AISC LRFD 1.pdf

202Comm. DCHAPTER DTENSION MEMBERSD1. DESIGN TENSILE STRENGTHDue to strain hardening, a ductile steel bar loaded in axial tension can resist, withoutfracture, a force greater than the product of its gross area and its coupon yield stress.However, excessive elongation of a tension member due to uncontrolled yielding ofits gross area not only marks the limit of its usefulness, but can precipitate failure ofthe structural system of which it is a part. On the other hand, depending upon thereduction of area and other mechanical properties of the steel, the member can failby fracture of the net area at a load smaller than required to yield the gross area.Hence, general yielding of the gross area and fracture of the net area both constitutefailure limit states. The relative values of t given for yielding and fracture reflectthe same basic difference in factor of safety as between design of members anddesign of connections in the AISC ASD Specification.The length of the member in the net area is negligible relative to the total length ofthe member. As a result, the strain hardening condition is quickly reached andyielding of the net area at fastener holes does not constitute a limit state of practicalsignificance.D2. BUILT-UP MEMBERSThe slenderness ratio L/rof tension members other than rods, HSS, or strapsshould preferably not exceed the limiting value of 300. This slenderness limit recommendedfor tension members is not essential to the structural integrity of suchmembers; it merely assures a degree of stiffness such that undesirable lateral movement(“slapping” or vibration) will be unlikely.See Section B7 and Commentary Section E4.D3. PIN-CONNECTED MEMBERS AND EYEBARSForged eyebars have generally been replaced by pin-connected plates or eyebarsthermally cut from plates. Provisions for the proportioning of eyebars contained inthe LRFD Specification are based upon standards evolved from long experiencewith forged eyebars. Through extensive destructive testing, eyebars have beenfound to provide balanced designs when they are thermally cut instead of forged.The somewhat more conservative rules for pin-connected members of nonuniformcross section and those not having enlarged “circular” heads are likewise based onthe results of experimental research (Johnston, 1939).Somewhat stockier proportions are provided for eyebars and pin-connected membersfabricated from steel having a yield stress greater than 70 ksi (485 MPa), inorder to eliminate any possibility of their “dishing” under the higher design stress.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION