Section 1811A Prestressed Rock and Soil Foundation Anchors
The requirements of this section address the use of vertical rock and soil anchors in resisting seismic or wind overturning forces resulting in tension on shallow foundations.
Geotechnical report for the prestressed rock and soil foundation anchors shall address the following:
- Minimum diameter and minimum spacing for the anchors including consideration of group effects.
- Maximum unbonded length and minimum bonded length of the tendon.
- Maximum recommended anchor tension capacity based upon the soil or rock strength/grout bond and anchor depth/spacing.
- Allowable bond stress at the ground/grout interface and applicable factor of safety for ultimate bond stress.
- Anchor axial tension stiffness recommendations at the anticipated anchor axial tension displacements, when required for structural analysis.
- Minimum grout pressure for installation and post-grout pressure.
- Class I Corrosion Protection is required for all permanent anchors. Geotechnical report shall specify the corrosion protection recommendations for temporary anchors.
- Performance test shall be at a minimum of 1.6 times the design loads. There shall be a minimum of two preproduction test anchors. Preproduction test anchors shall be tested to ultimate load or 0.80 times the specified minimum tensile strength of the tendon. A creep test is required for all prestressed anchors with greater than 10 kips of lock-off prestressing load.
- Lock-off prestressing load requirements.
- Acceptable drilling methods.
- Geotechnical observation and monitoring requirements.
- Tendons shall be thread-bar anchors conforming to ASTM A722.
- The anchors shall be placed vertical.
- Design loads shall be based upon the load combinations in Section 1605A.3.1 and shall not exceed 60 percent of the specified minimum tensile strength of the tendons.
- Ultimate load shall be based upon Section 1616A.1.16 and shall not exceed 80 percent of the specified minimum tensile strength of the tendons.
- The anchor shall be designed to fail in grout bond to the soil or rock before pullout of the soil wedge by group effect.
- Foundation design shall incorporate the effect of lock-off loads.
- Design shall account for as-built locations of soil anchors considering all the acceptable construction tolerances.
- Design shall account for both short and long term deformation.
- Enforcement agency may require consideration of anchor deformation in evaluating deformation compatibility or building drift where it may be significant.