PROFIS ENGINEERING
ACI 318-19 General Provisions for Case 1, Case 2 and Case 3
Figure 2.
Reference Figure 2 [ 4 ] above, which is taken from ACI 318-19 Fig. R17.7.2.1b. ACI 318-19 provisions for Case 1, Case 2 and Case 3 only reference two anchor rows with one anchor in each row. Understanding the general concept of these provisions is first necessary in order to understand how more complex anchor layouts can be considered with respect to concrete breakout in shear.
Determining which case is relevant for the anchorage shown in Figure 2 [ 4 ] requires checking the spacing( s) between the two anchor rows, in the direction of the applied shear load( V); versus the distance of the near anchor row, in the direction of V, to the nearest fixed edge being considered( c a1, 1). If s is greater than or equal to c a1, 1, Case 1 and Case 2 are relevant because a complete concrete breakout failure surface can develop from each row. Case 1 and Case 2 must be considered together. For design purposes, once a concrete breakout failure surface develops from the near row anchor( Case 1), all of the shear load is redistributed to the far row anchor( Case 2) because the spacing between rows is sufficient to permit formation of a complete failure surface from the far row. Therefore, in Figure 2 [ 4 ], the Case 1 shear load distribution showing one-half of the load( 0.5V) acting on the near row anchor is only relevant to design calculations for the near row anchor. Case 2 assumes concrete breakout has occurred from the near row anchor. Since s ≥ c a1, 1, all the shear load is redistributed to the far row anchor, so Case 2 design calculations are predicated on the total shear load( 1.0V) acting on the far row anchor.
If s < c a1, 1, the shear concrete breakout calculations default to Case 3 because a complete concrete breakout failure surface can only develop from the near row anchor. The total shear load( 1.0V) is assumed to act on the near row anchor.
CASE 1: two anchor rows, one anchor in each row
• Reference the Case 1 illustration in Figure 2 [ 4 ].
• The c a1-value for the anchor row nearest the fixed edge being considered equals c a1, 1.
• The spacing( s) between the near row and far row anchors in the direction of the applied shear load( V) is greater than or equal to c a1, 1.
• Since s ≥ c a1, 1, a complete concrete breakout failure surface can develop from both the near row anchor and the far row anchor. Therefore, both Case 1 and Case 2 must be checked.
• Case 1 assumes shear concrete breakout failure occurs from the near row anchor.
• The shear load resisted by the near row anchor is assumed to equally distributed between the near row and far row such that V near
row =( V total / 2 rows) = 0.5V.
• The shear nominal concrete breakout strength for the near row anchor( V cb, near row) is calculated using the distance of the near row anchor from the fixed edge( c a1, 1).
• The design concrete breakout strength for the near row anchor( ϕV cb, near row) is checked versus 0.5V.
• Case 1 is satisfied if the percent utilization defined as( 0.5V / ϕV cb, near row) ≤ 100 %.
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