Shaping of Axially Compressed Bipolarly Prestressed Closely Spaced Built-Up Members

The paper presents a method of shaping and describing the geometry of bipolarly prestressed closely spaced built-up member with symmetrical supports and a bisymmetrical cross-section. The following has been defined as a function dependant on the position along the length of the x section of the closely spaced built-up member with determined geometrical parameters: intial elastic y₀ (x) of the closely spaced built-up member chord in the prestressing zone, distance between the chords in the clear s_i(x), moment of inertia J_i(x) relative to main axes and eccentricity e_i(x) of compressive force in a single chord. The length of the extreme section L₁ and the prestressing zone L₂, the maximum distance between chords s_max in the clear and the geometric characteristics of a single chord section were assumed. A full and correct description of the geometry of bipolarly prestressed closely spaced built-up members is necessary to start the static and stress analysis. As a result of the introduction of a bipolar displacement prestressing into the closely spaced built-up member, the moment of inertia increases in the middle part with respect to the non-material axis z. It allows predicting the increase of the critical load bearing capacity of the closely spaced built-up member. The load bearing capacity of bipolarly prestressed closely spaced built-up members was estimated using the modified Engesser’s formula for two-chord closely spaced built-up member with rigid battens. For selected pair of channel sections, the analytical critical load estimation results were verified using FEM.