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Why does PROFIS Anchor calculate concrete breakout in shear using only the near edge distance?

Posted by Tim Freudensteinover 8 years ago
Why does PROFIS Anchor calculate concrete breakout in shear using only the near edge distance?

*Posted on behalf of a Hilti customer.

Anchor,Profis,Shearcalculations

1 Reply
Posted by Morganover 8 years ago
Hilti Verified

Dear Customer,

well this is a good question. A fixture being anchored to concrete may have holes in it that permit placement of the fixture over the anchors. These holes are typically greater than the diameter of the anchor, which results in an annular  space between the anchor and the fixture. When a shear load is applied to anchors with an annular space, it is reasonable to assume that these anchors will not all be in contact with the fixture when the load is applied. The anchors cannot then be assumed to simultaneously resist the shear load being applied. Therefore, when anchors are not rigidly attached to a fixture and a shear load acts on the fixture, PROFIS Anchor assumes the existence of an annular space, and considers only the near edge anchors as being effective in resisting the shear load prior to concrete edge breakout occurring.

PROFIS Anchor also assumes that the anchor(s) nearest to the edge of concrete must resist the full shear load when an annular space exists. PROFIS Anchor calculations assume existence of an annular space when ASTM F1554 cast-in headed bolts and Hilti post-installed anchors have been selected for design.

However, if the anchors are rigidly attached to a fixture (e.g. welded), all of the anchors can be assumed to simultaneously resist an applied shear load. For example, PROFIS Anchor considers this condition when cast-in-place AWS headed studs have been selected. If the ACI 318 anchoring-to-concrete provisions for anchors rigidly attached to a fixture are satisfied, PROFIS Anchor calculates Vb and then nominal concrete breakout in shear, assuming ca1 from the far row of anchors.

When multiple rows of anchors make up the connection, the question becomes how many of these rows are effective in resisting the shear load and how is this load distributed among each row? Do all of the rows take up the load or only some of them? How is the load distributed among the rows assumed to be taking up the shear load? Is the load distributed evenly among the rows (e.g. per ACI 318) or does it diminish as rows are located farther away from the edge? In lieu of more definitive code provisions, PROFIS Anchor design assumptions for shear concrete breakout default to the conservative assumption that only the near edge anchors are effective in resisting total shear load when an annular space exists.
Please feel free to reach out any time, if you need additional information.

All the best
Morgan