Developing Fatigue Resistant Tube-to-transverse Plate Welded Connections for Highway Sign, Signal, and Luminaire Support Structures

Fatigue resistances of various tube-to-transverse plate welded connections for highway sign, signal, and luminaire support structures were evaluated experimentally and analytically. The connections considered for this study were unstiffened fillet- and groove-welded, stiffened fillet-welded tube-to-transverse plate welded connections including round and multi-sided tube cross sections. Due to the influence of the local out-of-plane bending on the connection behavior, local stresses were used for the evaluations. Two local stresses, which were the geometric stress (DNV 2012) and the notch stress (Roy and Fisher 2005), were used for assessing respectively finite and infinite life fatigue resistances. Applicability of these stresses for fatigue resistance evaluation on tube-to-transverse plate welded connection was verified by experimental study.Ten types of 68 full-scale specimens were fatigue tested under constant amplitude loading. Most of the unstiffened fillet- and groove-welded tube-to-transverse plate connection experienced fatigue cracking at the weld on the tube. Some of the fatigue cracks in groove-welded connections initiated from the backing ring top weld due to the poor quality of the weld. All the fatigue cracks in the multi-sided tube-to-transverse plate welded connections were initiated from the bend corner and propagated to the flat face of the tube. Especially, the multi-sided tubular connections had sharp bend corner experienced very early fatigue cracking at the sharp bend corner compared to round tubular connections having similar geometries. In the stiffened tube-to-transverse plate fillet-welded connections, two fatigue cracking mode was observed, which were weld toes on the tubes of the stiffener termination and fillet-welded tube-to-transverse plate. Fatigue test results obtained from this study and other studies were collected and used for the fatigue resistance evaluation. Fatigue resistance of the connections in both finite and infinite life regimes were defined in terms of geometric and notch stress concentration factors.Analytical study was conducted to extend the experimental results over a broad range of structure sizes and geometric combinations. Geometric parameters which affected the behavior of the connections were selected and the ranges of parameters were determined based on the survey of the standard drawings of each state department of transportation. Influences of each geometric parameters on the behaviors of each connection were investigated. The results demonstrated that the interaction between the transverse plate and tube governed the behavior of unstiffened tube-to-transverse plate welded connections. Reducing transverse plate flexibility was the most effective method of decreasing stress concentration and improving fatigue resistance of the connections. Analysis for multi-sided tubular connection demonstrated that the bend corner in multi-sided tube cross section worked like a stiffener and developed stress concentration. Sharp bend corner made by less number of sides and small bend radius produced high stress concentration. For stiffened tube-to-transverse plate fillet-welded connections, the ratio between tube and stiffener thicknesses, height of stiffener, and distance between stiffeners were the important factors for determining fatigue resistances of two critical locations, which were the stiffener termination and the fillet-welded tube-to-transverse plate connection. Also, it was found that the tube thickness is the major geometric parameter influences the normalized notch stress, which the notch stress was normalized by the geometric stress.Based on the investigation of the influences of each geometric parameter on the behaviors of the connections, parametric equations for each connection were developed using nonlinear regression analysis. The parametric equations accurately represented the influences of each geometric parameters and provided accurate estimation of geometric and notch stress concentration factors.