Sections which can form a plastic hinge with the rotation capacity required for plastic global analysis are designated Class 1 or plastic'cross-sections. Eurocodes 3 and 4 introduce the concept of cross- section classification to determine whether or not local buckling limits the ability of the cross-section to develop its plastic moment resistance and the rotations necessary for the redistribution of internal moments. "Slender"elements:Class 3 and 4 sections in which elastic analysis and elastic stress distributions less than or equal to yield are adopted in design. "Compact"elements:Class 1 and 2 sections in which elastic analysis (with considerable moment redistribution)or plastic analysis (for Class 1 sections),and plastic stress-blocks are adopted in design.For analysis based on uncracked concrete,Eurocode 4(1992)allows 40%redistribution of moments for Class 1 sections and 30%for Class 2 sections
• Sections which can form a plastic hinge with the rotation capacity required for plastic global analysis are designated Class 1 or ‘plastic’ cross-sections. • Eurocodes 3 and 4 introduce the concept of crosssection classification to determine whether or not local buckling limits the ability of the cross-section to develop its plastic moment resistance and the rotations necessary for the redistribution of internal moments
Element Method of Method of AISC Euro- CSA Type analysing load calculating LRFD code3 16.1 effects moment- (1986) (1992) (1989) resistance plastic analysis yielded Compact Class 1 Class 1 or elastic stress Plastic analysis with blocks large redistribution Compact elastic analysis yielded Class 2 Class 2 with limited stress compact redistribution blocks elastic analysis elastic non- Class 3 Class 3 with limited stress compact non- redistribution limited to compact Slender yield stress elastic analysis elastic slender Class 4 Class 4 with no stress< slender redistribution yield stress
Class 1 cross-sections are those which may be considered capable of developing both the plastic moment resistance and the necessary rotation capacity before any local buckling. Class 2 cross-sections are able to develop their plastic moment resistance,but rotation capacity is limited by local buckling. The classification of a cross-section is determined by the classification of its constituent plate elements that are in compression,which in turn is determined by width-to-thickness ratios (or slenderness)
• Class 1 cross-sections are those which may be considered capable of developing both the plastic moment resistance and the necessary rotation capacity before any local buckling. • Class 2 cross-sections are able to develop their plastic moment resistance, but rotation capacity is limited by local buckling. • The classification of a cross-section is determined by the classification of its constituent plate elements that are in compression, which in turn is determined by width-to-thickness ratios (or slenderness)
e'ay 1.0 Class 1 0.8 Class 3 0.6 Class 4 Rotation, 0.4 约 0.2 Maximum Momenl.M End rotation ratio 8/e
By definition,Class2‘compact密实性’cross-sections can develop the plastic moment capacity of the section although local buckling limits the rotation capacity and prevents full redistribution of moment at such sections. bt<10 dw/t w <25 6t=14 4w=31 1.0 15<bt<17 0.8 33<d./w<40 4,>14 tw>46 002 D.04 0.08 0.0B 0.10 0.12 rotation of plastic hinge,radians
• By definition, Class 2 ‘compact 密实性’ cross-sections can develop the plastic moment capacity of the section although local buckling limits the rotation capacity and prevents full redistribution of moment at such sections