圧縮力が作用する冷間成形溝形断面部材の弾性局部座屈耐力と座屈挙動

抄録

　　Singly-symmetric open sections in pure compression reveals two fundamental buckling modes: distortional and local, except for overall buckling such as bending buckling. Though these buckling modes reveal independently or in combination in channel members, this paper focuses on only independent local buckling. Traditionally, local buckling strength is calculated as a simply supported plate or fixed supported plate because its methodology is easy and plain. However, when such a simple buckling strength calculation method is applied to a member whose purpose is to thin the member and make advanced use, such as light gauge steel member, the local buckling strength must be calculated precisely. There is a concern that the advantages of these members will be lost. Therefore, in this paper, the buckling displacements of the web/flange/lip are expressed by a series of displacement functions. The elastic local buckling evaluation formula, which considers the element interactions, is proposed via the energy method. Also, the relationship between the cross-sectional shape of the channel member and the buckling behavior is clarified. Finally, comparisons between the proposed and conventional formulae are described.

　　From this research, the following are found.

1)　To consider the restraint effect due to the element interaction of each plate that constitutes the member, the buckling waveform of the web/flange/lip is expressed by a series of displacement functions. Using this displacement function, the elastic local buckling strength formula for the channel member is proposed by the energy method.

2)　The lip width ratio's effect on the elastic local buckling strength differs depending on the difference in the flange width ratio b_f/b_w. When the flange width ratio becomes large, local buckling is relatively likely to occur in the flange; however, as the lip width ratio c/b_w increases, the flange's local buckling is restrained so that the lip restraining effect appears in a range where the flange width ratio is large. However, the effect of the lip width ratio on the local buckling resistance is not large.

3)　Since the plate thickness in the cross-section is uniform regardless of the presence or absence of lips when the flange width ratio decreases, the flange's width-thickness ratio decreases relatively, and local buckling of the flange is suppressed. As a result, the elastic local buckling strength increases.

4)　The flange coefficient is almost 4.0 or less in the channel member's realistic shape even when local buckling occurs in the channel member. The current design manual for light gauge steel members stipulates the required lip width for the flange buckling coefficient to satisfy 4.0 to suppress distortion buckling and cause local buckling. However, since the range in which the flange buckling coefficient satisfies 4.0 within the channel member's manufacturing range is limited, it is not appropriate as a basis for setting the required lip width.

5)　The average value of the finite element elastic eigenvalue analysis results against the elastic local buckling strength obtained from the proposed formula (19) is 0.967, the standard deviation is 0.00744, and the maximum error is 4.49 %. The approximate formula (20) can also obtain reasonably calculated results, which is a simple notation of the proposed formula (19).