: Recently such a member is often used to that of temporary structures, that is composed of steel tubes and built up to a truss-type member by welding.
    When there are no restraint for the transverse direction at any panel point, this member should be treated as an I-section member with open-web.
    For an I-section member with open-web, we scarcely have any practical investigation.
    On the lateral buckling of compression flange ( or compression chord ), we have to consider the cooperation of web and tension flange with compression flange.
    We have searched for the solution as a problem of least energy method.
    This report shows the method of theoretical analysis for a critical condition of elastic stability of this member, and the examination by experiments.
    Owing to compare with an I-section member, the following conditions are considered for the member objected : both chord members are parallel, each panel length is equal, type is Warren or Pratt, each chord member, each diagonal member and each vertical member are steel tube with equal section respectively, and members are rigidly jointed to each other.
    The critical loads of this member, when it buckles laterally, is expressed as following equations ;
    ( P_e - N )( P_w - N ) - [ 4 / h² ]·M² = O
    P_e = [π² / l ² ] ( 2 E I₀ + E I₁ cos ³α + G J₁ cosα sin ² α)
    P_w = [2 π² / l ²] E I₀ + 4 / h² ( 2 λ G J₀ + E I₁ cos α sin ² α + G J₂ tan α)
where N is an axial compressive load, M is a bending moment at each end, and h is a distance of both chord members.
    P_e and P_w are respectively corresponded with an Euler's buckling value and a Wagner's buckling value of an I-section member with close-web.
    And it has been proved that the critical loads calculated from above formulas are very nearly equal to those obtained on experiments.