Design of built-up compression members should comply with the basic requirement for prevention of local and overall buckling of compression members as summarized in Arts. To ensure, however, that individual components, such as plates and shapes, of a built-up member act together, the AISC ASD and LRFD specifications for structural steel buildings emphasize proper interconnection of the components. Many of the AISC requirements are the same for ASD and LRFD. The ASD specification, however, requires that all connections be welded or made with fully tightened, high-strength bolts.
Connections at Ends. For built-up columns bearing on base plates or milled surfaces, components in contact at or near the ends should be connected with rivets, bolts, or welds.
Bolts should be placed parallel to the axis of the member not more than four diameters apart for a distance of at least 11⁄2 times the maximum width of the member. The weld should be continuous and at least as long as the maximum width of the member.
Intermediate Connections. Between the end connections of built-up compression members, the longitudinal spacing of welds or bolts should be adequate to transfer applied forces.
Along an outside plate, when welds are used along the plate edges or when bolts are provided at all adjacent gage lines (not staggered) at each section, the maximum spacing should not exceed 12 in or 127t /F , where t is the thickness of the thinner outside plate and F is the y specified minimum yield stress of the steel. When bolts are staggered, the maximum spacing along each gage line should not exceed 18 in or 190tF . y For two rolled shapes in contact, the maximum longitudinal spacing of bolts or intermittent welds should not exceed 24 in. If two or more rolled shapes in compression members are separated by intermittent fillers, the shapes should be connected at the fillers by at least two intermediate connectors at intervals that limit local buckling. Accordingly, the AISC specifications restrict the slenderness ratio of each shape KL/ r, where L is the connector spacing, to a maximum of three-fourths the governing slenderness ratio of the built-up member.
The least radius of gyration r should be used in computation of the slenderness ratio of each component.
Components of a built-up compression member that has open sides may be tied together with lacing or perforated cover plates. Lacing may consist of flat bars, angles, channels, or other shapes inclined at an angle to the axis of the member of at least 60 for single lacing and 45 for double lacing. The lacing should terminate at tie plates that are connected across the ends of the member and that are at least as long as the distance s between the lines of connectors between tie plates and member components. Tie plates also should be installed at intermediate points where lacing is interrupted and should be at least s /2 long. Thickness of tie plates should be at least s / 50. A tie plate should be connected to each component by at least three bolts or by welds with a length of at least one-third that of the tie plate. Spacing of bolts in tie plates should not exceed six diameters.
Lacing should be capable of resisting a shear force normal to the axis of the built-up member equal to 2% of the total compressive stress in the member in ASD (compressive design strength in LRFD). Spacing of lacing should limit the L/ r of the flange, where L is the distance along the flange between its connections to the lacing, to a maximum of threefourths the governing slenderness ratio of the built-up member. Maximum permissible slenderness ratio for single lacing is 140, and for double lacing, 200. For lacing in compression, the unsupported length should be taken as the distance between lacing connections to the built-up member for single lacing and 70% of that for double lacing. When the distance between the lines of connectors across the open sides of the built-up member exceeds 15 in, angles or double lacing, joined at intersections, should be used.
Continuous cover plates perforated with access holes are an alternative to lacing. The plates should be designed to resist axial stress and local buckling . Length of holes in the direction of stress should not exceed twice the width, and the holes should have a minimum radius of 11⁄2 in. The clear distance between holes in the direction of stress should be at least the transverse distance between the nearest lines of connecting bolts or welds.
For built-up members made of weathering steels that will be exposed unpainted to atmospheric corrosion, designers should take precautions to ensure that corrosion is not accelerated by moisture entrapped between faying surfaces. Hence components should be held in tight contact. Spacing of fasteners between a plate and a shape or between two plate components in contact should not exceed 14 times the thickness of the thinnest part nor 7 in. Maximum edge distance should not exceed 8 times the thickness of the thinnest part nor 5 in.
Strength of Built-Up Compression Members. For ASD, the equations for allowable stresses for axially loaded compression members given in Art. 6.16.2 also may be used for built-up compression members.
If the buckling mode of a built-up member involves relative deformation or slip between the components that induces shear in the connections between components, resistance to buckling may be lowered. The AISC LRFD specification requires that the design strength of a built-up compression member composed of two or more shapes be computed from Eqs. (6.39) to (6.40b) with the slenderness ratio KL/ r in c replaced by a modified slenderness ratio (KL/ r)m given by Eqs. (6.95) and (6.96). When intermediate connectors are snug-tight bolted,