Anchor Design

Anchor Capacity

Although there is no theoretical limit to the capacity of a multistrand anchor, practical considerations such as drill hole size and the availability of material handling equipment limit the size of an anchor to 91-0.6" (15.2 mm) dia. strands in most cases. Larger anchors can be manufactured, but the practicality and economics of their use should be thoroughly evaluated before they are incorporated into a design.

Stress Levels

DYWIDAG Strand Anchor Systems may be stressed to the allowable limits of the American Concrete Institute Code No. ACI 318. The maximum jacking stress (test load) for anchors shall not exceed 0.80 f_pu of the prestressing steel. The lock off load depends on the specific requirements of the project. Initial load transfer force at lock-off shall not exceed 0.70 f_pu.

Wedges shall always be seated at a load that is greater than 0.50 f_pu.
The final (working) prestress level is dependent on:

Application
Installation procedure
Stressing sequence
Rigidity of the structural system
Seating losses

Anchor Length

No theoretical length limit exists, however, considerations should be made to allow for practical drilling and material handling.

The unbonded length of an anchor is determined by the location of the failure plane and/or the location of competent ground capable of resisting the anchor force. A minimum unbonded length of 15 ft is recommended for strand anchors, so that load losses associated with seating of the wedges will not result in major decrease of prestress force.

The bond length of the anchor depends on: