Role of local slenderness in the rotation capacity of structural steel members

• Local slenderness of elements determines both strength and strain capacity.
• FEM analyses show the connection between the slenderness and the strain capacity.
• Element strain capacity may be utilized to determine curvature in a member.
• Rotational ductility is connected to the strain capacity and member characteristics.
• Simplified and elaborated formulae are proposed to calculated rotational ductility.

The objective of this paper is to demonstrate how element (e.g., flange) local slenderness may be used to predict element strain capacity, and in turn, the element strain capacity may be used to predict member rotational capacity in structural steel members. Member plastic hinge rotation capacity has an important role in the design of steel structures, and while implicit understanding of the rotation capacity has sufficed in the past, as inelastic direct analysis methods are adopted in conventional as well as seismic design more explicit treatments are needed. Accordingly, a comprehensive series of material and geometric shell finite element collapse analyses are performed in ABAQUS on component elements (plates). The finite element analysis confirms the hypothesis that local slenderness of an element is intimately connected to the element's strain capacity. Utilizing element strain capacity to determine member rotational ductility demonstrates the importance of additional factors, such as depth-to-length and shape factor of the member in predicting the rotational capacity. The proposed method assumes Euler–Bernoulli beam theory, ignores interaction between local and lateral–torsional buckling, and presumes the flange (not the web) controls the section strain capacity. The analyses are compared to existing code provisions for both conventional and seismic design and recommendations for potential improvements are made.