Bridges

* Best Paper Winner Austroads Bridge Conference 2022

Highway sign support structures are fatigue sensitive structures which require careful structural assessment, particularly in reference to their connections. AS5100 provides ultimate and serviceability limit state design requirements and refers to AASHTO for all aspects of the fatigue limit state design. AASHTO provides nominal stress-based fatigue detail categories for circular hollow sections and multisided sections, however, fatigue detail categories for rectangular or square sections are not available in AASHTO. Instead, AASHTO suggested an alternate local stress-based and/or experimental based methodology as provided in Appendix C. In Australia, it is common practice to use SHS, RHS and plate fabricated box sections as sign support structures. The fatigue design of connections associated with these sections using nominal stress-based method is not possible as connection detail categories are not covered in AS5100 or AASHTO. Based on the literature review there are various gaps and inconsistencies exists with current design guidelines and there is no consistent, nor suitable singular method is available for the typical gantry design scenarios encountered on projects throughout Australia, particularly those that use square and rectangular sections.

To address these gaps and inconsistencies, three different fatigue design methodologies are presented, varying in complexity from nominal and geometric stress methods to the modified notch stress method. Given the need to simplify the design process and to better utilise the current Australian Standards, the first method proposed that a combination of AS5100.6 for fatigue categories and assessment, AS1170.2 for natural wind gust loads and cycles be adopted for sign structure design and supplemented with AASHTO, where required, for truck induced gust and galloping. Second method proposed is to use AASHTO for all fatigue loading and AS5100.6 for fatigue categories and assessment using 100 million cycles. A simplified notch stress method is also proposed, that reduces the design and computational effort required as per AASHTO. It is also proposed that geometric stress analysis using finite element modelling in accordance with IIW or DNVGL shall be allowed as many typical connection details being used are not available in AS5100.6 or AASHTO.