Aerodynamic Parameters on a Multisided Cylinder for Fatigue Design

Abstract

Cantilevered signal, sign, and light support structures are used nationwide on major interstates, national highways, local highways, and at local intersections for traffic control purposes. Recently, there have been a number of failures of these structures that can likely be attributed to fatigue. In Iowa, USA (Dexter 2004), a high-mast light pole (HMLP), which is typically used at major interstate junctions, erected for service in 2001 along I-29 near Sioux City collapsed in November 2003 (see Figure 1 (a)). Fortunately, the light pole fell onto an open area parallel to the interstate and injured no one. Figure 1 (b) shows another highmast lighting tower failure in Colorado, USA (Rios 2007) that occurred in February of 2007. Similar to the failure in South Dakota, fracture initiated at the weld toe in the base plate to pole wall connection, and then propagated around the pole wall until the structure collapsed. It appears that these structures may have been designed based on incomplete and/or insufficient code provisions which bring reason to reevaluate the current codes that are in place. A luminary support structure or HMLP is generally susceptible to two primary types of wind loading induced by natural wind gusts, or buffeting and vortex shedding, both of which excite the structure dynamically and can cause fatigue damage (AASHTO 2009). Vortex shedding is a unique type of wind load that alternatively creates areas of negative pressures on either side of a structure normal to the wind direction. This causes the structure to oscillate transverse to the wind direction. When the vortex shedding frequency (i.e., the frequency of the negative pressure on one side of the structure) approaches the natural frequency of the structure, there is a tendency for the vortex shedding frequency to couple with the frequency of the structure (also referred to as “lock-in” phenomenon) causing greatly amplified displacements and stresses.