AISC LRFD 1.pdf

Sect. E4.] BUILT-UP MEMBERS 29(a) For intermediate connectors that are snug-tight bolted:2 2⎛ Kl ⎞ Kl a⎜ ⎟ = ⎛⎝ r ⎠m ⎝ ⎜ ⎞⎟ + ⎛r ⎠o ⎝ ⎜ ⎞⎟(E4-1)ri⎠(b) For intermediate connectors that are welded or fully tensioned bolted:where⎛⎜⎝Klr2⎞ Kl⎟ = ⎛⎠ ⎝ ⎜ ⎞ α⎟ + 082 .r ⎠ 12( + α )⎛ a⎜⎝ rm o ib2⎞⎟⎠2(E4-2)⎛ Kl ⎞⎜ ⎟ = column slenderness of built-up member acting as a unit⎝ r ⎠o⎛ Kl ⎞⎜ ⎟ = modified column slenderness of built-up member⎝ r ⎠ma = distance between connectors, in. (mm)r i = minimum radius of gyration of individual component, in. (mm)r ib = radius of gyration of individual component relative to its centroidal axisparallel to member axis of buckling, in. (mm) = separation ratio = h / 2r ibh = distance between centroids of individual components perpendicular tothe member axis of buckling, in. (mm)2. Detailing RequirementsAt the ends of built-up compression members bearing on base plates or milled surfaces,all components in contact with one another shall be connected by a weld havinga length not less than the maximum width of the member or by bolts spaced longitudinallynot more than four diameters apart for a distance equal to 1 1 2 times themaximum width of the member.Along the length of built-up compression members between the end connectionsrequired above, longitudinal spacing for intermittent welds, bolts, or rivets shall beadequate to provide for the transfer of the required forces. For limitations on thelongitudinal spacing of connectors between elements in continuous contact consistingof a plate and a shape or two plates, see Section J3.5. Where a component ofa built-up compression member consists of an outside plate, the maximum spacingshall not exceed the thickness of the thinner outside plate times 075 . E/F y , nor 12in. (305 mm), when intermittent welds are provided along the edges of the componentsor when fasteners are provided on all gage lines at each section. When fastenersare staggered, the maximum spacing on each gage line shall not exceed thethickness of the thinner outside plate times112 . E/F y nor 18 in. (460 mm).Individual components of compression members composed of two or more shapesshall be connected to one another at intervals, a, such that the effective slendernessratio Ka / r i of each of the component shapes, between the connectors, does notexceed three-fourths times the governing slenderness ratio of the built-up member.The least radius of gyration r i shall be used in computing the slenderness ratio ofeach component part. The end connection shall be welded or fully tensioned boltedwith clean mill scale or blast-cleaned faying surfaces with Class A coatings.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION