| Hollow-structural sections (HSS) can be very economical and aesthetic for the design of cantilevered outdoor structures such as wind turbines, transmission towers, communications towers, and sign, signal and light supports. The HSS are chosen because of low wind drag and good torsional stiffness. Circular and rectangular hollow sections can be used, as well as multi-sided tapered tubes bent from steel sheet. The HSS is often welded into a relatively thick base plate, which is referred as a socket connection for bolting cantilevered arms to posts, as well as connections of tower bases to foundation anchor rods. Many states have reported failures or maintenance problems with socket connections from vibrational fatigue. This research included experimental and computational components to investigate the fatigue behavior and design of sign support structures. The results of the experimental study showed that eight-sided tube socket connections have fatigue resistance around AASHTO Category K2. However, doubling the base plate thickness allowed for this socket connection to attain Category E' resistance, although there were not enough specimens to make a definitive conclusion. The experimental data also suggested the tube diameter and thickness affect the fatigue resistance of socket connections. Since socket connections exhibit a variable fatigue resistance based on connection geometry, a parametric study was undertaken using finite element analysis to examine how the stress concentration factor (SCF) changes with geometry. In total, 222 models were analyzed with varied tube shape, tube thickness, tube diameter, bend radius of multi-sided shapes, number of anchor rods, and position of anchor rods. Round tubes had the lowest SCFs. The SCF of multi-sided tubes was exacerbated by using fewer anchor rods, sharper tube bends (both in angle and curvature), and proximity of the anchor rod to a tube bend. In all cases, the SCF increased by moving the anchor rods further from tube and using fewer of them. However, the SCF decreased exponentially by increasing the base plate thickness. Finally, a parametric equation was introduced that predicts the maximum SCF for any socket connection based on inputs defined by the geometry. This equation could then be used to predict fatigue life. |
