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 fpu 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 fpu.
Wedges shall always be seated at a load that is greater than 0.50 fpu.
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:
- Capacity of the ground (ground conditions)
- Hole diameter
- Installation practices
- Drilling method
- Grouting method
Post Grouting can significanly improve the holding capacities of anchors in soil and reduce the bond length.
Literature
Publications below are useful guides for selecting and designing temporary and permanant ground anchor systems.
Recommendations for Prestressed
Rock and Soil Anchors
published by the PTI (Post-Tensioning Institute)
www.post-tensioning.org
Geotechnical Engineering Circular No. 4 - Ground
Anchors and Anchored Systems
published by the Federal Highway Administration (FHWA)
www.fhwa.dot.gov