Experimental and analytical research was performed to develop guidelines for the fatigue design of cantilevered sign, signal, and luminaire support structures. Four wind-loading phenomena were identified as possible sources of large amplitude vibrations which could lead to fatigue failures, i.e. galloping, vortex shedding, natural wind gusts, and truck-induced wind gusts. Aerodynamic and aeroelastic wind tunnel tests characterised the potential susceptibility and dynamic response of cantilevered sign and signal support structures to the galloping and vortex shedding phenomena. Signs and signal attachments with backplates which are rigidly mounted to a cantilevered mast-arm produced large amplitude galloping-induced vibrations. These findings were consistent with the observed dynamic responses of cantilevered signal support structures in the field. The wind tunnel tests also confined that sign and signal support structures are not susceptible to vortex-induced vibrations from the attachments to these structures or, in most cases, from the members themselves. Vortex shedding induced loads are most significant in non-tapered structural members. Static and dynamic finite element analyses were performed to estimate the magnitude of galloping, vortex shedding, and natural wind gust equivalent static fatigue limit-state loads. In addition, calculations were performed to validate a very simple static load model for truck-induced gust loads. Fatigue-sensitive connection details (e.g. mast arm-to-column, column-to-base plate) were identified from state department of transportation standard drawings and manufacturer literature. The fatigue strengths of these details were categorised according to the AASHTO and AWS fatigue design curves. This review revealed that many cantilevered support structure connection details exhibit very low fatigue strengths (i.e AASHTO Category E' or lower). Additional emphasis was placed on determining the fatigue resistance of .anchor bolts and also the relationship of anchor bolt stresses to support structure forces. Static tests on full-scale, eight-bolt pattern, anchor bolt groups indicated that the relationship between support structure forces and anchor bolt stresses can be accurately predicted by applying the flexure equation using the moment of inertia of the bolt group. Full-scale fatigue tests were conducted on 47 anchor bolt specimens. These results indicated that the constant-amplitude fatigue limit corresponding to the AASHTO Category D design curve (48 MPa or 7 ksi) is a reasonable lower bound estimate for snug- and fully tightened axially loaded anchor bolts. Several fatigue tests on the full-scale foundations confirmed the validity of the proposed approaches. The fatigue limit-state wind loads and the fatigue detail categorisation were incorporated into a proposed specification for the fatigue-resistant design of cantilevered signal, sign, and luminaire support structures. In the proposed specification, Importance Factors are used to adjust the level of structural reliability for three categories of cantilevered support structures. The most important cantilevered support structures will be designed to resist rarely occurring wind-loading phenomena. Most existing support structure designs would require changes to comply with the provisions of the proposed specification for structures in the most important category. To meet the design specifications, increases in member section properties and/or the selection of more fatigue-resistant connection details will be required. No significant increase of section properties, on average, should be required to meet the proposed provisions for non-critical structures, which are calibrated to achieve the same average level of reliability as existing designs. (The evaluation and retrofit of existing structures in service were not part of the scope.) Sample fatigue design calculations for a sign, signal, and luminaire support structure are included in the report. Recommendations for future research are presented. (A)
Abstract