2.0 SHEAR
2.1 Concrete Breakout Failure Mode
Variables ψ c , V
Variables ACI 318-19 Chapter 17 Provision Comments for PROFIS Engineering
ψ c , V
17.7.2 . 5.1 Modification factor for the influence of cracking in anchor regions at service load levels and presence or absence of supplementary reinforcement , ψ c , V
, shall be determined as follows :
( a ) For anchors located in a region of a concrete member where analysis indicates no cracking at service load levels , ψ c , V shall be permitted to be 1.4 .
( b ) For anchors located in a region of a concrete member where analysis indicates cracking at service load levels , ψ c , V shall be in accordance with Table 17.7.2.5.1 .
Table 17.7.2.5.1 — Modification factor where analysis indicates cracking at service load levels , ψ c , V
Conditions
Anchors without supplementary reinforcement or with edge reinforcement smaller than a No . 4 bar
Anchors with reinforcement of at least a No . 4 bar or greater between the anchor and the edge
Anchors with reinforcement of at least a No . 4 bar or greater between the anchor and the edge , and with the reinforcement enclosed within stirrups spaced at not more than 4 in . ψ c , V
1.0
1.2
1.4 ψ c , V
, is a modification factor for cracked or uncracked concrete conditions . Concrete cracks when tensile stresses in the concrete imposed by loads or restraint conditions exceed its tensile strength . Concrete is typically assumed to crack under service load conditions . ACI 318 anchoring-to-concrete provisions assume cracked concrete as the baseline condition for designing cast-in-place and post-installed anchors , since cracks in the anchor vicinity can result in a reduced ultimate load capacity and increased displacement at ultimate load , compared to uncracked concrete conditions . Uncracked concrete conditions can be assumed if it can be shown that cracking of the concrete at service load levels will not occur over the anchor service life . PROFIS Engineering defaults to cracked concrete conditions , and the default ψ c , V
-value for this condition equals 1.0 . Uncracked concrete conditions are accounted for by increasing ψ c , V to a value of 1.4 .
When cracked concrete conditions are assumed , ACI 318 anchoring-to-concrete provisions also permit ψ c , V to be increased if supplementary edge reinforcement is present in the concrete member . If cracked concrete conditions exist , and existing reinforcement of # 4 bars or greater is present at the fixed edge where concrete breakout is being considered , ACI 318-19 Table 17.7.2.5.1 permits ψ c , V to be increased to a value of 1.2 . If cracked concrete conditions exist , and the existing edge reinforcement in a concrete member consists of # 4 bars or greater enclosed by a stirrup , Table 17.7.2.5.1 permits ψ c , V to be increased to a value of 1.4 . The figure below illustrates how ψ c , V can be assumed to equal 1.2 or 1.4 for cracked concrete conditions .
Use of additional , i . e . “ supplementary ” edge reinforcement for shear calculations permits an increased strength reduction factor ( ϕ-factor ) to be used when calculating design concrete breakout strength ( ϕ Vcb or ϕ Vcbg
).
Reference the Results section of the report for more information on the strength reduction factor ϕ concrete
.
181 NORTH AMERICAN PROFIS ENGINEERING ANCHORING TO CONCRETE DESIGN GUIDE — ACI 318-19 Provisions