S
l
z d 0
n
= elastic section modulus of anchor bolt at concrete surface ( a uniform cross section is assumed )
= internal lever arm adjusted for spalling of the surface concrete as follows :
= z + ( n ⋅ d o
)
= distance from center of base plate to surface of concrete ( standoff distance )
= anchor outside diameter at concrete surface
= 0 , for static loading with clamping at the concrete surface as provided by a nut and washer assembly ( required for mechanical anchors );
= 0.5 , for static loading without clamping at the concrete surface , e . g ., adhesive anchor without nut and washer at concrete surface
3.1.10 TORQUING AND PRETENSIONING OF ANCHORS
Application of torque is intended to induce a tension force in the anchor bolt . It is therefore important that the torquetension relationship associated with the anchor nut , washer and threaded anchor element be maintained as close to factory conditions as possible during anchor installation . This is best accomplished by keeping the anchor assembly in its packaging to prevent undue contamination with dust , oil , etc . prior to anchor installation . Note that damage to anchor threads as caused by attempts to re-straighten an anchor after installation , hammer impacts , etc ., can significantly alter the torque-tension relationship and result in improper anchor function under load , including failure . Likewise , application of lubricants to the threads may generate excessive pretension loads in the anchor during torquing , which can also result in failure .
There are three possible reasons to apply torque to an anchor bolt in concrete or masonry :
Determination of bolt bending — ASD
Note that stand-off installations of post-installed mechanical anchors require a nut and bearing washer at the concrete surface as shown above for proper anchor function .
INCREASE IN CAPACITY FOR SHORT-TERM LOADING — ASD
Some building codes allow a capacity ( stress ) increase of 1 / 3 when designing for short-term loading such as wind or seismic . The origin of the 1 / 3 increase is unclear as it relates to anchor design , but it is generally assumed to address two separate issues : 1 ) strain-rate effects , whereby the resistance of some materials is increased for transitory stress peaks , and 2 ) the lower probability of permanent and transitory loads occurring simultaneously .
While Hilti does not include the 1 / 3 increase in published capacities for anchors in concrete , it is the responsibility of the designer to determine the appropriateness of such a capacity increase under the applicable code .
1 . To produce a clamping force , therefore eliminating gaps and play within the connected parts . Note that this clamping force is not assumed to be sufficient to permit the shear resistance of the anchorage to be determined on the basis of baseplate friction ( i . e ., as a slip-critical condition ) owing to the relaxation of clamping forces over time .
2 . To produce a pretension force in the anchor bolt which is resisted by a corresponding pre-compression in the base material ( concrete or masonry ). Pretension force serves to reduce anchor displacements under service load and may also serve to reduce the fatigue effects of cyclic loading .
3 . To verify the anchorage will hold the tensile preload generated by the recommended torque . This helps reduce the likelihood of a grossly misinstalled anchor and / or completely unsuitable base material .
Anchor pretensioning forces dissipate over time due to relaxation in the concrete and , to a lesser degree , in the bolt threads . Re-torquing anchors can result in a higher level of residual prestress .
Anchor pretensioning should not be counted on for cases where cracking of the concrete may occur ( i . e ., earthquake loading ).
Anchor Fastening Technical Guide Edition 22 | 3.0 ANCHORING SYSTEMS | 3.1 ANCHOR PRINCIPLES AND DESIGN Hilti , Inc . 1-800-879-8000 | en español 1-800-879-5000 | www . hilti . com | Hilti ( Canada ) Corporation | www . hilti . ca | 1-800-363-4458
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