Anchor LoadsThere are 2 basic methods of calculating potential anchor holding forces, one by measuring the shear force of the screw in the soil and the other by calculating the 'holding cone' of the soil around the anchor. With both of these methods it is clear that if the depth of the screw anchor is doubled, there is a much greater ratio of holding force than double. By doubling the depth of the anchor it is posible to increase the holding down force by as much as a factor of 6.
The main consideration when specifying anchors is the holding capability of the ground being used. Many factors affect the ultimate holding force of the anchor, specifically the type of soil; loose sandy soil will displace much more than firm clay or dense soil. Waterlogged soil can affect the holding ability of the anchor by reducing the effectiveness by as much as half or more. With the wide variety of ground and soil conditions the variables multilpy, which is not acceptable for structural systems which rely on a firm base to support or anchor them. The only way to confirm the holding capability is to perform in situ tests to prove the installation.