3.1.4 ANCHOR BEHAVIOR UNDER LOAD
When loaded in tension to failure , anchors may exhibit one or more identifiable failure modes . These include :
• steel failure in tension
• anchor pullout or pull-through failure
• adhesive bond failure
• concrete breakout failure
• concrete splitting failure
• side-face blowout failure
Failure modes associated with anchors loaded to failure in shear may be characterized as follows :
• steel failure in shear / tension
• concrete edge breakout failure
• pryout failure
PRESTRESSING OF ANCHORS
In general , properly installed anchors do not exhibit noticeable deflection at the expected service load levels due to the application of the prescribed installation torque . External tension loading results in a reduction of the clamping force in the connection with little increase in the corresponding bolt tension force . Shear loads are resisted by a combination of bearing and friction resulting from the anchor preload forces .
At load levels beyond the clamping load , anchor deflections increase and the response of the anchor varies according to the anchor force-resisting mechanism . Expansion anchors capable of follow-up expansion show increased deflections corresponding to relative movement of the cone and expansion elements . Adhesive anchors exhibit a change in stiffness corresponding to loss of adhesion between the adhesive and the base material whereby tension resistance at increasing displacement levels is provided by friction between the uneven hole wall and the adhesive plug . In all cases , increasing stress levels in the anchor bolt / element result in increased anchor displacements .
LONG TERM BEHAVIOR
Following are some factors that can influence the long-term behavior of post-installed anchoring systems . Adhesive anchoring systems :
• Pretensioning relaxation
• Chemical resistance / durability
• Creep
• Freeze / thaw conditions
• High temperature
Mechanical anchoring systems :
• Pretensioning relaxation
• Fatigue
• Concrete cracking
• Fatigue
• Concrete cracking
• Corrosion
• Fire
• Seismic loading
• Corrosion
• Fire
• Seismic Loading
All Hilti adhesive anchor systems suitable for use with the Strength Design Method have been tested for sustained loading conditions as per ACI 355.4 and ICC-ES Acceptance Criteria AC308 .
3.1.5 ANCHOR DESIGN
The design of anchors is based on an assessment of the loading conditions and anchorage capacity . Strength design ( SD ), limit state design ( LSD ), and allowable stress design ( ASD ) methods are currently in use in North America for the design of anchors .
Strength Design : The Strength Design Method for anchor design has been incorporated into several codes such as IBC and ACI 318 . The method assigns specific strength reduction factors to each of several possible failure modes , provides predictions for the strength associated with each failure mode , and compares the controlling strength with factored loads . The Strength Design Method is a more accurate estimate of anchor resistance as compared to the ASD approach . The Strength Design Method , as incorporated in ACI 318 Chapter 17 , is discussed in Section 3.1.6 . Strength Design is state-of-the-art and Hilti recommends its use where applicable .
Limit State Design : The limit state design method for anchor design is described and included in the CSA A23.3 Annex D . In principle , the method follows the strength design concept with the application of different strength reduction factors . The limit states design method generally results in a more accurate estimate of anchor resistance as compared to the ASD approach . This approach is discussed further in 3.1.7 .
Allowable loads : Under the Allowable Stress Design Method , the allowable load , or resistance , is based on the application of a safety factor to the mean result of laboratory testing to failure , regardless of the controlling failure mode observed in the tests . The safety factor is intended to account for reasonably expected variations in loading . Adjustments for anchor spacing and edge distance are developed as individual factors based on testing of two- and four-anchor groups and single anchors near free edges . These factors are multiplied together for specific anchor layouts . This approach is discussed further in section 3.1.9 . Allowable Stress Design is typically used today for masonry applications .
3.1.6 ACI 318 Chapter 17 Strength Design — SD ( LRFD )
Strength Design of anchors is referenced in the provisions of ACI 355.2 , ACI 355.4 , ACI 318 Chapter 17 and the ICC- ES Acceptance Criteria AC193 for mechanical anchors and AC308 for adhesive anchors .
STRENGTH DESIGN ( SD ) TERMINOLOGY
Terminology used in the strength design provisions is consistent with the terminology of ACI 318 Chapter 2 .
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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