Study On The Stability Behavior Of Cold-Formed Steel Flexural Members

The stability is the key problem in the area of cold-formed steel structures. Although the design standards in different countries are not the exactly same, most of them are compiled around the stability problem. The instability of cold-formed steel members include local buckling, distortional buckling and lateral-torsional buckling. The previous research achievements in local buckling and lateral-torsional buckling are plentiful. Because the recognization of distortional buckling was late and its own complexity, many research results are not consentaneous and need further study. With the cold-formed steel members tending to higher strength, thinner thickness and more complex section shapes, the distortional buckling maybe become the dominating buckling mode.The effective section method was used for design of cold-formed steel members all over the world. This method is only suitable for local buckling of members, but is not suitable for distortional buckling. It is very fussy for calculate the effective width and its sectional properties. Since the section shapes trend toward complicated, the effective section method shows its shortages gradually. Considering the distortional buckling and the shortages of the effective section method, a new design method——Direct Strength Method (DSM) had emerged. This method uses full sectional properties of the member instead of its effective sectional properties, so long as acquiring its elastic buckling load, its load carrying capacity can be gained according to the DSM formulas. The calculating process is simple and convenient. But this method can only solve the problems of some certain section shape members within the limited boundary conditions such as the pin-ended columns and simply supported beams. To widen its application range and make it usable for more section shapes and more complex loading conditions is the main task at present. Previously, the deflection of cold-formed steel members was obtained also by the effective section method, so the estabilishment of the Direct Deflection Method (DDM) to accommodate the DSM is urgently required.This paper investigates the stability behavior of cold-formed steel flexural members by test investigation and finite element parametric analysis completely and systematically. Edge stiffener is a key factor that influencing the member's buckling behavior, so C- and Z-section members with upright, inclined and complex edge stiffener were selected as research objects to study their failure modes and bending strengths under pure bending and non-pure bending states. The DSM formulas had been regressed which can be used for pure bending and non-pure bending state respectively by the bending strengths got from parametric analysis. The DDM formulas for calculate the overall bending deflections also had been established by the deflection curves which obtained from parametric analysis.A total of 24 specimens that divided into 12 tests including 6 pure bending tests and 6 non-pure bending tests had been conducted. The C-section specimens with upright, inclined and complex edge stiffeners and the widths of edge stiffeners were varied. Local buckling, distortional buckling and interaction between local and distortional buckling were observed in the tests. By comparing the bending strengths under the two bending states of the specimens with the same sections, the rule of failure modes influencing specimens'bending strengths had been concluded. All the tests were simulated by the finite element program of ANSYS which considered geometric and material non-linearities and the geometric imperfections of specimens. The simulated results showed good agreement with the tests results, which established well foundation for further finite element parametric analysis.On the basis of test investigation, large quantity finite element parametric analysis were performed for cold-formed steel C- and Z-section flexural members which employ upright, inclined and complex edge stiffeners, and the failure modes and bending strengths under the two bending states were also compared. The results showed that the bending strength under non-pure bending was higher more than 10%~20% than which under pure bending when the members buckled in distortional buckling. But the bending strengths under the two bending states had little difference when the members buckled in interaction between local and distortional buckling, and the bending strengths under pure bending were little higher than which under non-pure bending for most of members. By comparing the load bearing efficency of the members with inside angled edge stiffener and inside hooked edge stiffener showed that the load bearing efficency of inside angled edge stiffener was higher than which of inside hooked edge stiffener under both the two bending states for most of members.Taking the original DSM formulas as standard, the revised DSM formulas which can be used for different edge stiffener members had been regressed by the analysis results of upright, inclined and complex edge stiffener members under pure bending and non-pure bending states. Because the original DSM formulas were obtained by the results of pure bending members, this paper widened its range of application and made it usable for non-pure bending members. The pure bending and non-pure bending memers were both divided into two groups which buckled in interaction between local and distortional buckling and distortional buckling, and the revised DSM formulas for the two buckling modes had been regressed respectively. The original and revised DSM formulas were used to calculate the bending strengths of some selected members, and the bending strengths were compared with the analysis results showed that the bending strengths got from the revised DSM formulas were more accurate than which got from the original one.The DDM for calculate the overall bending deflection about the plane which moment act on was carried out. Some members were selected, by comparing the thickness t, web height H, flange width B and edge stiffener types showed that the ratio of flange width to thickness was the key factor that influencing the overall bending deflection, but other parameters'effects were small. By analyzing the scattered points'distribution of every line, the DDM formulas for upright, inclined and complex edge stiffener members which with different thicknesses had been established.Due to the existing methods for calculate the elastic distortional buckling stress of flexural members are usually fussy and also considering the actual situation of our country, simplified formulas for calculate the elastic distortional buckling stress of upright and inclined edge stiffener members had been established on the basis of the results which got from the finite strip program of CUFSM and the effect rule of every parameter. The comparison between the results got from the simplified formulas and which got from CUFSM showed that the errors of the two simplified formulas were both small and can satisfy the calculating precision.