interconnector spacings are considered in the parametric study. The bearing capacity values obtained from the experimental tests and FE analyses are used to assess the performance of current design guidelines. The results show that the current DL/T 5154-2012 code is too conservative by an average value of 30%. Therefore, a kd coefficient method is proposed to improve the prediction accuracy of the bearing capacity, and the method is verified against the FE and test results of the compound member with the IC interconnector under eccentric compression tests.