PROFIS ENGINEERING
Equal Number of Anchors and Unequal Spacing
Figure 4.1.
Reference
Figure 4.1 [ 10 ]. It illustrates a hypothetical anchorage consisting of sixteen anchors. The shear load( V ua) will be assumed to act on only the first three columns( col 1, col 2 and col 3). Assume the anchor spacing in the y direction is less than 3c a1, col 1, so the anchors in each column act as a group( e. g. reference ACI 318-19 Section 17.5.1.3.1). For the design assumptions below, ACI 318 provisions for considering concrete breakout in shear could be extrapolated as follows.
• Assume the spacing in the direction of V ua between col 1 and col 2( s x12) is less than c a1, col 1.
• Since s x12 is less than c a1, col 1, the concrete failure surface for col 2 will merge with that of col 1 because a complete failure surface cannot develop from col 2.
• Case 3 is relevant for col 1 and col 2.
• Assume the spacing in the direction of the V ua between col 2 and col 3( s x23) is greater than c a1, col 1.
• Since s x23 is greater than c a1, col 1, a complete concrete failure surface can develop from col 3.
• Case 1 / Case 2 is relevant for col 1 and col 3.
• The applied shear load( V ua) is assumed to act on col 1, col 2 and col 3; however, any load acting on col 2 will be redistributed to col 1 because the concrete failure surface for col 2 is assumed to merge with that of col 1. One possible design assumption would be that the total shear load acts only on col 1. This is a conservative interpretation of ACI 318 provisions. An alternate design assumption would be that the shear load is distributed between col 1 and col 3 since a complete failure surface can develop from both columns. This is a reasonable interpretation of ACI 318 provisions. The design engineer must determine how to best interpret ACI 318 provisions for their anchoring application. See page 10 for more information.
Reference Figure 4.1( a) [ 11 ]. Since Case 3 is relevant for col 1 and col 2, the total shear load( V ua) could be assumed to act only on col 1, disregarding concrete breakout from col 3. This is a conservative assumption. The design premise being that the merging of the concrete breakout failure surface from col 2 with that of col 1 negates consideration of concrete breakout from col 3.
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