Cyclic performance of bolted cruciform and splice connectors in retrofitted transmission tower legs

Aging transmission towers are increasingly being found to have insufficient strength to meet increased design wind loads and additional electricity demands. One potential solution is to strengthen old tower members through adding new structural components. The bolted cruciform connector is an effective load-transferring component for members that combine new and old components. This paper addresses the cyclic performance of bolted cruciform and splice connectors in retrofitted transmission towers through a series of experimental tests. One type of 12-bolt splice connector and one type of 8-bolt cruciform connector were designed and two groups for each type were tested under monotonic and cyclic loading conditions. Key parameters including bolt pretension, bolt-slip load and dynamic stiffness with varying load amount, frequency and cycle number were investigated. Experimental results showed that the cyclic loadings reduce bolt-slip load and static structural stiffness of cruciform and splice connectors due to the surface smoothening and bolt pretension loss. The bolt pretension and bolt-slip load continuously reduced with the increase of loading cycle numbers and loading magnitudes. However, the static bolt-slip load was not found to be sensitive to varying loading frequencies. Finally, practical prediction equations were developed in terms of cyclic loading parameters for both connectors. The prediction equations showed strong agreement with experimental results in both low and high loading magnitudes under varying loading conditions.