AISC LRFD 1.pdf

224 FLEXURAL MEMBERS [Comm. I3.tested at Lehigh University and the results supplemented by the results of 58 testsperformed elsewhere. The range of stud and steel deck dimensions encompassed bythe 75 tests were limited to:(1) Stud dimensions: 3 4-in. dia. 3.00 to 7.00 in.(2) Rib width: 1.94 in. to 7.25 in.(3) Rib height: 0.88 in. to 3.00 in.(4) Ratio w r / h r : 1.30 to 3.33(5) Ratio H s / h r : 1.50 to 3.41(6) Number of studs in any one rib: 1, 2, or 3The strength of stud connectors installed in the ribs of concrete slabs on formedsteel deck with the ribs oriented perpendicular to the steel beam is reasonably estimatedby the strength of stud connectors in flat soffit composite slabs multiplied byvalues computed from Equation I3-1.The 1999 edition of the Specification includes a new upper limit of 0.75 on thereduction factor of Equation I3-1 for single studs located in deck ribs oriented perpendicularto the beam. This limit has been imposed as a temporary measure inresponse to a mounting set of test data (e.g., Easterling, Gibbings, and Murray,1993; Kemp and Trinchero, 1997) that indicates that stud strengths calculated bythe product of Equations I3-1 and I5-1 may be unconservative when a single studper rib is used. Research to further resolve this issue and to assess whether stud pairsare also affected is currently underway. Differences between recent test results andthose originally used to develop Equation I5-1 for ribbed decks (Grant et al., 1977)appear to be due to the fact that (1) most of the earlier tests reported by Grant et al.were for beams with studs placed in pairs centered within the ribs, (2) stud strengthsused to originally calibrate Equation I5-1 were back calculated from momentstrengths of beam specimens which tend to mask variations in the stud strengths,and (3) differences in modern steel deck profiles that affect the placement of studsin the rib. The last reason may be particularly important. As shown in Figure C-I3.4,modern steel deck profiles with stiffeners (reinforcing rib) located along the centerline of the rib require that studs be placed off-center in the rib. Depending on thelocation of the stud relative to the direction of shear transfer, for studs in the “weakposition”, the resulting reduction in edge distance between the stud and rib wall canlead to premature failure accompanied by punching of the stud through the steeldeck. Therefore, in addition to applying the required cap of 0.75 on the reductionfactor (Equation I3-1) for single studs in a rib, it is recommended to avoid situationswhere all the studs may be located in the “weak position” by either alternating studplacement between the “weak” and “strong” positions or coordinating placementof studs to ensure they are all installed in the strong position.For the case where ribs run parallel to the beam, limited testing (Grant et al., 1977)has shown that shear connection is not significantly affected by the ribs. However,for narrow ribs, where the ratio w r /h r is less than 1.5, a shear stud reduction factor,Equation I3-2, has been employed in view of lack of test data.The Lehigh study (Grant et al., 1977) also indicated that Equation C-I3-7 for effectivesection modulus and Equation C-I3-6 for effective moment of inertia werevalid for composite construction with formed steel deck.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION