The Continuous Strength Method for Stainless-Steel Built-up I-sections

Stainless steel alloys are now increasingly used in construction for their superior material properties such as corrosion resistance, higher strength, significantly lower maintenance cost and attractive appearance. Despite its obvious advantages over its traditional counterpart ordinary carbon steel, its use in structures has been limited primarily due to the lack of appropriate design guidance that hinders the optimum utilization of its beneficial properties. Effective width method, which was originally proposed considering the elastic, perfectly plastic material behavior of carbon steel, is the current codified approach to deal with the local buckling of stainless steel cross-sections although stainless steel shows significantly different nonlinear rounded stress-strain response. In effective width approach, gross cross-sectional area is reduced to an effective cross-section, and all geometric properties have to be calculated based on effective dimensions to account for the loss of effectiveness due to local buckling. This is a lengthy process and is not quite rational for stainless steel considering the continuous nature of its stress-strain response.

The Continuous Strength Method (CSM), a strain-based design method, has recently been developed to incorporate appropriately the beneficial effects of material nonlinearity. Primary component of CSM is a base curve, which relates the deformation capacity of a given thin-walled section to the cross-section slenderness and a material model that explicitly recognizes strain hardening. The Continuous Strength Method (CSM) is a recently proposed technique that attempts to utilize the benefits of stain hardening through a continuous design curve, which does not rely on distinct cross-section classification. Gardner and Nethercot first developed the concept for stainless steel hollow sections. Gardner and Ashraf extended the technique for other nonlinear metallic materials such as aluminum and high strength steel showing its general applicability, and Ashraf et al. proposed generalized design rule “Resistance based on Deformation Capacity” for both hollow and open stainless-steel cross-sections. Gardner first introduced the term “Continuous Strength Method” and made the technique more generalized for easy applications. Recently, Afshan and Gardner proposed CSM guidelines using a simple elastic, linear hardening material model, which is shown to produce accurate and consistent predictions at the cross-section level for stocky sections. This recent modification makes CSM more straightforward in exploiting the benefits of strain hardening.

Now research is going on for stainless-steel built-up I-sections. Those I-sections were consisted of two C-sections connected back-to-back to each other with bolts spaced at a specific interval. The approach includes the calculation of individual capacity of C-sections using the CSM base curve. Then using the section properties of I-sections, the individual section capacity was converted to that of built up sections. The study was conducted over 21 collected test results to determine the axial capacity according to the proposed design method. The predicted capacities and the test results shows that the methodology offers improved accuracy and reduced scatter relative to the current design methods.

It needs to be noted that bolt impacts are now being researched for better result and the method is currently under consideration for inclusion in European and North American design standards for stainless steel structures.

The basic curve of CSM is given below-