7.3 Behavior of composite columns As a combination of the response of plain concrete and the response of bare steel,composite interaction is synergistic;it enhances the performance of the whole superior to the sum of the component parts. Encased composite columns: Prior to flexural cracking,the flexural stiffness El can be estimated as the product of the Young's modulus E and the moment of inertia l for the gross concrete of the section
7.3 Behavior of composite columns As a combination of the response of plain concrete and the response of bare steel, composite interaction is synergistic; it enhances the performance of the whole superior to the sum of the component parts. Encased composite columns: Prior to flexural cracking, the flexural stiffness EI can be estimated as the product of the Young’s modulus Ec and the moment of inertia Ig for the gross concrete of the section
The Elastic Behavior of the Section The stiffness of a uncracked composite section is the sum of the stiffness of each component part,such as: for axial force: A As +Asr+Ac/n (1) for flexural force:I Is Isr Ic/n (2) n=Es/Ee The long term effects can be taken into account Under sustained loading,the creep of concrete in effect reduces E,and any flexural tension cracking reduces the effective amount of concrete in a section
The Elastic Behavior of the Section The stiffness of a uncracked composite section is the sum of the stiffness of each component part, such as： for axial force: (1) for flexural force: (2) n = Es/Ec The long term effects can be taken into account Under sustained loading, the creep of concrete in effect reduces Ec , and any flexural tension cracking reduces the effective amount of concrete in a section
Both the maximum and minimum stress in concrete, steel profile,and steel rebars can be computed by: N M Oc.max(min) n·A ni 2 ds 土 noc.max Os.max(min) A ) Osr.max s.mgx N M dsr o. Osr.max(min) A 2 0sr.min noe.min Composite column (a)Composite section fully effective. (b)Cracked composite section
Both the maximum and minimum stress in concrete, steel profile, and steel rebars can be computed by: Composite column (a) Composite section fully effective. (b) Cracked composite section
If the section is in a cracked condition,only the concrete in compression has to be considered,and the approach is similar to the one used for reinforced concrete sections.The elastic neutral axis,xe,can be evaluated by =0 where S the first moment of the cross-section effective area(steel and concrete assumed to be under compression)with respect to the elastic neutral axis I=the inertia of the effective section respect to the same line d the overall dimension of the cross-section N Oc n.S 4+ds Os S d+dsr Osr 2
If the section is in a cracked condition, only the concrete in compression has to be considered, and the approach is similar to the one used for reinforced concrete sections. The elastic neutral axis, xe, can be evaluated by
Concret filled composite columns > Compression strength of the column is at least as great as the sum of the strength of the bare shell and the strength of the unconfined concrete. Triaxial confinement from steel shell can increase the effective strength of concrete. Some design codes permit stocky column compression strength estimates that recognize values of f'effectively higher than the specified cylinder strength
Concret filled composite columns Compression strength of the column is at least as great as the sum of the strength of the bare shell and the strength of the unconfined concrete. Triaxial confinement from steel shell can increase the effective strength of concrete. Some design codes permit stocky column compression strength estimates that recognize values of f c ’ effectively higher than the specified cylinder strength