Experimental testing and evaluation of real-scale lap-splice bolted connections used in typical lattice steel transmission towers

Abstract

Typically, lattice steel transmission towers (LSTTs) are built using bolted connections, although the performances of these connections are complicated. Generally, lap-splice bolted connections are used for long primary (leg) members to ensure their continuity. Herein, 17 lap-splice bolted joints, with real-scale dimensions as used in three LSTTs, are experimentally tested under compression, with the bolts being under shear. The considered three LSTTs carry 33 kV double-circuits overhead transmission lines, where the loading conditions and design are complied with the relevant requirements of the technical specification of ASCE-10/97 (2000). The main objective of this paper is to study the behaviour and failure mode of the lap-splice bolted joints of such three LSTTs, which are often considered either pinned or fully-rigid in practice. The joints relationships by means of the axial load versus joint deformation under shear forces on bolts are measured and evaluated, where the lap-splice bolted joints are found to show pre-slippage, slippage, bearing and plastic stages. The joints are found to fail due to bearing of either inner angle members or the splice plates. The axial stiffness of the lap-splice bolted joints is then obtained from these load–displacement curves. Moreover, stiffness and shear strength of the current joints are compared with the component based method (CBM), as presented in EN 1993−1−8 (2005). In this method, the structural steel joint under shear loading is treated as a multi-spring model composed of in-series springs, which enables obtaining its force–displacement relationship. The initial stiffness and strength comparison based on this simple method shows a good agreement with the experimental results. Additionally, the strength obtained by CBM is shown to conform to the experimental failure modes. Based on the results, the laboratory experimental compression tests as well as CBM estimations of bolted lap-splice joints connections are useful tools to evaluate the behaviour of these joints. Hence, they are suggested to be taken into consideration during the overall elastic design of the towers by using well calibrated finite element modelling instead of using expensive full-scale loading prototype testing method in accordance to IEC 60652 (2002).