Hysteretic Model of Single-Bolted Angle Connections for Lattice Steel Towers

Single-bolted angle joints are widely used on secondary bracings of lattice steel towers due to their low cost and ease of construction. It has been observed that the hysteretic behavior of these joints has a significant impact on the dynamic performance of the tower structure and the nonstructural components supported by the tower. However, the various phases of this hysteretic cycle have never been investigated in detail. Therefore, there are no accurate numerical models for these types of joints in the popular software packages for structural analysis. This in turn hinders advanced studies on the performance of these critical infrastructure components (utility towers) under strong wind or seismic loads. This paper first explains the mechanics and the various stages of the hysteretic behavior of the joint, including friction, slippage, bolt bearing, and plasticity. Finite-element models are built and validated for the analytical modeling of the joint under monotonic loading. The model for hysteretic behavior is then presented, considering cyclic joint slippage and bolt-hole elongation (damage accumulation). A novel algorithm is developed to efficiently incorporate the analytical model into a computer program. In particular, the analytical model is the basis for a new zero-length finite element that can be used in the OpenSees framework. The methodology is applied to single-bolted angle joints with 10 typical configurations. The proposed analytical zero-length element is shown to agree well with brick element simulation results under both monotonic and cyclic loading. Therefore, the hysteretic behavior of the single-bolted angle connections can be incorporated into the dynamic analysis of lattice steel towers by inputting easily obtained physical properties, for example, plate thickness and width, bolt and bolt hole diameter, and material strength. This proposed element will enable engineers and researchers to efficiently study the cyclic performance of lattice tower structures capturing well the joint behavior.