E6Built-up members

E6.1 Compressive Strength

This section applies to built-up members composed of two shapes either (a) interconnected by bolts or welds or (b) with at least one open side interconnected by perforated cover plates or lacing with tie plates. The end connection shall be welded or connected by means of pretensioned bolts with Class A or B faying surfaces.

User Note: It is acceptable to design a bolted end connection of a built-up compression member for the full compressive load with bolts in bearing and bolt design based on the shear strength; however, the bolts must be pretensioned. In built-up compression members, such as double-angle struts in trusses, a small relative slip between the elements can significantly reduce the compressive strength of the strut. Therefore, the connection between the elements at the ends of built-up members should be designed to resist slip.

The nominal compressive strength of built-up members composed of two shapes that are interconnected by bolts or welds shall be determined in accordance with Sections E3, E4, or E7, subject to the following modification. In lieu of more accurate analysis, if the buckling mode involves relative deformations that produce shear forces in the connectors between individual shapes, $L_{c} / r$ is replaced by $\left(L_{c} / r\right)_{m}$, determined as follows:

(a) For intermediate connectors that are bolted snug-tight

(E6-1)

• (b) For intermediate connectors that are welded or are connected by means of pre-tensioned bolts with Class A or B faying surfaces

(1) When $\frac{a}{r_{i}} \leq 40$ ${\left({\frac{L_{c}}{r}}\right)}_{m} = {\left({\frac{L_{c}}{r}}\right)}_{o}$ (E6-2a)

a (2) When >40 ri

$\left(\frac{L_{c}}{r}\right)_{m}=\sqrt{\left(\frac{L_{c}}{r}\right)_{o}^{2}+\left(\frac{K_{i} a}{r_{i}}\right)^{2}}$

(E6-2b)

E6.2 General Requirements

Built-up members shall meet the following requirements:

• (a) Individual components of compression members composed of two or more shapes shall be connected to one another at intervals, $a$, such that the slenderness ratio, $a / r_{i}$, of each of the component shapes between the fasteners does not exceed three-fourths times the governing slenderness ratio of the built-up member. The minimum radius of gyration, $r_{i}$, shall be used in computing the slenderness ratio of each component part.
• (b) At the ends of built-up compression members bearing on base plates or finished surfaces, all components in contact with one another shall be connected by a weld having a length not less than the maximum width of the member or by bolts spaced longitudinally not more than four diameters apart for a distance equal to 1½ times the maximum width of the member.

Along the length of built-up compression members between the end connections required in the foregoing, longitudinal spacing of intermittent welds or bolts shall be adequate to provide the required strength. For limitations on the longitudinal spacing of fasteners between elements in continuous contact consisting of a plate and a shape, or two plates, see Section J3.6. Where a component of a built-up compression member consists of an outside plate, the maximum spacing shall not exceed the thickness of the thinner outside plate times $0.75 \sqrt{E / F_{y}}$, nor 12 in. (300 mm), when intermittent welds are provided along the edges of the components or when fasteners are provided on all gage lines at each section. When fasteners are staggered, the maximum spacing of fasteners on each gage line shall not exceed the thickness of the thinner outside plate times $1.12 \sqrt{E / F_{y}}$, nor 18 in. (450 mm).

• (c) Open sides of compression members built up from plates or shapes shall be provided with continuous cover plates perforated with a succession of access openings. The unsupported width of such plates at access openings, as defined in Section B4.1, is assumed to contribute to the available strength provided the following requirements are met:
• (1) The width-to-thickness ratio shall conform to the limitations of Section B4.1.

User Note: It is conservative to use the limiting width-to-thickness ratio for Case 7 in Table B4.1a with the width, b, taken as the transverse dis- tance between the nearest lines of fasteners. The net area of the plate is taken at the widest hole. In lieu of this approach, the limiting width-to- thickness ratio may be determined through analysis.

• (2) The ratio of length (in direction of stress) to width of hole shall not exceed 2.
• (3) The clear distance between holes in the direction of stress shall be not less than the transverse distance between nearest lines of connecting fasteners or welds.
• (4) The periphery of the holes at all points shall have a minimum radius of 1½ in. (38 mm).
• (d) As an alternative to perforated cover plates, lacing with tie plates is permitted at each end and at intermediate points if the lacing is interrupted. Tie plates shall be as near the ends as practicable. In members providing available strength, the end tie plates shall have a length of not less than the distance between the lines of fasteners or welds connecting them to the components of the member. Intermediate tie plates shall have a length not less than one-half of this distance. The thickness of tie plates shall be not less than one-fiftieth of the distance between lines of welds or fasteners connecting them to the segments of the members. In welded construction, the welding on each line connecting a tie plate shall total not less than one-third the length of the plate. In bolted construction, the spacing in the direction of stress in tie plates shall be not more than six diameters and the tie plates shall be connected to each segment by at least three fasteners.
• (e) Lacing, including flat bars, angles, channels, or other shapes employed as lacing, shall be so spaced that $L / r$ of the flange element included between their connections shall not exceed three-fourths times the governing slenderness ratio for the member as a whole. Lacing shall be proportioned to provide a shearing strength normal to the axis of the member equal to $2 \%$ of the available compressive strength of the member. For lacing bars arranged in single systems, $L / r$ shall not exceed 140 . For double lacing, this ratio shall not exceed 200 . Double lacing bars shall be joined at the intersections. For lacing bars in compression, $L$ is permitted to be taken as the unsupported length of the lacing bar between welds or fasteners connecting it to the components of the built-up member for single lacing, and $70 \%$ of that distance for double lacing.

User Note: The inclination of lacing bars to the axis of the member shall pref- erably be not less than 60º for single lacing and 45° for double lacing. When the distance between the lines of welds or fasteners in the flanges is more than 15 in. (380 mm), the lacing should preferably be double or made of angles.

For additional spacing requirements, see Section J3.6.