Inelastic Lateral-torsional Buckling of High-strength Steel I-shaped Flexural Members

Abstract

This research focuses on the inelastic behavior of structural I-shaped members fabricated from 800MPa high strength steel (HSA800). HSA800, a new generation of high performance steel, produced by thermo-mechanical controlling (TMC) process, has the advantages to acquire the high strength as well high toughness and relatively low carbon equivalent value (CEV). Due to these features, the high strength steel has attracted considerable attention in the construction industry for its use in new structural applications with the help of the appropriate design and fabrication methods. However, due to the lack of sufficient research on the effects of the different post-yield range characteristics of the high strength steel to the structural behavior, the current code directly or indirectly restricts high strength steel by adopting limiting parameters such upper yield strength limit, upper yield-to-tensile (Y/T) strength ratio limit or a certain level of ductility capacity (rotation capacity). The primary target of this research is to experimentally and analytically quantify the effects of the post-yield range characteristics of mild and high strength steel on I-shaped flexural members. In addition, this study attempts to establish a methodology to provide adequate slenderness ratios to ensure inelastic lateral torsional buckling (LTB) strength and a certain level of rotation capacity, related to the existing AISC-LRFD specification and EC 3 code. This work consists of four major sections: stress-strain curve idealizations, estimation of in-plane rotation capacities, and analytical and experimental studies of inelastic LTB behaviors. In the stress-strain curve idealizations, tensile coupons of HSA800, SM570, and SM490 plates are tested and statistical regression curves were proposed to predict the Y/T strength ratio and tensile-to-yield (T/Y) strain ratio according to yield strength, which are pivotal values to idealize the initial portion of the stress-strain curve up to tensile strength. Four idealized material models (the traditional model (#1) and the Haaijer model (#2) for mild steel

the Ramberg-Osgood model (#3) for high strength steel

and the Piecewise linear model (#4) for both steel grades) with properly assumed parameter values are suggested and numerically verified with the tensile coupon data. In the estimation of in-plane rotation capacities, a simplified method is proposed to calculate the in-plane rotation capacity at maximum moment of HSA800, SM570, and SM490 I-shaped members under uniform and moment gradient loading conditions by adopting the piecewise linear models. Under uniform moment loading condition, the in-plane rotation capacity is directly proportional to the T/Y strain ratio. On the other hand, at the moment gradient loading condition, three parameters including Y/T strength ratio, T/Y strain ratio, and yield plateau length together influence on the in-plane rotation capacity. Due to high Y/T strength ratio and low T/Y strain ratio of HSA800, the in-plane rotation capacity of HSA800 I-shaped member is inevitably low as compared to other grades (SM570 and SM490) of steel under moment gradient loading condition. Parametric studies were conducted to increase the rotation capacity level of the HSA800 I-shaped member under the moment gradient condition, demonstrating that lowering the Y/T strength ratio to 0.80 levels only does not ensure the satisfactory rotation capacity of the existing AISC-LRFD specification assumed

the increase of T/Y strain ratios is thus inevitably required. In the analytical studies of inelastic LTB behaviors, the methods to quantify the inelastic section stiffness (effective flexural, warping, and torsional rigidities of I-shaped member fabricated from mild and high strength steel), including the presence of the residual stresses, are proposed by applying the tangent modulus theories. This inelastic section stiffness is crucial to develop the LTB strength and rotation capacity curves of the I-shaped member under uniform and moment gradient loadings. After verifications of the derived strength and rotation capacity curves in current studies with the previous experimental data, parametric studies are conducted to evaluate the geometrical and material effects on the LTB capacity of I-shaped member. By comparing the results of the parametric models with the existing unbraced length limits specified in AISC-LRFD specifications, a methodology to design appropriate I-section geometry depending on the material selection of I-shaped member is proposed. In the experimental studies, three types of welded I-shaped specimens (type A-[G:H-250x150x15x15]-[M:Ho-775], type B-[G:H-400x150x15x15]-[M:Ho-775], and type C-[G:H-400x150x15x15]-[M:Hy-349-822], where [G:] indicates the cross section geometry

[M:Ho] and [M:Hy] indicates the homogeneous and hybrid I-section respectively) were fabricated and tested under uniform moment to examine the geometrical and material effects of the I-section on LTB behaviors. All specimens failed by LTB, triggering a sinusoidal shape failure mode. The measured critical buckling strength and rotation capacity of both type A and type C specimens, where the effective section rigidities to plastic moment ratio are relatively high, satisfy the existing AISC-LRFD specification and EC 3 code. However, type B specimens, where a high height-to-width ratio is applied or the effective section rigidities to plastic moment ratio are relatively low, the current AISC-LRFD unbraced length limit would not give conservative rotation capacity values. By comparing the experimental data with the analytically developed buckling curves, it is shown that the curves well predict experimental LTB strength and rotation capacity values.