ditional field monitoring data collected by Foley and Ginal [10]were used to develop a procedure to predict the service lives of full-span overhead sign support trusses. Rice Lafave and Mehuys [11] investivated end connection effects on the susceptibility of welded aluminum truss tubular web members to vortex shedding induced resonant vibrations. A field test method was developed to reliablely determine the fundamental natural frequency of web components and these data were used to compute vortex-shedding frequencies and estimated the critical wind speeds calibrated to in-situ structural components. These component characterististic were then correlated with wind-related fatigue damaged components identified in field inspections and design recommendations limiting the maximum web member slenderness ratio were developed to minimize vibration problems in new designs. Constantinescu, Bhatti, and Tokyay [12, 13] performed a detailed CFD study to analytically determine the wind loads and pressure distributions through numerical simulation on large highway sign structures. Parameters studies included; aspect ratio and sign spacing for regular panels, sign depth for the DMS cabinets, presence of back-to-back signs, presence of trucks underneath the signs, and reduction in the mean pressure force on the panels due to presence of small air holes. For complex multi-panel combinations pressure distributions were highly non-symmetrical and not accurately addressed in current design provisions. Authors recommend that CFD simulations be used to improve these wind load estimates and to develop more accurate load and response characteristics of DMS truss structures. Kacin, Rizzo, and Tajari [14] performed and analitical study to develop an algorithm to determine the fatigue life of an overhead four-chord truss sign structures. The algorithm uses a time varying natural wind loading and a finite element model to extract stress histories of selected critical elements. Complete stress ranges are counted and a linear damage accumulation method is used to find the fatigue life of some critical members. Current study A detailed understanding of the wind forces on large highway signs is crucial for the safe and economical design of the supporting truss structures. Large DMS systems are increasingly employed to manage highway traffic flow and to provide accurate, timely information to highway users. Recent field inspection experiences in Iowa indicate that truss structures supporting DMS signs may be subject to more complex and extreme wind loads that are not accounted for in current code driven design procedures and are resulting in increasing maintenance and fatigue problems in relatively new sign installations. The Iowa DOT has commissioned a multi-institution 24 month analytical and field monitoring study to better understand the response and critical design characteristics of these new sign structures. This study will utilize the technical expertise of several universities to execute an extensive analytical and field monitoring test plan using CFD simulations and field observations of several in-service structures to better quantify the environmental wind loads on DMS sign structures and the resulting dynamic response of these structures with particular focus on characterizing cumulative stress cycling envelope of fracture critical components and connections. Data from prior steady-state CFD simulations at the University of Iowa will be extended to include more detailed unsteady air-flow simulations around DMS sign panels. These simulations will provide quantitative data related to unsteady forces (dynamic loads) on the DMS panels that will be input to structural models of several in-service DSM truss installations to study the vibration characteristics of the supporting truss structures. Project field observations will consist of short-term monitoring of four DMS cabinet/truss installations and long-term monitoring of one DMS cabinet/truss installation. Short-term monitoring objectives are designed primarily to characterize and bound the dynamic response and properties of the individual test structures and critical components subject to transient gusting events from trucks and limited background wind exposure. These structures will be extensively instrumented (approximately 50 data channels) to capture global and component level strain and acceleration response data along with environment wind and temperature measurements. Data from these short-term monitoring tests will be used to establish a sparser long-term monitoring plan designed to characterize features of extended stress cycling envelope for critical components and to capture a wider range of responses to ambient background and extreme wind loadings. The long-term monitoring will extend over a three to six month period. During this monitoring, strain (a minimum of 8 locations or components), acceleration (approximately 4 locations), temperature (ambient plus 2 structure temperatures), and wind information (speed and direction) information will be collected. Level triggering may be used later in the monitoring period to limit the collection of redundant data. Analysis of these data will focus identifying important dynamic behaviors of the test structure under both ambient and truck induced winds. The numerical simulations developed to characterize structural loads and resulting dynamic re413
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