Pitch is the distance (in) along the line of principal stress between centers of adjacent fasteners.
It may be measured along one or more lines of fasteners. For example, suppose bolts are staggered along two parallel lines. The pitch may be given as the distance between successive bolts in each line separately. Or it may be given as the distance, measured parallel to the fastener lines, between a bolt in one line and the nearest bolt in the other line.
Gage is the distance (in) between adjacent lines of fasteners along which pitch is measured or the distance (in) from the back of an angle or other shape to the first line of fasteners.
The minimum distance between centers of fasteners should be at least three times the fastener diameter. (The AISC specification, however, permits 22â„3 times the fastener diameter.)
Limitations also are set on maximum spacing of fasteners, for several reasons. In builtup members, stitch fasteners, with restricted spacings, are used between components to ensure uniform action. Also, in compression members, such fasteners are required to prevent local buckling. In bridges, sealing fasteners must be closely spaced to seal the edges of
plates and shapes in contact to prevent penetration of moisture. Maximum spacing of fasteners is governed by the requirements for sealing or stitching, whichever requires the smaller spacing.
For sealing, AASHTO specifications require that the pitch of fasteners on a single line adjoining a free edge of an outside plate or shape should not exceed 7 in or 4 4t in, where t is the thickness (in) of the thinner outside plate or shape (Fig. 5.9a). (See also the maximum edge distance, Art. 5.12). If there is a second line of fasteners uniformly staggered with those in the line near the free edge, a smaller pitch for the two lines can be used if the gage g (in) for these lines is less than 11â„2 4t. In this case, the staggered pitch (in) should not exceed 4 4t 3â„4g or 7 in but need not be less than half the requirement for a single line (Fig. 5.9b). See AASHTO specifications for requirements for stitch fasteners.
Bolted joints in unpainted weathering steel require special limitations on pitch: 14 times the thickness of the thinnest part, not to exceed 7 in (AISC specification).
