Direct Fastening Technical Guide , Edition 24
3.2.1.7 BENDING MOMENTS ON POWER-ACTUATED FASTENERS IN CONCRETE
Bending moments on power-actuated fasteners installed in concrete and masonry base materials can be minimized through proper design detailing and installation practice . Proper design should include redundancy with multiple fastening points instead of single-point fastenings , in order to distribute bending moment effects among multiple fasteners . Installation of threaded rod coupler hangers on powder-actuated threaded studs should be done with the coupler run down over the stud shank and in contact with the concrete or masonry base material as depicted in the figure below . Care should be taken not to exceed the maximum torque discussed in Section 3.2.1.5 .
Although no design equations are provided for determining equivalent bending moment loads on power-actuated fasteners , recommended allowable bending moments for threaded stud fasteners are provided in Section 3.2.11 based on testing with a safety factor of 2:1 for static loading .
Relatively small bending moments can contribute to a significant reduction in the overall fastening capacity and must be checked by the design engineer .
Arrangements to reduce or prevent moment on shank :
Arrangement causing moment to act on shank :
Coupler not tight against concrete
Non-symmetric arrangement :
• Moment on fastened part
• Prying effect must be considered in determining loads acting on fastener
Coupler tight against concrete
3.2.1.8 COMBINED LOADING OF POWER-ACTUATED FASTENERS IN CONCRETE
Combined loading of power-actuated fasteners installed in concrete can be treated with exponent α = 1 unless otherwise provided for the specific application ( e . g . ceiling clips with power-actuated fasteners subjected to 45 ° loading ).
( N s / N rec
) α + ( V s / V rec
) α ≤ 1.0 where :
N s
= Applied tension load
V s
= Applied shear load
N rec
= Allowable tension load
V rec
= Allowable shear load
3.2.1.9 FASTENER CLAMPING AND NAILHEAD STAND-OFF
Power-actuated fastenings must be thought of in terms of a fastening system consisting of the power-actuated tool , cartridges , battery or gas canister energy source and the fastener itself . Not all power-actuated fastening systems can achieve adequate embedment and proper clamping of the fastened part to the base material . The installer should start with the lowest power regulation and cartridge and work up until tight clamping is achieved . During installation of the fastener , the plastic guidance washer may be removed completely , partially or remain intact . Any of these scenarios would be considered acceptable , provided the attached material is clamped tightly to the base material and the specified embedment is achieved .
Installers should never “ double-shoot ” fastenings with excessive nail head stand-off in order to drive them deeper . This can create a safety hazard and break the bond or sintering between the fastener shank and the base material .
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