AISC LRFD 1.pdf

42 COMPRESSION MEMBERS [Sect. I2.used in calculating the strength of a composite column shall not exceed 60 ksi(415 MPa).(5) The minimum wall thickness of structural steel pipe or HSS filled with concreteshall be equal to b Fy /3Efor each face of width b in rectangular sectionsand D Fy /8Efor circular sections of outside diameter D.2. Design StrengthThe design strength of axially loaded composite columns is c P n ,where c = 0.85P n = nominal axial compressive strength determined from Equations E2-1through E2-4 with the following modifications:(1) A g is replaced by A s , the gross area of steel shape, pipe, or HSS, in. 2 (mm 2 )(2) r is replaced by r m , the radius of gyration of the steel shape, pipe, or HSS exceptthat for steel shapes it shall not be less than 0.3 times the overall thicknessof the composite cross section in the plane of buckling, in. (mm)(3) F y is replaced by F my , the modified yield stress from Equation I2-1F my = F y + c 1 F yr (A r / A s ) + c 2 f c (A c / A s )(I2-1)(4) E is replaced by E m , the modified modulus of elasticity from Equation I2-2.E m = E + c 3 E c (A c / A s )(I2-2)whereA c = area of concrete, in. 2 (mm 2 )A r = area of longitudinal reinforcing bars, in. 2 (mm 2 )A s = area of steel, in. 2 (mm 2 )E = modulus of elasticity of steel, ksi (MPa)E c = modulus of elasticity of concrete. E c is permitted to be computed15 .from Ec= w fc′(metric: Ec= 0 041w 15 .. fc′) where w, the unitweight of concrete, is expressed in lbs./cu. ft (kg/m 3 ) and f c is expressedin ksi (MPa).F y = specified minimum yield stress of steel shape, pipe, or HSS, ksi(MPa)F yr = specified minimum yield stress of longitudinal reinforcing bars, ksi(MPa)f c = specified compressive strength of concrete, ksi (MPa)c 1 , c 2 , c 3 = numerical coefficients. For concrete-filled pipe and HSS: c 1 = 1.0, c 2= 0.85, and c 3 = 0.4; for concrete-encased shapes c 1 = 0.7, c 2 = 0.6,and c 3 = 0.23. Columns with Multiple Steel ShapesIf the composite cross section includes two or more steel shapes, the shapes shall beinterconnected with lacing, tie plates, or batten plates to prevent buckling of individualshapes before hardening of concrete.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION