These are proportioned so that their gross and net areas are large enough to resist imposed loads. The criteria for determining the net area of a tension member with bolt holes is the same for allowable stress design and load-and-resistance-factor design. In determination of net area, the width of a bolt hole should be taken 1â„16 in larger than the nominal dimension of the hole normal to the direction of applied stress. Although the gross section for a tension member without holes should be taken normal to the direction of applied stress, the net section for a tension member with holes should be chosen as the one with the smallest area that passes through any chain of holes across the width of the member. Thus, the net section may pass through a chain of holes lying in a plane normal to the direction of applied stress or through holes along a diagonal of zigzag line.
Net section for a member with a chain of holes extending along a diagonal or zigzag line is the product of the net width and thickness. To determine net width, deduct from the gross width the sum of the diameters of all the holes in the chain, then add, for each gage space in the chain, the quantity
s^2 /4g
where s longitudinal spacing (pitch, in) of any two consecutive holes and g transverse spacing (gage, in) of the same two holes.
The critical net section of the member is obtained from that chain with the least net width.
When a member axially stressed in tension is subjected to nonuniform transfer of load because of connections through bolts to only some of the elements of the cross section, as in the case of a W, M, or S shape connected solely by bolts through the flanges, the net area should be reduced as follows: 10% if the flange width is at least two-thirds the beam depth and at least three fasteners lie along the line of stress; 10% also for structural tees cut from such shapes; 15% for any of the preceding shapes that do not meet those criteria and for other shapes that have at least three fasteners in line of stress; and 25% for all members with only two fasteners in the line of stress.
ASD of Tension Members
Unit tensile stress Ft on the gross area should not exceed 0.60Fy, where Fy is the minimum yield stress of the steel member (see Table 7.11). Nor should Ft exceed 0.50Fu, where Fu is the minimum tensile strength of the steel member, when the allowable stress is applied to the net area of a member connected with fasteners requiring holes. However, if the fastener is a large pin, as used to connect eyebars, pin plates, etc., Ft is limited to 0.45Fy on the net area. Therefore, for the popular A36 steel, the allowable tension stresses for gross and net areas are 22.0 and 29.0 ksi, respectively, and in the case of pin plates, 16.2 ksi.
LRFD of Tension Members
Design tensile strength Pn, kips, of the gross area Ag, in2, should not exceed 0.90Fy, where Fy is the minimum yield stress of the steel (Table 7.9) and Pn AgFy. Nor should the design tensile strength Pn, kips, exceed 0.75Fu on the net area Ae, in2, of the member. Other criteria control the design tensile strength of pinconnected members. (Refer to the AISC specification for LRFD.)
