AISC LRFD 1.pdf

216 COMPRESSION MEMBERS [Comm. I2.MPa) for normal weight concrete. A lower limit of 3 ksi (21 MPa) is specifiedfor normal weight concrete and 4 ksi (28 MPa) for lightweight concrete toencourage the use of good quality, yet readily available, grades of structuralconcrete.(4) In addition to the work of Bridge and Roderick (1978), SSRC Task Group 20(1979), and Galambos and Chapuis (1980), recent work by Kenny, Bruce, andBjorhovde (1994) has shown that due to concrete confinement effects, the previouslimitation of 55 ksi (380 MPa) for the maximum steel yield stress ishighly restrictive. Further, the most commonly used reinforcing steel grade hasa yield stress of 60 ksi (415 MPa). The increase is therefore a rational recognitionof material properties and structural behavior.The 60 ksi (415 MPa) limitation for the yield stress is very conservative fortubular composite columns, where the concrete confinement provided by thetube walls is very significant. Kenny et al. have proposed raising the value of F yfor such columns to whatever the yield stress is for the steel grade used, but nothigher than 80 ksi (550 MPa).(5) The specified minimum wall thicknesses are identical to those in the 1995 ACIBuilding Code (1995). The purpose of this provision is to prevent buckling ofthe steel pipe or HSS before yielding.2. Design StrengthThe procedure adopted for the design of axially loaded composite columns isdescribed in detail in Galambos and Chapuis (1980). It is based on the equation forthe strength of a short column derived in Galambos and Chapuis (1980), and thesame reductions for slenderness as those specified for steel columns in Section E2.The design follows the same path as the design of steel columns, except that theyield stress of structural steel, the modulus of elasticity of steel, and the radius ofgyration of the steel section are modified to account for the effect of concrete andlongitudinal reinforcing bars. A detailed explanation of the origin of these modificationsmay be found in SSRC Task Group 20 (1979). Galambos and Chapuis(1980) includes comparisons of the design procedure with 48 tests of axially loadedstub columns, 96 tests of concrete-filled pipes or tubing (HSS), and 26 tests of concrete-encasedsteel shapes. The mean ratio of the test failure loads to the predictedstrengths was 1.18 for all 170 tests, and the corresponding coefficient of variationwas 0.19.3. Columns with Multiple Steel ShapesThis limitation is based on Australian research reported in Bridge and Roderick(1978), which demonstrated that after hardening of the concrete the composite columnwill respond to loading as a unit even without lacing, tie plates, or batten platesconnecting the individual steel sections.4. Load TransferTo avoid overstressing either the structural steel section or the concrete at connections,a transfer of load by direct bearing, shear connectors, or a combination ofboth is required. When shear connectors are used, a uniform spacing is appropriatein most situations, but when large forces are applied, other connector arrangementsLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION