A tension member and all its components must be proportioned to meet the requirements for maximum slenderness ratio given in Table 11.24. The member also must be designed to ensure that the allowable tensile stress on the net section is not exceeded.
The net section of a high-strength-bolted tension member is the sum of the net sections of its components. The net section of a component is the product of its thickness and net width.
Net width is the minimum width normal to the stress minus an allowance for holes. The diameter of a hole for a fastener should be taken as 1â„8 in greater than the nominal fastener diameter. The chain of holes that is critical is the one that requires the largest deduction for holes and may lie on a straight line or in a zigzag pattern. The deduction for any chain of holes equals the sum of the diameters of all the holes in the chain less, for each gage space in the chain, s2/4g, where s is the pitch, in, of any two successive holes and g is the gage, in, of those holes.
For angles, the gross width should be taken as the sum of the widths of the legs less the thickness. The gage for holes in opposite legs is the sum of the gages from back of angle less the thickness. If a double angle or tee is connected with the angles or flanges back to back on opposite sides of a gusset plate, the full net section may be considered effective.
But if double angles, or a single angle or tee, are connected on the same side of a gusset plate, the effective area should be taken as the net section of the connected leg or flange plus one-half the area of the outstanding leg. When angles connect to separate gusset plates, as in a double-webbed truss, and the angles are interconnected close to the gussets, for example, with stay plates, the full net area may be considered effective. Without such interconnection, only 80% of the net area may be taken as effective.
For built-up tension members with perforated plates, the net section of the plate through the perforation may be considered the effective area.
In pin-connected tension members other than eyebars, the net section across the pinhole should be at least 140%, and the net section back of the pinhole at least 100% of the required net section of the body of the member. The ratio of the net width, through the pinhole normal to the axis of the member, to thickness should be 8 or less. Flanges not bearing on the pin should not be considered in the net section across the pin.
To meet stress requirements, the section at pinholes may have to be reinforced with plates.
These should be arranged to keep eccentricity to a minimum. One plate on each side should be as wide as the outstanding flanges will allow. At least one full-width plate on each segment should extend to the far side of the stay plate and the others at least 6 in beyond the near edge. These plates should be connected with fasteners or welds arranged to distribute the bearing pressure uniformly over the full section.
Eyebars should have constant thickness, no reinforcement at pinholes. Thickness should be between 1â„2 and 2 in, but not less than 1â„8 the width. The section across the center of the pinhole should be at least 135%, and the net section back of the pinhole at least 75% of the required net section of the body of the bar. The width of the body should not exceed the pin diameter divided by 3â„4 + Fy /400, where Fy is the steel yield strength, ksi. The radius of transition between head and body of eyebar should be equal to or greater than the width of the head through the center of the pinhole.
Eyebars of a set should be symmetrical about the central plane of the truss and as nearly parallel and close together as practicable. But adjacent bars in the same panel should be at least 1â„2 in apart. The bars should be held against lateral movement.
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. The pitch of stitch fasteners on any single line in the direction of stress should not exceed 24t, 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 24t or 30t + 3g/4. The gage between adjacent lines of stitch fasteners should not exceed 24t.
Cover Plates. When main components of a tension member are tied together with cover plates, the shear normal to the member in the planes of the plates should be assumed equally divided between the parallel plates. The shearing force should include that due to the weight of the member plus other external forces.
When perforated cover plates are used, the openings should be ovaloid or elliptical (minimum radius of periphery 11â„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 of a perforated plate should be at least 1â„50 of the distance between nearer lines of connection.
When stay plates are used to tie components together, the clear distance between them should be 3 ft or less. Length of end stay plates between end fasteners should be at least 1.25l, and length of intermediate stay plates at least 0.563l. Thickness of stay plates should not be less than l /50 in main members and l /60 in bracing. They should be connected by at least three fasteners on each side to the other components. If a continuous fillet weld is used, it should be at least 5â„16 in.
Tension-member components also may be tied together with end stay plates and lacing bars like compression members. The last fastener in the stay plates preferably should also pass through the end of the adjacent bar.
