port structure. Support structures for large DMS cabinets such as the 4-chord trusses depicted in Figure 1 designed per current provisions may have an adequate factor of safety against strength failure but little data is available on the additional aeroelastic and dynamic load effects caused by the added depth, larger projected area, and larger eccentrically positioned mass of DMS cabinets. Figure 2 shows a close-up view a typical DMS cabinet installation that highlights the upstream cabinet offset. This cabinet and the supporting arched truss are the subject of the field data described later in the paper. Fatigue and vibration of non-cantilevered sign support structures were formally addressed by Fouad, et al. in NCHRP Report 494 [2] which included recommendations for new fatigue provisions using equivalent static load design procedure to address individual aeroelastic wind load effects experience by in-service structures. Selected prior work Several investigations document fatigue related deterioration of non-cantilevered truss support structures and have developed mitigation strategies to monitor or repair damaged structures. Pantelides,Nadauld and Cercone [3] developed a glass fiber reinforced polymer composites (GFRPs) repair process for cracked aluminum overhead sign structural components. The static load carrying capacity of the undamaged welded connection, and the fatigue damaged (through cracking) connection repaired with GFRP composites are established and resulting in retrofitted connections with GFRP reinforcement achieving between 1.17 to 1.25 times the capacity of the original welded aluminum connection. Wipf and Phares [4] utilized field load tests to estimate the residual load capacity of a fatigue damaged sign support truss and compared these results to an analysis of an undamaged truss. These data were used to develop a set of management recommendations to monitor, repair, or replace similar structures for the Iowa DOT. Huckelbridge and Metzger [5] preformed an extensive failure analysis on the near collapse of a 4-chord square sign support truss spanning IR 75 in Ohio caused by the complete fracture of one lower chord member and an adjacent diagonal strut. The assessment utilized data from in-situ traffic-induced vibration measurements near the failure, finite-element simulation estimating the expected dynamic response of the truss, in-service time and inspection history, an estimate of the cumulative vibrational effect of truck traffic, and a metallurgical examination of the failed components. The failure was attributed to extremely high-cycle fatigue of the chord-web diagonal welded connection (possibly exceeding 1 billion cycles over the structure’s 40 year in-service life) even though the estimated effective stress range of the Category ET connection was below current AASHTO constant amplitude fatigue limits for the detail. Park, McLean and Stallings [6, 7] studied the effect of wind induced fatigue on the performance of two VMS truss support structures using simualted dynamic response data and field monitored vibration data. One of these structures exhibited noticable in-service vibration levels and was found to have a finite fatigue life per provisions in the 4th edition of the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals that required a long-term monitoring/inspection plan. The researchers reported mixed results in establishing the relative importance of attributing stress levels to natural and truck induced wind gusting. Other research, both analytical and laboratory based, has been performed to investigate specific aspects of the fatigue problems in standard and DMS sign truss support structures. Foley and Peronto [8, 9] performed an extensive laboratory study to quantify the fatigue-life variability of the AASHTO ET fatigue detail category for welded circular hollow section (CHS) connections commonly used in sign support structures. In the investigation fatigue tests were performed on field harvested and fabricated Y-joint specimens and fatigue lives were predicted via several methodologies. The researchers noted that banding in the microstructure of CHS sections can cause bilinear behavior in the material’s stressstrain response in the elastic region resulting in a marked decrease in ductility and fatigue life. These data along with adFig 2 Typical offset DMS cabinet installation 412
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