B4Member properties

B4.1 Classification of Sections for Local Buckling

For members subjected to axial compression, sections are classified as nonslender-element or slender-element sections. For a nonslender-element section, the width-to-thickness ratios of its compression elements shall not exceed $\lambda_{r}$ from Table B4.1a. If the width-to-thickness ratio of any compression element exceeds $\lambda_{r}$, the section is a slender-element section.

For members subjected to flexure, sections are classified as compact, noncompact, or slender-element sections. For all sections addressed in Table B4.1b, flanges must be continuously connected to the web or webs. For a section to qualify as compact, the width-to-thickness ratios of its compression elements shall not exceed the limiting width-to-thickness ratios, $\lambda_{p}$, from Table B4.1b. If the width-to-thickness ratio of

one or more compression elements exceeds $\lambda_{p}$, but does not exceed $\lambda_{r}$ from Table B4.1b, the section is noncompact. If the width-to-thickness ratio of any compression element exceeds $\lambda_{r}$, the section is a slender-element section.

For cases where the web and flange are not continuously attached, consideration of element slenderness shall account for the unattached length of the elements and the appropriate plate buckling boundary conditions.

User Note: The Commentary discusses element slenderness when the web and flange are not continuously attached.

B4.1a Unstiffened Elements

For unstiffened elements supported along only one edge parallel to the direction of the compression force, the width shall be taken as follows:

• (a) For flanges of I-shaped members and tees, the width, $b$, is one-half the full-flange width, $b_{f}$.
• (b) For legs of angles and flanges of channels and zees, the width, $b$, is the full leg or flange width.
• (c) For plates, the width, $b$, is the distance from the free edge to the first row of fasteners or line of welds.
• (d) For stems of tees, $d$ is the full depth of the section.

User Note: Refer to Table B4.1 for the graphic representation of unstiffened element dimensions.

B4.1b Stiffened Elements

For stiffened elements supported along two edges parallel to the direction of the compression force, the width shall be taken as follows:

• (a) For webs of rolled sections, $h$ is the clear distance between flanges less the fillet at each flange; $h_{c}$ is twice the distance from the centroid to the inside face of the compression flange less the fillet or corner radius.
• (b) For webs of built-up sections, $h$ is the distance between adjacent lines of fasteners or the clear distance between flanges when welds are used, and $h_{c}$ is twice the distance from the centroid to the nearest line of fasteners at the compression flange or the inside face of the compression flange when welds are used; $h_{p}$ is twice the distance from the plastic neutral axis to the nearest line of fasteners at the compression flange or the inside face of the compression flange when welds are used.
• (c) For flange plates in built-up sections, the width, $b$, is the distance between adjacent lines of fasteners or lines of welds.

B4.2 Design Wall Thickness for HSS

The design wall thickness, $t$, shall be used in calculations involving the wall thickness of hollow structural sections (HSS). The design wall thickness, $t$, shall be taken equal to the nominal thickness for box sections and HSS produced according to ASTM A1065/A1065M or ASTM A1085/A1085M. For HSS produced according to other standards approved for use under this Specification, the design wall thickness, $t$, shall be taken equal to 0.93 times the nominal wall thickness.

User Note: A pipe can be designed using the provisions of this Specification for round HSS as long as the pipe conforms to ASTM A53/A53M Grade B and the appropriate limitations of this Specification are used.

B4.3 Gross and Net Area Determination

B4.3a Gross Area

The gross area, $A_{g}$, of a member is the total cross-sectional area.

B4.3b Net Area

The net area, $A_{n}$, of a member is the sum of the products of the thickness and the net width of each element computed as follows:

In computing net area for tension and shear, the width of a bolt hole shall be taken as 1/16 in. (2 mm) greater than the nominal dimension of the hole.

For a chain of bolt holes extending across a part in any diagonal or zigzag line, the net width of the part shall be obtained by deducting from the gross width the sum of the diameters or slot dimensions as provided in this section, of all holes in the chain, and adding, for each gage space in the chain, the quantity s²/4g,

where

• g = transverse center-to-center spacing (gage) between fastener gage lines, in. (mm)
• s = longitudinal center-to-center spacing (pitch) of any two consecutive bolt holes, in. (mm)

For angles, the gage for bolt holes in opposite adjacent legs shall be the sum of the gages from the back of the angles less the thickness.

For slotted HSS welded to a gusset plate, the net area, $A_{n}$, is the gross area minus the product of the thickness and the total width of material that is removed to form the slot.

In determining the net area across plug or slot welds, the weld metal shall not be considered as adding to the net area.

For members without holes, the net area, $A_{n}$, is equal to the gross area, $A_{g}$.