Criteria for Built-Up Compression Members

Compression members should be designed so that main components are connected directly to gusset plates, pins, or other members. Stresses should not exceed the allowable for the gross section. The radius of gyration and the effective area of a member with perforated cover plates should be computed for a transverse section through the maximum width of perforation. When perforations are staggered in opposite cover plates, the effective areashould be considered the same as for a section with perforations in the same transverse plane.
Solid-Rib Arches. A compression member and all its components must be proportioned to meet the requirements for maximum slenderness ratio in Table 11.24. The member also must satisfy width-thickness requirements (Table 11.25). In addition, for solid-rib arches, longitudinal stiffeners are required when the depth-thickness ratio of each web exceeds

Stitching. In built-up members, welds connecting plates in contact should be continuous.
Spacing of fasteners should be the smaller of that required for sealing, to prevent penetration of moisture (Art. 5.11), or stitching, to ensure uniform action and prevent local buckling.
The pitch of stitch fasteners on any single line in the direction of stress should not exceed 12t, where t  thickness, in, of the thinner outside plate or shape. If there are two or more lines of fasteners with staggered pattern, and the gage g, in, between the line under consideration and the farther adjacent line is less than 24t, the staggered pitch in the two lines, considered together, should not exceed 12t or 15t  3g/8. The gage between adjacent lines of stitch fasteners should not exceed 24t.

Fastener Pitch at Ends. Pitch of fasteners connecting components of a compression member over a length equal to 1.5 times the maximum width of member should not exceed 4 times the fastener diameter. The pitch should be increased gradually over an equal distance farther from the end.
Shear. On the open sides of compression members, components should be connected with perforated plates or by lacing bars and end stay plates. The shear normal to the member in the planes of the plates or bars should be assumed equally divided between the parallel planes. The shearing force should include that due to the weight of the member, other external forces, and a normal shearing force, kips, given by

where P  allowable compressive axial load on member, kips
L  length of member, in
r  radius of gyration, in, of section about axis normal to plane of lacing or perforated plate
Perforated Plates. When perforated cover plates are used, the openings should be ovaloid or elliptical (minimum radius of periphery 11⁄2 in). Length of perforation should not exceed twice its width. Clear distance between perforations in the direction of stress should not be less than the distance l between the nearer lines of connections of the plate to the member.
The clear distance between the end perforation and end of the cover plate should be at least 1.25l. For plates groove-welded to the flange edge of rolled components, l may be taken as the distance between welds when the width-thickness ratio of the flange projection is less than 7; otherwise, the distance l should be taken between the roots of the flanges. Thickness should meet the requirements for perforated plates given in Table 11.25.


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