On the rotation capacity of structural steel circular hollow sections in plastic analysis

Abstract

In BS5950 (BS5950. Part 1, Structural use of steelwork in building. British Standards Institution, 1990), it is recommended that in order to analyse a structure by plastic methods the section classification of its members must be “plastic”. This condition is necessary so that “large” rotations can be attained without local buckling of the member. For tubular members of circular hollow section the ratio of D/ t must be ≤40 ε 2 for plastic classification. Similar limiting values of D/ t are recommended in EC3 (Eurocode 3. Design of steel structures, Part 1.1. General Rules and Rules for Buildings, British Standards Institution, 1993) as 50 ε 2 and AISC (American Institue of Steel Construction. Load and resistance factor design specification for structural steel buildings. 1986) as, the BS equivalent of, 32 ε 2. The above limitation ensures that the value of the plastic moment of resistance is maintained during sufficiently large rotations and local buckling in the form of longitudinal waves does not occur [Seide P, Weingarten VI. On the buckling of circular cylindrical shells under pure bending. Journal of Applied Mechanics 1961;28(March)]. But it does not take into account the secondary P– δ effect which takes place in the plane of the cross section of the tube. This effect will reduce the plastic section (ovalisation) modulus due to the deformation in the form of flattening of the cross section. The present paper attempts to investigate the behaviour of circular hollow sections beyond the stage of development of the plastic moment of resistance where large rotations will take place in order to achieve load distibution in the structure. It was found that the reduction in the theoretical plastic moment of resistance can be significant, and this was confirmed by experimental evidence.