Building Design and Construction Handbook - Merritt - Ventech!

7.100 SECTION SEVEN
axis equals the sum of the tensile forces on the fasteners above the axis. Then, with
these assumptions, the tensile force on the fastener farthest from the neutral axis is
where d � distance of each fastener from the neutral axis
d max � distance from neutral axis of farthest fastener
A � nominal area of each fastener
dmaxPl2 ƒt � (7.80)
2 �Ad
The maximum resultant stresses ƒ t and ƒ v � P/n are then plotted as an ellipse
and R is determined graphically. The allowable stress is given as the tensile stress
F t as a function of the computer shear stress ƒ v. (In Tables 7.24 and 7.26, allowable
stresses are given for the ellipse approximated by three straight lines.)
Note that the tensile stress of the applied load is not additive to the internal
tension (pretension) generated in the fastener on installation. On the other hand,
the AISC Specification does require the addition to the applied load of tensile
stresses resulting from prying action, depending on the relative stiffness of fasteners
and connection material. Prying force Q (Fig. 7.42b) may vary from negligible to
a substantial part of the total tension in the fastener. A method for computing this
force is given in the AISC Manual.
The old method for checking the bending strength of connection material ignored
the effect of prying action. It simply assumed bending moment equal to P/n times
e (Fig. 7.42). This procedure may be used for noncritical applications.
7.33 SLIP-CRITICAL BOLTED CONNECTIONS
Design of this type of connection assumes that the fastener, under high initial
tensioning, develops frictional resistance between the connected parts, preventing
slippage despite external load. Properly installed A307 bolts provide some friction,
but since it is not dependable it is ignored. High-strength steel bolts tightened nearly
to their yield strengths, however, develop substantial, reliable friction. No slippage
will occur at design loads if the contact surfaces are clean and free of paint or have
only scored galvanized coatings, inorganic zinc-rich paint, or metallized zinc or
aluminum coatings.
The AISC ‘‘Specification for Structural Steel for Buildings,’’ ASD and LRFD,
lists allowable shear for high-strength bolts in slip-critical connections. Though
there actually is not shear on the bolt shank, the shear concept is convenient for
measuring bolt capacity.
Since most joints in building construction can tolerate tiny slippage, bearingtype
joints, which are allowed much higher shears for the same high-strength bolts
when the threads are not in shear planes, may, for reasons of economy, lessen the
use of slip-critical joints.
The capacity of a slip-critical connection does not depend on the bearing of the
bolts against the sides of their holes. Hence, general specification requirements for
protection against high bearing stresses or bending in the bolts may be ignored.
If the fasteners B in Fig. 7.41b are in a slip-critical connection, the bolts above
the neutral axis will lose part of their clamping force; but this is offset by a compressive
force below the neutral axis. Consequently, there is no overall loss in
frictional resistance to slippage.